A comprehensive, integrated computer model for basin-wide water resources management

The Rattlesnake Creek Basin in south-central Kansas has been experiencing substantial increase in water demand in recent years. Streamflow depletions and ground-water level declines are evidence of significant stress on the water resources in the area. A comprehensive, integrated numerical model was developed to simulate surface-water flow, ground-water flow, and stream-aquifer interactions on a continuous basis at a regional scale to serve as a tool for evaluating long-term management strategies. The surface-water flow model SWAT (Soil and Water Assessment Tool) and the ground-water flow model MODFLOW (MODular three-dimensional finite-difference ground-water FLOW) were utilized for constructing the comprehensive combined model, SWATMOD. Modeling of the surface-water flow system using the SWAT model is presented in detail. Data related to weather, land use conditions, and topographic features were collected from various sources and used as input for the model. A weighted-average procedure was implemented to overcome the quasi-lumped nature of SWAT and to better represent heterogeneity within the basin. Approximately 62 percent of the area in the basin is non-contributing. The pond option in the SWAT model was used to represent non-contributing areas. Extensive sensitivity analyses on model parameters were conducted, and model limitations and parameter uncertainties were emphasized. The model was calibrated (for the period 1955 to 1980) and validated (for the period 1981-1994) using historical data. A trial-and-error technique was employed to calibrate the model based on measured ground-water levels, streamflows, and reported irrigation water use amounts. The verification results indicated a satisfactory match between the observed and simulated ground-water levels and streamflows.; To facilitate the use of the complex SWATMOD model, a graphics-based, user-friendly, pre- and post-processor program, the Decision Support System (DSS), was developed. The DSS enables the user to modify specific hydrologic conditions and water withdrawal patterns, perform simulations, and view the impact of such modifications on streamflows and ground-water levels. The DSS was used to implement and evaluate a number of hypothetical management scenarios involving changes in water withdrawal rates and results are presented. Model results were used to identify management scenarios that could improve streamflows and ground-water levels in the future.