Site-specific analysis of a droughted corn crop: II. Water use and stress

In the southeastern USA Coastal Plain, spatial variation in soils causes ex treme spatial variation in grain yield, as seen in yield maps. Corn (Zea ma ys L.) appears to be particularly susceptible to soil variation, especially during periods of drought. Our objectives were to compare variation in wat er use and stress of corn within and among soil map units. In one field, at two sites in each of four map units, we measured site-specific effects of soil variation on crop water use from 40 d after planting until after matur ity using a time-domain reflectometer (TDR). On 4 d during vegetative growt h, drought stress was evaluated on eight transects using infrared thermomet er (IRT) measurements of canopy temperature (T-c). During the most severe d rought, visibly stressed areas had canopy-air temperature differences (T-c - T-a) > 10 degrees C, yet other areas remained <2 degrees C. Two days afte r a 46-mm rain, T-c - T-a was near zero over the whole field, indicating li ttle water stress. The time series of TDR measurements produced estimates o f daily evapotranspiration, runoff, and infiltration; site-to-site differen ces in these dominated the water balance. Water stress, inferred from water use, matched that inferred earlier from yield components. In sum, corn at the eight sites arrived at final water use via fundamentally different path s. Further, variation between sites within soils was significant, indicatin g that soil map units are not homogenous with respect to water relations. T hese results underscore the need for within-season observations of crop wat er use and stress to augment interpretation of site-specific yield maps.