Hydrologic information on the variability of streamflow during low-flow periods is needed for the effective management of surface-water resources in Maryland and Delaware. Low-flow characteristics derived from streamflow under natural conditions are presented for 94 continuousrecord gaging stations in Maryland, Delaware, and surrounding States, and for 131 low-flow partial-record gaging stations in Maryland and Delaware. Methods are developed to estimate low-flow characteristics at ungaged stream sites for average 7-, 14-, and 30-consecutive-day lowflow discharges for recurrence intervals of 2, 10, and 20 years. The methods are applicable to unregulated streams in Maryland and Delaware for watersheds having drainage areas ranging from 2.0 to 875 square miles.
Maryland and Delaware are divided into four regions on the basis of physiography and the results of regression analyses. The regions are the Eastern Shore region in Maryland and Delaware, the eastern-Piedmont region in Maryland, the western region in Maryland, and the southern region in Maryland. Equations are developed from regression analyses to estimate lowflow characteristics at ungaged sites on ungaged streams for the Eastern Shore region, the easternPiedmont region, and the western region. A generalized least-squares multiple-regression technique is used to develop the estimation equations that relate average 7-, 14-, and 30- consecutive-day low-flow discharges for recurrence intervals of 2, 10, and 20 years to physical and climatological features of drainage basins in the study area. Adjustments are necessary to low-flow discharges estimated from the equation for the western region because of the effects of carbonate rocks on low flows. The adjustment method is based on analysis of the residuals of regional estimation equations and the percentage of the basin underlain by carbonate rocks. An equation is developed on the basis of a distance-weighted average of low-flow discharges at gaging stations to estimate low flow characteristics at ungaged sites on ungaged streams for the southern region. Estimates of the accuracy of low-flow characteristics determined from all regional equations are provided.
Two methods are presented for estimating low-flow characteristics at ungaged sites on gaged streams. One method combines low-flow characteristics determined from gaging station data with results from the regional regression equation at the gaging station and at the ungaged site to estimate the low-flow characteristics either upstream or downstream from the gaging station. The other method estimates low-flow characteristics at ungaged sites between gaging stations using a graphical solution. Estimates of accuracy of the low-flow characteristics determined by these methods also are provided.
