A WEATHER-SOIL VARIABLE FOR ESTIMATING SOIL-MOISTURE STRESS AND CORN YIELD PROBABILITIES

Agricultural technology has increased crop yield potentials, but on ra in-fed crops yields are still severely reduced with the normal climati c frequency of drought. Objectives were (1) to determine an interactio n regression of county average corn (Zea mays L.) yield on a soil mois ture stress variable and technology trend and (2) to estimate the prob ability of soil moisture stress and resulting average corn yield in Ti ppecanoe County, Indiana. The soil moisture stress variable (S-c) was the sum of modeled daily ratios of actual to potential evapotranspirat ion [Sigma(ET/PET)] over critical corn growth and development periods. The interaction regression model of corn yield on Sc and technology t rend (T = year) for Tippecanoe County was associated with 70% of the v ariance in the 1961 to 1992 average county corn yields when S-c was a 90-d period (S-90) from 39 d before corn silking to 50 d after. With n o moisture stress (S-90 = 90), the technology trend over the last 32 y r was 0.17 t ha(-1) yr(-1) (2.7 bu acre(-1) yr(-1)). With 1992 technol ogy, each deficit unit of S-90 reduced the yield 0.19 t ha(-1) (3.1 bu ac re(-1)). The distributions of S-90 and predicted corn yield were h ighly negatively skewed. The probability of having an S-90 less than 8 5 (at least some moisture stress), and a county corn yield less than 9 .5 t ha(-1) (152 bu acre(-1)) is 69%, but the probability of severe st ress (S-90 < 75) and corn yield less than 7.5 +/- 0.8 t ha(-1) (139 +/ - 13 bu acre(-1)) is 22%. For the same weather regime, the probability of moisture stress and resulting corn yields differs greatly for indi vidual soils. For a poorly drained soil (Typic ArgiaquolI) the probabi lity of having an S-90 less than 85 is 41%, but for a well-drained soi l (Typic Argiudoll) the probability is 90%.