Modeling and Analysis

Data and hydrologic models can help scientists and water resource managers better understand the effects sustainable groundwater use may have on the future of both groundwater and surface-water sources. The USGS uses various modeling software systems to analyze data and anticipate potential future scenarios that may be beneficial to creating a sustainable groundwater management plan.

• USGS Water Resources Groundwater Software (free, open-source)

Simulation of Aquifer Storage and Recovery in Pajaro Valley, Monterey Bay

Simulation of Groundwater and Surface-Water Resources for the Santa Rosa Plain Watershed

In response to growing water demands in Sonoma County, the USGS developed a fully coupled surface-water and groundwater flow model of the Santa Rosa Plain watershed using the USGS groundwater and surface-water flow model, GSFLOW. The model was used to define hydrologic budget components and to evaluate the potential hydrologic effects of future climate and projected groundwater development on the basin.

GSFLOW consists of two integrated model components: (1) a watershed-component model developed by using Precipitation Runoff Modeling System (PRMS), and (2) a groundwater-component model developed by using the Modular Groundwater Flow Model MODFLOW-NWT. Development of a calibrated, coupled groundwater and surface-water flow model allows for analysis of the movement of water through the aquifer system, and the simulated effects of hydrologic stresses or potential stresses on the groundwater and surface-water systems. By using groundwater, precipitation, surface water, and streamgaging data, scientists were able to use GSFLOW software to create a groundwater and surface-water model to better understand and help manage the hydrologic resources in the Santa Rosa Plain watershed. The model was able to simulate potential future-climate scenarios to estimate the long-term effects of climate change and variability, with and without the estimated effects of groundwater pumping.

Scientists use hydrologic models like the Santa Rosa Plain model to determine hydrologic changes given climate change, with and without pumping. In regard to the Santa Rosa Plain model, for water years 2011 to 2040, model results generally show that higher pumping rates based upon future-climate scenarios may cause further groundwater level declines, and associated reductions in groundwater discharge to streams and groundwater evapotranspiration. The model also shows an increase in the number of stream reaches with streamflow losses. Model results for different climate conditions without pumping for water years 2011 to 2099 indicated greater variability in total streamflow, higher frequency of low streamflows, a shift in the timing of maximum streamflows from December to February, and a decrease in evapotranspiration due to depletion of groundwater resources compared to historical conditions.

Simulation of Aquifer-Stream Interaction in the San Joaquin River Restoration Program Study Area

To better understand the potential effects of restoration flows on existing drainage problems, anticipated as a result of the San Joaquin River Restoration Program (SJRRP), the USGS in cooperation with the U.S. Bureau of Reclamation developed a groundwater flow model (SJRRPGW) of the SJRRP study area that is within 5 miles of the San Joaquin River and adjacent bypass system from Friant Dam to the Merced River. The primary goal of the SJRRP is to reestablish the natural ecology of the river to a degree that restores salmon and other fish populations. Increased flows in the river, particularly during the spring salmon run, are a key component of the restoration effort. A potential consequence of these increased river flows is the exacerbation of existing irrigation drainage problems along a section of the river between Mendota and the confluence with the Merced River. Historically, this reach typically was underlain by a water table within 10 feet of the land surface, thus requiring careful irrigation management and (or) artificial drainage to maintain crop health. The SJRRPGW is designed to meet the short-term needs of the SJRRP; future versions of the model may incorporate potential enhancements, several of which are identified in this report.

The SJRRPGW was constructed using the USGS groundwater flow model MODFLOW and was built on the framework of the USGS Central Valley Hydrologic Model (CVHM) within which the SJRRPGW model domain is embedded. The Farm Process (FMP2) was used to simulate the supply and demand components of irrigated agriculture. The Streamflow-Routing Package (SFR2) was used to simulate the streams and bypasses and their interaction with the aquifer system. The 1,300-square mile study area was subdivided into 0.25-mile by 0.25-mile cells. The sediment texture of the aquifer system, which was used to distribute hydraulic properties by model cell, was refined from that used in the CVHM to better represent the natural heterogeneity of aquifer-system materials within the model domain. In addition, the stream properties were updated from the CVHM to better simulate stream-aquifer interactions, and water-budget subregions were refined to better simulate agricultural water supply and demand. External boundary conditions were derived from the CVHM.