Adera et al. (2026) Assessing future hydrologic extremes using an integrated hydrology and river operations model in the Russian River watershed

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2026
• Date: 2026-01-06
• Authors: Saalem Adera, Ayman H. Alzraiee, Richard G. Niswonger, Enrique Triana, Derek W. Ryter, John A. Engott
• DOI: 10.1016/j.ejrh.2025.103016

Research Groups

• U.S. Geological Survey, California Water Science Center
• U.S. Geological Survey, Water Resources Mission Area
• RTI International, Center for Water Resources

Short Summary

This paper introduces an integrated hydrology and river operations model for the Russian River watershed to assess future hydrologic extremes. The model projects significantly longer and more severe streamflow droughts, lower seasonal low flows, and higher peak streamflows under future climate and water use scenarios, underscoring the critical role of reservoir operations in mitigating these impacts despite decreased reliability at Lake Mendocino.

Objective

• To analyze streamflow depletion due to groundwater pumping under historical conditions in the Russian River watershed.
• To simulate and compare three historical (1990–2015) and eight future (2016–2099) water use and climate change scenarios to assess their impacts on water use, reservoir stages, groundwater storage change, stream-aquifer exchange, and streamflow (including droughts, low flows, and high flows).

Study Configuration

• Spatial Scale: The Russian River watershed in coastal, northern California, draining approximately 3850 square kilometers (km²). The active model domain covers 3186 km², discretized into 300 meter (m) x 300 m grid cells (253 columns, 411 rows, and 4 layers).
• Temporal Scale: Historical scenarios cover 1990–2015 (comparisons for water years 1991–2015). Future scenarios cover 2016–2099. The model uses daily time steps and monthly stress periods.

Methodology and Data

• Models used:
  • MODSIM-GSFLOW (integrated modeling system)
  • GSFLOW (Coupled Groundwater and Surface-water Flow hydrologic model, version 2.0)
  • PRMS (Precipitation-Runoff Modeling System)
  • MODFLOW-NWT (Newton-Raphson formulation of the Modular Groundwater-Flow Model)
  • MODSIM (River Basin Management Decision Support System, river operations model)
  • PEST (Parameter ESTimation) for automated calibration.
• Data sources:
  • Gridded elevation, climatology, soils, land cover, and land use data.
  • Hydrogeologic characteristics (conceptualization of five geologic units generalized into three model layers).
  • Climate forcings: Daily precipitation and temperature from four Global Climate Models (CanESM2, CNRM-CM5, HadGEM2-ES, MIROC5) for Representative Concentration Pathway (RCP) 4.5 and RCP 8.5 scenarios, bias-corrected.
  • Projected surface water inflows from Mark West Creek (from Santa Rosa Plain Integrated Hydrologic Model).
  • Future population estimates for Sonoma and Mendocino Counties (Hauer, 2019) for municipal, industrial, and rural domestic water use.
  • Historical observations for calibration: streamflow, groundwater levels, reservoir stages, municipal/industrial and rural domestic groundwater pumpage, and agricultural water use.
  • Publicly available U.S. Geological Survey data releases (ScienceBase) for various input data and observations.

Main Results

• Streamflow Depletion: The mean streamflow depletion factor (SDF) was 0.13, with higher values in alluvial aquifers (mean 0.19) compared to upland areas (mean 0.09). Alluvial aquifers in Potter, Ukiah, Alexander, Santa Rosa, and Lower Russian River Valleys showed the greatest depletion, with maximum SDFs ranging from 0.66 to 1.
• Water Use: Future mean annual water use was projected at 72 million cubic meters (M m³) for municipal and industrial, 26 M m³ for agricultural, and 9 M m³ for rural domestic. The elimination of interbasin transfers reduced total agricultural water use by approximately 10%, while climate change scenarios projected an 11% increase in total agricultural water use due to rising temperatures and evapotranspiration.
• Reservoir Reliability: Lake Mendocino's stage was projected to be much lower and more variable in future scenarios (mean 219 m vs. 225 m historical), dropping below deadpool stage in 0.4–10% of days in climate change scenarios (up to 12.3% in the no-interbasin-transfer scenario). Both Lake Mendocino and Lake Sonoma reached flood stage more frequently in the wettest climate change scenarios (Lake Mendocino: 9.8–11.7% of days vs. 9.3% historical; Lake Sonoma: 10.1–10.8% of days vs. <3% historical), indicating increased future flood risk.
• Groundwater Storage Change: Watershed-scale groundwater storage remained stable or slightly increased (up to 2.3% by 2099 in the wettest scenarios). However, localized declines were observed in some subbasins in the driest scenarios (e.g., an 11 M m³ decrease in Subbasin 18 by 2099).
• Stream-Aquifer Exchange: Watershed-aggregated stream leakage into aquifers was greatest during November-January. The peak groundwater contribution to streamflow shifted from October historically to February-April in future scenarios, with dry season (July-October) groundwater contribution dropping from 23% to 15%. The elimination of interbasin transfers favored gaining conditions for the river upstream of the Dry Creek confluence.
• Streamflow Extremes:
  • Droughts: Streamflow droughts are projected to be 59% longer (184 days vs. 116 days historical) and 54% more severe (524 cubic meters per second [m³/s] vs. 340 m³/s historical) in climate change scenarios.
  • Low Flows: Annual minimum 7-day streamflows declined by 26% (2.3 m³/s vs. 3.1 m³/s historical) in climate change scenarios.
  • High Flows: Mean annual peak streamflows increased by 125% in the wettest climate change scenarios (1232 m³/s vs. 876 m³/s historical), suggesting increased flood risk.

Contributions

• Introduces the Russian River Integrated Hydrologic Model (RRIHM), an integrated hydrology and river operations model (MODSIM-GSFLOW) that uniquely represents surface-groundwater interactions and uses climate forcings to estimate dynamic, soil-moisture dependent water use demands superimposed on reservoir operations and water supply constraints.
• First study to examine future climate-dependent river operations, water use, and their interactions with surface water and groundwater using a high-resolution, spatially distributed model in the Russian River watershed.
• Provides novel insights into the dynamics of the interconnected surface water and groundwater system, demonstrating the resilience of watershed-scale groundwater storage but also localized vulnerabilities.
• Highlights the critical role of reservoir operations in modulating hydrologic extremes, showing that neglecting reservoir operations may overestimate climate change impacts on water resources.

Funding

• California State Water Resources Control Board (Water Rights Division)
• Sonoma Water
• USGS Water Availability and Use Science Program

Citation

@article{Adera2026Assessing,
  author = {Adera, Saalem and Alzraiee, Ayman H. and Niswonger, Richard G. and Triana, Enrique and Ryter, Derek W. and Engott, John A.},
  title = {Assessing future hydrologic extremes using an integrated hydrology and river operations model in the Russian River watershed},
  journal = {Journal of Hydrology Regional Studies},
  year = {2026},
  doi = {10.1016/j.ejrh.2025.103016},
  url = {https://doi.org/10.1016/j.ejrh.2025.103016}
}