Drinking Water Supply

Vulnerability Assessment of Drinking Water Supplies

The most probable long-term climate projections indicate New York State and the Northeastern U.S. will have the same or slightly higher mean annual precipitation in the coming decades. However, even in the likely scenario of slightly increased annual precipitation, other factors may still lead to additional droughts. Namely, extended growing seasons, higher temperatures, and the possibility of more intense, less frequent summer rainfall may lead to increased short-term drought periods (but not necessarily long-term droughts that last a year or more). The definition of drought differs depending whether one is looking at it from a meteorological, hydrologic, agricultural perspective. As discussed here, droughts occur only when precipitation deviations are of sufficient spatial and temporal scope to cause deficits in surface and/or groundwater supplies used for drinking water.

To assess water supplies in the face of climate change in New York State, we assume that long-term average supply will remain largely the same but that the duration and/or frequency of dry periods may increase. To develop a relative level of vulnerability, water systems have been classified based on the span of time over which they can handle a temporary, but sizable, decrease in supply. For both surface water supplies and groundwater supplies, the systems are divided into three categories: sensitive to short droughts (2-3 months), sensitive to moderate and longer droughts (greater than 6 months), and relatively insensitive to any droughts. This provides a basic sense of the population and the characteristic of communities likely to be most vulnerable to yet uncertain changes in water supply. For this vulnerability assessment, we used a list of permitted public water supplies serving populations greater than 3,000 people available from the New York State Department of Health.

Table 2 summarizes our vulnerability analysis (additional details are available in the full report). This summary provides an inter-comparison of potential vulnerabilities in drinking water supplies across the state, giving perspective on how limited resources should potentially be targeted to build resilience to climate variability and change. Table 2 lists each water source category, identifies the categories' sensitivity to climate change, and enumerates the population served by each source. Sensitivity is related to the length of drought that a water system could endure without being severely stressed, as estimated from system storage and demands. Systems with low sensitivity have sizable storage relative to demand while systems with high sensitivity have minimal storage relative to demand.

The populations with little sensitivity to climate change are those that draw from larger waterbodies such as the Great Lakes, Finger Lakes, or large rivers (Category 1). However, most of the State's population has a water supply that falls into the category of moderate sensitivity to climate change. Major reservoir systems (Categories 2 and 3) are presumed to have moderate sensitivity because there is increased likelihood of decreases in summer and fall water availability. If such systems have extensive storage capacity (i.e. large reservoirs relative to demand), they should be relatively insensitive, but such systems may also face increasing demands from expanding populations and changing future water uses. Major aquifer (Categories 5 and 6) are presumed to have moderate sensitivity because there remains uncertainty in how changes in the timing of annual precipitation may change recharge in a changing climate.

  Category Sensitivity to Climate Change Population Served
1 Withdrawal from Large Waterbodies Low 2,000,000
2 NYC System Moderate 8,300,000
3 Other Reservoir Systems Moderate 1,300,000
4 Run-of-the-river on small drainage High 62,000
5 Long Island GW Moderate 3,200,000
6 Other Primary Aquifers Moderate 650,000
7 Homeowner Well Water Moderate to High 1,900,000
8 Other Small Water Supply Systems (GW or SW) Moderate to High 1,600,000
      Total = 19,000,000

Table 2. Overview of vulnerability of water supply for NYS populations due to climate change. Sensitivity is related to the length of drought that a water system could endure without being severely stressed, as estimated from system storage and demands.

Finally, systems with little storage are likely to prove most sensitive to the more variable and drought-prone conditions that may be experienced in New York State. Namely run-of-the-river systems on small streams (Category 4), shallow wells (a portion of Category 7), wells in only moderately productive aquifers (a portion of category 8), and systems with small reservoirs relative to demand (a portion of category 8) will be the most affected by climate change. Since these types of systems serve small communities or individual homeowners, these entities may lack the expertise or resources to make proactive management decisions prior to actually running out of water. However, due to the dependence of such small systems on unique, localized conditions, it is unlikely these systems will fail simultaneously or that all systems in a given geographic region will run dry.

Suggested Adaptations

Many water supply systems lack the type of formal operating rules that are useful for mitigating the risk of shortages. Accordingly, one adaptation strategy that would be useful for managing contemporary and future climate risks would be to require public water suppliers to establish rule curves for water supply reservoirs and aquifers [e.g. a rule curve can set specific guidelines for reservoir releases given the amount of stored water at different times of the year]. The development of rule curves would provide a systematic, unbiased protocol for managing water supplies under drought.

Additionally, while there are numerous stream gauging stations throughout New York, there are few routine measurements of reservoir, aquifer, or lake levels. Consideration should be given to developing an automated gauging network or, at a minimum, a formal reporting network (e.g. routine manual measurements submitted to a central online clearinghouse) of water levels in public water supply reservoirs and aquifers. This would provide the basis for an improved early warning system for recognizing supply shortages while also establishing a long-term record for better understanding the link between the hydrology of specific watersheds and climate.

Possible adaptations could involve creating new water management commissions to oversee water allocations in regions with multiple competing users. The Delaware and Susquehanna River Basins already have commissions that oversee water allocations. Basin-level commissions should be established for other major rivers in the state, in particular the Hudson and Mohawk Rivers where population density and growth are greatest. The Hudson and Mohawk Rivers would be the highest priority as other major rivers (the Genesee River, Black River, Oswego River/Finger Lakes Region) have fewer users and already fall into the Great Lakes Basin.

The greatest likelihood of future water shortages is likely to occur on small water systems. The New York State Department of Health currently maintains a stockpile of equipment (mobile pumps, water tanks, filters, etc.) that can be used by municipalities to assist in supplementing critically low water supplies. Given the potential vulnerability of small water supply systems with climate change, the New York State Department of Health should consider updating and possibly enlarging its stockpile of drought emergency equipment.

Finally, across the State the threat posed by less certain water supplies can be most readily addressed by reducing consumption. Comprehensive water conservation plans have been developed for regions of the state (e.g. New York City) as well as other parts of the country. New York City's efforts during the 1980s probably served to avoid drought emergencies during some years of the 1990s. Additionally, in many cases a large portion of water usage is related to non-essential uses such as landscape irrigation, pools, and car washing. For example, in Rockland County water demand rises to upwards of 37 mgd in the summer from around 27 mgd during the winter (Haverstraw Water Supply Project, 2009; DEIS, United Water NYS). Thus, if demands must be cut, water usage could be greatly reduced in some locales without directly affecting basic activities essential for proper hygiene and sanitation.

Learn More

Climate Change Links

Intergovernmental Panel on Climate Change (IPCC) (link)

Northeast Climate Choices (UCS Reports) (link)

Climate Change and Northeast Agriculture (link)

Climate Change and Water Resources (NCAR) (link)

USDA Global Change Program Office (GCPO) (link)