Calibration and Groundwater Management Scenario Analysis with the Scott Valley Integrated Hydrologic Model

Abstract:

Late summer streamflow for the Scott River in northern California has decreased approximately 50% since the mid 1960’s, resulting in increased water temperatures and disconnection of certain portions of the stream which negatively impacts aquatic habitat of fish species such as coho and fall-run Chinook salmon. In collaboration with local stakeholders, the Scott Valley Integrated Hydrologic Model has been developed, which combines a water budget model and a groundwater-surface water model (MODLFOW) of the 200 km² basin. The goal of the integrated model is to better understand the hydrologic system of the valley and explore effects of different groundwater management scenarios on late summer streamflow. The groundwater model has a quarter-hectare resolution with aggregated monthly stress periods over a 21 year period (1990-2011). The Scott River is represented using either the river (RIV) or streamflow routing (SFR) package. UCODE was used for sensitivity analysis and calibration using head observations for 52 wells in the basin and gain/loss observations for two sections of the river. Of 32 model parameters (hydraulic conductivity, specific storage, riverbed conductance and mountain recharge), 13 were found significantly sensitive to observations. Results from the calibration show excellent agreement between modeled and observed heads and to seasonal and interannual variations in streamflow. The calibrated model was used to evaluate several management scenarios: 1) alternative water budget which takes into account measured irrigation rates in the valley, 2) in-lieu recharge where surface-water instead of groundwater is used to irrigate fields near the river while streamflow is sufficiently high, and 3) managed recharge on agricultural fields in gulches on the eastern side of the valley in the winter months. Preliminary results indicate that alternative water management scenarios (in-lieu and managed recharge) significantly increase late summer streamflow by keeping groundwater levels near the stream higher, and the stream gaining longer into the dry summer period.