Upstream Water Diversions Not Always Bad for Downstream Users
Authors:
- Reed Palmer - Hazen and Sawyer
The standing assumption on diversions of water from a river is that they invariably deplete the supply available to downstream users. While true in the case of irrigation and other diversions where little if any of the water is returned to the river, the reverse may be true—diversions may actually increase the quantity of water downstream during periods of low natural river flow—for municipal and other withdrawals where (a) storage is involved, and (b) a significant fraction of the water is returned to the river as treated wastewater effluent.
This paper discusses the results of water resource modeling of the Upper Neuse Basin which demonstrated that increase water use by Durham and Raleigh can beneficially impact water conservation storage in Falls Lake. Hazen and Sawyer completed this modeling using the Neuse River Basin Model (NRBM), which the NCDENR, Division of Water Resources (DWR) commissioned and recently released, to assist the cities of Raleigh and Durham in evaluating the supply capacity of their existing water supply reservoirs and alternatives to meet future water demands. The NRBM, developed by Hydrologics., Inc., for the DWR using its OASIS platform, is the first basinwide model of the Neuse River and thus the first tool to provide the capability to evaluate the impacts of water use in one area of the basin on other areas.
The NRBM demonstrates that higher levels of water use in Durham result in a net increase in the yield of Falls lake—notwithstanding that higher refill requirements at the upstream reservoirs translates to less water available to refill Falls lake. An even more interesting finding is that increased withdrawals from the Falls Lake water supply storage pool translate to increases in the volume of storage remaining in the lake’s water quality pool during periods of drought. These findings have broad implications to other municipal diversions and surface water resource systems. An important corollary to “the more water you use the more you return,” is that, while water recycling and reuse have undeniable benefits, they may not be the bonanza to overall water conservation that is widely assumed, especially in terms of impacts to downstream users.
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