THE IMPACT OF STORM CHARACTERISTICS AND LAND USE ON NUTRIENT EXPORT IN TWO GLACIATED WATERSHEDS IN CENTRAL INDIANA, USA

Abstract

This study investigated nutrient export during three spring storm events in two different land use watersheds (agricultural and mixed land use) in a glacial till landscape of the Midwestern, USA. The objectives of the study were: (1) to determine how land use affects water, nitrate, soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC) delivery (timing, amount) to streams during spring storms in two central Indiana watersheds with contrasting land use; and (2) to determine nitrate, SRP and DOC flow pathways to streams during spring storms. High frequency stream sampling of nutrients and cations, coupled with hydrograph separations using δ18O, was used to identify water flow pathways and event and pre-event water contributions to the streams. Data indicate land use and storm characteristics play a role in the export of water and nutrients. In the agricultural watershed (Watershed A), the storm hydrograph is dominated by pre-event water, whereas the mixed land use watershed (Watershed M) storm hydrograph is more event water dominated. Watershed A also contains higher nutrient concentrations, especially nitrate. High bulk precipitation and greater maximum intensity export more nitrate, SRP, and DOC to the streams. Results also indicate nitrate, DOC, and SRP concentrations display distinct temporal patterns during spring storm events. DOC concentration increased with stormflow and peaked on the rising limb/with maximum discharge regardless of land use or storm event. In Watershed A, SRP concentration followed a similar pattern to DOC during small storms; therefore they are believed to be exported together with flushing of saturated near-surface soil waters via macropores/overland flow. However, SRP likely has multiple flowpaths, one dominated over another depending on the storm. Nitrate concentrations matched Ca2+, Mg2+, and Na+ trajectories and decreased with stormflow, suggesting a tile drain/subsurface flowpath. Nitrate and SRP peak concentrations are delayed relative to DOC in Watershed M. The wet retention ponds in the headwaters are believed to delay the stormflow response, and therefore, the delivery of nutrients to the stream.