DETERMINATION OF SOIL-WATER EVAPORATION AND TRANSPIRATION FROM ENERGY-BALANCE AND STEM-FLOW MEASUREMENTS

Frequent measurements of soil water evaporation (E) and transpiration (T) are needed to quantify energy and water balances of sparse crops. Field experiments were conducted in Lubbock, TX to examine the feasibility of partitioning evapotranspiration (ET) from a cotton crop (Gossypium hirsutum L.) during periods of partial cover. The Bowen ratio energy balance method and heat balance stem flow measurements were used to make near-instantaneous measurements of ET and T, respectively. Transpiration on a unit land area basis was determined by normalizing stem flow measurements by leaf area or plant density. Soil water evaporation was computed as the difference between ET and T. The accuracy of the method was evaluated by comparing calculated values of E with measured values obtained from soil microlysimeters. Measurements over an 8-day period following an irrigation indicated that daily values of calculated E were within 0.5 mm of measured values in six out of seven comparisons when stem flow measurements were normalized on a leaf area basis. On average, daily calculated E was within ±11% of measured values. Calculated and measured cumulative E agreed to within 0.6 mm at the end of the evaluation period. Computing T by normalizing stem flow on a plant density basis resulted in overestimates of T and underestimates of E. Error analysis indicates that the precision of the E estimate decreases rapidly as evaporation becomes a smaller fraction of ET, and is influenced equally by the resolution of the stem flow and leaf area measurements. This study demonstrates that high frequency, independent measurements of soil and canopy evaporation can be obtained by measurement of ET and stem flow. © 1990.