Estimation of Specific Yield for Regional Groundwater Models: Pitfalls, Ramifications, and a Promising Path Forward

Abstract

Specific yield (Sy) is one of the most important aquifer parameters in groundwater models with large storage changes. However, it has received limited attention from the modeling community and there are few, if any, approaches that can be widely applied for determination of Sy at the scale required for regional aquifer studies. Despite the limitations of parameter estimation methods and of direct measurements of aquifer storage properties in the field, the large uncertainty in model Sy values is typically ignored and Sy is incorrectly assumed to be a known quantity. These practices can introduce errors into model predictions of aquifer budget components, such as recharge and storage depletion. In turn, aquifer management and planning can be greatly impacted, as estimates of aquifer responses to pumping changes are highly dependent on those simulated budget components. In this work, we use a groundwater model of a portion of the High Plains aquifer in the central United States to illustrate the impacts of inappropriately large Sy values on model predictions and the related water management ramifications. Building on our recent work, we propose a data‐driven approach for determining specific yield for regional groundwater models. Results demonstrate that this new approach can improve model reliability and lead to more robust predictions of aquifer responses to management scenarios.

Net Inflow: An Important Target on the Path to Aquifer Sustainability

“…If a model cannot reproduce the magnitude and temporal behavior of the field‐calculated net inflow, then further calibration should be considered. As we have shown elsewhere, the specific yield estimate obtained from this plot can also serve as an important model constraint (Butler et al 2020a; Liu et al 2022).…”

“…This calculation is typically done with water‐level data, the best available estimate of pumping, and an estimate of specific yield (Konikow 2013). Recent work, however, has shown that the common values used for specific yield in many modeling studies may not be representative of conditions in often highly heterogeneous aquifers (Butler et al 2020a; Liu et al 2022). Under certain conditions, stream depletion, a major component of capture in interconnected stream‐aquifer systems, can be estimated directly (Barlow and Leake 2012).…”

Quantifying the Impact of Lagged Hydrological Responses on the Effectiveness of Groundwater Conservation

“…In groundwater modeling, parameter estimation often seeks to find a local or global optimum to match limited observations while minimizing an objective function using software such as PEST (Doherty, 2015) or UCODE (Poeter & Hill, 1999). However, the hunt for an ideal parameter set that results in simulated values closely matching observed values can ignore other possible parameter sets that perform nearly equally well (Liu et al., 2022; Savenije, 2001). This is true for our surrogate model of the SD‐6 area as the lack of vadose zone observation data paired with an exploration of a wide parameter space resulted in two possible and equally valid mechanisms, or combination of mechanisms, that affect the long term performance of pumping reduction‐based groundwater conservation initiatives.…”

Influence of Shallow Groundwater Evapotranspiration on Recharge Estimation Using the Water Table Fluctuation Method

“…Generally, these ratios at annual scale should be lower than one. Uncertainty in specific yield estimates though, can introduce great errors to aquifer budget terms (Liu et al., 2022). For both sites, groundwater recharge was likely overestimated because specific yield values estimated at sites where the water‐table is very close to the land surface are typically higher than the actual field specific yields (Childs, 1960).…”