ABSTRACTS
The Role of the Biosphere in
the Climate of Tropical Regions
West Africa:
Drought frequency analysis of annual rainfall
series in central and western Sudan
Elfatih A. B. Eltahir
Abstract: Rainfall is the most important water resource in central and
western Sudan, a region affected by the recent drought in Africa. A general methodology
for studying the annual rainfall process is presented and applied to data from central and
western Sudan. It is assumed that certain time series models adequately describe the
annual rainfall process in the region. Based on this assumption, the drought frequencies
are calculated in the subregions with stationary series. The theory of runs is applied in
calculating drought frequencies using a data generation method.
Dynamics of wet and dry years in West Africa
Elfatih A. B. Eltahir and Guiling Gong
Abstract: This paper proposes a theoretical framework for
describing interannual climatic variability over West Africa. The dynamical theory of
zonally symmetrical thermally direct circulations suggests that a meridional monsoon
circulation must develop over any tropical region (off the equator) when the absolute
vorticity near the tropopause reaches a threshold value of zero. However, for a moist
atmosphere that satisfies a quasi-equilibrium balance between moist convection and the
radiative forcing, the absolute vorticity at upper-tropospheric levels is a function of
both latitude and the meridional distribution of boundary-layer entropy. Hence, the onset
of a monsoon circulation depends in a nonlinear fashion on these two factors. The theory
predicts that a flat distribution of entropy does not drive any circulation and that a
relatively large gradient of entropy should drive a strong monsoon circulation. The
location of the region of West Africa, relatively close to the equator, dictates that the
dynamics of a monsoon over that region are relatively sensitive to interannual
fluctuations in the meridional gradient of boundary-layer entropy. Here, we present
observations on entropy and wind over West Africa during the monsoon seasons of 1958 and
1960. The following observations were consistent with the proposed relationship between
boundary-layer entropy and the monsoon circulation: a large meridional gradient of
boundary-layer entropy, a healthy monsoon, and wet conditions over the Sahel region were
observed in 1958; and a nearly flat distribution of entropy, very weak circulation, and
relatively dry conditions were observed in 1960. Moreover, the proposed theoretical
relationship between the meridional gradient of boundary-layer entropy and the monsoon
circulation over West Africa is consistent with the empirical observations of sea surface
temperature anomalies (SSTAs) in the tropical Atlantic and rainfall in the Sahel region.
Theoretically, a cold (warm) SSTA in the region located south of the West African coast
should favor a large ( small ) meridional gradient of entropy, a strong (weak) monsoon
circulation, and wet (dry) conditions in the Sahel. A large body of observations confirms
that cold (warm) SSTAs off the southern coast of West Africa are associated with wet (dry)
years in the Sahel region.
Role of vegetation in sustaining large-scale
atmospheric circulations in the tropics
Elfatih A. B. Eltahir
Abstract: The focus of this paper is the role of rain forests in
large-scale atmospheric circulations. The significance of this role is investigated by
studying the response of the tropical atmosphere to a perturbation in the state of
vegetation (deforestation) over three regions: the Amazon, Congo, and Indonesia. A theory
is developed to relate tropical deforestation and thc resulting changes in the large-scale
atmospheric circulation. Field observations and numerical simulations support the argument
that tropical deforestation reduces the total net surface radiation, including terrestrial
and solar forms. However, the energy balance at the land-atmospheric boundary dictates
that for equilibrium conditions, any reduction in net surface radiation has to be balanced
by a similar reduction in the total flux of heat, including sensible and latent forms.
Since these fluxes supply heat as well as entropy from the forest into the atmospheric
boundary layer, a reduction in the total flux of heat reduces the boundary layer entropy.
In a moist atmosphere, that satisfies a quasi equilibrium between moist convection and
radiative forcing, the equilibrium temperature profile is uniquely related to the boundary
layer entropy. Under such conditions, large-scale deforestation reduces boundary layer
entropy relative to the surroundings, cools the upper troposphere, and results in
subsidence, divergent flow in the boundary layer, and weakening of the large-scale
circulation. These changes are simulated using a simple linear model of atmospheric flow.
The comparison of the model predictions with observations of atmospheric circulations over
the Amazon, Congo, and Indonesia suggests a significant role for vegetation in maintaining
large-scale atmospheric circulations in the tropics.
Sources of moisture for rainfall in west Africa
Guiling Gong and Elfatih Eltahir
Abstract: The objective of this study is to identify the sources of
moisture for rainfall in west Africa. A model of precipitation recycling is developed and
applied to the region of west Africa to obtain quantitative estimates of the moisture
contributed by local evaporation as well as the moisture contributed by the zonal and
meridional fluxes from the surrounding regions. We estimated the recycling ratio for the
entire region by specifying three subregions where evaporation is treated as the source of
moisture: west Africa, central Africa, and the tropical Atlantic Ocean. We find that
evaporation from the tropical Atlantic Ocean, west Africa, and central Africa contribute
about 23, 27, and 17% of rainfall in west Africa, respectively. Moisture fluxes from the
tropical Atlantic are almost in phase with rainfall in west Africa. However, we find that
moisture supply from central Africa is strongly regulated and limited by the westerly flow
associated with the monsoon circulation. Hence the large-scale monsoon circulation is not
only the main forcing of rainfall over west Africa, but the dynamics of this circulation
exert significant control on where the moisture comes from.
The response to deforestation and desertification in a model
of West African monsoons
Xinyu Zheng and Elfatih A.B. Eltahir
Abstract. Since Charney proposed his theory on the dynamics of deserts
and droughts in the Sahel [Charney, 1975], there has been significant scientific interest
in the interaction between vegetation and climate in this region. The essence of this
interaction is that the atmospheric circulation, and therefore rainfall, over this region
may be sensitive to changes in vegetation cover near the desert border. Here we describe
simulations of the West African monsoons with a simple zonally-symmetric model. The
results suggest that the potential impact of human induced change of land cover on
regional climate depends critically on the location of the change in vegetation cover.
That is, desertification along the border with the Sahara (e.g., in Chad, Niger, Mali and
Mauritania) leaves a relatively minor impact on monsoon circulation and regional rainfall;
deforestation along the southern coast of West Africa (e.g., in Nigeria, Ghana and Ivory
Coast) may result in complete collapse of monsoon circulation, and a significant reduction
of regional rainfall.
The role of vegetation in the dynamics
of West African monsoons
Xinyu Zheng and Elfatih A.B. Eltahir
Abstract: The focus of this paper is the role of meridional
distribution of vegetation in the dynamics of monsoons and rainfall over West Africa. We
develop a moist zonally symmetric atmospheric model coupled with a simple land surface
scheme to investigate these processes. Four primary experiments have been carried out to
examine the sensitivity of West African monsoons to perturbations in vegetation patterns.
