General Model Construction
Models will be constructed using MODFLOW-NWT which is the Newton formulation of MODFLOW 2005.
- The Newton formulation extends the applicability of MODFLOW 2005 to solve non-linear problems representing unconfined aquifer conditions and evapotranspiration of groundwater as well as wetting and drying of cells related to surface water and groundwater interactions.
- Models will vary stresses on a semi-annual basis to coincide with irrigation and non-irrigation seasons and will simulate conditions using monthly time steps.
Humboldt River streamflow will be simulated using the Streamflow Routing Package (SFR).
- Humboldt River inflows entering the middle HRB model domain at the Carlin streamflow gage will be specified based on historic streamflow data.
- Inflow of the Humboldt River into the lower HRB model domain near the Comus streamflow gage will be specified during calibration based on historic streamflow data, but will be simulated output from middle HRB model for scenario simulations.
- Major perennial tributaries to the Humboldt River will be represented using the River Package upstream of streamgages closest to confluence with Humboldt River.
- For the Lower Humboldt:
- The Lower Humboldt River Basin model covers a system that is naturally in dynamic state of equilibrium (steady state), and as such will represent the Humboldt River differently than the upstream models.
- Humboldt River streamflow will be simulated using the River Package (RIV) for the lower Humboldt River Basin.
- Below the streamgages closest to confluence with Humboldt River, perennial tributary flow will be specified using historically observed streamflows.
Surface-water diversions from the Humboldt River will be simulated using the Streamflow Routing Package, similar to how diversions were handled in lower Walker River basin model.
- Seasonal diversion rates for each ranch will be specified based on the median right in priority for the season and Humboldt River priority tables (Hennen) for each irrigation season for model calibration.
- When the models are used to evaluate streamflow depletion, seasonal surface-water diversions will be demand driven based on median priority dates as determined from simulated Humboldt River flows at Palisade gage and water distribution tables by Hennen.
- The consumptive use portion of diversions will be removed from the model (i.e., is not simulated as evapotranspiration) and the remaining water will be evenly recharged to groundwater beneath fields served by irrigation to represent irrigation return flows.
- Consumptive use and recharge from diversions will be determined according to project efficiencies for each diversion (project efficiency is defined here as the ratio of crop consumptive use to total diversion at point of diversion).
Groundwater withdrawals in the models will be simulated using the well package (WEL) in MODFLOW-NWT.
- Only individual groundwater withdrawals or aggregated groundwater withdrawals greater than 5 acre-feet per year will be included in the models.
- Groundwater withdrawals for domestic, stock water, and other minor uses are considered negligible in comparison with irrigation, municipal, and industrial uses but may be considered if aggregated withdrawals for a particular area exceed 5 acre-feet per year.
- Specified rates of groundwater withdrawals for lands irrigated only by groundwater (primary groundwater) will be equivalent to the consumptive use portion for crops being irrigated.
- When groundwater is used to supplement surface-water use (supplemental groundwater), seasonal groundwater withdrawals will be equivalent to the crop consumptive use less water delivered from surface-water irrigation.
- For mine dewatering, the non-consumptive portion of withdrawals will be routed either to the Humboldt River or injected into basin-fill sediments according to mine-specific water disposal methods.
The magnitude of precipitation derived groundwater recharge originating within each hydrographic area will be specified in the model using the MODFLOW Recharge package.
- Recharge will be distributed to mountain blocks, mountain fronts, and alluvial fans and valleys similar to methods employed by
Halford and Plume.
- Magnitude of recharge will be allowed to vary from existing estimates during model calibration in areas with sufficient hydrologic data.
Evapotranspiration (ET) from groundwater will be simulated in the model using the MODFLOW Evapotranspiration package.
- ET is simulated in the model based on simulated depth to water with groundwater ET occurring at a maximum potential rate when depth to water is at land surface, and a linearly decreasing rate with depth until a specified extinction depth is reached in which no ET of groundwater occurs.
- The maximum potential rate of ET from groundwater and extinction depths will be determined through model calibration.
- These estimates will be constrained by new estimates of ET discharge being developed by the Desert Research Institute.