THE IMPORTANCE OF HYDROLOGIC TRANSIENCE IN CONCEPTUAL AND NUMERICAL GROUNDWATER MODELING IN THE GREAT BASIN

Currently, importance of transience in a groundwater system is determined by timeframe of management decisions. Periods shorter than the aquifer (watershed) response time, or the time it takes a watershed to recover from a change in hydrologic state, would not experience the new state and thus are treated as steady-state. This however, doesn’t mean that they won’t experience some transient response between the two hydrologic states. As a watershed’s response time is a function of its total size, the flat, regional watersheds characteristic of the Great Basin have long response times. Defining watershed extents as the area within which the water budget is balanced means inputs equal outputs. Steady-state budgets in arid regions are often balanced by extending watershed boundaries to include more area for recharge; however, the length and age of flow-paths necessary to balance these budgets are poorly constrained and often unrealistic. Inclusion of stored water in hydrologic budget calculations permits water balance within smaller contributing areas. To observe how transience affects response time a refined (transient) version of the USGS steady-state groundwater flow model is evaluated. This model is used to assess transient changes in contributing area for Clayton Valley, a lithium-brine producing endorheic basin in southwestern Nevada. Model runs of various recharge, discharge and storage bounds are created from conceptual models based upon historical climate data. Comparing results of the refined model to USGS groundwater observations allows for model validation and comparison against the USGS steady-state model. Though the transient contributing area to Clayton Valley is 85% smaller than that calculated from the steady-state solution, several long flow paths important to both water and solute budgets at Clayton Valley are preserved. Interbasin flow to Clayton Valley from stored waters in upgradient basins is an essential means of balancing Clayton Valley’s water budget. Management decisions in the Great Basin must be informed by conceptual models set within transient, regional context to be realistic and sustainable.