Each perturbation experiment is identical to the control experiment except that a change
in vegetation cover is imposed for a latitudinal belt of 10° in width. The numerical
experiments demonstrate that West African monsoons and therefore rainfall depend
critically on the location of the vegetation perturbations. While the magnitude of local
rainfall is sensitive to changes in local vegetation, the location of the Inter-Tropical
Convergence Zone (ITCZ) is not sensitive to changes in the vegetation northward or
southward from the location of ITCZ in the control experiment. However, the location of
the ITCZ is sensitive to changes of the vegetation distribution in the immediate vicinity
of the location of the ITCZ in the control experiment. The modeling results indicate that
changes in vegetation cover along the border between the Sahara desert and West Africa
(desertification) have a minor impact on the simulated monsoon circulation. On the other
hand, coastal deforestation may cause the collapse of the monsoon circulation and have a
dramatic impact on the regional rainfall. The observed deforestation in West Africa is
then likely to be a significant contributor to the observed drought.
A Mechanism Relating Tropical Atlantic Spring Sea Surface
Temperature and West African Rainfall
Xinyu Zheng, Elfatih A.B. Eltahir, and Kerry Emanuel
Abstract: In this paper, we describe a mechanistic study on the
role of tropical Atlantic sea surface temperature (SST) variability in the dynamics of
West African monsoon. A hypothesis that warm spring (April-June) SST results in a wet
monsoon is explored using a moist, zonally-symmetric model. A positive spring rainfall
anomaly has been simulated over the ocean, in response to the warm SST, which then
propagates onto the land and persists two to three months, even after the SST anomaly
vanishes. While the ocean-atmosphere interaction is crucial for the initial development of
the rainfall anomaly over land, the interactions between the ocean, land, and atmosphere
are found to be important for relating tropical Atlantic spring SST to West African
rainfall. Furthermore, the positive feedback between rainfall and soil moisture is
responsible for some of the persistence in the rainfall anomaly. We present a case study
for the wettest (1994) and driest (1992) years of the 1990s so far. The observations show
that a warm spring SST anomaly in 1994 was associated
with abundant summer rainfall over West Africa. These empirical observations are
consistent with the proposed physical mechanism.
The Role of Ecosystem Dynamics in Biosphere-Atmosphere
Interaction over the Coastal Region of West Africa
Julie E. Kiang and Elfatih A.B. Eltahir
In this study, we develop a one-dimensional model of the tropics which includes two-way
interaction between the biosphere and the atmosphere, including ecosystem dynamics. The
model integrates an atmospheric model, a biospheric model and a monsoon circulation model
and is applied to coastal West Africa to test the sensitivity of the coupled system to
changes in vegetation cover. We perform three sets of simulations - one with a fixed
monsoon circulation, one with an interactive monsoon circulation, and one with modified
boundary conditions at the northern edge of the domain. Our control
simulations show that the model is able to reasonably approximate observed conditions. The
model simulates a single stable forest equilibrium in the first two sets of simulations,
those which correspond most readily with present conditions in West Africa. These
simulations indicate that the monsoon plays an important role in modulating the climate of
the region and in shaping the response of the system to vegetation changes. Changes in the
monsoon which allowed hot and dry air to penetrate into the model domain from the north
strongly modified the equilibrium climate towards drier conditions. This finding motivated
further testing of the system assuming degraded conditions to the north, which revealed
the possibility of two different equilbria - one forest and one grassland. The
existence of multiple equilibria in the biosphere-atmosphere system depends not only on
the magnitude of the vegetation-induced climate perturbation, but also on whether or
not the perturbation extends across the threshold in moisture conditions controlling
competition between trees and grasses.
Biosphere-Atmosphere
Interactions Over West Africa: 1. Development and Validation of a Coupled Dynamic Model
Guiling Wang and Elfatih A. B. Eltahir
Abstract: In this study we develop a zonally symmetric,
synchronously coupled biosphere atmosphere model including ecosystem dynamics, and apply
this model to study biosphere-atmosphere interactions in the region of West Africa.
The atmospheric model is zonally symmetric, and includes representation of atmospheric
dynamics, a radiation scheme, a moist convection scheme, a boundary layer scheme, and a
cloud parameterization scheme. The biosphereic model is the Integrated BIosphere
Simulator (IBIS), which includes representation of the water, energy, momentum, and carbon
balance, vegetation phenology, and vegetation dynamics. We modified the
representation fo canopy hydrology in IBIS to account for the impact of rainfall sub-grid
variability. The biospheric model and atmospheric model are separately tested
against observations. The synchronously coupled model is then used to simulate the
biosphere atmosphere system of West Africa. A study on the role of
biosphere-atmosphere interactions, including ecosystem dynamics in the climate variability
over West Africa using this model will be presented in a companion paper.
Biosphere-Atmosphere
Interactions Over West Africa: 2. Multiple Climate Equilibria
Guiling Wang and Elfatih A. B. Eltahir
Abstract: This paper presents both theoretical and numerical
analyses on the multiple-equilibrium nature of the regional climate system in West Africa.
Based on simple analyses on how the coupled biosphere atmosphere system responds to
vegetation perturbations within the scope of a dynamic ecosystem, we propose that the
regional climate system may have multiple equilibrium states coexisting under the same
precessional forcing. Using a synchronously coupled biosphere-atmosphere model which
includes explicit representation of ecosystem dynamics, we show that the equilibrium state
of the model is sensitive to initial vegetation distribution. This modeling result
supports the existence of multiple climate equilibria. Using the same model, further
experiments are carried out to investigate how the coupled system responds to
non-permanent vegetation perturbations. Our results demonstrate how transitions
between different climate equilbria can take place when governed by the two way
biosphere-atmosphere feedback. These findings advance our understanding regarding
teh mechanisms of climate variability over West Africa.
Modeling the Biosphere-Atmosphere System: The Impact of Sub-Grid
Variability in Rainfall Interception
Guiling Wang and Elfatih A. B. Eltahir
Abstract: Sub-grid variability in rainfall distribution has been widely recognized
as an important factor in include in the representation of land surface hydrology within
climate models. In this paper, using West Africa as a case study, we investigate how
the sub-grid variability in rainfall distribution affects the modeling of rainfall
intersection and other processes within a coupled dynamic biosphere0atmosphere
model, According to our results, even the evapotranspiration is tuned to be
consistent with observations while neglecting spatial variability of rainfall,
significant errors may result in the presentation of surface hydrological processes and
surface energy balance. However, the extent of the resulting errors ins not limited
to the surfaces processes. They extend to the atmosphere via the low-level clouds
feedback to impact solar radiation, boundary layer energy, atmospheric circulation and the
distribution of precipitation. The same errors also propagate into the biosphere
through vegetation dynamics and can eventually lead to a significantly different
biosphere-atmosphere equilibrium state. This study provides a good example for the
need to have physical realism in modeling most of the details of the complex
biosphere-atmosphere-ocean system.
