Spatial Analysis to Retrieve SWAT Model Reservoir Parameters for Water Quality and Quantity Assessment

Owing to their capacity to conserve water and regulate streamflow, small reservoirs are useful for agriculture, domestic water supply, energy production, industry, flood control, recreation, fisheries, and ecosystem conservation. The presence of these small reservoirs often affects the natural water pathways, but the use of a hydrological model such as the Soil and Water Assessment Tool (SWAT) can help to better apprehend these effects at the watershed scale. Indeed, the SWAT model allows modelers to represent and operate reservoirs by inputting the related parameters while setting the model. However, these reservoir parameters are not automatically generated by the SWAT model algorithms. Subsequently, SWAT users are left alone and must sort out the adequate approach to separately obtain or determine the reservoir parameters. Traditionally, reservoir parameters such as the volumes and surface areas are obtained through in situ hydrographic surveys which are costly and labor demanding. To help SWAT modelers retrieve the input parameters needed for modeling small reservoirs, this paper explicitly presents a spatial analysis procedure using the case study of a small watershed reservoir. In this procedure, the digital elevation model of the watershed is transformed into a triangulated irregular network and turned into contour lines which are used to identify the reservoir surface and volume at the principal and emergency spillways. The retrieved parameters were successfully used to calibrate and validate SWAT simulations of the watershed hydrological behavior. The spatial analysis procedure reported here is a cost-effective alternative to traditional in situ hydrographic surveys and it is useful for addressing watersheds with small reservoirs. The procedure eases the inclusion of reservoirs in SWAT and reduces the risk of model overfitting. Furthermore, the procedure could be useful for developing reservoir elevation-capacity-area curves.