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 (ET_⁰) such as green grass and a dimensionless coefficient for the specific plants (K_C).

ET₀ = A + BR_G + CR_GT_A
ET_M = K_CET₀

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 (R_n), surface temperature (T_s), and air temperature (Ta).

ET = R_n + A - B (T_s - T_a),

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

(I_w + E)
p = ---------------------
(I_w + E + I_O)

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