Water Loss from an Irrigated Sorghum Field: II. Evapotranspiration and Root Extraction1

Abstract Techniques for estimating evapotranspiration from water depletion within the soil profile frequently do not account for flux below the root zone. A method using tensiometers for obtaining evapotranspiration rates and root extraction patterns within an actively growing sorghum (Sorghum bicolor (L.) Moench) crop is discussed. Profile water depletion was calculated from water content profiles determined using tensiometer readings and laboratory‐measured soil water characteristics. Profile water depletion minus flux below the root zone yielded the daily evapotranspiration rate. During the 31‐day study, approximately 65% of the total water loss was due to evapotranspiration and approximately 35% was due to flux loss from the root zone. These data clearly show the importance of considering flux below the root zone when attempting to determine evapotranspiration rates using depletion methods. The method of determining evapotranspiration rates illustrated in this paper provides an alternative to the microclimatological and lysimeter methods.