The Role of Vegetation Dynamics in Enhancing the Low-Frequency Variability
of the Sahel Rainfall
Wang, G. and E. A. B. Eltahir
Rainfall observations in the Sahel region of West Africa show significant variability
at the time scale of decades. Here we explore the mechanisms of this low-frequency
variability using a coupled biosphere-atmosphere model which includes explicit
representation of vegetation dynamics. By forcing the model with the observed sea surface
temperature (SST) of the tropical Atlantic Ocean during the period 1898-1997, numerical
experiments on the climate variability of West Africa have been carried out. The results
of these experiments suggest that vegetation dynamics is a significant process in shaping
the natural variability of the Sahel rainfall. The response of the regional climate system
to large-scale forcings is significantly regulated by vegetation dynamics. The relatively
slow response of vegetation to changes in the atmosphere acts to enhance the low-frequency
rainfall variability. The regional climate system over West Africa has several climate
regimes coexisting under the current precessional forcing. Climate transitions between
different regimes act as another mechanism contributing to the low-frequency rainfall
variability. Climate persistence at one regime and climate transition towards another
collectively compose a distinct type of multi-decadal variability.
Ecosystem Dynamics and the Sahel Drought
Wang, G. and E. A. B. Eltahir
The Sahel region of Africa has been experiencing a persistent drought throughout the
last three decades. Here, we present a new perspective on the underlying physical
mechanism behind this phenomenon. We use a coupled biosphere-atmosphere model including
explicit representation of ecosystem dynamics to demonstrate that, regardless of the
nature of the initial forcing, the natural response of the local grass ecosystem to the
dry conditions of the late 1960s played a critical role in maintaining the drought through
the following decades. The onset of the drought has been marked by a forced shift from a
self-sustaining wet climate equilibrium to a similarly self-sustaining but dry climate
equilibrium.
Role of topography in facilitating coexistence of trees and grasses within savannas
Kim, Y. and E. A. B. Eltahir
The factors and processes that may explain the observed coexistence of trees and grasses in savannas are not well understood. Here we propose a new hypothesis that addresses this issue. We hypothesize that "variations in elevation at relatively short horizontal scales of similar to1 km force similar variations in soil moisture and thus create significantly different hydrologic niches within any large area. Under water-limited conditions the relatively wet valleys favor trees, while the relatively dry hills favor grasses. This coexistence of trees and grasses is only possible for a window of climatic conditions that are characteristic of savannas.'' To test this hypothesis, numerical simulations are performed for the region of West Africa using a model that simulates vegetation dynamics, the Integrated Biosphere Simulator ( IBIS), and a distributed hydrologic model, Systeme Hydrologique Europeen ( SHE). IBIS is modified to include the groundwater table (GWT) as a lower boundary. The spatial distribution of GWT is simulated by SHE. At 9degreesN the model simulates trees even when the GWT is assumed to be infinitely deep; at 13degreesN the model simulates grasses even when the capillary fringe of the GWT reaches the surface. However, for the transitional climate, at 11degreesN, trees are simulated when the GWT is at similar to2.5 m from the surface, but grasses are simulated when the GWT is deeper than 2.5 m. These results suggest that the variability of soil moisture forced by topography can be a determinant factor of vegetation distribution within savannas. Furthermore, they confirm that this role of topography can be significant only in a certain climatic window characteristic of savannas.
The Amazon:
Estimation of the fractional coverage of rainfall
in climate models
Elfatih A. B. Eltahir and Rafael L. Bras
Abstract: The fraction of the grid cell area covered by rainfall, 1,
is a very important parameter in the descriptions of land surface hydrology in climate
models. A simple procedure is presented for estimating this fraction, based on extensive
observations of storm areas and rainfall volumes. It is often observed that storm area and
rainfall volume are linearly related. This relation is utilized in rainfall measurement to
compute rainfall volume from radar observations of the storm area. The authors suggest
that the same relation be used to compute the storm area from the volume of rainfall
simulated by a climate model. A formula is developed for computing y, which describes the
dependence of the fractional coverage of rainfall on the season of the year, the
geographical region, rainfall volume, spatial resolution of the model, and the temporal
resolution of the model.
The new formula is applied in computing y over the Amazon region. Significant temporal
variability in the fractional coverage of rainfall is demonstrated. The implications of
this variability for the modeling of land surface hydrology in climate models are
discussed.
A description of rainfall interception over large
areas
Elfatih A. B. Eltahir and Rafael L. Bras
Abstract: A new scheme is developed for describing interception at
spatial scales comparable to the typical resolution of climate models. The scheme is based
on the Rutter model of interception and statistical description of the subgnd-scale
spatial variability of canopy storage and rainfall. The interception loss simulated by the
new scheme is significantly smaller than those simulated by other schemes that do not
include considerations for spatial variability. The explanation of this result is partly
in the enhancement of spatially averaged canopy drainage due to the large local drainage
from the few buckets of large canopy storage.
The relative reduction in interception loss simulated by the new scheme may explain the
overestimation of interception loss by climate models that do not include the effects of
spatial variability on interception processes.
On the response of the tropical atmosphere to
large-scale deforestation
Elfatih A. B. Eltahir and Rafael L. Bras
Abstract: Recent studies on the Amazon deforestation problem predict
that removal of the forest will result in a higher surface temperature, a significant
reduction in evaporation and precipitation, and possibly significant changes in the
tropical circulation. Here, we discuss the basic mechanisms contributing to the response
of the tropical atmosphere to deforestation. A simple linear model of the tropical
atmosphere is used in studying the effects of deforestation on climate. It is suggested
that the impact of large-scale deforestation on the circulation of the tropical atmosphere
consists of two components: the response of the tropical circulation to the negative
change in precipitation (heating), and the response of the same circulation to the
positive change in surface temperature. Owing to their different signs, the changes in
predicted temperature and precipitation excite competing responses working in opposite
directions.
The predicted change in tropical circulation determines the change, if any, in
atmospheric moisture convergence, which is equivalent to the change in run-off. The
dependence of run-off predictions on the relative magnitudes of the predicted changes in
precipitation and surface temperature implies that the predictions about run-off are
highly sensitive, which explains, at least partly, the disagreement between the different
models concerning the sign of the predicted change in Amazonian run-off.
Precipitation recycling in the Amazon basin
Elfatih A. B. Eltahir and Rafael L. Bras
Abstract: Precipitation recycling is the contribution of evaporation
within a region to precipitation in that same region. The recycling rate is a diagnostic
measure of the potential for interactions between land surface hydrology and regional
climate. In this paper we present a model for describing the seasonal and spatial
variability of the recycling process. The precipitation recycling ratio, p, is the basic
variable in describing the recycling process. p is the fraction of precipitation at a
certain location and time which is contributed by evaporation within the region under
study. The recycling model is applied in studying the hydrologic cycle in the Amazon
basin. It is estimated that about 25% of all the rain that falls in the Amazon basin is
contributed by evaporation within the basin. This estimate is based on analysis of a data
set supplied by the European Centre for Medium-range Weather Forecasts. The same analysis
is repeated using a different data set from the Geophysical Fluid Dynamics Laboratory.
