DOI: 10.5066/p1mccaom
title: Santa Rosa Plain integrated hydrological model: simulating the hydrological system of the Santa Rosa Plain, California with analysis of future climate scenarios

description: The U.S. Geological Survey (USGS) conducted a study in cooperation with the Sonoma County Water Agency to assess the water resources of the SRPW and develop tools to better understand the groundwater and surface-water systems in the watershed. The foundation of these tools is the development and application of the Santa Rosa Plain hydrologic model (SRPHM) using GSFLOW to simulate historical groundwater and surface-water conditions, estimate groundwater budgets, and assess hydrologic conditions under possible future climate scenarios. In the publication (Alzraiee and others, 2025) this model is referred to as SRPHM 2.0. The original model simulated hydrological conditions and water use from water years 1975 to 2010 (Woolfenden and Nishikawa, 2014). Recently (2023), the USGS, in cooperation with Sonoma Water and the California State Water Resources Control Board, updated the SRPHM model to extend its simulation period to the end of the 2018 calendar year, incorporate new estimates of rural and agricultural water use, and use efficient input format for climate variables. The updated model was recalibrated, and evaluation of the new model calibration is included in the report (Alzraiee and others, 2025). The report presents the results of comparing the hydraulic heads, streamflow, and groundwater budget simulated by the updated model with those generated by the original model and observed data. The main difference in the simulated budget between the original (SRPHM 1.0) and updated SRPHM 2.0 is the estimates of agricultural pumping, rural domestic pumping, and return flow generated from rural water use that was not simulated in the original model. The revised agricultural pumping is simulated using the agricultural package, which constrains pumping to available groundwater. The use of the agricultural package leads to a more realistic estimation of agricultural water use, with revised agricultural pumping being one-third less than that in the original model. The updated model was then used to generate stream depletion maps, simulate climate change scenarios during 2019-99, and simulate water rights allocation using the Model for Decision Support in Integrated River Basin Management (MODSIM). The results from simulating eight future climate scenarios indicated either an increase in groundwater storage or no significant change in the next 80 years, along with an increase in recharge, an increase in actual evapotranspiration in six out of eight climate projections, and an increase in surface runoff. The increases in the simulated future groundwater storage, recharge, evapotranspiration, and runoff in most climate projections are mainly driven by the projected increase in precipitation in most of the future climate scenarios. The updated model also was used to test a pilot case study demonstrating water-resource allocation among different users with different water rights using the integrated MODSIM-Groundwater and Surface-Water Flow Model (GSFLOW) platform. The updated SRPHM serves as a valuable tool for analyzing historical and future hydrologic conditions in the Santa Rosa Plain watershed and preparing for future uncertainties.