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Energy and Water Balance of a Sparse Crop: Simulated and Measured Soil and Crop Evaporation¹

ABSTRACT

Dryland crops grown in semiarid environments often do not completely cover the soil, leaving a portion of the soil surface exposed to a condition of rapid soil-water evaporation. Quantitative separation of soil evaporation and crop transpiration is important if cultural practices or cultivars are to be evaluated. This study was designed to evaluate a combined energy and water balance model, ENWATBAL, to describe the concurrent heat and water fluxes in a row crop. Inputs to the model include soil and plant variables and daily weather data. Measurements were made for a period of 74 d over a cotton (Gossypium hirsutum L.) canopy during 1985 on an Olton soil (fine, mixed, thermic Aridic Paleustolls) at Lubbock, TX. Data collected included soil-water content, soil temperature, root distribution, soil evaporation with microlysimeters, and leaf area index, for both an irrigated and a dryland plot. The values for daily evaporation and evapotranspiration calculated with the model were within 1 standard deviation of the measured values. Cumulative evaporation and evapotranspiration from the model agreed with measured values within 7% for the dryland and 8% for the irrigated plot. Estimated soil-water and temperature profiles also agreed closely to measured values. Soil evaporation was found to be 30% of evapotranspiration, for both the irrigated and the dryland plot. The ENWATBAL model provides a reliable method of evaluating the effects of management practices and crop selection on the water-use efficiency of crop production in a semiarid area.

¹ Joint contribution from the Texas Agric. Exp. Stn., Texas A&M Univ., and USDA.

² Assistant Professor, Texas Agric. Exp. Stn., Route 3, Box 219, Lubbock, TX 79401; Professor, Dep. of Soil & Crop Sciences, Texas A&M Univ., College Station, TX 77843; Research Leader and Soil Physicist, Cropping Systems Res. Lab., USDA-ARS, Lubbock, TX 79401, respectively.

Received for publication November 10, 1986.

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