FIGURE 5 - uploaded by William J. Elliot
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Thirty-Year Average Runoff Depth (mm/yr), Average Lateral Flow Depth (mm/yr), and Number of Days of Runoff Simulated Along (a) a Concave Hillslope, (b) a Convex Hillslope, and (c) an S-Shaped Hillslope, Each Using 1 OFE vs. 10 OFEs.
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We incorporated saturation excess overland flow processes in the Water Erosion Prediction Project (WEPP) model for the evaluation of human disturbances in watersheds. In this presentation, we present results of the modified WEPP model to two watersheds: an agricultural watershed with mixed land use, and a forested watershed. The agricultural waters...
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... output from WEPP-UI of average annual surface runoff depth and average number of days per year of surface runoff using 10 OFEs as compared to a single OFE are presented in Figure 5 for three slope configurations: concave slope, convex slope, and S-shaped slope. The average annual hydrologic response at the outlet for all three slope configurations for various approaches ranging from a single 150 m OFE to a hillslope broken down into 19, 8 m OFEs is summarized in Table 3. ...
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... flow over the entire hillslope (Figures 5a-5c). In contrast, the hydrology of a hillslope is greatly controlled by the shape of a hillslope when using multiple OFEs. ...
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... concave hillslope generates more runoff for a longer period of time than the other two slope shapes (Table 3 and Figure 5a). Compared to the single OFE simulation, when using multiple OFEs the model accounts for the convergence of subsurface lateral flow in the downslope direction. ...
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... important distinction is that a single OFE implies that the surface runoff depth is uniform over the entire length of the slope. As seen in Figure 5a, both the depth of runoff and number of days of runoff per year is smaller than that predicted by the single OFE for the upper six OFEs (i.e., the upper 60% of the hillslope). In other words, the majority of the surface runoff is generated on the lower 40% of the hillslope, as expected for saturation-excess runoff generation (Figure 5a). ...
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... seen in Figure 5a, both the depth of runoff and number of days of runoff per year is smaller than that predicted by the single OFE for the upper six OFEs (i.e., the upper 60% of the hillslope). In other words, the majority of the surface runoff is generated on the lower 40% of the hillslope, as expected for saturation-excess runoff generation (Figure 5a). ...
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... hillslopes where surface runoff generation is dominated by saturation-excess processes (i.e., variable source area hydrology), there tends to be an inverse relationship between the generation of lateral flow and surface runoff. Figure 5 shows the distribution of both lateral subsurface flow and surface runoff along the three slope configurations (lateral flow for a single OFE is not shown because it is constant along the slope; see Table 3). Notice that subsurface lateral flow is greatest and surface runoff is smallest at the top of the slope and with increasing distance downslope subsurface lateral flow decreases and surface runoff increases. ...
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... that subsurface lateral flow is greatest and surface runoff is smallest at the top of the slope and with increasing distance downslope subsurface lateral flow decreases and surface runoff increases. By the end of the slope, the average annual surface runoff is greater than the average annual subsurface lateral flow (Figure 5a and Table 3). According to the simulation using a single OFE much more of the water leaves the concave hillslope as subsurface lateral flow (186 mm/yr), than as surface runoff (48 mm/yr). ...
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... contrast to the concave slope, surface runoff in a convex-shaped slope is much less affected by downslope convergence of subsurface lateral flow, and, therefore, simulated surface runoff is much more uniform along the slope (Figure 5b). The hydraulic gradient increases with distance downslope resulting in better downslope drainage. ...
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... are much fewer runoff events on a convex slope as compared to a concave slope. On average, surface runoff only occurs 5 days/yr on a convex slope as opposed to 82 days/yr for a concave slope (Figures 5a and 5b). Runoff is greatest on the upper flat slopes at the top of a convex hillslope. ...
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... is greatest on the upper flat slopes at the top of a convex hillslope. This upper section of the hillslope has low lateral flow gradients and, therefore, soils remain at greater soil moisture content (Figure 5b). As the slope begins to increase, lateral flow gradients increase and surface runoff generated upslope reinfiltrates into the soil, illustrated with the decrease in surface runoff. ...
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... S-shaped slope configuration shows the effects of the convex hillslope in the upper sections, and the effects of the concave hillslope in the lower sections (Figure 5c). Surface runoff increases downslope as the hydraulic gradient is reduced, and storage is exceeded, resulting in saturation-excess runoff at the bottom of the slope. ...
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... 7 illustrates the distribution of cumulative sediment yield and net erosion for the S-curve shape. Although average annual runoff at the outlet of the S-curve hillslope is greater using multiple OFEs, along the slope the 10 OFEs simulation shows greater surface runoff than the single OFE simulation only on the last two OFEs (i.e., the bottom 30 m of the slope; Figure 5c). From Figure 7, WEPP simulated net deposition over the last 30 m rather than net soil detachment. ...
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... Each hillslope can be divided into up to 19 overland flow elements (OFEs) with unique combinations of vegetation and management characteristics. Mathematically, this characterization based on the OFEs is important to better represent the variablesource-area hydrology in complex topography (Boll et al., 2015;Crabtree et al., 2006). We used the GeoWEPP model (Renschler, 2003) to create hillslope files for each of the five study watersheds based on the 10-m digital elevation model (DEM) of the Tahoe basin. ...
... Over the last decade this team has made significant improvements to the WEPP model and has developed interfaces for easier use of the model. Among the improvements are the addition of Penman-Montieth evapotranspiration algorithms (Wu et al., 2004a), subsurface converging lateral flow to represent variable source area runoff (Wu et al., 2004b;Boll et al., 2006a;Crabtree et al., 2006), improving canopy biomass routines for forested applications (Dun et al., 2006), and developing algorithms to simulate the effect of structures and impoundments on runoff and sediment delivery (Wu and Dun, 1998). The WEPP model now produces reliable, continuous simulations of streamflow and sediment loading at the hillslope and watershed scales in mixed land use watersheds (Boll et al., 2006a,b;Crabtree et al. 2006; see attached Figures 1 and 2). ...
... Among the improvements are the addition of Penman-Montieth evapotranspiration algorithms (Wu et al., 2004a), subsurface converging lateral flow to represent variable source area runoff (Wu et al., 2004b;Boll et al., 2006a;Crabtree et al., 2006), improving canopy biomass routines for forested applications (Dun et al., 2006), and developing algorithms to simulate the effect of structures and impoundments on runoff and sediment delivery (Wu and Dun, 1998). The WEPP model now produces reliable, continuous simulations of streamflow and sediment loading at the hillslope and watershed scales in mixed land use watersheds (Boll et al., 2006a,b;Crabtree et al. 2006; see attached Figures 1 and 2). In Dr. Boll's study funded by the Conservation Effectiveness Assessment Program of the USDA-CSREES, WEPP and CONCEPTS models are used to evaluate the cumulative effects of erosion control management practices in large watersheds. ...
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