Calculation of Domestic Well GPCD Rates for the

Upper San Pedro Basin Active Management Area Review Report (March 2005)

 

 

            The methodology used by the Arizona Department of Water Resources (ADWR) to calculate the domestic well GPCD rate for the Upper San Pedro AMA Review Report (AMA Report) is discussed on pages 4-18 and in Appendix J.  More specific information, including actual calculations is presented here.

 

 

Domestic Well Use Estimates

           

            There are various estimates of domestic well use from a number of reports.  A document related to the Special Master’s De Minimus Report evaluated domestic well use in the San Pedro River Watershed.[1] The report recommended .17 to .25 acre-feet per year for indoor domestic use and .83 to .75 acre-feet per year for irrigation associated with domestic use; or 1 acre-foot per year per well. The irrigation number was based on 4 acre-feet per acre to irrigate about .2 acres. The Special Masters memorandum decision, findings of fact and conclusions of law in the Sands Group of Cases (W1-11-19) and Other Related Cases (consolidated), November 14, 1994, accepted the report’s recommendations, finding that the 1 acre-foot per well estimate was a “reasonable estimate of individual self-supplied residential uses…based on a typical household of three persons and irrigation and the watering of pets and farmyard animals on an adjoining 0.2 ac of land…”.

 

            A New Mexico study estimated domestic well use at .35 acre-feet per residence per year assuming 114 GPCD and 2.74 pphu.[2]  An ADWR analysis of domestic well use, using Phoenix AMA assumptions, estimated that the demand for a four family household could vary from .44 acre-feet/yr to 10 acre-feet/yr. The higher number assumed irrigation of a two acre pasture at the rate of 4.9 acre-feet per acre.[3]

 

            The Hydrographic Survey Report for the San Pedro River Watershed (San Pedro HSR) estimated domestic well use at .17 acre-feet per person per year (page C-88 and C-89).  This figure was based on annual report data from the Phoenix AMA of urban water use.

 

 

Upper San Pedro Domestic Well Use Methodology

 

            Although these reports and others were evaluated, researchers chose to develop a methodology to reflect, to the extent possible, the actual water use characteristics of domestic well users in each of the Upper San Pedro sub-areas as described below. Total use is composed of interior use, landscaping use and irrigated pasture/garden use.

 

  1. Calculate the domestic well population to generate a population-based rather than well-based domestic well demand estimate.

                  Census population estimates were considered the most reliable and are a widely                           accepted number. The service area population was subtracted from the sub-area                                  population to generate a domestic well population. See Appendix J of the AMA                                  Report for a complete explanation of the population methodology.

 

  1. Find an example of metered and reported water use by a large lot service area, without pasture/garden irrigation, that is considered representative of residential domestic well use in the Upper San Pedro Basin.

                  The Avra Water Coop in the Tucson AMA is a large rural water company located                       in the Avra Valley. An analysis of 898 lots in the service area using Pima County                         “Mapguide” found an average lot size of 1.3 acres.  A landscape survey of the                                     service area in 1987 found that landscaping was primarily low water use although                  fruit trees and livestock were observed[4]. Irrigated pasture was not observed and                                     exterior water use characteristics are not believed to have changed significantly                      since the survey. The population of the service area in 2000 was 7,086.  The                                average residential per capita rate from ADWR Annual Water Withdrawal and                          Use Reports for the period 1992-2002 was 108 GPCD or .12 af/person/year.[5]

 

  1. Analyze the San Pedro HSR to determine the demand of irrigated acreage of pasture/gardens (<2 acres in size) associated with domestic wells.

                  < 2 acre irrigation was derived by subtracting average acres irrigated (Table C-                            17) and deficit irrigated acres from total irrigated acres (Table C-15); 572 acres                                  in the Sierra Vista sub-area and 307 acres in the Benson sub-area. (See                                     discussion pg. C-77). Demand was estimated using the crop consumptive use,                            assuming deficit irrigation.  This demand was divided by the domestic well                               population in 1990, calculated by subtracting the water provider population                                    from the sub-area population, to yield an acre-foot per person use.

 

Benson sub-area

Residential use                          =   .12 af/person/year

< 2 acre irrigation acres             =   307 acres x 3.43 af/acre x .86 = 906 acre-feet

                                                (3.43 is the crop mix consumptive use estimated from a 2002 field survey and .86 is a deficit irrigation factor calculated for deficit irrigated turf in the Tucson area)

irrigation use per person             906 af / 2087 domestic well pop. in 1990 = .435 af/person

Total domestic well use              =    .12 + .435 = .55 af/person/year

 

Sierra Vista sub-area

Residential use                          =   .12 af/person/year

< 2 acre irrigation acres             =    572 acres x 3.04 af/acre x .86 = 1490 acre-feet

                                                (3.04 is the crop mix consumptive use estimated from the San Pedro HSR and .86 is a deficit irrigation factor calculated for deficit irrigated turf in the Tucson area)

irrigation use per person             1490 af / 6339 domestic well pop. in 1990 = .23 af/person

            Total domestic well use              =    .12 + .23 = .35 af/person/year

 

[1] “Identification and Quantification of Domestic, Stockwatering and Stockpond Water Right Entitlements San Pedro River Watershed,” January 11, 1994 prepared by Gregg S. Ten Eyck, Leonard Rice Consulting Water Engineers, Inc.

2 Domestic Wells in New Mexico, Prepared by the Office of the State Engineer in response to the Resolution of Senate Joint Memorial 5, Forty Fourth Legislature, Second Special Session, 2000, Hydrology Bureau December 2000.

3 Internal ADWR memo, December 19, 2002.

4 ADWR Provider Profile; Avra Water Coop, March 7, 1988.

5 All water providers in AMAs are required to meter withdrawals and submit an Annual Water Withdrawal and Use Report to ADWR which includes data on the volume of water pumped and the number of residential and non-residential service connections. Large providers (those serving over 250 acre-feet per year) must also report the volume of water delivered to customers. ADWR calculates a service area population and person per household (pphu) figure for each provider for the U.S. Census years. The annually reported residential connection figure is multiplied by the appropriate pphu to generate a service area population. ADWR analyzes each water provider’s annual report and calculates an annual per capita water use rate by dividing the volume of water pumped by the service area population. This rate is a point of regulatory compliance. Annual reports have been collected since 1984 so the Department has a long period of per capita water use information for many providers in AMAs. The Department also has information about housing age, general landscaping characteristics, lot size, etc.