Physically Based Estimation of Maximum Precipitation over Three Watersheds in Northern California: Relative Humidity Maximization Method

Abstract

Maximum precipitation (MP) was estimated by means of a regional atmospheric model over three watersheds in northern California [(1) the American River watershed (ARW), (2) the Yuba River watershed (YRW), and (3) the Upper Feather River watershed (UFRW)], based on the reconstruction and analyses of the historical severe storms that were recorded over these target watersheds, and where the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis data were available for regional atmospheric modeling of the historical storms. Since severe storm events are mainly caused by atmospheric rivers (ARs) in northern California, the contribution of an AR on precipitation over each of the targeted watersheds is maximized to estimate the 72-h MP. For this purpose, the moisture of ARs is maximized after shifting the regional atmospheric model’s atmospheric boundary conditions (BCs) in space with respect to latitude and longitude so that the AR strikes each of the targeted watersheds in the optimized location. To maximize the moisture of the AR, the relative humidity at the boundaries of the modeling domain is set at 100%. The 72-h MPs that were estimated by this method are 738, 690, and 513 mm over the ARW, YRW, and UFRW, respectively. The estimated MP values are respectively 1.74, 1.50, and 1.36 times larger than the historical maximum values at the previously mentioned basins.