Mapping zones of aquifer recharge and discharge based on correlation of naturally occurring hydrologic features, Central Texas

Defining vertical flow of groundwater is important for developing conceptual hydrologic/hydrogeologic models and water budgets. Defining vertical flow components can be difficult with sparse well data. This concept is of particular interest when attempting to predict the flow path (local-, intermediate- or regional-scale) that additional recharging surface water will take when phreatophytes (water intensive species) are removed in support of landscape restoration. A GIS-based analysis of dynamic pressure increments (DPI) and proximity of wells to streams of different Strahler order maximizes the use of sparse data to map upward- and downward-directed flow. The method was tested in a part of the Pedernales River watershed in Central Texas to study recharge and discharge and movement of groundwater between the Trinity Hill Country and the Llano Uplift aquifers.

A dynamic pressure increment (DPI) as defined by Tóth measures departure from hydrostatic and indicates where groundwater flow is directed upward (+DPI; discharge) or downward (-DPI; recharge). Posting DPI on a GIS-based topographic map showed an obvious correlation--positive values in tributary valleys and negative values in upland settings. Correlation of DPI and well distance is statistically significant for 2nd - to 5th-Strahler order streams. Recharge and discharge areas for individual streams were mapped as a function of distance from each stream and considered the linear regression's prediction interval.

In an attempt to further understand the link between stream order and local-, intermediate- and regional-scale flow paths, an elevation-depth-DPI plot was made of all wells within the study area. Patterns showed a regional hydrostatic (DPI = 0) divide between recharging (downward vertical component) and discharging (upward vertical component) at an elevation of approximately 1450 ft. Intermediate-scale systems of groundwater flow were identified by the occurrence of hydrostatic (DPI = 0) conditions as subsets of the regional system.