Dryland Evaporative Flux in a Subhumid Climate: III. Soil Water Influence¹

Abstract

Abstract This investigation was conducted to determine the relationship between evaporative rates of field‐grown cotton ( Gossypium sp.) and grain sorghum [ Sorghum bicolor (L.) Moench] and the soil water status for use in predicting evaporation on watersheds. Soil water content and soil water matric potential of Houston Black clay were measured throughout complete growing seasons for cotton and grain sorghum concurrently with measurements of the daily evaporation rate. Evaporation rates were independent of the soil water status until soil water was depleted beyond a threshold value. This threshold, termed the lower limit for potential evaporation (LLE 0 ), was reached when approximately 18.2 cm of water had been removed from a soil profile initially wet. Another 6.5 cm of soil water was extracted at a decreasing rate before evaporation practically stopped. Evaporation rates after the LLE 0 threshold was reached were practically independent of the energy available for evaporation and depended on the rooting distribution and the water movement to the roots. An analysis of the soil water transmission characteristics after the threshold LLE 0 was reached showed that practically all the water extracted by plant roots was moving from the volume of soil immediately surrounding the roots. This result points out the need for growing crops with deep, dense root systems in dryland for maximum utilization of stored soil water.