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Saturation-excess runoff is the major runoff mechanism in humid well-vegetated areas where infiltration rates often exceed rainfall intensity. Although the Soil and Water Assessment Tool (SWAT) is one of the most widely used models, it predicts runoff based mainly on soil and land use characteristics, and is implicitly an infiltration-excess runoff type of model. Previous attempts to incorporate the saturation-excess runoff mechanism in SWAT fell short due to the inability to distribute water from one hydrological response unit to another. This paper introduces a modified version of SWAT, referred to as SWAT-Hillslope (SWAT-HS). This modification improves the simulation of saturation-excess runoff by redefining hydrological response units based on wetness classes and by introducing a surface aquifer with the ability to route interflow from 'drier' to 'wetter' wetness classes. Mathematically, the surface aquifer is a nonlinear reservoir that generates rapid subsurface stormflow as the water table in the surface aquifer rises. The SWAT-HS model was tested in the Town Brook watershed in the upper reaches of the West Branch Delaware River in the Catskill region of New York, USA. SWAT-HS predicted discharge well with a Nash-Sutcliffe Efficiency of 0.68 and 0.87 for daily and monthly time steps. Compared to the original SWAT model, SWAT-HS predicted less surface runoff and groundwater flow and more lateral flow. The saturated areas predicted by SWAT-HS were concentrated in locations with a high topographic index and were in agreement with field observations. With the incorporation of topographic characteristics and the addition of the surface aquifer, SWAT-HS improved streamflow simulation and gave a good representation of saturated areas on the dates that measurements were available. SWAT-HS is expected to improve water quality model predictions where the location of the surface runoff matters. [ABSTRACT FROM AUTHOR]

Despite observational evidence of lateral preferential flow paths in northeastern U.S. watersheds, their effects on the sources of runoff remain unclear. An intense field survey was undertaken during the 2007 growing season to determine the sources of stream runoff from a 2.51 km² watershed in the Catskill Mountains, New York State. Lateral preferential flow paths are caused by groundwater springs and soil piping in this region. A two-component hydrograph separation using δ18O showed that event water (rain water) was a significant source of runoff during nine rainfall events (from July to October). With these rainfall events, 14 to 37% of the volume and 18 to 49% of the peak streamflow was attributable to event water. Further, end-member mixing analysis (EMMA), using δ18O, Si, and dissolved organic carbon (DOC), showed that saturated areas accounted for 2 to 24% of the total volume and 4 to 59% of peak streamflow but that groundwater was the dominant source of runoff volume during all events. Field surveys of saturated areas also suggested that near-stream areas were insufficient to generate the observed stream chemistry during rainfall events larger than 8 mm. A connection with the hillside saturated areas was therefore required to explain the results of the hydrograph separations, which were corroborated by the timing of the transient (perched) groundwater and overland flow. The hydrometric measurements confirmed that hillside lateral preferential flow paths rapidly transported water to near-stream saturation areas during runoff events under relatively dry antecedent conditions. A qualitative comparison with conventional techniques for distributing variable saturation areas (VSA) using surface topography and soil transmissivity (i.e., topographic index and soil topographic index), which do not consider the effects of lateral preferential flow paths, demonstrated that typical parameterizations (on the order of <10−1 m) would not have the spatial resolution to represent the measured lateral preferential flow paths (on the order of <10−3 m). Overall, the results suggest that the lateral redistribution of water from hillside areas reduces the influence of surface topography and channel topology on the sources of stream runoff, a finding that is consistent with recent ones from other landscapes where glacial soils have coevolved with the terrestrial hydrology. [ABSTRACT FROM AUTHOR]

Identifying phosphorus (P) source areas and transport pathways is a key step in decreasing P loading to natural water systems. This study compared the effects of two modeled runoff generation processes - saturation excess and infiltration excess - on total phosphorus (TP) and soluble reactive phosphorus (SRP) concentrations in 10 catchment streams of a Catskill mountain watershed in southeastern New York. The spatial distribution of runoff from forested land and agricultural land was generated for both runoff processes; results of both distributions were consistent with Soil Conservation Service-Curve Number (SCS-CN) theory. These spatial runoff distributions were then used to simulate stream concentrations of TP and SRP through a simple equation derived from an observed relation between P concentration and land use; empirical results indicate that TP and SRP concentrations increased with increasing percentage of agricultural land. Simulated TP and SRP stream concentrations predicted for the 10 catchments were strongly affected by the assumed runoff mechanism. The modeled TP and SRP concentrations produced by saturation excess distribution averaged 31 percent higher and 42 percent higher, respectively, than those produced by the infiltration excess distribution. Misrepresenting the primary runoff mechanism could not only produce erroneous concentrations, it could fail to correctly locate critical source areas for implementation of best management practices. Thus, identification of the primary runoff mechanism is critical in selection of appropriate models in the mitigation of nonpoint source pollution. Correct representation of runoff processes is also critical in the future development of biogeochemical transport models, especially those that address nutrient fluxes. [ABSTRACT FROM AUTHOR]

The TOPMODEL framework was used to derive expressions that account for saturated and unsaturated flow through shallow soil on a hillslope. The resulting equations were the basis for a shallow-soil TOPMODEL (STOPMODEL). The common TOPMODEL theory implicitly assumes a water table below the entire watershed and this does not conceptually apply to systems hydrologically controlled by shallow interflow of perched groundwater. STOPMODEL provides an approach for extending TOPMODEL's conceptualization to apply to shallow, interflow-driven watersheds by using soil moisture deficit instead of water table depth as the state variable. Deriving STOPMODEL by using a hydraulic conductivity function that changes exponentially with soil moisture content results in equations that look very similar to those commonly associated with TOPMODEL. This alternative way of conceptualizing TOPMODEL makes the modelling approach available to researchers, planners, and engineers who work in areas where TOPMODEL was previously believed to be unsuited, such as the New York City Watershed in the Catskills region of New York State. [ABSTRACT FROM AUTHOR]

The Soil Water Assessment Tool (SWAT) is employed throughout the world to simulate watershed processes. A limitation of this model is that locations of saturation excess overland flow in hilly and mountainous regions with an impermeable layer at shallow depth cannot be simulated realistically. The objective of this research is to overcome this limitation with minor changes in the original SWAT code. The new approach is called SWAT-with-impervious-layers (SWAT-wil). Adaptations consisted of redefining the hillslope length, restricting downward percolation from the root zone, and redefining hydrologic response units (HRUs) such that they are associated with the landscape position. Finally, input parameters were chosen such that overland flow from variable saturated areas (VSAs) corresponds to the variable source interpretation of the Soil Conservation Service (SCS) curve number runoff equation. We tested the model for the Town Brook watershed in the Catskill Mountains. The results showed that the discharge calculated with SWAT-wil agreed with observed outflow and results simulated with the original SWAT and SWAT-hillslope (SWAT-HS) models that had a surface aquifer that transferred water between groups of HRUs. The locations of the periodically saturated runoff areas were predicted by SWAT-wil at the right locations. Current users can utilize the SWAT-wil approach for catchments where VSA hydrology predominates. [ABSTRACT FROM AUTHOR]