Based on this data set, the recycling ratio is estimated to be 35%. The seasonal
variability of the recycling ratio is small compared with the yearly average. The new
estimates of the recycling ratio are compared with results of previous studies, and the
differences are explained.
Sensitivity of regional climate to deforestation
in the Amazon basin
Elfatih A. B. Eltahir and Rafael L. Bras
Abstract: Deforestation results in several adverse effects on the
natural environment. The focus of this paper is on the effects of deforestation on
land-surface processes and regional climate of the Amazon basin. In general, the effects
of deforestation on climate are likely to depend on the scale of the deforested area. In
this study, we are interested in the effects due to deforestation of areas with a scale of
about 250km. Hence, a meso-scale climate model is used in performing numerical experiments
on the sensitivity of regional climate to deforestation of areas with that size. It is
found that deforestation results in less net surface radiation, less evaporation, less
rainfall, and warmer surface temperature. The magnitude of the change in temperature is of
the order of 0 5°C, the magnitudes of the changes in the other variables are of the order
of 10%.
In order to verify some of the results of the numerical experiments, the model
simulations of net surface radiation are compared to recent observations of net radiation
over cleared and undisturbed forest in the Amazon. The results of the model and the
observations agree in the following conclusion: the difference in net surface radiation
between cleared and undisturbed forest is, almost equally partitioned between net solar
radiation and net long-wave radiation. This finding contributes to our understanding of
the basic physics in the deforestation problem.
Relationship between surface conditions and
subsequent rainfall in convective storms
Elfatih A. B. Eltahir and Jeremy S. Pal
Abstract: This paper describes the relationship between surface
conditions (temperature and humidity) and subsequent rainfall. The focus is on convective
storms that are forced and maintained locally due to conditional instability in the
vertical distribution of atmospheric temperature. These storms are described using two
probabilistic measures: (1) the probability of occurrence of storms given surface
conditions and (2) the average storm rainfall. The surface conditions are described by a
single variable: surface wet-bulb temperature. The proposed theoretical relationships are
tested using an hourly data set on rainfall and wet-bulb temperature from the Amazon
region. These observations confirm that both measures increase linearly with wet-bulb
temperature. However, for the occurrence of any storm the wet-bulb temperature has to
exceed a threshold of about 22°C. The sensitivity of the frequency of storms to changes
in the climatology of surface wet-bulb temperature is larger than the corresponding
sensitivity of the average storm rainfall. These general concepts are applied in
discussing the potential impact of changes in land cover on rainfall patterns using two
specific examples: deforestation in the Amazon region and development of irrigation
projects in the Columbia River basin.
Stochastic modeling of the thermally induced atmospheric
flow at mesoscale
Jingfeng Wang, Rafael L. Bras and Elfatih A. B. Eltahir
Abstract: This paper presents a three-dimensional stochastic linear
model of the atmospheric flow induced by the variability of heat flux over land surface.
The primitive equations relating perturbation terms of wind field, geopotential and
buoyancy are formulated as a system of stochastic partial differential equations and
solved analytically. The solution is based on spectral representations of the homogeneous
random fields. The flow intensity is found to be proportional to the standard deviation of
the heat flux into the atmosphere. The intensity of the vertical motion becomes more
sensitive to the differential heating with a larger length scale as altitude goes higher.
Stability and synoptic wind inhibit the development of the flow. The proposed theory
improves the understanding of the role that heterogeneous land surface plays in
atmospheric circulations at the mesoscale.
A stochastic linear theory of mesoscale
circulation induced by the thermal heterogeneity of the land surface
Jingfeng Wang, Rafael L. Bras, and Elfatih A. B. Eltahir
Abstract: This paper presents a three-dimensional stochastic linear
model of the mesoscale circulation induced by the variability of turbulent sensible heat
flux over land surface. The primitive equations relating wind field, geopotential, and
potential temperature are formulated as a system of stochastic partial differential
equations and solved analytically. The solution is based on spectral representations of
homogeneous random fields. The flow intensity is found to be proportional to the standard
deviation of the turbulent sensible heat flux into the atmosphere. Large (small) scales of
spatial variability in the surface heating preferably impact circulations at high (low)
altitudes. The mesoscale fluxes associated with the atmospheric flow are related to
explicit functions of atmospheric stability, variance of turbulent heat flux, and synoptic
wind. The authors find that the vertical momentum flux is significant in the presence of
synoptic wind and that the flow perpendicular to the direction of the synoptic wind is
responsible for this momentum flux. The proposed linear theory identifies the synoptic
conditions under which the land-surface heterogeneity may play a role in atmospheric
circulations at the mesoscale.
Numerical Simulation of
Nonlinear Mesoscale Circulation Induced by Thermal Heterogeneity
of Land Surface
Jingfeng Wang, Elfatih A. B. Eltahir, and Rafael
L. Bras
Mesoscale circulations forced by a random distribution of surface
sensible heat flux have been investigated using a three-dimensional numerical model.
The complex land surface is modeled as a homogeneous random field characterized by
a spectral distribution. Standard deviation and length scale of the sensible heat
flux at the surface have been identified as the important parameters that describe the
thermal variability of land surface. The form of the covariance of the random
surface forcing is not critical in driving the mesoscale circulation. The thermally
induced mesoscale circulation is significant, and extends up to about five kilometers when
the atmosphere is neutral. It becomes weak and is suppressed when the atmosphere is
stable. The mesoscale momentum flux is much stronger than the corresponding
turbulent momentum flux in the neutral atmosphere, while the two are comparable in the
stable atmosphere. The mesoscale heat flux has a different vertical profile than
turbulent heat flux, and may provide a major heat transport mechanism beyond the planetary
boundary layer. The impact of synoptic wind on the mesoscale circulations is
relatively weak. Nonlinear advection terms are responsible for momentum flux in the
absence of synoptic wind.
The role of
clouds in the surface energy balance over the Amazon Forest
Elfatih A.B. Eltahir and E. James Humphries Jr.
Abstract: Deforestation in the Amazon region will initially impact the
energy balance at the land surface through changes in land cover and surface hydrology.
However, continuation of this human activity will eventually lead to atmospheric feedback,
including changes in cloudiness which may play an important role in the final equilibrium
of solar and terrestrial radiation at the surface. In this study, the different components
of surface radiation over an undisturbed forest in the Amazon region are computed using
data from the Amazon region micrometerological experiment (ARME). Several measures of
cloudiness are defined: two estimated from the terrestrial radiation measurements, and one
from the solar radiation measurements. The sensitivity of the surface fluxes of solar and
terrestrial radiation to natural variability in cloudiness is investigated to infer the
potential role of the cloudiness feedback in the surface energy balance. The results of
this analysis indicate that a 1% decrease in cloudiness would increase net solar radiation
by ca. 1.6 W/m2. However, the overall magnitude of this feedback, due to total
deforestation of the Amazon forest, is likely to be of the same order as the magnitude of
the decrease in net solar radiation due to the observed increase in surface albedo
following deforestation. Hence, the total change in net solar radiation is likely to have
a negligible magnitude. In contrast to this conclusion, we find that terrestrial radiation
is likely to be more strongly affected; reduced cloudiness still decrease net terrestrial
radiation; a 1% decrease in cloudiness induces a reduction in net terrestrial radiation of
ca. 0.7 W/m2; this process augments the similar effects of the predicted warming and
drying in the boundary layer. Due to the cloudiness feedback, the most significant effect
of large-scale deforestation on the surface energy balance is likely to be in the
modification of the terrestrial radiation field rather than the classical albedo effect on
solar radiation fields. The net effect of clouds is to reduce net radiation; a 1% increase
in cloudiness induces a reduction in net radiation of ca. I W/m2. The implications of this
negative feedback on large-scale land-atmosphere interactions over rainforests are
discussed. c 1998 Royal Meteorological Society.
