ABSTRACT
Normalized base-flow (NBF), base-flow discharge per unit area [cubic feet/sec/mile2 (cfsm) or liters/sec/km2 (Lsk)], is a useful, easily-calculated parameter that is characteristic of various terranes. As applied to karst terranes, it can be used to estimate the approximate recharge-area of springs, characterize their basins, and assess hydrogeologic relationships. The basic concept of NBF, under various names, has commonly been applied to regional hydrology and occasionally to springs. Nevertheless, the NBF concept has not been widely applied to the study of karst aquifers. It should be.
Comparison of data on recharge areas that have been determined by tracer testing and potentiometric mapping with data on base-flow discharge of nearby basins shows that springs of all sizes in the same stratigraphic, structural, and physiographic setting tend to have near-identical NBFs. Base-level springs in the Mammoth Cave area may be placed into three groups having mostly autogenic recharge, each with a characteristic mean NBF. These groups are autogenic recharge with: 1. Up to 25% allogenic recharge from sandstone-capped ridges or terrane with a near-surface, leaky, chert aquitard: Geometric Mean=0.21 cfsm 2. Up to 30% allogenic recharge from carbonate terranes on the Glasgow Upland that support spring-fed surface streams, most of which are perennial: Geometric Mean=0.17 cfsm 3. Locally-thick, areally-significant sand and gravel cover: Geometric Mean=0.85 cfsm
In contrast, the NBF for springs with mostly autogenic recharge in the Bluegrass Region is 0.11 cfsm. The differences between Groups 1 and 2 are a consequence of the relatively rapid runoff and lesser storage in the extensive headwaters area of carbonate-terrane-derived allogenic recharge. The differences between Group 3 and combined Group 1 + 2 are a consequence of relatively high storage and discharge-attenuating properties of a thick mass of sand and gravel in a dissolutionally-lowered, disaggregated, Pennsylvanian channel deposit. The NBF of springs in the Bluegrass Region is about 60 to 70% that of Groups 1 and 2 in the Mammoth Cave Region. The difference is a consequence of the shaley, silty beds in the Bluegrass Region inhibiting extensive, well-integrated dissolution of the carbonates; storm runoff is higher and storage within the aquifer is less than in the Mammoth Cave Region. The NBF of stream flow in the Bluegrass Region is about half that of streams in the Mammoth Cave Region—for the same reasons that the NBF of their springs is different.
Interpretation of field data shows that the groundwater divide separating the Green River watershed from the Bacon Creek watershed, immediately north of the river, is locally almost 3 miles (4.8 km) north of the topographic divide separating them. Green River is pirating the headwaters of the Bacon Creek watershed on a regional scale.
If the amount and statistical significance of data justify doing so, a single equation relating the recharge area of an undelineated spring, spring group, or stream to both its base-flow and its regional NBF for the same setting can be easily written. This is a powerful, extremely useful procedure for estimating recharge area of undelineated springs and spring groups.
The concepts of degree of delineation of a basin boundary by tracing, as measured by T, and the accuracy of delineation of abasin boundary, A, mearnred as apercentage of the probable area of the basin, are introduced and defined quantitatively.
