Dryland Evaporative Flux in a Subhumid Climate: IV. Relation to Plant Water Status

This investigation was conducted to determine the relations between plant water status and plant evaporation rates for use in defining critical limits of soil water availability to field crops. Seasonal trends in relative water content (RWC) of grain sorghum [Sorghum bicolor (L.) Moench] were measured concurrently with evaporation rates during a prolonged drying cycle in 1969. When the available soil water supply was depleted beyond a critical threshold, called the lower limit for potential evaporation (LLE₀), daily evaporation rates began to decline substantially below the level of energy available for evaporation. Before the LLE₀ threshold was reached, midday leaf RWC values were approximately 0.90. Afterwards, these values declined in proportion to measured decreases in daily evaporation. Early morning RWC values did not decrease significantly until 8 days after evaporation began to decline. Leaf stomatal response to soil water deficit was evident when daylight leaf diffusion resistances changed from 2 sec/cm before the LLE₀ threshold to about 25 sec/cm 10 days after the threshold was reached. These results indicate that plant water status can be used in conjunction with measured changes in soil water content to define the amount of soil water extractable by plant roots for a particular crop, soil, and climate between the maximum water-holding capacity of the soil and the LLE₀ threshold. These critical soil water parameters can be applied to predicting evaporation in two stages: (i) a freely evaporating stage when evaporation is limited by the amount of energy available for evaporation; and (ii) a falling rate stage when evaporation declines below the potential rate. An equation is proposed for predicting evaporation in the falling stage as an exponentially decreasing function of time after the LLE₀ threshold is reached.