Document Type


Publication Date

January 1992


Introduction: The objective of this research to develop a spatially distributed water balance model based on the integration of spatially distributed data. Progress this year has consisted of model development, instrument acquisition, installation and development of experimental procedures, and baseline data collection. The original research plan called for detailed observations related to the water balance over the year September 1991 to August 1992. The detailed measurements were to start with accumulation of the snowpack followed by melt and evapotranspiration measurements from March to August. The objective was to measure the energy balance parameters starting with the peak accumulation, through the melt and infiltration phases, the greenup of vegetation, the peak evapotranspiration period and the dry-down and senescence of grasses and other species in the upper Sheep Creek sub-basin of the Reynolds Creek Experimental Watershed. Groundwater depths as well as run-off in the stream were to be measured and samples of the snowpack water, soil water, groundwater and run-off water were to be taken for isotopic tracing. Unfortunately, average snow accumulation was well below average last winter. April 1st, snow course measurement indicated that the snowpack at the Reynolds mountain sub-basin was only 30% of normal. In the Upper Sheep Creek sub-basin, which usually has a 10-m high drift during this time of the year there was less than 0.5 m of snow. After consultation with the USDA scientists from the Northwest Watershed Research Center, we decided in February to postpone the field campaign for one year. In retrospect this was wise because there was no runoff reponse from Upper Sheep Creek (the basin where we plan to do our detailed model development) and few of the groundwater wells had measureable response, we would have had nothing to measure. The intensive field campaign will be conducted in Spring 1993. Although a repeat of last year's condition is possible, the chances are very low and we are hopeful of better snowfall. A revised project time schedule is shown on the next page. This has us finished the field measurement in August 1993. Given this it is unlikely that all data reduction and modeling will be complete by August 15, 1993 so we will need until May 1993, a 9-month no-cost extention to complete the data analysis. In this report we describe our progress in terms of data base development, model development, and modeling based on data from earlier years. Some of this will be presented at the AGU Fall meeting in San Francisco in Devember (see abstract on page 3). We also developed snow-isotope fractionation studies, evapotranspiration model development, installation of soil moisture measurement equipment, and database development. Abstract: A distributed mass balance appraoch is being developed to model spatially variable hydrologic processes in an arid mountain watershed. The model will be applied to Upper Sheep Creek, a 26 ha catchment within the Reynolds Creek ARS Experimental Watershed, near Boise, ID. The model is based on a DEM representation of basin topography. A mass balance equation relating moisture inflow, outflow, and the change in storage is resolved to give the moisture defecit in each DEM cell. Moisture input is subsurface flow from up-gradient DEM cells and surface influx from rain or a spatially distributed energy-balance snowmelt model. Subsurface outlfow is determined from topographic slope and transmissivity, which is a function of moisture content. We illustrate the effects of topography on the areal distribution of soil moisture and the time variation of streamflow in Upper Sheep Creek and compare our results with field observations and streamflow measurements.