Abbreviations: DEM, digital elevation model; E NS , Nash-Sutcliffe coefficient of efficiency; HRU, hydrologic response unit; MUSLE, modified universal soil loss equation; NPS, nonpoint[r]
(1)TECHNICAL REPORTS
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 km2) to 7% for the Sunrise
watershed (991 km2) Sediment delivery ratios calculated from
published relations based on watershed area alone were about to 6%, closer to pre-riverine–scale yields in our watersheds
Use of the Soil and Water Assessment Tool to Scale
Sediment Delivery from Field to Watershed in an
Agricultural Landscape with Topographic Depressions
James E Almendinger,* Marylee S Murphy, and Jason S Ulrich
N
onpoint-source (NPS) pollution is considered to be the largest threat to water quality in the United States (USEPA, 2009), yet we have limited under-standing of how NPS pollution is transported from source to receiving water (Walling, 1983) Transport mechanisms based on rainfall-runoff processes, including runoff generation and soil detachment, are well studied at the plot scale (Wischmeier and Smith, 1978) However, aquatic-resource impacts, such as chan-nel instability or lake eutrophication, emerge at the watershed scale, where catchment inputs are cumulative and integrated Application of plot-scale rates without modification to the watershed scale can be inappropriate For example, extrapolation of plot-scale erosion rates to whole watersheds can overestimate sediment loads by several orders of magnitude (Trimble and Crosson, 2000) Watershed computer models based on runoff and transport mechanisms derived from plot-scale studies have the challenge of appropriately scaling these mechanisms up to the watershedThe sediment delivery ratio (SDR) attempts to account for the differences between plot and watershed scales We here define the SDR as the area-specific sediment yield (t km−2 yr−1
or t ha−1 yr−1) measured at a watershed outlet divided by an
area-weighted gross, or plot-scale, erosion (same units) The SDR may be expressed as a fraction or as a percentage Gross erosion is commonly calculated with the universal soil loss equation (USLE) or its revised variant (RUSLE) The USLE methods have been criticized as being arbitrary, and other estimates of gross erosion could be used (Parsons et al., 2006); nonetheless, USLE-calculated gross erosion rates provide a critical, repli-cable benchmark against which other erosion rates may be con-trasted Empirical studies have shown that watershed sediment yield decreases substantially as watershed area increases (Dendy and Bolton, 1976), so much so that the SDR may drop below 10% as watershed area reaches 100 km2 (Roehl, 1962; Shen
and Julien, 1993) This relation is well known but poorly con-strained: it encompasses a huge amount of variability and has a
Abbreviations: DEM, digital elevation model; ENS, Nash-Sutcliffe coefficient of efficiency; HRU, hydrologic response unit; MUSLE, modified universal soil loss equation; NPS, nonpoint-source; SDR, sediment delivery ratio; SWAT, Soil and Water Assessment Tool; USLE, universal soil loss equation
J.E Almendinger, St Croix Watershed Research Station, Science Museum of Minnesota, 16910 152nd St N., Marine on St Croix, MN 55082; M.S Murphy, Three Rivers Park District, 3000 Xenium Ln N., Plymouth, MN 55441; J.S Ulrich, Dep of Bioproducts and Biosystems Engineering, Univ of Minnesota, 1390 Eckles Ave., St Paul, MN 55108 Assigned to Associate Editor Ali Sadeghi
Copyright © 2012 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America All rights reserved No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher J Environ Qual
doi:10.2134/jeq2011.0340 Received 19 Sept 2011
*Corresponding author (dinger@smm.org) © ASA, CSSA, SSSA
5585 Guilford Rd., Madison, WI 53711 USA
Journal of Environmental Quality
APPLICATIoNS oF THE SWAT MoDEL