Conventional Approaches For Measuring Evaporation

Theoretical developments such as those described in the previous section are generally dependent upon experimental data for verification. There are a variety of conventional approaches for measuring evaporation, ranging from simple to complex and having a range of accuracies and spatial scales.

Most simply, evaporation can be measured under field conditions by monitoring the change in soil water storage over a period of time. Though this can be accomplished fairly easily with a neutron soil water probe, this method does not account y* = T(1 + rc/ra)

for the drainage from the zone sampled or the upward movement of water from a saturated zone into the zone sampled. Discussions of the problems encountered in determining evaporation by soil sampling were presented by Robins et al. (1954) and Jensen and Wright (1978).

Weighing lysimeters are open-top tanks filled with soil in which crops are grown under natural conditions. Evaporation from the contained soil and plants is generally determined either by weighing the entire unit with a mechanical scale or with a counterbalanced scale and load cell; the reduction in the unit's weight over time equals the rate of water transfer to the atmosphere by evaporation. For accurate results, the soil conditions within the lysimeter should be the same as those without, and the lysimeter must be surrounded by the same vegetation that is growing in the lysimeter for a desired radius of about 100 m. A detailed summary of the use of lysimeters for estimation of evaporation can be found in publications by van Bavel and Myers (1962) and Howell et al. (1985).

Commercial instrumentation is available for determining evaporation using an energy balance approach (Bowen ratio) and a mass transfer method (eddy correlation). The Bowen ratio method [based on Eqs. (1) to (3)] allows values of evaporation to be obtained hourly during daylight hours. The accuracy of the method decreases with decreasing flux of water vapor, or when there is low evaporative demand (e.g., at night). A description of the Bowen ratio equipment was provided by Spittlehouse and Black (1980) and Gay and Greenberg (1985).

The eddy correlation method was proposed by Swinback (1951) based on the theoretical description of the mean vertical flux of water vapor:

where P is atmospheric pressure (kPa), w' is the instantaneous deviation of vertical wind speed from the mean vertical wind (w) at height z, and e' is the instantaneous deviation of the partial water vapor pressure from the mean at height z. Evaluation of Eq. (9) is accomplished using vertical anemometers and vapor pressure sensors with short sampling intervals (hundredths of seconds) to determine w' and e' in short, successive periods of time (tenths of seconds). This method is amenable to field use in routine measurements for extended periods, e.g., months or years (Kanemasu et al., 1979).

Other approaches that have been used to measure evaporation rates include the inflow-outflow method for monitoring evaporation from catchments (Holmes, 1984) and portable gas assimilation chambers (Reicosky, 1981). A limitation of all the techniques described in this section is that they yield essentially point values of evaporation and, therefore, are applicable only to a homogeneous area surrounding the equipment that is exposed to the same environmental factors. An evaluation of the spatial distribution of evaporation over large heterogeneous areas would be prohibitive using these conventional point measurement techniques. There are advantages and disadvantages of these conventional methods and the remote-sensing techniques discussed in the following sections. Conventional methods yield data at one location but operate continuously over time. Techniques that utilize remotely sensed inputs yield data for each resolution element of the sensor, thus spatially distributed values of evaporation, but at only an instant in time.

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