Reduced complexity model to simulate reductions in sediment delivery from a agricultural watershed in southern Minnesota

The Greater Blue Earth River Basin (GBERB), a tributary to the Minnesota River, produces very large loads of suspended sediment. Most of the watershed is flat, poorly drained, till and glacial lake deposit that are now almost entirely in row crops with pervasive subsurface drainage tiling. The lower part of the watershed is deeply incised in response to a baselevel fall of 60 m on the Minnesota River. Agricultural conversion of the uplands in the past 200 years is correlated with accelerated sediment delivery from the watershed. The balance of sediment sources has shifted from field erosion to near-channel erosion with the advent of soil conservation and increased river discharge over the past 70 years. Solution of the sediment loading problem will require some combination of direct reduction in sediment erosion and reduction in peak river discharges via distributed water storage. To support decision-making by watershed stakeholders, we developed a watershed simulation model to evaluate reductions in GBERB sediment loading in response to different water and sediment management. The model uses a reduced complexity approach to (i) provide real-time evaluation of model uncertainty, (ii) accommodate existing information from soil mapping, stream gaging, sediment fingerprinting, and high-resolution topography, and (iii) incorporate near-channel sediment supply, which is the largest source of sediment and is not adequately represented by other watershed models. We apply a topographic filter to spatially distribute information on sediment sources to quantify the fraction of sediment sources delivered to the watershed outlet. A corresponding sediment delivery ratio is then used to discount reductions in sediment erosion from management actions. We use stream gaging records to develop a relation between peak river discharges and sediment loading from near-channel sources. This relation is used to indicate the reduction in sediment supply from upland water storage and reduce peak flows through the incised valleys in the lower watershed. The model uses as input the extent and effectiveness of management actions to reduce soil erosion, store water to reduce river peak flows, and directly address sediment inputs from near-channel sources.