Relationship Between Plant Available Water and Subsequent Rainfall in Oklahoma.

Soil moisture has been shown to affect subsequent precipitation in transition zones between wet and dry climates, including the Great Plains region of North America. We hypothesized that soil moisture is positively correlated with the 21-d subsequent precipitation in Oklahoma during the growing season. Precipitation and 14-year soil moisture data were compiled from 110 Mesonet sites across Oklahoma. From this data we calculated the plant available water (PAW) in the soil for three different depths (0-10 cm, 0-40 cm, and 0-80 cm) to determine the strength of correlation between PAW and the subsequent 21-d precipitation.  A positive correlation existed between PAW and subsequent 21-day precipitation during June-July, with r² = 0.33 (0-10 cm), 0.29 (0-40 cm) and 0.29 (0-80 cm). These results suggest a weaker relationship exists in Oklahoma between PAW and subsequent 21-d rainfall than that which has been previously reported between soil moisture and subsequent rainfall in Illinois.  We also discovered unexpected, significant negative correlations between PAW and subsequent rainfall outside the growing season. The correlations documented here suggest that real-time PAW observations may be useful in the prediction of short-term drought or flood probabilities in Oklahoma and may possibly assist farmers in their crop management strategies.