Estimated Non-reservoir Streamflows of Esopus Creek at Coldbrook and Mount Marion, New York

Problem Statement

More than nine million people rely on the New York City Water-Supply System for their daily-drinking water needs. Approximately 40 percent of this water comes from the Schoharie and Ashokan Reservoirs (fig. 1). This water is transported from the Catskill Area to New York City through Esopus Creek and a series of man-made tunnels and aqueducts built starting in the early 1900s (fig. 1).  The U.S. Geological Survey (USGS) has been measuring streamflow continuously in the Upper and Lower Esopus Creeks for many decades. Specifically, streamflow has been measured in the Upper Esopus Creek at Coldbrook (station number 01362500) for about 80 years and in the Lower Esopus Creek at Mount Marion (station number 01364500) for about 40 years (fig. 1). Knowledge of the non-reservoir flow regimes of the Upper and Lower Esopus Creeks could be beneficial in some water resource management applications.

Objectives

With the above in mind, the USGS, in cooperation with the NYCEP, propose to estimate the non-reservoir flow regimes of the Upper and Lower Esopus Creeks at the two existing USGS gages at Coldbrook and Mount Marion (fig. 1). Streamflows measured by USGS together with reservoir operations records supplied by the NYC-DEP will be used to estimate non-reservoir streamflow at these two stations for water years 1931 to 2010[1].  The resulting non-reservoir daily streamflow records will be used to calculate statistics such as mean daily flow, mean monthly flow, mean annual flow, and flow durations at both gages.  Indicators of hydrologic alteration (IHA) parameters such as the magnitude of monthly water conditions, magnitude and duration of annual extreme water conditions, timing of annual extreme water conditions, frequency and duration of high and low pulses, and the frequency and duration of water conditions changes will also be calculated for non-reservoir flows at the Coldbrook and Mount Marion gages (The Nature Conservancy, 2009). 

Benefit Statement

The analysis proposed herein will assist the NYCEP in evaluating how the Schoharie and Ashokan Reservoirs affect the flow regimes of the Esopus Creek. This project also fulfills the USGS Water Resources Division’s mission to provide reliable, impartial, timely information that is needed to understand the Nation’s water resources through a program of shared efforts and funding with State and local partners.

Approach

Streamflow data have been collected by the USGS in the Upper Esopus Creek at Coldbrook since 1931 and in the Lower Esopus Creek at Mount Marion since 1970. At these continuous-record gages, stream stage is measured every 15 minutes (near-real-time). Streamflow is computed with a stage-discharge relation (or rating), which is developed on the basis of periodic manual measurements of streamflow. Data from these two gages will be retrieved from the USGS National Water Inventory System Web Interface (NWIS Web). In addition to the aforementioned streamflow data, records of reservoir diversions, releases, and spillage are also needed.  The USGS began electronically recording diversions from the Schoharie Reservoir at the outlet of the Shandaken tunnel in 1996.  Prior to 1996 the NYCEP kept written records of these diversions.  Records of spillage and releases from the Ashokan Reservoir are also needed.  The NYC-DEP will supply the USGS with Schoharie Reservoir diversions and Ashokan Reservoir releases and spillage (in electronic format) so that the data can be used to estimate non-reservoir streamflow and calculate flow statistics such as the magnitude, frequency, duration, timing, and rate of change of daily hydrologic conditions.  

Non-reservoir streamflow at Coldbrook will be estimated by subtracting Shandaken tunnel outflows (Schoharie Reservoir diversions) from measured streamflow at the Coldbrook gage.  There are many days when the Ashokan Reservoir was not spilling or releasing; on these days measured streamflows at the Mount Marion are assumed to represent non-reservoir streamflow for the drainage area downstream of the reservoir.  These non-reservoir flows at Mount Marion will be regressed against estimated non-reservoir flows at Coldbrook for the same time period using USGS computer programs (for example, Hydcomp or Miste).  This relation will be used to estimate non-reservoir flows at Mount Marion on days when the Ashokan Reservoir was spilling or releasing and from 1931 to 1970, when there is no streamflow record at Mount Marion.  Discharge for a small portion of the Mount Marion watershed located downstream of the Coldbrook gage and upstream of the Ashokan Reservoir is not measured by either gage and will be estimated using runoff models (Randall, 1996).  Estimated non-reservoir daily streamflow at Mount Marion for 1931 to 2010 will be calculated by summing estimated non-reservoir streamflow at Coldbrook, estimated (or measured) non-reservoir streamflow at Mount Marion downstream of the Ashokan Reservoir, and the runoff estimated discharge for the ungaged portion of the watershed upstream of the Ashokan Reservoir.

IHA parameters (see above) will be calculated at the Coldbrook and Mount Marion gages using non-reservoir flows.  This analysis will be done using downloadable software developed by The Nature Conservancy (TNC) and is intended to summarize a long period of hydrologic data into a more manageable series of ecologically relevant hydrologic parameters (The Nature Conservancy, 2009).

Deliverables

            Data analysis procedures and results will be published in an online-only USGS Report.  In addition to the online report, USGS will provide twelve hard copies to NYCDEP.  A spreadsheet of the digitized daily non-reservoir streamflow record at both gages and graphical and tabular outputs from the IHA analysis will also be made available to NYCEP.  A technical review draft of this report will be available 6 to 8 months after Schoharie Reservoir diversion and Ashokan Reservoir releases and spillage data are received by USGS from NYCEP.

References

Randall, A.D., 1996, Mean annual runoff, precipitation, and evapotranspiration in the glaciated northeastern United States, 1951-80, U.S. Geological Survey Open-File Report 96-395, 2 pls

The Nature Conservancy, Indicators of hydrologic alteration version 7.1 user’s manual, with rPurview LLC-Ted Rybicicki, Totten software design, Smythe scientific software, http://conserveonline.org/workspaces/iha/documents/download/view.html, 81 p.

Project
Location by County

Catskill Region: Delaware County, NY, Greene County, NY, Schoharie
County, NY, Sullivan County, NY, Ulster County, NY