Evapotranspiration Measurement Schemes

 Direct (Lysimeter) This device consistes of a block of soil covered with vegetation.  The block of soil is initially removed from the forest and placed into a container.   Next the block of soil is returned to its original location so that the container as well as the soil is set into the ground.  Over time, the input of precipitation is measured via rain aguages and the drainage output is recorded.  During this same interval, the block of soil is frequently massed to estimate the amunt of water loss via evapotranspiration.   evapotranspiration. ∝ precipitation - drainage - ∆mass Indirect (Water balance) Although lysimeters may be effective in accurately determining evapotranspiration levels, on a large scale it would be impossible to implement such a design.  Researchers therefore have come to use large scale measurements of rainfall to determine evapotranspiration levels.  Typically rainfall data is gathered from satellites and then using a particular algorith, evapotranspiration is determined. Adding energy balance considerations, one can derive more accurate predictions of evapotranspiration and evaporation.  For specific plants, a simple equation can be written to express the maximum evapotranspiration (ET M ) for that plant.  This value is related to the maximum evapotranspiration for a reference plant (ET0) such as green grass and a dimensionless coefficient for the specific plants (KC). ET 0 = A + BRG + CRGTA ETM = KCET0 This however, is just one estimation of evapotranspiration.  Countless other studies have developed estimations based on similar principles.  Another such equation relates evapotranspiration to net radiation (Rn), surface temperature (Ts), and air temperature (Ta). ET = Rn + A - B (Ts - Ta), where A and B are constants.

Evapotranspiration mechanisms

Mechanism controlling changes in evapotranspiration are primarily driven by changes in albedo, roughness and the depth of water available to plant roots.  Increased albedo inhibits absorption of the incoming solar radiation, reducing the available energy for latent-heat exchanges.

Evaporation

Evaporation can be indicated by a measure called the precipitation recycling ration (p).  This ratio is the contribution of evaporation within a region to precipitation in the same regio.  A high precipitation recycling ratio estimate is not sufficient to conclude a strong role for land surface hydrology in the regional climate.  Rather, it suggests a strong potential for significant changes in surface hydrology to impact regional climate.

The following model makes two assumptions: 1) atmospheric water vapor is well-mixed, and 2) the rate of change of storage of water vapor is negligible compared with water vapor fluxes at the time-scale for which the model is applicable.  The model gives two distinct relationships for water vapor evporation, that within the region, and that outside the region, yielding the equation

(Iw + E)
p =  ---------------------
(Iw + E + IO)

where inflow is represented by I, evaporation is represented by E, and the subscripts o and w represent outside the region and inside the region respectively.

Careful observation of evaporation data has led to the conclusion that the atmosphere above the Amazon basin is not a closed sytem.  Data suggests that theere is a significant migration of moisture out of the basin.  Futhermore, this flux out of the basin accounts for only 68% of the flux into the system.  This means that the outflow of atmospheric moisture from the basin may contribute important input to the hydrologic cycles of the surrounding regions.  Further, changes in the Amazon basin evaporation may potentially affect the moisture supply and rainfall of surrounding regions.

Evapotranspiration Data

The Amazon rainforest is highly efficient in recycling water vapor back into the atmosphere .  Measuring this parameter however, is has proved extremely difficult. Evapotranspiration levels are highly variable across the Amazon basin as evidenced by the following data:
• 610mm in the semi-arid Rio Grande basin
• 1520mm in the Orthon River basin
• 780mm in Andean part of Beni River basin
• 1220mm in oriental basins of Mamoré River
• 800mm in the Bolivian andean part of the upper Madeira River basin
• 63-68% of precipitation , 33-37% is runoff
Results of evapotranspiration are summarized below:

Hyrdologic cycle of the Amazon Region

 Research Rainfall Transpiration Evapotranspiration Runoff mm mm % mm/day mm % mm/day mm % Marques et al. 1980 2328 1260 54.2 3.5 1068 45.8 23289 1000 43.0 2.7 1328 57.0 2328 1330 57.1 3.6 998 42.9 Villa Nova et al. 1976 2000 1460 73.0 4.0 540 27.0 1168 58.4 3.2 832 41.6 2105 1569 73.4 4.3 532 26.6 Molion 1975 2379 1146 48.2 3.2 1233 51.8 Ribeiro et al. 1979 2478 1536 62.2 4.2 942 38.0 1508 60.8 4.1 970 39.2 Ipean 1978 2179 1475 67.5 4.0 704 32.5 1320 60.6 3.6 859 39.4 Dmet 1978 2207 1452 65.8 4.0 755 34.2 1306 59.2 3.6 901 40.8 Jordan et al. 1981 3664 1722 47.0 4.7 1905 52.0 5.2 1759 48.0 Leopolo et al. 1981 2089 1014 48.5 2.7 1542 74.1 4.1 5441 25.9 Leopolo et al. 1982 2075 1287 62.0 3.5 1675 80.7 4.6 400 19.3 Shuttleworth 1988 2636 992 37.6 2.7 1320 50.0 3.6 Able-2B 1987 (1 month) 290 157 54.1 5.2

Summary of Surface Variables for Control (C) and Deforested (D) Simulations Averaged over 3 years for Amazonia

 Surface Variable Control Deforested Precent Difference Evapotranspiration (m/d) 3.12 2.27 -27.2% Precipitation (m/d) 6.60 5.26 -20.3% Soil Moisture (cm) 16.13 6.66 -58.7% Runoff (m/d) 3.40 3.00 -11.9% Net Raditation (W/m^2) 147.29 125.96 -14.4% Temperature (C) 23.55 25.98 10.3% Sensible Heat (W/m^2) 57.19 60.15 5.2% Bowen Ratio 0.85 1.50 76.5%

Mean water budget for Amazonia. The data re 12-month mean (January to December) values

 Total Precipitation (P) (mm/year) Evapotranspiration (E) (mm/year) E-P E/P Precipitable Water (mm) Control 2464 1657 -807 0.67 37.7 Deforestation 1821 1161 -661 0.63 35.4 Difference -642 -496 +146 -0.04 -2.3 Change (%) -26.1 -30.0 +18.0 -5.9 -6.1

Evapotration Data

The contribution to rainfall of precipitation recycling increases westward and southward.  The maximum rate of recylcing occurs at the south-western corner of the basin, where more than 50% of the precipitation is contributed to by evaporation.

Sources

Possible climatic impacts of amazonia deforestation
Authors: Nobre, Carlos A.
Source: Water Management of the Amazon Basin, (245-260)
Editors: Braga, Benedite P. F., Jr., and Fernandez-Jauregui, Carlos A.

Date: August 1991

Water and salt balances of the Bolivian Amazon

Authors: Roche, M. A., et al.
Source: Water Management of the Amazon Basin, (83-94)
Editors: Braga, Benedite P. F., Jr., and Fernandez-Jauregui, Carlos A.

Date: August 1991

A GCM simulation of the impact of Amazonian deforestation on climate using an improved capony representation
Authors: Lean, J. and Rowntree, P. R.
Source: Quarterly Journal of the Royal Meteorological Society 119 pp 509-530
Date: 1993

Precipitation recycling in the Amazon basin
Authors: Eltahir, E. A. B. and Bras, R. L.
Source: Quarterly Journal of the Royal Meteorological Society 120 P861-880
Date: 1994