Precipitation Uncertainty and Raingauge Network Design within Folsom Lake Watershed

The present study examines the estimation of hourly precipitation over the Folsom Lake watershed in mountainous California. The objectives are to (1) quantify the uncertainty associated with the estimation of precipitation by the existing operational real-time network of gauges, and (2) identify possible sites for the placement of additional gauges in order to reduce the precipitation interpolation error. To reach these goals, historical hourly data are used from a dense network operated during the period 1980–1986. The approach consists of developing a series of surrogate operational networks from the dense network using an increasing number of gauges and quantifying the reduction of estimation error in each case. Kriging is used to interpolate the point measurements to the nodes of a rectangular 3.7 km grid superimposed over the catchment. The variance of the kriging estimates is considered in the analysis. The recommended additional gauge sites are mainly located in the upper parts of the forks of the American River, which drain the Folsom Lake watershed. The estimated reduction in local variance due to the placement of the additional gauges is up to 70%, with substantial reduction of the estimation bias, in the headwaters of the Middle and North Forks of the American River, with a smaller reduction for the South Fork. The proposed gauge network is most appropriate for short-term flood forecasting applications.