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Using hydrologic models such as the Soil and Water Assessment Tool (SWAT) to quantify the conveyance, storage, and retention functions of wetlands is a cost-effective means of generating the scientific information required for watershed management and wetland protection/restoration. However, it has been a challenge to appropriately represent wetlands in models, and few SWAT applications reported in the literature have considered wetlands. The objectives of this study were to (1) demonstrate how to incorporate wetlands into a SWAT model using a "hydrologic equivalent wetland" (HEW) concept and (2) use the SWAT model to simulate the streamflows in the 4506 km² upper portion of the Otter Tail River watershed in northwestern Minnesota. The HEWs were defined in terms of six calibrated parameters: the fraction of the subbasin area that drains into wetlands (WET_FR), the volume of water stored in the wetlands when filled to their normal water level (WET_NVOL), the volume of water stored in the wetlands when filled to their maximum water level (WET_MXVOL), the longest tributary channel length in the subbasin (CH_L1), Manning's n value for the tributary channels (CH_N1), and Manning's n value for the main channel (CH_N2). The model performance was judged in terms of three statistics: the Nash-Sutcliffe coefficient (Ej²), the coefficient of determination (R²), and the performance virtue (PVk). For the study area, the HEWs were determined to have a value for WET_FR ranging from 50% to 96%, i.e., draining more than 50% of the subbasins, depending on the spatial distribution and types of the wetlands in each subbasin. A wetland with a minimal alteration tended not to contribute water to the streams until it reached its maximum capacity, as indicated by identical calibrated values for WET_NVOL and WET_MXVOL for all the subbasins except for the three that included altered wetlands. The results indicated a strong linear relationship between WET_NVOL and the corresponding surface area (R² = 0.98) and an overall average depth of 0.85 m for the HEWs. The parameters WET_NVOL and WET_MXVOL may primarily represent the storage function of the wetlands, whereas the conveyance and retention functions would be reflected by the latter five parameters. In addition, the SWAT model incorporating the HEW assumption had an acceptable or satisfactory performance in simulating the streamflows for an evaluation period from 1 December 1969 to 31 May 1975 at daily, monthly, seasonal, and annual time steps (Ej² > 0.36, PVk > 0.75), although its performance varied from one evaluation station to another and might be weak for spring and fall in some evaluation years. Nevertheless, additional model runs with the "no wetland" and "synthetic wetland" assumptions, which are two approaches typically used to deal with wetlands for modeling purposes, indicated that the HEW concept is superior in incorporating wetlands into SWAT for the study area. [ABSTRACT FROM AUTHOR]

The Soil and Water Assessment Tool (SWAT), a widely used watershed hydrology and water quality model provides three different methods (Hargreaves, Priestley-Taylor, and Penman-Monteith) for estimating potential evapotranspiration (PET) and the corresponding actual evapotranspiration (AET). Although these methods have been extensively tested, the effects of using them within SWAT's framework are largely unknown. The objective of this study was to test the three PET methods within SWAT's framework using data collected in the Wild Rice River watershed, located in northwestern Minnesota. The performance of the SWAT models was measured using three statistics: the Nash-Sutcliffe coefficient (Ej²), coefficient of determination (R⊃2), and performance virtue (PVk). The three models were independently calibrated and validated using the observed daily stream flows at two USGS gauging stations. The simulated stream discharges were compared with the corresponding observed values and the estimated evapotranspiration examined in accordance with the wet-environment areal evapotranspiration (ETw) derived from the evaporation data for Williams Lake, located about 100 km southeast of the study watershed. The use of the three PET methods resulted in different values for two calibration parameters, namely the soil evaporation compensation factor and SCS curve number. At the lower station, which is near the watershed outlet, the observed annual mean discharge (8.33 m³/s) during the model validation period was predicted to be 10.25, 10.87, and 9.69 m³/s by SWAT-Penman, SWAT-Priestley, and SWAT-Hargreaves, respectively. The annual mean discharge (10.83 m³/s) was more accurately predicted during the model calibration period, with an absolute error of less than 0.5 m³/s. The prediction errors for the upper station were comparable with those for the lower station. In addition, all three models exhibited good performance when simulating the monthly, seasonal and annual mean discharges (Ej² >0.75 and PVk >0.80) and satisfactory performance when predicting the daily stream flows (Ej² >0.36 and PVk >0. 70). In estimating evapotranspiration for the study watershed, SWAT-Hargreaves seemed to be slightly superior to the other two models, while SWAT-Priestley might be more appropriate for an Erw value greater than 8.0 mm/d. Nevertheless, the AET values estimated by the three models shared a concurrent spatial pattern and temporal trend, and were insignificantly different from each other at a 5% significance level (p-values > 0.05). The results indicated that after calibration, using the three ET methods within SWAT produced very similar hydrologic (AET and discharge) predictions for the study watershed. [ABSTRACT FROM AUTHOR]

Wetlands are one of the most important watershed microtopographic features that affect hydrologic processes (e.g., routing) and the fate and transport of constituents (e.g., sediment and nutrients). Efforts to conserve existing wetlands and/or to restore lost wetlands require that watershed-level effects of wetlands on water quantity and water quality be quantified. Because monitoring approaches are usually cost or logistics prohibitive at watershed scale, distributed watershed models such as the Soil and Water Assessment Tool (SWAT), enhanced by the hydrologic equivalent wetland (HEW) concept developed by Wang [Wang, X., Yang, W., Melesse, A.M., 2008. Using hydrologic equivalent wetland concept within SWAT to estimate streamflow in watersheds with numerous wetlands. Trans. ASABE 51 (1), 55-72.], can be a best resort. However, there is a serious lack of information about simulated effects using this kind of integrated modeling approach. The objective of this study was to use the HEW concept in SWAT to assess effects of wetland restoration within the Broughton's Creek watershed located in southwestern Manitoba, and of wetland conservation within the upper portion of the Otter Tail River watershed located in northwestern Minnesota. The results indicated that the HEW concept allows the nonlinear functional relations between watershed processes and wetland characteristics (e.g., size and morphology) to be accurately represented in the models. The loss of the first 10-20% of the wetlands in the Minnesota study area would drastically increase the peak discharge and loadings of sediment, total phosphorus (TP), and total nitrogen (TN). On the other hand, the justifiable reductions of the peak discharge and loadings of sediment, TP, and TN in the Manitoba study area may require that 50-80% of the lost wetlands be restored. Further, the comparison between the predicted restoration and conservation effects revealed that wetland conservation seems to deserve a higher priority while both wetland conservation and restoration may be equally important., (Copyright 2010 Elsevier Ltd. All rights reserved.)