New Jersey Water Science Center
Great Falls of the Passaic River at Paterson, N.J.
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Statistics for Statewide Water Capacity Analysis
Project Title: Statistics for Statewide Water Capacity Analysis
The objectives of this project are to:
Statement of Problem
The New Jersey Department of Environmental Protection (NJDEP) is undertaking an update of the Statewide Water-Supply Master Plan for New Jersey. As part of this process, the water supply planners want to perform an assessment of the potential water capacity of the surficial aquifers of the state. The approach that they envision is to examine the low streamflow statistics for each Hydrologic Unit Code-11 (HUC-11) watershed. Streamflow at dry times is primarily composed of groundwater discharge, and of course supports the in-stream aquatic habitat. The NJDEP wants to examine the streamflow during a "normal" dry period and that of a drought. The difference between these flows has been termed the "low flow margin of safety" and can be viewed as the water "capacity" of the watershed. Their planning and management would focus on comparing consumptive groundwater and surface-water uses to this capacity amount and possibly to regulate use to some proportion of that amount. The specific low flow indicators that they have targeted are the September median flow, representing "normal" dry conditions, and the 7Q10 flow representing drought conditions (figs. 1 & 2). NJDEP has asked USGS to play a role in this process by determining the low-flow statistics for gaging stations in New Jersey and to develop the means to correlate confidently to ungaged basins, thereby covering the state. There are 150 HUC-11 watersheds within the State ranging in size from approximately 25 to 143mi2 (fig. 3). In addition, USGS will also provide NJDEP with checked water-use data to support the comparison with consumptive use. NJDEP will make that comparison and will define any water capacity limits.
Strategy and Approach
The approach to accomplish this investigation is to use low-flow statistics (September median flow and 7Q10) calculated from the record of those gaging stations that are sufficiently unregulated and un-impacted by inflows and withdrawals to develop a correlation to certain watershed characteristics. The low flow statistics for ungaged HUC-11 watersheds will be estimated using this correlation.