Dryland Evaporative Flux in a Subhumid Climate: I. Micrometeorological Influences
1971
Ritchie, Joe T.
Daily values of the major energy balance components were determined throughout complete growing seasons of dryland cotton and grain sorghum in Central Texas. Daily net radiation was found to be partitioned primarily between evaoprative flux and sensible heat flux. When crop canopies covered at least 45% of the ground surface and plant evaporation was not limited by soil water, daily net radiation was approximately equal to daily measured evaporation. Before a crop cover was established, evaporation was highly dependent on the water content of the surface soil and the fractional crop cover. For both crops, when water was depleted from the soil to the extent that plant evaporation began to be limited, daily evaporative flux decreased within 10 days to approximately half the daily net radiation. Diurnal variation in the evaporative flux during plant water stress was approximately proportional to net radiative flux, with the proportionality principally determined by the degree of soil water availability and the magnitude of the net radiation. Seasonal variation in the net radiation was influenced primarily by the solar radiation and the surface reflectance (albedo). The seasonal albedo variation depended primarily on degree of plant cover, ranging from 0.055 over practically bare, freshly plowed, black clay soil to approximately 0.23 over a practically complete cover cotton canopy. This result suggests a possibility for water conservation by increasing the surface albedo during the part of the growing season when wet topsoil is exposed to direct radiation. Closer row spacing and higher plant populations should help in achieving this effect.
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