Hydrological models have been increasingly used for the effect of land cover change and forest management operations on hydrological processes. In the Sierra Nevada, where timber harvest and prescribed fire are commonly employed for forest management, hydrological models have rarely been used, especially in small watersheds. In this research, the SWAT model (Soil Water and Assessment Tool) was used to simulate streamflow on a daily time-step in P301, a small headwater mountain watershed located in the southern Sierra Nevada. The watershed is 1 km2, where about 72% of the land is covered by a dense mixed-conifer forest. SWAT performs satisfactorily with a coefficient of determination (R2) of 0.59 and a Nash-Sutcliffe efficiency value (NSE) of 0.59. This is important to know given the complexity arising from model uncertainty and the intricacies of Sierra Nevada hydrology. Although SWAT performed "satisfactory", the model still missed two key hydrological processes: the timing of snowmelt and isolated peak flow events. In addition, simulating streamflow on the daily time-step is good for understanding watershed processing and functioning but is not as useful for forest and land management. SWAT will need further model adjustments as well as monthly and yearly water yield estimates in order to be considered for the evaluation of forest management operations in P301.

The U.S. Geological Survey, in cooperation with the Iowa Department of Natural Resources, used the Soil and Water Assessment Tool to simulate streamflow and nitrate loads within the Cedar River Basin, Iowa. The goal was to assess the ability of the Soil and Water Assessment Tool to estimate streamflow and nitrate loads in gaged and ungaged basins in Iowa. The Cedar River Basin model uses measured streamflow data from 12 U.S. Geological Survey streamflowgaging stations for hydrology calibration. The U.S. Geological Survey software program, Load Estimator, was used to estimate annual and monthly nitrate loads based on measured nitrate concentrations and streamflow data from three Iowa Department of Natural Resources Storage and Retrieval/Water Quality Exchange stations, located throughout the basin, for nitrate load calibration.

Spatial variability of rainfall is a significant factor in hydrologic and water quality modeling. In recent years, characterizing and analyzing the effect of spatial variability of rainfall in hydrologic applications has become vital with the advent of remotely sensed precipitation estimates that have high spatial resolution. In this study, the effect of spatial variability of rainfall in hourly runoff generation was analyzed using the Soil and Water Assessment Tool (SWAT) for Big Sandy Creek and Walnut Creek Watersheds in North Central Texas. The area of the study catchments was 808 km2 and 196 km2 for Big Sandy Creek and Walnut Creek Watersheds respectively. Hourly rainfall measurements obtained from raingauges and weather radars were used to estimate runoff for the years 1999 to 2003. Results from the study indicated that generated runoff from SWAT showed enormous volume bias when compared against observed runoff. The magnitude of bias increased as the area of the watershed increased and the spatial variability of rainfall diminished. Regardless of high spatial variability, rainfall estimates from weather radars resulted in increased volume of simulated runoff. Therefore, weather radar estimates were corrected for various systematic, range-dependent biases using three different interpolation methods: Inverse Distance Weighting (IDW), Spline, and Thiessen polygon. Runoff simulated using these bias adjusted radar rainfall estimates showed less volume bias compared to simulations using uncorrected radar rainfall. In addition to spatial variability of rainfall, SWAT model structures, such as overland flow, groundwater flow routing, and hourly evapotranspiration distribution, played vital roles in the accuracy of simulated runoff.

Water balance studies for large and small river basins are the subject of this book. Here, the specific focus is on the soil and water assessment tools (SWAT) model coupled with geographic information system (GIS) remote sensing data for a comprehensive study. Some books available in the market provide an overview of different hydrological models for water balance but not specifically for the SWAT model. This book effectively utilizes the SWAT model to study the water balances in small and large catchments with consistent competence and excellent accuracy for yearly and monthly water balance modules along with suspended sediment yield over several slope classes of the catchments. The approach is new and has been successfully utilized, as discussed in a case study on the Taleghan Catchment in Iran. These implementation models may assist as advantageous techniques for incorporated management of catchments in the direction of sustainable development. This book will help readers who wish to study all the changes related to those in water balances.