Water Resources in Rockland - Planning in a Changing World

Rockland County Water Use

washing dishesDifferent ways of measuring the amount of water used serve different purposes. The highest daily use in a year, or peak demand, is important in understanding requirements for fire protection and increased seasonal demand. It's only necessary to meet peak demand for a short period of time, however, and average demand, measured in millions of gallons/day (mgd) is used to assess the adequacy of a water provider's sustainable water supply. In assessing the potential role of increased water conservation in long-term water supply plan it's useful to look at per capita usage. We take all three perspectives below. Click here for revised April 25, 2010 presentation on Rockland County Water Use and Conservation Potential (2/16/11). Click here for April 2, 2013 public talk at Rockland Community College on Water Conservation Opportunities in Rockland County.

Annual Average Demand (MGD)
mgd = million gallons per day
1970 19.7
1971 19.9
1972 20.8
1973 21.8
1974 22.5
1975 23.2
1976 24.2
1977 25.6
1978 25.2
1979 25.3
1980 25.9
1981 22.4
1982 23.1
1983 25.4
1984 26.1
1985 24.3
1986 24.6
1987 26.6
1988 27.7
1989 26.7
1990 27.4
1991 29.9
1992 28.3
1993 28.8
1994 28.6
1995 28.4
1996 28.0
1997 27.5
1998 28.9
1999 28.8
2000 28.6
2001 29.7
2002 26.7
2003 28.5
2004 29.2
2005 31.7
Maximum Day Demand (MGD)
mgd = million gallons per day
1970 29.8
1971 33.0
1972 28.6
1973 36.2
1974 37.9
1975 35.7
1976 33.4
1977 43.0
1978 37.5
1979 37.1
1980 40.8
1981 30.7
1982 31.4
1983 37.0
1984 37.3
1985 27.7
1986 34.4
1987 38.9
1988 41.6
1989 33.1
1990 34.2
1991 44.9
1992 35.7
1993 40.7
1994 38.6
1995 40.0
1996 36.8
1997 39.2
1998 42.9
1999 44.6
2000 39.1
2001 46.5
2002 32.1
2003 37.4
2004 40.3
2005 43.5
Source: Dr. Daniel Miller, Rockland County Hydrologist, testimony to the Public Service Commission, 2006.
Minor typos in original testimony corrected here.
Drought. Not included in max demand analysis because max demand artificially reduced. Not representative.
Drought. However, included in analysis since water use restrictions were declared in Sept, well after max-day demand occurred.

Historical Average Demand in Rockland County

United Water data from 1970-2005 show a general increase in average demand from the low of 19.7mgd in 1970 to the high of 31.7mgd in 2005. To put this in perspective, Rockland County sustainable water supply was assessed by UWNY to be 32-34 mgd in 2006, barely above annual demand in 2005.

Historical Peak Demand in Rockland County

United Water data from 1970-2005 show a general increase from 29.8mgd in 1970 to 43.5mgd in 2005. The low was 27.7mgd in 1985 and the highest daily demand in the period was 46.5mgd in 2001. To put this in perspective, Rockland County peak day capacity is from 44.5 - 47.5mgd. In other words, Rockland County peak demand in the summer is approaching peak day capacity. Peak day capacity is defined as the maximum amount of water that can be delivered in a day, given the currently developed resources (wells and surface waters).

In looking at the historical peak demand it's helpful to know that requests for voluntary conservation were made in 1981, 1982 and 1985 before mandatory drought restrictions were enacted as part of the Rockland Sanitary Code, and mandatory water restrictions were invoked in 1995, 1999 and 2002.


Rockland County Seasonal Water Usage

updated 2/16/2011

Single-family residences are the largest group of water users in Rockland County, with average daily per capita usage between 2000 and 2009 ranging from 62.49 gallons per capita per day (gpcd) in 2009 to 72.89 in 2001. Looking at residential usage data on a per capita basis provides us with a way of assessing conservation potential in the future. In the table to the right, estimated indoor usage is an average of daily per capita usage from mid-October through mid-April. Indoor and outdoor use is an average of daily per capita usage for mid-April through mid-October. The estimated outdoor use is the difference between the two.

