Vapor‐Budget Measurement of Irrigation Evaporative Losses

Abstract

A vapor‐budget technique for measurement of short‐term evaporation from moist, rough surfaces (such as irrigated cropped areas) is developed based on the control volume concept. The control volume is specified so that the difference in the horizontal fluxes of outgoing and incoming atmospheric water vapor equals the evaporation rate. The fluxes are estimated by measuring vertical profiles of wind speed and water vapor. A height of 3 m is sufficient for the present field application. Simplifications used in actual application of the technique are discussed and evaluated. This approach is field tested (square test site, 30 m on each side) and compared with simultaneous water‐budget measurements of the evaporation. The vapor budget is within 12% of the water budget for steady wind direction so long as an average of five profile sets is used for each 15‐min evaporation value. One of the primary advantages of the control volume approach is that it requires measurements only at the periphery, thus providing integrated evaporation values for complex surfaces.