Updating Low-Flow Statistics at USGS Streamgages and Regionalization of Selected Low-Flow Characteristics for Alabama Streams Active

Updating Low-Flow Statistics at USGS Streamgages and Regionalization of Selected Low-Flow Characteristics for Alabama Streams

INTRODUCTION

As part of their mission to protect public health and aquatic ecosystems, Alabama state agencies need accurate and representative streamflow statistics to establish realistic and applicable criteria for both water quality and water quantity (fig. 1). Historically, low-flow statistics, such as the annual minimum 7-day average flow that likely will occur, on average, once every 10 years (7Q10), have been used by water-resource managers and planners as a threshold criterion for applying the chronic aquatic life criteria for such things as determining waste-load allocations for point sources, total maximum daily loads (TMDL) for streams, and the quantity of water that can be safely withdrawn from a particular stream (Alabama Department of Environmental Management, 2012). Because of the importance of these applications, it is critical to effectively measure and document base-flow data for use in updating low-flow frequency relations on a regular basis, preferably every 10 years, and especially after periods of extreme low flow, such as have occurred in the Southeast in recent years. Low-flow statistics in Alabama have not been updated on a statewide base since 1990 (Atkins and Pearman, 1994). To provide better documentation of flow characteristics for Alabama streams, the U.S. Geological Survey (USGS) is leading a cooperative investigation to update annual low-flow statistics at USGS continuous-record streamgaging stations in Alabama, hereafter referred to as streamgages.

OBJECTIVES

The objectives for the investigation will be completed in two phases:

• Phase I will be to update low-flow statistics at selected active and discontinued streamgages on unregulated, and where appropriate, regulated streams that are not tidally influenced, and
• Phase II will be to develop regional regression equations for selected low-flow characteristics for unregulated streams.

APPROACH

Phase I

Phase I of the investigation will cover a period beginning on or about May 1, 2014, and ending September 30, 2017. The low-flow statistics will be updated at active and discontinued streamgages if at least 10 years of record are available. In addition, a flow-duration analysis will be completed for the streamgages and selected exceedance percentiles will be provided.

For unregulated stations with short-term records (those which have at least 10 years of record but less than about 30 years), the low-flow statistics may be improved by using record extensions or augmentation methods (Hirsch, 1982) based on correlations with long-term gages. This approach is particularly beneficial if the streamflow data at the short-term record station were collected during an unusually dry, wet, or otherwise unrepresentative period. For this study, streams for which multiple USGS streamgages are located and represent a mix of long-term and short-term gages, assessments will be made to determine if suitable correlations exist, and conditions warrant, for the record augmentation of the short-term gages.

The Phase I results will be published in a USGS Scientific Investigation Report. In addition, the statistics will be included in the USGS StreamStats application for Alabama making them easily accessible for the gaged locations and assisting in transferring the statistics to appropriate ungaged locations (Ries and others, 2008; Welcome to StreamStats, Alabama, accessed on March 22, 2013, at http://streamstats.usgs.gov/alabama.html).

Phase II

Phase II of the study will cover a period beginning on or about October 1, 2017, and ending September 30, 2019. The focus of Phase II of this investigation will be the generation of regional regression equations for ungaged locations on unregulated streams to estimate the annual minimum 1- and 7-day average flows that likely will occur, on average, once every 10 years (1Q10 and 7Q10 respectively), and the annual minimum 7-day flow that likely will occur, on average, once every 2 years (7Q2). For the streamgages from Phase I that are located in unregulated basins, a set of explanatory (independent) variables will developed using GIS data layers for basin characteristics such as drainage area, slope, and elevation, various geology and soils indices, hydrologic characteristics such as streamflow variability or base-flow recession indices, and climatic characteristics, such as mean annual precipitation (Smakhtin, 2001; Eash and Barnes, 2012). The explanatory variables will then be tested in the regression analysis using a combination of ordinary least squares (OLS), weighted least squares (WLS) and generalized least squares (GLS) regression techniques. The OLS techniques will be used to select the initial set of explanatory variables and to determine the regionalization scheme for the state. The WLS and (or) GLS techniques will be used to compute the final regression coefficients and to compute the accuracy statistics for the final equations (Eng and others, 2009). Once the regional regression equations are finalized, the Alabama StreamStats application will be updated to include the capability of generating the explanatory variables included in the low- flow regression equations. The low-flow regression equations will be incorporated into StreamStats and tested for accuracy.

PLANNED REPORTS

For Phase I, a USGS Scientific Investigations Report (SIR) will be prepared describing the methods, assumptions, and results from the low-flow analysis of CR streamgaging station records. The tentative title of the report will be “Low-flow frequency and flow duration of selected Alabama streams at continuous-record streamgaging stations”. For Phase II, a USGS SIR will be prepared describing the methods, assumptions, and results from the regional regression analysis of low-flow statistics for unregulated basins. The tentative title of the report will be “Methods for estimating selected low-flow frequency statistics for unregulated streams in Alabama”. Both reports will be peer-reviewed and approved by the USGS prior to publication.