A See-Saw Oscillation Between the
Amazon and Congo Basins
Elfatih A.B. Eltahir, Brian Loux, Teresa
K. Yamana, and Arne Bomblies
Abstract: The climate of Earth is shaped to
a significant degree by the occurrence of intense storms over
three regions: the Amazon and Congo basins and the Pacific Ocean.
However, little is known about natural oscillations in the amounts
of rainfall over the Amazon and Congo basins. Here, we present
new satellite observations on tropical rainfall distribution
and historical river flow observations to document a natural
see-saw oscillation across the Atlantic Ocean: floods over the
Amazon basin tend to coincide with droughts over the Congo basin
and vice versa. This phenomenon is most significant during the
southern hemisphere summer, and was observed most clearly during
the decades of 1945-1955, 1960s, and 1970s. The mechanism responsible
for this see-saw phenomenon is based on the Gill model of tropical
circulations since rising motions associated with floods over
either of the two basins is likely to force subsidence and droughts
over the other basin.
Soil Moisture-Rainfall Feedbacks
A feedback mechanism in annual
rainfall, Central Sudan
Elfatih A. B. Eltahir
Abstract: Annual rainfall in many parts of the world is an independent
process. Yet annual rainfall series in some regions of Africa show characteristics
incompatible with such an hypothesis.
The annual rainfall process in Central Sudan is weakly dependent. The hypothesis that
the dependence is due to a "feedback mechanism" is investigated using a
mathematical model based on the water balance of the neighboring region, Bahr Elghazal.
Precipitation Recycling
Elfatih A. B. Eltahir and Rafael L. Bras
Abstract. The water cycle regulates and reflects natural variability
in climate at the regional and global scales. Large-scale human activities that involve
changes in land cover, such as tropical deforestation, are likely to modify climate
through changes in the water cycle. In order to understand, and hopefully be able to
predict, the extent of these potential global and regional changes, we need first to
understand how the water cycle works. In the past, most of the research in hydrology
focused on the land branch of the water cycle, with little attention given to the
atmospheric branch. The study of precipitation recycling, which is defined as the
contribution of local evaporation to local precipitation, aims at understanding hydrologic
processes in the atmospheric branch of the water cycle. Simply stated, any study on
precipitation recycling is about how the atmospheric branch of the water cycle works,
namely, what happens to water vapor molecules after they evaporate from the surface, and
where will they precipitate? Estimation of precipitation recycling over any large
basin, such as the Mississippi or the Amazon, is a necessary step before developing a
quantitative description of the regional water cycle. This paper reviews the research on
the concept of precipitation recycling and emphasizes the basic role of this process in
defining the different components of the atmospheric branch in any regional water cycle.
To illustrate the assumptions and limitations involved in estimation of precipitation
recycling, we present and discuss a general formula for estimation of precipitation
recycling. The recent estimates of annual precipitation recycling ratio from different
regions are reviewed and compared. Finally, the dependence of precipitation recycling over
any region on the spatial scale is discussed and illustrated by the example of the Amazon
basin.
An analysis of the
soil moisture-rainfall feedback, based on direct observations from Illinois
Kirsten L. Findell and Elfatih A. B. Eltahir
Abstract. Many global and regional climate modeling studies have demonstrated
the importance of the initial soil water condition in their simulations of regional
rainfall distribution. However, none of these modeling studies has been tested against
directly observed data. This study tests the hypothesis that soil saturation is positively
correlated with subsequent precipitation by analyzing a 14-year soil moisture data set
from the state of Illinois. The linear correlation between an initial soil saturation
condition and subsequent rainfall is significant during the summer months, reaching a peak
of r2 > 0 4 in mid-June. This result is consistent with the
hypothesis that knowledge of late spring/ early summer soil moisture conditions can aid in
the prediction of drought or flood years, but it does not necessarily prove that feedback
from anomalous soil moisture reservoirs is the cause of anomalous summer conditions.
Further analyses indicate that from early June to mid-August, persistence in rainfall
cannot fully account for the observed correlations, suggesting the likelihood of a
physical feedback mechanism linking early summer soil saturation with subsequent
precipitation. However, spatial and temporal data limitations restrict the potential for
drawing strong new conclusions from the Illinois Study.
A soil moisture-rainfall feedback mechanism 1. Theory and
observations
Elfatih A. B. Eltahir
Abstract. This paper presents a hypothesis regarding the fundamental
role of soil moisture conditions in land-atmosphere interactions. We propose that wet soil
moisture conditions over any large region should be associated with relatively large
boundary layer moist static energy, which favors the occurrence of more rainfall. Since
soil moisture conditions themselves reflect past occurrence of rainfall, the proposed
hypothesis implies a positive feedback mechanism between soil moisture and rainfall. This
mechanism is based on considerations of the energy balance at the land-atmosphere
boundary, in contrast to similar mechanisms that were proposed in the past and that were
based on the concepts of water balance and precipitation recycling. The control of soil
moisture on surface albedo and Bowen ratio is the fundamental basis of the proposed soil
moisture-rainfall feedback mechanism. The water content in the upper soil layer affects
these two important properties of the land surface such that both variables decrease with
any increase in the water content of the top soil layer. The direct effect of soil
moisture on surface albedo implies that wet soil moisture conditions enhance net solar
radiation. The direct effect of soil moisture on Bowen ratio dictates that wet soil
moisture conditions would tend to enhance net terrestrial radiation at the surface through
cooling of surface temperature, reduction of upwards emissions of terrestrial radiation,
and simultaneous increase in atmospheric water vapor content and downwards flux of
terrestrial radiation. Thus, under wet soil moisture conditions, both components of net
radiation are enhanced, resulting in a larger total flux of heat from the surface into the
boundary layer. This total flux represents the sum of the corresponding sensible and
latent heat fluxes. Simultaneously, cooling of surface temperature should be associated
with a smaller sensible heat flux and a smaller depth of the boundary layer. Whenever
these processes occur over a large enough area, the enhanced flux of heat from the surface
into the smaller reservoir of boundary layer air should favor a relatively large magnitude
of moist static energy per unit mass of the boundary layer air. The dynamics of localized
convective storms as well as the dynamics of large-scale atmospheric circulations have
been shown to be sensitive to the distribution of boundary layer moist static energy by
several previous studies. These theoretical concepts are tested using field observations
from Kansas and explored further in a companion paper [Zheng and Eltahir, this issue]
using a simple numerical model.
