An analysis of the soil moisture-rainfall feedback, based on direct observations from Illinois

Many global and regional climate modeling studies have demonstrated the importance of the initial soil water condition in their simulations of regional rainfall distribution. However, none of these modeling studies has been tested against directly observed data. This study tests the hypothesis that soil saturation is positively correlated with subsequent precipitation by analyzing a 14-year soil moisture data set from the state of Illinois. The linear correlation between an initial soil saturation condition and subsequent rainfall is significant during the summer months, reaching a peak of r(2)>0.4 in mid-June. This result is consistent with the hypothesis that knowledge of late spring/early summer soil moisture conditions can aid in the prediction of drought or flood years, but it does not necessarily prove that feedback from anomalous soil moisture reservoirs is the cause of anomalous summer conditions. Further analyses indicate that from early June to mid-August, persistence in rainfall cannot fully account for the observed correlations, suggesting the likelihood of a physical feedback mechanism linking early summer soil saturation with subsequent precipitation. However, spatial and temporal data limitations restrict the potential for drawing strong new conclusions from the Illinois study.