TRANSIENT WATER-STRESS IN A VEGETATION CANOPY - SIMULATIONS AND MEASUREMENTS

Measurements of evapotranspiration and radiometric surface temperature, supplemented by model simulations, illustrate the effect of transient water stress on the evapotranspiration and radiometric surface temperatures over a corn (Zea Mays L) canopy. Transient stress is manifested by a plateau in the evapotranspiration and a warmer radiometric temperature of the canopy. Model simulations suggest that the cause of the evapotranspiration plateau is not solely due to reduced soil water content but to excessive atmospheric demand in the face of internal plant constraints. One of these constraints is a threshold epidermal leaf water potential, below which stomatal resistance increases very rapidly with decreasing leaf water potential. Accordingly the evapotranspiration plateau can occur at relatively high soil water content. The sensitivity of the canopy temperature to changing soil water content is weak at high soil water content. The evapotranspiration plateau and its effect on radiometric surface temperatures would not be detectable until, in this case, the soil water content decreases to about half of field capacity. Since evapotranspiration collapse (wilting) would occur at about 35% of field capacity, it may be possible to remotely monitor the onset of crop water stress over dense vegetation before catastrophic damage occurs.