The relationship between base flow recession characteristics in steep watersheds and geomorphologic and soil parameters is investigated. The formulation for the groundwater outflow was obtained by means of a hydraulic approach applied to a simple conceptual model for a hillslope. Long-term flow data of 19 representative basins in the Allegheny Mountain section of the Appalachian Plateaus were analyzed on the basis of this formulation. Results showed that the reaction factor, which is a time scale of base flow recession, is dependent on the mean land slope, the drainage density, and the ratio (K/f) of the hydraulic conductivity and the drainable porosity. On account mainly of the nonuniform distribution of the physical characteristics within a basin, the reaction factor for a given watershed is somewhat variable with time, but the adoption of a constant value is useful to represent average conditions for a recession period. Analysis of the (K/f) dependency showed that macropores and other structural features may greatly affect the watershed base flow. Evaporation from groundwater appears to constitute only a minor portion of overall basin evaporation. INTRODUCTIONDuring rainless periods, the flow in a basin's stream system, which is often referred to as base flow is sustained by groundwater discharge. But even during wet periods, base flow constitutes an important and sometimes dominant streamflow component particularly in humid forested areas. Therefore an understanding of the behavior of groundwater outflow is essential in studies of water budgets and the response of catchments to various hydrologic and climatic inputs.The natural groundwater discharge in a catchment as a function of time is largely controlled by the physical and hydrologic properties of the aquifer materials, many of which are reflected in the morphology of the basin. Since geomorphologic characteristics can be readily obtained from maps and air photos, it is of interest to establish reliable relationships between the groundwater outflow rate and the controlling geomorphologic parameters of a basin. As noted elsewhere [Ze-&arias and Brutsaert, this issue], previous hydrogeomorphic studies have been primarily statistical in nature. On the other hand, in groundwater hydrology, most investigations that deal with the problem of drainage from hillslopes have focused on the hydraulics and almost no attempts have been made to relate the outflow to basin-wide geomorphologic or aquifer parameters. The objective of this paper is to investigate the effect of geomorphologic and soil characteristics on base flow recession by means of a simple conceptual model and on the basis of basin scale parameters. The parameters in question are mainly land slope and drainage density of watersheds, but the hydraulic conductivity and the drainable porosity of the aquifer materials are also considered. The approach is then applied to data obtained for some basins located in the Appalachian Plateaus. Mountain section of the Appalachian Plateau physiographic province. The s...
Twenty geomorphologic parameters which constitute a well-balanced representation of the area, length, elevation, form, and shape aspects of watersheds as well as the texture of their drainage networks were considered. On the basis of groundwater theory and the results of previous related studies, eight of these parameters were identified as being relevant to the process of groundwater discharge. Values of the selected parameters were obtained for representative watersheds located in a section of the Appalachian Plateaus. The data, which were generated from U.S. Geological Survey topographic maps, were then subjected to a factor analysis to assess the relative importance of the parameters with respect to their influence on groundwater outflow behavior. The analytical results and their interpretation showed that among the parameters that are related to groundwater outflow, total length of perennial streams, average basin slope, and drainage density are the ones that are most closely related to the process. The influences of these parameters on groundwater outflow behavior are independent of each other. Thus in relating groundwater outflow characteristics to basin morphology, only these parameters need be considered and the inclusion of additional parameters does not necessarily yield a better relationship and may result in redundancy. The findings of the study are consistent with existing concepts regarding base flow (or drought flow) and have been confirmed by an independent study that investigated the process based on a more mechanistic approach.
A simple and relatively rapid method of determining the characteristic or typical slope of a basin is proposed. The expression for the parameter which is given by Savg = [L(H•-H•o2)]/(A cos /•) was developed from considerations of basin geometry and basic geomorphologic facts and involves only quantities that can be easily extracted from topographic maps. Characteristic basin slope values of 19 Appalachian Plateau watersheds which range in size from 7.4 to 178.2 km 2 were determined by the proposed method. These values were compared with corresponding values obtained by Horton's intersection line method and with those derived from county soil survey reports and maps. The comparisons showed very close agreement between the three sets of values indicating the high accuracy of the method developed here. Similar comparisons involving values determined by previous slope methods indicated that these values are at best poorly related to actual average basin slope, and that most of them are incapable of detecting small differences in slope between similar watersheds. Moreover, the proposed method significantly reduces the amount of work required by other slope methods of comparable accuracy. 1895
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