A soil moisture-rainfall feedback mechanism 2. Numerical
experiments
Xinyu Zheng and Elfatih A. B. Eltahir
Abstract. Here we develop a numerical model to investigate the
hypothesis proposed by a companion paper [Eltahir, this issue], which describes a soil
moisture-rainfall feedback mechanism. The model is designed to describe the seasonal
evolution of the West African monsoon rainfall and is used to perform numerical
experiments that elucidate the mechanisms of the response of rainfall to soil moisture
anomalies. A significant rainfall anomaly is simulated by the model in response to a
hypothetical soil moisture anomaly that has been imposed during early summer. However, the
magnitude of this anomaly almost vanishes when the net radiation at the surface is not
allowed to respond to the soil moisture anomaly. Hence the results of the numerical
experiments support the proposed hypothesis and highlight the crucial importance of the
radiative and dynamical feedbacks in regulating the rainfall anomalies that result from
the soil moisture anomalies.
Hydroclimatology of Illinois: A comparison of monthly
evaporation estimates based on atmospheric water balance and soil water balance
Pat Jing-Feng Yeh, Michelle Irizarry and Elfatih A. B. Eltahir
Abstract: Here we describe the regional-scale hydrological cycle of
Illinois, including both the land and atmospheric branches, using a dataset on most of the
hydrological variables i.e., precipitation, streamflow, soil water content, snow depth,
groundwater table level, and atmospheric flux of water vapor. Since direct observations on
evaporation are not available, two different approaches-soil water balance and atmospheric
water balance were applied to estimate the regional evaporation over Illinois from 1983 to
1994. The availability of a comprehensive hydrological dataset covering the large area of
Illinois facilitated a comparison between these two approaches for estimation of
evaporation. To our knowledge, this is the first time such a comparison has been made. The
climatologies of the monthly evaporation estimates from the two approaches agree
reasonably well, and within a 10% error; however, substantial differences exist between
the two estimates of evaporation for individual months. The seasonal variability of the
evaporation estimates based on soil water balance is largely balanced by the seasonal
pattern of subsurface storage, whereas the seasonal variability of evaporation estimates
from the atmospheric water balance is almost entirely balanced by the seasonal pattern of
lateral fluxes of water vapor. This contrast reflects a fundamental difference in the
hydrology of the land and atmospheric branches of the regional water cycle. In light of
the fact that independent datasets were used in the two approaches, our results are
encouraging: the atmospheric water balance approach has the potential for the accurate
estimation of the climatology of regional evaporation, at least for humid regions at a
scale similar to that of Illinois (~ 105 Km2 ). However, sensitivity analysis suggests
that the accuracy of atmospheric water.
On the Asymmetric Response of Aquifer Water Level to
Droughts and Floods in Illinois
Pat J.-F. Yeh and Elfatih A. B. Eltahir
Abstract: Here we analyze observed characteristics of the natural
variability in the regional-scale hydrological cycle of Illinois, including the soil and
atmospheric branches. This analysis is based on a consistent data-set that describes
several hydrological variables: the flux of atmospheric water vapor, incoming solar
radiation, precipitation, soil moisture content, aquifer water level, and river flow.
The climatology of the average regional hydrological cycle has been estimated.
Variability in incoming solar radiation, not precipitation, is the main forcing of
the seasonal variability in evaporation, soil moisture content, aquifer water level, and
river flow. While precipitation plays a minor role in shaping the natural
variability in the regional hydrological cycle at the seasonal time scale; variability in
precipitation is the major factor in shaping the natural variability in the regional
hydrological cycle at the inter-annual time scale. The anomalies in the different
variables of the regional hydrological cycle have been computed and the persistence
patterns of extreme floods and droughts have been compared. The 1988 drought left a
signature in the aquifer water level that is significantly more persistent than the
corresponding signature for the 1993 summer flood. The discharge from unconfined
groundwater aquifers to streams (base-flow) provides an efficient dissipation mechanism
for the wet anomalies in aquifer water level (groundwater rating curve), which may explain
why droughts leave a significantly more persistent signature on groundwater hydrology, in
comparison to the signature of floods. The non-linearity has been attributed to the
increasing degree by which the unconfined aquifers get connected to the channels network,
as the aquifer water level rises leading to higher drainage density. The potential
implications of these results regarding the impact on regional water resources due to any
future climate change are discussed.
Analysis of the Pathways Relating Soil Moisture and
Subsequent Rainfall in Illinois
Kirsten Findell and Elfatih A.B. Eltahir
Abstract: This study is a continuation of an earlier work [Findell and Elahir,
1997] on the soil moisture-rainfall feedback using a data set of biweekly neutron probe
measurements of soil moisture at up to 19 stations throughout Illinois. This earlier
work showed that soil moisture can play a significant role in maintaining drought or flood
conditions during the summer. Results of a linear correlation analysis between
initial soil saturation and rainfall in the subsequent three weeks showed that a positive
correlation between these two variables was present from early June to mid-August.
This correlation was more significant than the serial correlation within precipitation,
suggesting the likelihood of a physical mechanism linking soil moisture to subsequent
rainfall. Further investigations probed the nature of such a physical pathway
linking soil moisture to subsequent rainfall: these analyses are presented in this
paper. Near-surface hourly observations of pressure, temperature, wet-bulb
temperature, and relative humidity from 13 stations in and close to Illinois were used as
indicators of near-surface air conditions. Time series of the spatial average of
each of these and other quantities were then calculated by averaging data from the 13
stations at each hour. An analysis of the connections between an average daily soil
saturation time series for the whole state of Illinois with these state wide average air
conditions did not yield the anticipated positive correlation between soil moisture and
moist static energy (MSE). It is not clear if this is due to limitations of the data
or of the theory. Other factors, such as clouds, could potentially be masking the
impacts of soil moisture on the energy of the near-surface air. There was evidence ,
however, that moisture availability (or lack thereof) at the surface has a very strong
impact on the wet-bulb depression of near-surface air, particularly from mid-May to the
end of August, showing good correspondence to the period of significant soil
moisture-rainfall association. The final set of analyses performed included an
investigation of hourly boundary layer and rainfall data. Data from 82 hourly
rainfall stations were averaged to compare state wide hourly rainfall to state wide hourly
boundary layer conditions. A link between high MSE and high rainfall was noted
during some summer months, and a link between low wet-bulb depression and high rainfall
was evident for all of the months analyzed (April through September). These analyses
then suggest that the significant but weak correlation between soil moisture and rainfall
in Illinois summers is at least partially due to soil moisture controls on the wet-bulb
depression of near-surface air.
