|Risser, D - USGS|
Submitted to: Government Publication/Report
Publication Type: Government Publication
Publication Acceptance Date: February 11, 2005
Publication Date: April 1, 2005
Citation: Risser, D.W., Gburek, W.J., Folmar, G.J. 2005. Comparison of methods for estimating ground-water recharge and base flow at a small watershed underlain by fractured bedrock in the eastern united states. U.S. Geological Survey Scientific Investigations Report. 31 p. Interpretive Summary: Ground-water recharge is one of the least understood components of the hydrologic cycle, and its relation to watershed-scale production of base flow is equally ill-defined, especially within the fractured bedrock environment. Data from a ground water recharge site and nearby experimental watershed established in east-central PA by Agricultural Research Service, USDA, was used in a cooperative study conducted by USGS and ARS to compare a variety of methods for estimating both recharge and base flow, thereby improving our ability to routinely quantify both hydrologic components. Recharge was estimated using lysimeter data, water balance techniques, observed ground-water fluctuations, and analytical equations, and base flow was estimated by stream hydrograph separation based on conventional USGS computer techniques (PART and HYSEP). Both recharge and base flow estimates were compared for an 8-year period (1994-2001) coinciding with operation of the lysimeters at the Masser recharge site and a longer 34-year period (1968-2001) during which climate and streamflow data were available on the 2.8-square-mile watershed WE-38. Estimates of mean-annual recharge at the two sites for 1994-2001 ranged from 9.9 to 14.0 inches, and mean-annual base flow ranged from 9.0 to 11.6 inches. Estimates of recharge and base flow using the variety of methods illustrated a general consistency in magnitude, especially when contrasted to the approximately 44 inches of annual rainfall, but the inherent differences of the methods must be considered when comparing results. For example, although drainage from the lysimeters provide the most direct measure of potential recharge, it does not reflect the spatial variability incorporated in watershed-based estimates of recharge or base flow. In summary, to bracket the range of recharge and base flow expected, the study showed that recharge derived from application of the watershed-scale analytical equations should be compared with base flow derived from the HYSEP computer program. In general terms though, all methods proved useful for estimating the two important flow components of ground-water recharge and watershed base flow.
Technical Abstract: This study by the U.S. Geological Survey, in cooperation with the Agricultural Research Service (ARS), U.S. Department of Agriculture, compared multiple methods for estimating ground-water 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 Ground-Water 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) gravity lysimeters, (2) a daily water balance, (3) water-table fluctuations, 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: gravity lysimeters (12.2), daily water balance (12.3), Rorabaugh equations with PULSE (10.2) or RORA (14.0), and water-table fluctuations (9.9) inches. Mean-annual base flow from streamflow-hydrograph separation ranged from 9.0 to 11.6 (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), HYSEP Fixed Interval (11.6), which corresponds to between 21 and 33 percent of precipitation. Estimating recharge from multiple methods is useful, but the inherent differences of the methods must be considered when comparing results. For example, although unsaturated-zone 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.