3. Marco Conceptual
3.7. La adolescencia
No satisfactory or consistent estimates of current outdoor residential water use are available for California. CDWR provides a variety of indi- rect estimates in different studies, mostly for a baseline of 1990. Given the uncertainties in the data, we felt a range of estimates would better capture the wide variation in the data and allow us to examine different scenarios. We initially developed five separate baseline estimates of out- door residential water use for 1990, described in detail in Appendix B (http://www.pacinst.org/reports/urban_usage/). Table 3-1 summarizes the results of four of those estimates (we exclude here the “winter watering” estimate, because of inconsistency in the results), together with three sep- arate estimates from the Department of Water Resources. The results of our calculations ranged from 850,000 to 1,650,000 AF/yr – a factor of nearly two – showing the high uncertainties about actual outdoor resi- dential water use. One of CDWR’s estimates is even higher: 2.23 million acre-feet (Table 3-1).
We used the “average month” method result to represent the low end of our range, and we offer results based on the low and high estimates and on the average of the high and low estimates. The 1990 estimates were then projected to generate an initial 2000 and 2020 baseline using the CDWR assumption that per capita use remains constant (Table 3-2).
“Average month” 850,000 “Minimum month” 910,000 “Hydrologic region” 1,090,000 “Representative city” 1,650,000 CDWR Bulletin 160-93 1,520,000 (a) CDWR Bulletin 160-93 1,340,000 (b) CDWR Bulletin 166-4 2,230,000 (c)
Institute Method Result (AF/year) Table 3-1
Estimates of 1990 Outdoor Residential Water Use
Notes: Estimates are rounded. For details see Appendix B
(http://www.pacinst.org/reports/urban_usage/). (a) This estimate uses CDWR’s applied urban
demand of 7.8 MAF in 1990, assumed ratio of residential use-to-total urban use (0.57), and assumed ratio of outdoor-to-total (0.34). (b) This estimate uses CDWR assumed outdoor
per capita value (40 gpcd) and 1990 population of 30 million.
(c) CDWR 1994b lists total residential use as 4.55 MAF (Table 2-7) and indoor residential use as 2.32 MAF (Table 2-9), leaving 2.23 MAF of outdoor use.
66 Outdoor Residential Water Use and Conservation Potential
Lack of good data has greatly hindered progress in both capturing and measuring efficiency improvements in the residential landscape sector. There is agreement that the potential for saving water is substantial, but the tools to quantify and evaluate specific savings in specific landscapes are only beginning to be developed. Most agencies know little about the characteristics of their residential landscapes; they do not always have reliable estimates of outdoor water use, let alone landscape acreage, type of plantings, or irrigation methods. Residential customers typically do not have dedicated irrigation meters, so site-specific information can be a challenge to collect. Because of the expense involved and because it is difficult for agencies to quantify savings, outdoor water-use data collection and analysis has traditionally been considered a low priority.1
Few districts have collected data on residential landscapes. Statewide estimates are even less reliable.
One estimate of conservation potential is the difference between an effi- cient water budget and current water use. To establish a water budget we need weather data and information on the nature and extent of irrigated acreage. Weather data are available from the CIMIS weather stations throughout the state (Gleick 1999). The latter is more difficult to obtain. In order to develop baseline estimates of residential landscape areas, we contacted agencies, irrigation and landscape associations, and various organizations and individuals working on landscape issues. The only statewide estimates available come from the Department of Water Resources, which estimates that in 1995 there were 1.2 to 1.4 million acres of urban landscape, most of which is irrigated.2This value is modi-
fied from preliminary estimates made during the 1980s of the ratio of landscape acreage to total urban acreage derived from land-use surveys (CDWR 1998). These ratios differ widely by county and can vary up to 40 percent (CDWR 1998). CDWR projections also assume that landscape acreage will increase proportionately to projected population growth. Implicit in this assumption is that current conditions, such as housing density and type, will remain constant in the future. CDWR staff suspect that the 1.2 to 1.4 million acres estimate may be high because the amount of water one million acres would require (based on the product of land- scape area, reference evapotranspiration, and crop coefficients) is consid- erably higher than most urban water budgets (Matyac, personal commu- nications, 2000). Another possibility is that the estimate of water use per unit area is too high, an assumption we explore below.
While preparing Bulletin 160-98, CDWR staff conducted a telephone survey of landscape experts to ask whether they knew of any studies done to estimate statewide landscape acreage. That survey yielded widely varying estimates: 673,000 acres of turf according to a 1995 USEPA study; 1.4 million acres of turf according to a 1980s UC Riverside study; and 1.8 million acres of irrigated landscape according to an estimate made by the Council for a Green Environment. However, most of the
Low 983,000 1,290,000
High 1,900,000 2,510,000
Average 1,450,000 1,890,000
Estimate Water Use 2000 (AF/yr) Water Use 2020 (AF/yr)
Table 3-2
Projections of Outdoor Residential Water Use (2000 and 2020)
1 There are a handful of agencies, such as the EBMUD and IRWD, that have been trying to collect information on outdoor water use by landscapes. There has also been increased interest in obtaining this information and research and the most appropriate methods to do so. For these studies see the Landscape Area Measuring Study Final Evaluation Report, October 1999. Prepared for the U.S. Bureau of Reclamation by the Contra Costa Water District,http://watershare.usbr.gov/. See also the Annual Water Allocation and Methodology, Pilot Project Executive Summary. May 1998. Prepared for MWDOC, MWDSC, USBR, and the Moulton Niguel Water District by Psomas and Associates.
2 http://wwwdpla.water.ca.gov/urban/land/irrigate- dland.html.
respondents said that they were unaware of reliable data on statewide landscape acreage (Matyac, personal communications, 2000).
Other estimates of outdoor residential water use are derived from simple assumptions of the proportion of indoor to outdoor use, differences between certain types of billing periods, and other approaches using data that water agencies collect more directly. The latest estimates are that out- door water use ranges from 30 percent of residential use in coastal areas up to 60 percent in hot inland areas (CDWR 1998). In some parts of the state, more than twice as much water is used in the summer than in the winter (Figure 3-1). In the latest California Water Plan (Bulletin-160-98) CDWR estimates urban outdoor use (including commercial, industrial, and institutional sites; parks; and other large landscapes) at 2.4 million acre-feet per year, about 60 percent of which (1.4 million acre-feet) is assumed to be residential. CDWR then assumes that per capita use will remain constant as the population grows, forecasting that 2020 outdoor urban use will increase to about 3.6 MAF. The assumption behind these numbers is that in 2020 irrigation rates will be 0.8 and 1.0 ETofor new and existing landscapes respectively (CDWR 1998).