Calibration is the process of adjusting hydraulic properties (parameters) in the model and model construction to improve simulation of observed conditions. The middle and lower HRB models will be calibrated through coupled steady-state and transient simulations.
An automated parameter estimation routine known as PEST will be used to calibrate model parameters. PEST systematically adjusts properties to minimize differences between simulated and observed data. Existing hydrogeologic frameworks will be used to distribute and constrain properties in hydrogeologic units. Transmissivity and storage properties will be allowed to vary within units to better represent variability of the properties within hydrogeologic units.
Evaluating Humboldt River streamflow depletion resulting from groundwater withdrawals requires some reasonable assumptions related to the future stresses and streamflows entering the middle HRB model at the Carlin streamgage. Future stresses and streamflows are associated with natural future climate variations within the Humboldt River Basin that are unknown. A common approach for simulating future stresses and streamflows is to repeat historical climate conditions. Future stresses and stream inflow at the Carlin streamgage will be specified in the calibrated transient models by repeating the stresses specified in the calibrated model for the 50-year period from 1966 to 2015. This 50-year period will be repeated in order to simulate conditions 100 years into the future. This reference scenario is hereafter referred to as the “historical reference scenario”.
An alternative reference scenario will be developed that will simply repeat the mean annual conditions during the historical reference scenario. This alternative reference scenario is hereafter referred to as the “mean historical reference scenario”. This alternative reference scenario may be desirable for use with the streamflow depletion analysis as it will generate smooth results that are not directly influenced by interannual climate variability. The mean annual stresses and stream inflow at the Carlin streamgage will be repeated over 50- and 100-year simulations. Results from historical and mean historical reference scenarios will be compared and if found to be similar, the mean historical reference scenario will be used for evaluating streamflow depletion and developing streamflow depletion maps.