A GIS-based variable source area hydrology model

Abstract

Abstract:Eective control of nonpoint source pollution from contaminants transported by runo requires information about the source areas of surface runo. Variable source hydrology is widely recognized by hydrologists, yet few methods exist for identifying the saturated areas that generate most runo in humid regions. The Soil Moisture Routing model is a daily water balance model that simulates the hydrology for watersheds with shallow sloping soils. The model combines elevation, soil, and land use data within the geographic information system GRASS, and predicts the spatial distribution of soil moisture, evapotranspiration, saturation-excess overland¯ow (i.e., surface runo), and inter¯ow throughout a watershed. The model was applied to a 170 hectare watershed in the Catskills region of New York State and observed stream¯ow hydrographs and soil moisture measurements were compared to model predictions. Stream¯ow prediction during non-winter periods generally agreed with measured¯ow resulting in an average r 2 of 0 . 73, a standard error of 0 . 01 m 3 /s, and an average Nash-Sutclie eciency R 2 of 0 . 62. Soil moisture predictions showed trends similar to observations with errors on the order of the standard error of measurements. The model results were most accurate for non-winter conditions. The model is currently used for making management decisions for reducing non-point source pollution from manure spread ®elds in the Catskill watersheds which supply New York City's drinking water.

Hydrologic assessment of an urban variable source watershed in the northeast United States

“…[14] SMDR is a physically based, fully distributed, water balance model for shallow, sloping soils underlain by a restrictive layer [Steenhuis and van der Molen, 1986;Zollweg et al, 1996;Kuo et al, 1999;Frankenberger et al, 1999;Mehta et al, 2004;Gérard-Marchant et al, 2005]. The SMDR framework is adapted to run on the geographic information system (GIS) GRASS [U.S. Army Construction and Engineering Laboratory, 1997;Neteler and Mitasova, 2002].…”

“…Presented below are the crucial component processes in the order they occur in the model. For greater detail on SMDR formulation, one is referred to Zollweg et al [1996], Frankenberger et al [1999], and Gérard-Marchant et al [2005].…”

“…The rate at which water percolates out of the soil profile is controlled by the effective hydraulic conductivity of the underlying restricting layer. Effective conductivities used were 3 mm d À1 for bedrock and 0.2 mm d À1 for fragipan, within the published ranges for New York soils [Maidment, 1993;Frankenberger et al, 1999]. Below q FC percolation is negligible.…”

“…The initial portion of the event (29 March to 1 April 2005, Figure 11), consisted almost entirely of snowmelt, and was under predicted on all but one landscape sampling location (landscape 8). As noted by other researchers [Frankenberger et al, 1999;Mehta et al, 2004;Gérard-Marchant et al, 2005] SMDR is most prone to errors when the temperature fluctuates around freezing, which occurred during this period. On the following days (2 -4 April 2005), where there was minimal snowmelt and the majority of the precipitation occurred, the runoff losses were generally well captured (Figure 11).…”

“…Since the soil permeability of these areas is generally greater than the precipitation rate, runoff generation is predominantly by saturation excess. A saturated area may form during extended rainfall events where subsurface flow converges with the soil surface [Frankenberger et al, 1999], the slope flattens [Peters et al, 1995], the depth to the impervious layer decreases [Ogden and Watts, 2000], or the soil otherwise has little additional storage capacity [Dunne, 1978]. However, the spatial-temporal extent of these areas, termed variable source areas (VSA), is dynamic and not easily predicted.…”