El Niņo and the Flow in Tropical Rivers
El Nino and the natural variability in the flow of the Nile
River
Elfatih A. B. Eltahir
Abstract. Natural variability in the annual flow of the Nile River has been the
subject of great interest to the civilizations that have historically occupied the banks
of that river. Here we report results from analysis on two extensive data sets describing
sea surface temperature of the Pacific Ocean, and the flow of water in the Nile River. The
analysis suggests that 25% of the natural variability in the annual flow of the Nile is
associated with El Nino oscillations. A procedure is developed for using this observed
correlation to improve the predictability of the Nile flood. A simple hypothesis is
presented to explain physically the occurrence of the Hurst phenomenon in the Nile flow.
ENSO and the natural
variability in the flow of tropical rivers
Kishan N. Amarasekera, Robert F. Lee, Earle R. Williams, Elfatih A.B.
Eltahir*
Abstract: This paper examines the relationship between the annual
discharges of the Amazon, Congo, Parana, and Nile rivers and the sea surface temperature
(SST) anomalies of the eastern and central equatorial Pacific Ocean, an index of El
Nino-southern Oscillation (ENSO). Since river systems are comprehensive integrators of
rainfall over large areas, accurate characterization of the flow regimes in major rivers
will increase our understanding of large-scale global atmospheric dynamics. Results of
this study reveal that the annual discharges of two large equatorial tropical rivers, the
Amazon and the Congo, are weakly and negatively correlated with the equatorial Pacific SST
anomalies with 10% of the variance in annual discharge explained by ENSO. Two smaller
subtropical rivers, the Nile and the Parana, show a correlation that is stronger by about
a factor of 2. The Nile discharge is negatively correlated with the SST anomaly, whereas
the Parana river discharge shows a positive relation. The tendency for reduced
rainfall/discharge over large tropical convection zones in the ENSO warm phase is
attributed to global scale subsidence associated with major upwelling in the eastern
Pacific Ocean.
Nilometers, El Niņo, and Climate
Variability
Elfatih A. B. Eltahir and Guiling Wang
Abstract: Nilometers have been used for gauging the level of
water in the Nile river for more than five millennia. The written records describing
some of these measurements represent the longest written records for any hydrological
phenomenon. They describe interannual fluctuations in the Nile river flow which are
closely associated with El Niņo phenomenon. Here, we use information about
long-term variability in El Niņo occurrences that has been extracted from the Nilometers
records to test this significance of the recent trend in the frequency of El Niņo years.
We show that the observed frequency of El Niņo years during the last two decades
is rather high compared to the long-term statistics that are computed from about a
thousand years of Nilometers data; however similar levels of activity have been observed
during the first millennium.
Use of ENSO Information for Medium- and Long-range
Forecasting of the Nile Floods
Wang, G., and E. A. B. Eltahir
Abstract: The Nile river flow is negatively correlated with the Sea
Surface Temperature (SST) in the tropical eastern Pacific Ocean (TEP), an index of El Nino
- Southern Oscillation (ENSO). In this paper, we combine several sources of information,
including ENSO, rainfall over Ethiopia and the recent history of river flow in the Nile,
in order to obtain accurate forecasts of the Nile flood at Aswan. Bayesian theorem is used
in developing the discriminant forecasting algorithm. We use conditional categoric
probability to describe the flood forecasts, and define a synoptic index to measure the
forecasts skill. Our results show that ENSO information is the only valuable predictor for
the long-range forecasts (lead time longer than the hydrological response time scale,
which is 2-3 months in this study). However, the incorporation of the rainfall and river
flow information in addition to the ENSO information significantly improves the quality of
the medium-range forecasts (lead time shorter than the hydrological response time scale).
Topography and the Distribution of Soil
Moisture
Stochastic analysis of the relationship between topography
and the spatial distribution of soil moisture
Pat J.-F. Yeh and Elfatih A. B. Eltahir
Abstract. This paper deals with the issue of the spatial
horizontal variability of soil moisture in the root zone of
a shallow soil at the large scale. The problem of water flow
in the unsaturated zone is formulated so that topography appears
explicitly as a forcing for the movement and redistribution
of soil moisture. This formulation emphasizes the role of the
lateral redistribution of water that is induced by topography.
A stochastic theory is developed to relate the statistical distribution
of soil moisture to that of elevation. This approach will ultimately
facilitate the use of the readily available data sets describing
topography for the purpose of defining the large-scale distribution
of soil moisture. The steady state horizontal distribution of
soil moisture under homogeneous bare soil conditions is regulated
by three distinct factors: topography, climate, and soil properties.
First, topography, forces a distribution of soil moisture that
tends to mimic the elevation field at large scales. The other
two factors are the vertical divergence of water in response
to the climate forcing (evaporation) and the capillary resistance
to water movement. The climate forcing tends to smooth the spatial
distribution of soil moisture. However, the capillary forces
exerted by the soil matrix tend to resist displacement of water
and hence exert adverse effects against the topography and climate
forcings. The variance of the soil moisture distribution increases
with the variance of the elevation field and decreases with
the correlation scale of the elevation field and the magnitude
of the climate forcing. The impact of capillary forces on the
vertical fluxes of water is more significant than their impact
on the topographically induced horizontal fluxes, owing to the
larger hydraulic gradient in the vertical direction resulting
from the disparity in scale between the vertical and horizontal
directions.
Role of topography in facilitating coexistence
of trees and grasses within savannas
Kim, Y. and E. A. B. Eltahir
The factors and processes that may explain the observed coexistence
of trees and grasses in savannas are not well understood. Here
we propose a new hypothesis that addresses this issue. We hypothesize
that "variations in elevation at relatively short horizontal
scales of similar to1 km force similar variations in soil moisture
and thus create significantly different hydrologic niches within
any large area. Under water-limited conditions the relatively
wet valleys favor trees, while the relatively dry hills favor
grasses. This coexistence of trees and grasses is only possible
for a window of climatic conditions that are characteristic
of savannas.'' To test this hypothesis, numerical simulations
are performed for the region of West Africa using a model that
simulates vegetation dynamics, the Integrated Biosphere Simulator
( IBIS), and a distributed hydrologic model, Systeme Hydrologique
Europeen ( SHE). IBIS is modified to include the groundwater
table (GWT) as a lower boundary. The spatial distribution of
GWT is simulated by SHE. At 9degreesN the model simulates trees
even when the GWT is assumed to be infinitely deep; at 13degreesN
the model simulates grasses even when the capillary fringe of
the GWT reaches the surface. However, for the transitional climate,
at 11degreesN, trees are simulated when the GWT is at similar
to2.5 m from the surface, but grasses are simulated when the
GWT is deeper than 2.5 m. These results suggest that the variability
of soil moisture forced by topography can be a determinant factor
of vegetation distribution within savannas. Furthermore, they
confirm that this role of topography can be significant only
in a certain climatic window characteristic of savannas.
Hydrology and Hydroclimatology
Aggregation-disaggregation
properties of a stochastic rainfall model
Bo Zhiquan, Shafiqul Islam and Elfatih A. B.
