Effect of Spatial Distribution of Rainfall on Temporal and Spatial Uncertainty of SWAT Output

Abstract

Accurate rainfall data are critical for accurate representation of temporal and spatial uncertainties of simulated watershed-scale hydrology and water quality from models. In addition, the methods used to incorporate the rainfall data into the simulation model can significantly impact the results. The objectives of this study were to (1) assess the hydrologic impacts of different methods for incorporating spatially variable rainfall input into the Soil and Water Assessment Tool (SWAT) in conjunction with subwatershed delineation level and (2) assess seasonal and spatial uncertainty in hydrologic and water quality simulations of SWAT with respect to rain gauge density. The study uses three different methods to incorporate spatially variable rainfall into the SWAT model and three levels of subwatershed delineation. The impacts of ten different gauge-density scenarios on hydrology and water quality were subsequently evaluated by using the highest gauge-density scenario as a baseline for comparison. Through the centroid method, which is currently used by the AVSWAT-X interface, variations in the representation of measured annual rainfall as model input and corresponding simulated streamflow increased as subwatershed delineation level decreased from high-density to low-density. The rainfall input by the Thiessen averaging method for each subwatershed (Thiessen method) and the inverse-distance-weighted averaging method for the entire watershed (average method) were not sensitive to subwatershed delineation. The impacts of delineation on streamflow were also less with these two methods. The Thiessen method is recommended for SWAT simulation of a watershed with high spatial variability of rainfall. The currently used AVSWAT-X centroid method will also accurately represent spatially variable rainfall if a subwatershed delineation is used that sufficiently incorporates the density of observed rainfall stations. As the number of rain gauges used for the simulation decreased, the uncertainty in the hydrologic and water quality model output increased exponentially. Total phosphorus was most sensitive to the changes in rain gauge density, with an average coefficient of variation of root mean square difference (CVRMSD) of 0.30 from three watersheds, followed by sediment, total nitrogen, and streamflow, showing CVRMSD values of 0.24, 0.18, and 0.17, respectively. Seasonal variations in simulated streamflow and water quality were higher during summer and fall seasons compared to spring and winter seasons. These seasonal and temporal variations can be attributed to the rainfall patterns within the watershed.