Role of 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 soybeans (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 was the major driver of increased streamflow instead of LULC changes. 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 thus higher flows are mainly driven by climate (precipitation). Additional analysis showed that streamflows in any given year are also influenced by the precipitation in proceeding two to three years and this effect is manifested through increased soil moisture which likely increases infiltration and surface runoff. In this presentation, we will also show how preceding year’s precipitation and in turn soil wetness impacts daily hydrographs. In the past, these changes in daily hydrographs have been mistakenly attributed to tile drainage impacts.  We will further discuss how climate change has similarly impacted baseflow in the Upper Midwestern United States.