Effects of Groundwater Development—Not That Uncertain

Capture results in the reduction of groundwater discharge to streams, springs, wetlands, and phreatophytic plants.  Uncertainty in the timing and magnitude of capture from groundwater development can be attributed primarily to uncertainty in hydraulic diffusivity estimates.  Hydraulic diffusivity is transmissivity divided by storage coefficient. Transmissivity is the primary source of uncertainty in hydraulic diffusivity estimates.  Characterizing aquifers using hydraulic diffusivity assumes differences in drawdown with depth are minor.  This simplification is reasonable for estimating capture that occurs during decades of groundwater development.  Groundwater discharge locations and aquifer system extent also affect capture estimates, but these are mappable features with relatively well-defined surface expressions.

The effects of most groundwater development in the western United States can be quantified with minimal uncertainty because development occurs primarily in basin-fill aquifers.  Transmissivity of most basin fill in Nevada ranges between 1,000 and 30,000 ft²/d, without considering depositional environment.  Transmissivity estimates vary less than tenfold where basin fill is differentiated into playa, alluvial-fan, and fluvial-channel sediments.  Aquifer systems are better characterized with aquifer tests using pumped wells because transmissivity is estimated.  These estimates typically represent the full aquifer thickness where transmissivity exceeds 1,000 ft²/d.