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

In the southeastern USA Coastal Plain, spatial variation in soils causes extreme spatial variation in grain yield, as seen in yield maps. Corn (Zea mays L.) appears to be particularly susceptible to soil variation, espedally during periods of drought. Our objectives were to compare variation in water 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 maturity using a time-domain reflectometer (TDR). On 4 d during vegetative growth, drought stress was evaluated on eight transects using infrared thermometer (IRT) measurements of canopy temperature (T c ). During the most severe drought, visibly stressed areas had canopy-air temperature differences (T c - T,) > 10°C, yet other areas remained <2°C. Two days after a 46-mm rain, T c - T, was near zero over the whole field, indicating little water stress. The time series of TDR measurements produced estimates of daily evapotranspiration, runoff, and infiltration; site-to-site differences 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 paths. Further, variation between sites within soils was significant, indicating that soll map units are not homogenous with respect to water relations. These results underscore the need for within-season observations of crop water use and stress to augment interpretation of site-specific yleid maps.