Comparison of methods for estimating ground-water recharge and base flow at a small watershed underlain by fractured bedrock in the Eastern United States

Abstract

This study by the U.S. Geological Survey (USGS), in cooperation with the Agricultural Research Service (ARS), U.S. Department of Agriculture, compared multiple methods for estimating groundwater recharge and base flow (as a proxy for recharge) at sites in east-central Pennsylvania underlain by fractured bedrock and representative of a humid-continental climate. This study was one of several within the USGS GroundWater Resources Program designed to provide an improved understanding of methods for estimating recharge in the eastern United States. Recharge was estimated on a monthly and annual basis using four methods-(1) unsaturated-zone drainage collected in gravity lysimeters, (2) daily water balance, (3) water-table fluctuations in wells, and (4) equations of Rorabaugh. Base flow was estimated by streamflow-hydrograph separation using the computer programs PART and HYSEP. Estimates of recharge and base flow were compared for an 8-year period (1994-2001) coinciding with operation of the gravity lysimeters at an experimental recharge site (Masser Recharge Site) and a longer 34-year period (1968-2001), for which climate and streamflow data were available on a 2.8-square-mile watershed (WE-38 watershed). Estimates of mean-annual recharge at the Masser Recharge Site and WE-38 watershed for 1994-2001 ranged from 9.9 to 14.0 inches (24 to 33 percent of precipitation). Recharge, in inches, from the various methods was: unsaturated-zone drainage, 12.2; daily water balance, 12.3; Rorabaugh equations with PULSE, 10.2, or RORA, 14.0; and water-table fluctuations, 9.9. Mean-annual base flow from streamflowhydrograph separation ranged from 9.0 to 11.6 inches (21-28 percent of precipitation). Base flow, in inches, from the various methods was: PART, 10.7; HYSEP Local Minimum, 9.0; HYSEP Sliding Interval, 11.5; and HYSEP Fixed Interval, 11.6. Estimating recharge from multiple methods is useful, but the inherent differences of the methods must be considered when comparing results. For example, although unsaturatedzone drainage from the gravity lysimeters provided the most direct measure of potential recharge, it does not incorporate spatial variability that is contained in watershed-wide estimates of net recharge from the Rorabaugh equations or base flow from streamflow-hydrograph separation. This study showed that water-level fluctuations, in particular, should be used with caution to estimate recharge in low-storage fractured-rock aquifers because of the variability of water-level response among wells and sensitivity of recharge to small errors in estimating specific yield. To bracket the largest range of plausible recharge, results from this study indicate that recharge derived from RORA should be compared with base flow from the Local-Minimum version of HYSEP.

Comparison of local- to regional-scale estimates of ground-water recharge in Minnesota, USA

“…Application of multiple methods is recommended in estimating recharge because of the limitations inherent in each method Scanlon et al, 2002;Nimmo et al, 2005). For example, Risser et al (2005) had sufficient data to estimate recharge using four methods within a small watershed in Pennsylvania. It is rare, however, to have sufficient good-quality data to allow recharge estimation using more than two or three methods.…”

Estimation of flow components by recursive filters: case study of Paracatu River Basin (SF-7), Brazil

“…Nevertheless, because the hydrograph separation methods focus only on the base flow of watercourses, they do not incorporate the quantitative loss concerning river evapotranspiration and deep percolation that crosses beneath the gauging station (Risser et al, 2005). These two factors are important for the recharge analysis of the aquifers since they interfere in the water budget of the basin.…”

“…However, Risser et al (2005) noticed that conventional hydrograph separation by numerical methods (PART, local minimums fixed intervals, fluctuating intervals) might create difficulties for comparing base flow estimates of different-sized basins (including the nested ones). An increase in basin area implies: greater runoff time, flood lessening, bed friction on the flow, partially simultaneous flow contributions from tributaries, and regional hydrogoeological flow, among other factors, which can rarely be automatically calibrated by the available algorithms.…”

Water Balance to Recharge Calculation: Implications for Watershed Management Using Systems Dynamics Approach

“…Scanlon et al [69] categorized the techniques for estimating recharge for surface water, unsaturated zones, and saturated zones. Risser et al [70] used unsaturated-zone drainage collected in gravity lysimeters, daily water balance, water-table fluctuations in wells, and equations of Rorabaugh for estimating ground-water recharge at a humid-continental climate in east-central Pennsylvania. Lee et al [71] discussed the standard techniques for regional groundwater recharge calculation based on water-balance model and parameter-value adjustment of groundwater flow models.…”