Use of the Soil and Water Assessment Tool to Scale Sediment Delivery from Field to Watershed in an Agricultural Landscape with Topographic Depressions

Abstract

For two watersheds in the northern Midwest United States, we show that landscape depressions have a significant impact on watershed hydrology and sediment yields and that the Soil and Water Assessment Tool (SWAT) has appropriate features to simulate these depressions. In our SWAT models of the Willow River in Wisconsin and the Sunrise River in Minnesota, we used Pond and Wetland features to capture runoff from about 40% of the area in each watershed. These depressions trapped considerable sediment, yet further reductions in sediment yield were required for calibration and achieved by reducing the Universal Soil Loss Equation (USLE) cropping-practice (P) factor to 0.40 to 0.45. We suggest terminology to describe annual sediment yields at different conceptual spatial scales and show how SWAT output can be partitioned to extract data at each of these scales. These scales range from plot-scale yields calculated with the USLE to watershed-scale yields measured at the outlet. Intermediate scales include field, upland, pre-riverine, and riverine scales, in descending order along the conceptual flow path from plot to outlet. Sediment delivery ratios, when defined as watershed-scale yields as a percentage of plot-scale yields, ranged from 1% for the Willow watershed (717 km) to 7% for the Sunrise watershed (991 km). Sediment delivery ratios calculated from published relations based on watershed area alone were about 5 to 6%, closer to pre-riverine-scale yields in our watersheds.

Applications of the SWAT Model Special Section: Overview and Insights

“…These studies describe a wide range of factors that impacted sediment yield and transport results, including watershed delineation and input data resolution, variations in landscape features, and sediment routing methods. Zabaleta et al (2014), Almendinger et al (2014), and Beeson et al (2014) report mostly successful calibration and validation of sediment yield results, with R 2 and NSE values ranging from 0.47 to 0.94 for daily and/or monthly time-step comparisons. Bonumá et al (2014), Bieger et al (2014), and Lu et al (2014), on the other hand, report weaker statistical results, especially NSE values, in their respective validation periods, which ranged between -12.1 and 0.38.…”

“…Zabaleta et al (2014) and Watson and Putz (2014) also discuss problems with precipitation data inaccuracies. Almendinger et al (2014) modified SWAT to allow seepage from ponds, wetlands, and reservoirs to be added to lateral and groundwater flow paths, rather than be lost from the system, for the watersheds they simulated in Minnesota and Wisconsin. Beeson et al (2014) modified SWAT to better represent pothole systems (described further in Beeson et al, 2011).…”

“…Eight of the special collection papers report sediment yield and/or transport results (Almendinger et al, 2014;Beeson et al, 2014;Bieger et al, 2014;Bonumá et al, 2014;Lu et al, 2014;Santhi et al, 2014;White et al, 2014c;Zabaleta et al, 2014). These studies describe a wide range of factors that impacted sediment yield and transport results, including watershed delineation and input data resolution, variations in landscape features, and sediment routing methods.…”

SWAT Modeling for Depression-Dominated Areas: How Do Depressions Manipulate Hydrologic Modeling?

“…In most studies, the depression storage is considered as a lumped volume of water by incorporating the wetlands and ponds tools in SWAT [20,21]. Specifically, depressions are incorporated at the sub-basin scale and linked to the routing process in SWAT.…”

“…Specifically, depressions are incorporated at the sub-basin scale and linked to the routing process in SWAT. However, depression storage is a function of infiltration and other hydrologic processes, and thus the ponds and wetlands tools are not able to represent the real characteristics of depressions [20][21][22].…”