Eltahir
Abstract: A statistical approach based on the modified Bartlett-Lewis rectangular
pulses model is presented to disaggregate rainfall statistics
from daily data. Six model parameters are estimated from 24-
and 48-hour accumulated rainfall data. Based on these estimated
parameters, in addition to reproducing 24- and 48-hour statistics,
the model is shown to infer 1-, 2-, 6-, and 12-hour historical
statistics satisfactorily. An upper limit for disaggregation
scale (about 2 days) for this model has been identified.
This characteristic behavior of the model is related to the
power law dependence of the power spectrum for timescales smaller
than 2 days. A detailed comparison between observed and modeled
statistics of rainfall data is presented for two rain gages,
one from central Italy and the other from the midwestern United
States.
On the sensitivity of drainage density to climate change
Glenn E. Moglen, Elfatih A. B. Eltahir and Rafael L. Bras
Abstract: Drainage density reflects the signature
of climate on the topography and dictates the boundary conditions
for surface hydrology. Hence defining the relationship between
drainage density and climate is important in assessing the sensitivity
of water resources and hydrology to climate change. Here we
analyze the equilibrium relationship between drainage density
and climate and estimate the relative sensitivity of drainage
density to climate change. We conclude that the sign of the
resulting change in drainage density depends not only on the
direction of the change in climate but also on the prevailing
climatic regime.
Geomorphic
controls on regional base flow
M. Marani, E. A. B. Eltahir and A. Rinaldo
Abstract: The subsurface hydrological response plays an important
role in the hydrology of humid regions. In particular, the physical
relationship between base flow dynamics and the fluctuations
in spatially averaged water table depth, as described by the
groundwater rating curve, determine to a significant extent
the nature of statistical persistence of hydrological anomalies
in the unconfined aquifers level and river flow. In this paper,
we propose that the scale and shape of the groundwater rating
curve reflect some of the geomorphological characteristics of
the region such as relief, drainage density, and the hypsometric
distribution of the elevation field. These connections between
geomorphology and hydrology of river basins are investigated
using a simple model of unconfined groundwater flow applied
to synthetic basins as well as observed basins from Illinois.
Prediction
of regional water balance components based on climate, soil,
and vegetation parameters, with application to the Illinois
River Basin
J. D. Niemann and E. A. B. Eltahir
Abstract: This paper presents a framework for studying regional
water balance in which the physical processes are first described
at the local instantaneous scale and then integrated to the
annual, basin-wide scale. The integration treats the relative
soil saturation (i.e., the soil moisture divided by the porosity)
and precipitation intensities as stochastic variables in space
and time. A statistical equilibrium characterizes the annual
water balance, resulting in a specific relation that predicts
the space-time average of soil saturation in terms of soil,
climate, and vegetation parameters. Specific relationships are
proposed to relate the space-time average soil saturation to
runoff, groundwater recharge, and evapotranspiration. This framework
is applied to the Illinois River Basin. The shape of the spatial
and temporal distributions of soil saturation are determined
from observations. The other parameters are determined from
the physical characteristics of the basin and calibration procedures.
The resulting model is able to reproduce an observed relation
between the space-time average soil saturation and precipitation.
It is also able to reproduce observed relations between space-time
average soil saturation and space-time average evapotranspiration,
surface runoff, and groundwater runoff.
Representation
of Water Table Dynamics in a Land-Surface Scheme: 1. Model Development
Pat J. -F Yeh and Elfatih A. B. Eltahir
Abstract: Most of the current land surface
parameterization schemes lack any representation of regional
groundwater aquifers. Such a simplified representation of subsurface
hydrological processes would result in significant errors in
the predicted land-surface states and fluxes especially for
the shallow water table areas in humid regions. This study attempts
to address this deficiency. To incorporate the water table dynamics
into a land surface scheme, a lumped unconfined aquifer model
is developed to represent the regional unconfined aquifer as
a nonlinear reservoir, in which the aquifer simultaneously receives
the recharge from the overlying soils, and discharges runoff
into streams. The aquifer model is linked to the soil model
in the land surface scheme LSX through the soil drainage flux.
The total thickness of the unsaturated zone varies in response
to the water table fluctuations, thereby interactively couples
the aquifer model with the soil model. The coupled model (called
LSXGW) has been tested in Illinois for an 11-year period from
1984-1994. The results show reasonable agreements with the observations.
However, there are still secondary biases in the LSXGW simulation
partially resulting from not accounting for the spatial variability
of water table depth. The issue of sub-grid variability of water
table depth will be addressed in a companion paper [Yeh and
Eltahir, this issue].
Representation
of Water Table Dynamics in a Land-Surface Scheme: 2. Sub-grid
Variability
Pat J. -F Yeh and Elfatih A. B. Eltahir
Abstract: A lumped unconfined aquifer model
has been developed and interactively coupled to a land surface
scheme [Yeh and Eltahir, this issue]. Here, we address the issue
of the representation of sub-grid variability of water table
depths (WTD). A statistical-dynamical (SD) approach is used
to account for the effects of the unresolved sub-grid variability
of WTD in the grid-scale groundwater runoff. The dynamic probability
distribution function (PDF) of WTD is specified as a two-parameter
Gamma distribution based on observations. The grid-scale groundwater
rating-curve (i.e., aquifer storage-discharge relationship)
is derived statistically by integrating a point groundwater
runoff model with respect to the PDF of WTD. Next, a mosaic
approach is utilized to account for the effects of sub-grid
variability of WTD in the grid-scale groundwater recharge. A
grid-cell is categorized into different sub-grids based on the
PDF of WTD. The grid-scale hydrologic fluxes are computed by
averaging all the sub-grid fluxes weighted by their fractions.
This new methodology combines the strengths of the SD approach
and the mosaic approach. The results of model testing in Illinois
from 1984-1994 indicate that the simulated hydrologic variables
(soil saturation and WTD) and fluxes (evaporation, runoff, and
groundwater recharge) agree well with the observations. Due
to the paucity of the large-scale observations on WTD, the development
of a practical parameter estimation procedure is indispensable
before the global implementation of the developed LSXGW in climate
models.
Forest
on the edge: Seasonal cloud forest in Oman creates its own ecological
niche
Anke Hildebrandt and Elfatih A. B. Eltahir
Abstract: Cloud forests usually grow in the
moist tropics where water is not a limiting factor to plant growth. Here, for the
first time, we describe the hydrology of a water limited
seasonal cloud forest in the Dhofar mountains of Oman. This
ecosystem is under significant stress from camels feeding on
tree canopies. The Dhofar forests are the remnants of a moist
vegetation belt, which once spread across the Arabian
Peninsula. According to our investigation the process of
cloud immersion during the summer season creates within
this desert a niche for moist woodland vegetation.Woodland
vegetation survives in this ecosystem, sustained through
enhanced capture of cloud water by their canopies (horizontal
precipitation). Degraded land lacks this additional water
source, which inhibits re-establishment of trees. Our
modeling results suggest that cattle feeding may lead to
irreversible destruction of one of the most diverse ecosystems
in Arabia.
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