Compared to the study locations in the Residential Water Usage Study cited below, Rockland County's average increased summer usage is relatively low during this period at 11.54 gpcd. Rockland County's average non-summer usage at 62.15 gpcd is roughly 10% below the national indoor usage average of 69 gpcd. Comparing this number to the standard for an EPA WaterSense conserving home of 40 gpcd suggests indoor water conservation measures could be an important component in Rockland County's water supply planning going forward. In order to meet peak demand requirements, however, the peak daily demand in relationship to average daily demand is critical, and that ratio in Rockland County is approximately 1.6.

Single family residential per capita use by year (gpcd)
Year Average daily use Average estimated indoor use Summer indoor + outdoor use Estimated average outdoor use
2000 69.21 64.29 73.71 9.42
2001 72.89 64.95 80.72 15.77
2002 64.84 63.06 66.57 3.51
2003 66.49 61.72 71.21 9.49
2004 66.70 61.91 71.10 9.19
2005 71.36 62.14 80.45 18.31
2006 69.37 62.29 76.30 14.01
2007 69.90 60.50 79.12 18.62
2008 66.96 60.39 73.05 12.66
2009 62.49 60.24 64.67 4.43
Average 2000-2009 68.02 62.15 73.69 11.54
gcpd = gallons per capita per day
How does the new (2/2011) set of calculations for single-family residential water usage differ from the old calculations?

The people/ household factor used to convert single-family residential accounts to people was changed from 3.0 to 3.24. The old factor was an average of single-fanily residential, multi-family and apartment while the new factor represents single-family residential only. In addition the assumption about which months represent indoor use only was changed in order to match UWNY assumptions for easier discussion and comparison.

Will the water usage calculations and indoor/outdoor estimates change again?

We expect to refine the indoor/outdoor estimates in the future as more data becomes available and we are beginning to look at the development of weather-adjusted usage estimates.


Future Water Use

In order to determine requirements for future water supply both the Rockland County Bureau of Water Supply and United Water of New York estimate what demand for water will be in the future. They look at both peak day demand (the highest water use day of the year) and average water use (average daily use for a year).

Peak Demand

Rockland County's approach to projecting future demand was to start first with a straight line regression, predicting essentially that water use in the county will continue to increase in the future at the same rate that it has increased in the past. When making this estimate Rockland County removed the years when demand was lower than normal due to mandatory conservation restrictions during declared droughts. In this way they estimate the future growth of unconstrained demand during non-drought years, resulting in the following peak day demand projections:

2010 - 43.5MGD
2015 - 44.7MGD
2020 - 45.9MGD

It turns out however that because of the considerable year-to-year variability the actual maximum day demand was greater than predicted by linear regression 50% of the time etween 1970 and 2005. As a result, when Rockland County projects future demand for water it identifies a range so that it can estimate a maximum day demand number that will be as high or higher than the actual number 80, 90, or 99% of the time.

By excluding drought years, these projections account for existing regulations (Article V of the Sanitary Code) that can be used to reduce demand during drought.

UWNY uses a different methodology to estimate future water usage with numbers that are generally higher than the Rockland County projections but generally within the 99% confidence interval. UWNY projections show an increased rate of increase in water usage out to 2020, in contrast to the Rockland County projections that predict a stable rate of increase. In his testimony to the Public Service Commission the Rockland County hydrologist speculated that the projected increased rate of increase is "likely due to the style of development recently experienced in Rockland, i.e., large single-family homes with extensive landscaping and irrigation systems." (Miller, 2006)

Average Demand

For average demand Rockland County and UWNY take the same approaches they did with peak demand with two differences:

1. Rockland County began its regression to estimate future average demand in 1981 rather than 1970 because water conserving plumbing fixtures were introduced in 1980 and the rate of increase in average water use significantly slowed.

2. UWNY's average demand projections are lower than Rockland County's 80% confidence level, suggesting that 10% of the time actual demand will be exceeded.

New York State Laws and Regulations
NYS Regulations 3.1.1 Recommended Standards for Water Works, 2003 Edition, incorporated by reference in 10 NYCRR, Part 5, Subpart 5-1.222 require that.

1. For a surface water supply the quantity of water at the source shall be adequate to meet the maximum projected water demand of the service area as shown by calculations based on a one in fifty year drought or the extreme drought of record and should include considerations of multiple year droughts and provide a reasonable surplus for anticipated growth.

2. Total developed groundwater source capacity, unless otherwise specified by the reviewing authority shall equal or exceed maximum day demand with the largest producing well out of service.



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