Bruce M. Thomson, a former colleague of mine at the University of New Mexico who succeeded me as Director of the University's Water Resources Program is quite the Renaissance Man - environmental engineer extraordinaire, musician (fiddle, etc.), pilot, exceptional teacher, and so on, just published this paper in an ASCE journal: Stormwater Capture in the Arid Southwest- Flood Protection versus Water Supply. It deals with an important issue - what's the bigger need in the desert: flood protection or water supply? With much interest in Flood-MAR (Managed Aquifer Recharge), the question looms large.
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Click on the graphics to enlarge them or display them. I pasted half the 'Introduction' and the entire 'Conclusions' sections below.
Introduction
Communities in the arid southwestern US are especially vulnerable to water shortages due to the limited supply of available water. These problems are exacerbated by the inherent uncertainties of desert hydrology in which long dry periods are interspersed by brief but intense precipitation events. The water supply challenges for these communities are compounded by increased demand to meet the needs of growing communities and diminishing water resources resulting from climate change. These factors have increased awareness of the impacts of limited water supplies and is generating a search for new supplies to meet future demands.The options that are usually considered for meeting future demand include: (1) conservation, (2) interbasin transfer, (3) brackish water desalination, (4) wastewater reuse, and (5) rain or stormwater capture. Each has its own challenges and/or limitations. In recent years considerable attention has been given to water conservation with remarkable success. For example, in Albuquerque, New Mexico, voluntary conservation programs have reduced the per capita daily (pcd) water demand from greater than 950 Lpcd (250 gpcd) in 1995 to less than 500 Lpcd (130 gpcd) in 2019 (ABCWUA 2019). This has caused a 25% decrease in the community’s total water demand even though the population served has increased by more than 50% over this period.
Interbasin transfers have public appeal as there is the common misconception that neighboring basins have excess water that can be acquired and diverted to a neighboring basin experiencing chronic shortages. While this may have been true decades ago, it is certainly not the case at present where water resources in virtually all basins in the western US are overallocated, meaning the rights to divert water exceed the actual supply of wet water. Fleck (2016) and Kuhn and Fleck (2019) have recently summarized the conflicts associated with interbasin transfers in the Colorado River basin and the resulting social, political, economic, and environmental consequences. Though it’s a bit dated, the US Bureau of Reclamation published a study of potential regional water shortages in the western US (BOR 2005), which found that water conflicts are occurring or are likely to occur in every western metropolitan area south of the 42nd parallel (i.e., the Oregon–California border).
Cutting to the chase...
Conclusions
This paper suggests that there are five major constraints that limit the ability to capture urban stormwater and use it to augment a community’s water supply in arid regions: (1) acquiring water rights to allow capture, retention, and use of stormwater; (2) the hydrology of arid regions result in infrequent but intense runoff events that provide a limited volume of water compared to a large urban utility’s demand; (3) capturing and storing urban runoff requires additional storage volume beyond that used for flood control and dams would require rebuilding to allow long-term stormwater retention and control release; current dry dams cannot serve either function; (4) urban stormwater in arid regions is of very poor quality and would require treatment to remove large amounts of sediment, debris, and dissolved organic and inorganic pollutants; and (5) the cost of infrastructure to capture, convey, store, and treat urban runoff to address the previous four constraints will be very expensive. This paper did not consider the economics of stormwater retention and reuse because the uniqueness of every such project would be entirely location-dependent and generalized cost estimates cannot be developed.
These challenges were discussed in the context of the water laws, hydrology, watershed characteristics, and stormwater quality of Albuquerque, New Mexico, but they would also apply to almost every inland municipality located in the arid southwest.
Instead of community-scale stormwater retention and reuse, onsite retention and reuse may be a more realistic strategy for urban stormwater capture. In addition to recovering the value of water and reducing the demand on the community’s water supply, it would allow a reduction in the size of stormwater management systems. However, there are negative aspects as well. They require significant land area for on-site ponds or an investment in water tanks; land owners must maintain the retention system; and there is potential mosquito growth in standing water. If it can be successfully implemented, on-site retention will decrease the volume of water returned to the river, which may impact downstream water supplies and/or have environmental consequences by diminishing in-stream flows.
Perhaps the most important conclusion of this discussion is that it draws awareness to the multiple regulatory, hydrologic, infrastructure, and environmental factors associated with stormwater management. The discussion shows that though large-scale stormwater capture and reuse concept has public appeal, the regulatory and infrastructure challenges are so great that a project to recover the comparatively small volume of water available is not likely to be feasible. As communities seek to address the conflicting challenges of water supply shortages and need for flood protection, a holistic evaluation of all constraints is needed to develop best management practice.
Enjoy!
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