Regionalization of low-flow characteristics of Arkansas streams

Abstract

Equations for estimating the 7-day 2-year and 7-day 10-year low flow at sites on ungaged streams are presented. Regression analysis was used to develop equations relating basin characteristics and low-flow characteristics at 58 continuous-record gaging stations and 151 partial-record sites. Significant basin characteristics in the equations are drainage area and percentage of drainage basin underlain by permeable bedrock units. The study area, which encompasses the western two-thirds of the State, is divided into three regions based on the similarity of basin characteristics within each region. The analysis includes records for only those stations that are not highly regulated and have drainage areas less than 1,000 square miles. A three-step method is used to estimate low-flow characteristics at ungaged sites. The first step involves the use of a logistic regression to determine the probability that the 7-day annual minimum flow is zero at the site of interest. The second step uses this estimated probability of 7-day annual zero flow to determine if the estimated value of the 7day 2-year or the 7-day 10-year low flow is zero or needs to be estimated from one of the regional equations, which are based on a generalized-least-squares model for sites with non-zero flow. The third step involves the use of the regional equation, if flow is indicated, to determine the 7-day 2-year and 10-year low-flow values. Computer software has been written to facilitate the computation of low-flow characteristics at sites of interest The software is provided in written form and on a disk. INTRODUCTION Knowledge of low-flow characteristics of Arkansas streams is important in water-resource planning, particularly when questions related to draft storage requirements, dilution of contaminants, navigation, and maintenance of fish and wildlife habitat are concerned. From data collected at continuous and partial-record gaging stations operated by the U.S. Geological Survey (USGS) and other Federal agencies, low-flow characteristics (for example, 7-day 2-year and 7-day 10-year low flows) have been computed (Ludwig, 1992) and are available for helping make water-resource management decisions at these sites. Low-flow characteristics are commonly needed at sites for which daily discharge records or miscellaneous discharge measurements do not exist This report, a product of a study by the USGS, in cooperation with the Arkansas Soil and Water Conservation Commission, is the first to describe a method for the determination of low-flow characteristics at ungaged sites within the State of Arkansas. Purpose and Scope This report briefly describes the regression analysis techniques selected to effect the transfer of low-flow characteristics from gaged to ungaged stream sites, and provides the resulting predictive equations to estimate values of 7-day low flow for recurrence intervals of 2 and 10 years (Q7 2 and Q7 j Q) at ungaged stream sites in the study area. The study area includes the western two-thirds of the State (fig. 1). The eastern one-third of the State, which lies within the Mississippi Alluvial Plain, has been excluded from the study because withdrawals and return flows associated with irrigation throughout the area make the determination of predictable low-flow characteristics impossible.