Role of soil storage/soil wetness on streamflow in the upper midwestern United States

Increased streamflow and its associated impacts on water quality have frequently been linked to Land Use and Land Cover (LULC) changes such as increased tile drainage, cultivation of prairies, and adoption of soybean (Glycine max) in modern day cropping systems. However, recent analysis of streamflow records from 29 HUC 8 (Hydrologic Unit Code 008) watersheds in Iowa and Minnesota showed climate instead of LULC change was the major driver of increased streamflow. The analysis was done through comparison of annual streamflow vs. annual precipitation relationships for the periods prior to 1975 (pre-change period) and after 1976 (post-change period). A statistical shift in annual relationships from the pre- to post-change period was assumed to be an indication of LULC changes whereas a lack of statistical shift suggested no change in the relationship and higher flows were mainly driven by increased precipitation. In this paper, we further show that annual streamflow and annual base flow were influenced by precipitation in proceeding two to three years and this effect was manifested through increased soil moisture/soil storage which in turn influenced base flow and surface runoff processes. Recently, changes in daily hydrographs for years with similar annual precipitation have been attributed to tile drainage and cropping system changes. We also show that changes in daily streamflow hydrographs are also linked to soil wetness through preceding years' precipitation. We conclude that evaluation of LULC change impacts through comparison of daily, seasonal, or annual streamflow or base flow records between two periods needs to be done at comparable soil wetness conditions or previous years' precipitations.