Document Type


Publication Date

January 1990


Executive Summary: This report summarizes work conducted during the initial funding period (November 1, 1989 through June 30, 1990) of a Cooperative Agreement between the United States Forest Service (USFS) and the Utah Water Research Laboratory (UWRL), Utah State University. The purpose of the agreement is to develop a procedure for incorporating western mountain climate into the existing Climate Generator (CLIGEN), which is part of the Water Erosion Prediction Project (WEPP) procedure. In the Western U.S., few meteorological observations exist in high elevation areas where Forest Service properties are located. Therefore, a procedure for estimating climatological variables in mountainous areas is needed to apply WEPP in these regions. A physically-based approach, an expanded and improved orographic precipitation model, is proposed in this report. It will use radiosonde data and also lightning data to simulate convective storms. Climatological sequences thus estimated at ungaged locations will be represented using stochastic models, similar to the approach used in the existing CLIGEN, and their parameters will be available to users through maps. By using these stochastic models, WEPP users can synthesize climate sequences for input to WEPP. Several alternative approaches to developing the Mountain Climate Generator (MCLIGEN) have been formulated and evaluated. These options vary in their spatial resolution. Some will provide synthetic climate inputs whereas others will provide synthetic sequences of water delivery to the ground surface or overland flow delivery. The latter will reduce the user's responsibility for judging adequate snowpack or hydrological simulations, but will enormously increase the effort required for parameterization during the developmental phase. Based on our evaluation, we recommend that Option 2 for generating fine scale climate sequences be adopted. This option appears to satisfy the WEPP spatial resolution requirements of the USFS and requires a reasonable level of developmental effort. We also recommend that Option 3 be available to the users. We recomment that under this option snowpack initial conditions at a specified date be available based on a return period or exceedance probability. Under this option discontinuous simulation periods could be considered. The data, models, and parameters needed to implement the recommended approach can be divided into three parts: 1) climatological process models, 2) a snowpack imulation model, and 3) stochastic models of climatological variables and parameter regionalization. A chapter of the report is devoted to each of these three parts. Each chapter includes a literature review and a description of the proposed methodology and work plan for its development. We further recommend that a comprehensive plan for data collection for validation of the entire WEPP methodology applied to the mountainous Western U.S. be developed. Also, we propose that UWRL take the lead in settin gup a user group for orographic precipitation modelers.