4 Flow Systems
Numerical models have been used to simulate groundwater flow, changes in groundwater storage (water levels), and significantly, spring flow from the Edwards Aquifer. Estimating water budgets has been a central objective of numerical modeling. A key outcome of the conceptual and numerical modeling is the nearly one-to-one relationship between increases in pumping and decreases in spring flow under low-flow conditions (Brune and Duffin, 1983; Smith and Hunt, 2004; Hunt et al., 2011). Water resource managers rely on these groundwater flow models to characterize groundwater-flow conditions in the Edwards Aquifer to serve as the basis for predicting the effects of water-resource management decisions, particularly on spring flow. There are recognized limitations and shortcomings in all numerical groundwater models, such as the particular modeling software selected, uncertainty about the underlying conceptual model and its implementation in the ensuing numerical model. Nonetheless, models are the preferred tool for groundwater resource management (De Marsily, 1986; Anderson and Woessner, 2015; Scanlon et al., 2003; Hartmann et al., 2014; Green et al., 2019d).
The Edwards Aquifer is regional in scale with properties and characteristics that vary spatially and often temporally. Capturing these extreme heterogeneities imposes a large cost with regard to computation time and data-input requirements. At the same time, aquifer management concerns are usually defined at the local scale, requiring sufficient high resolution to account for local-scale characteristics of the aquifer. For these reasons and others, no single numerical model has been developed to simulate flow for the entirety of the Edwards Aquifer. Numerical models have been developed for segments and subsets of the Edwards Aquifer, including the San Antonio segment (Maclay and Land, 1988; Lindgren et al., 2004; Fratesi et al., 2015; Liu et al., 2017), the Barton Springs segment (Scanlon et al., 2001; Winterle et al., 2009; Hutchison and Hill, 2011), Kinney County (Hutchison et al., 2011a), and the Devils River watershed (Toll et al., 2017). In addition, there are regional-scale models, referred to as Groundwater Availability Models, which span large areas of the Edwards Aquifer and related aquifers, such as the Edwards-Trinity and Trinity aquifers (Anaya and Jones, 2004, 2009; Hutchison et al., 2011b; Jones et al., 2009). Although lumped-parameter (Wanakule and Anaya, 1993; Barrett and Charbeneau, 1996; Loáiciga et al., 2000) and solute-transport (Lindgren et al., 2011) models of the Edwards Aquifer have been developed, this discussion focuses on distributed groundwater-flow models. Post-2000 models of the greater Edwards Aquifer area were mostly developed as part of the Texas Water Development Board’s (TWDB) Groundwater Availability Model (GAM) program (Mace et al., 2008).