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Abstract
An agrometeorological study was conducted to determine soil water balance and to test for specific drought effects in a sorghum crop at Mead, NE.
Field experiments were conducted with eight different water treatments in a randomized factorial block irrigation design during 1990 and 1991 growing seasons. Pertinent meteorological, plant, and soil variables were monitored through the season. The components of the water balance equation for the sorghum crop, especially evapotranspiration and soil water content, were calculated using a model that describes the processes of crop transpiration, soil evaporation, and the hydrological balance of the root zone. The simulated (model) and observed (neutron probe readings) soil water content in the root zone compared well for all water treatments with average d-index of agreement 0.90.
A crop specific drought index (CSDI) for sorghum was also developed. This index is based upon the daily soil water balance. The major cause of grain yield data variability was associated with timing of water use during the season and frequency, timing, and amount of irrigation and rainfall. The relative impact of water stress during each of the three periods of growth is reflected by sensitivity coefficients ($\lambda$) where (ET$\sb{\rm a}$/ET$\sb{\rm p}\rbrack\sp\lambda$ was determined for the three stages of growth. These ratios were used to form a multiplicative model for estimating sorghum grain yield using field data in 1990 and 1991. The CSDI using $\lambda$s derived from ET$\sb{\rm a}$/ET$\sb{\rm p}$ ratios gave better results than when T$\sb{\rm a}$/ET$\sb{\rm p}$ ratios were used. The resulting model estimates of yield were in close agreement with observed values when tested using an independent data set, and presented better performance when compared with two other models.
Examination of ET from a study involving all combinations of irrigation and nonirrigation over three growth stages appears to realistically differentiate coefficients describing sensitivity of sorghum yield to time and amount of water use. We conclude that the inclusion of CSDI-sorghum to extract the yield signal from evapotranspiration as estimated within the soil water balance model will improve monitoring of the impact of drought in relation to sorghum production.
