• Mark L. Johnson

Tucson Water-One Water 2100 Analysis

Updated: Oct 9


Tucson Water (TW) is preparing an 80-year Water Master Plan called One Water 2100. This is an update to their previous Water Plan 2000-2050 (2012 Update).


The One Water 2100 Population Projections and Water Use Projections are now available on the TW website. The information and figures used in this analysis come from these two documents and responses to questions submitted to TW staff.


Note this analysis only considers potable water. It is assumed that TW will continue to supply recycled wastewater (effluent) to meet the demands of large recreational areas like golf courses, parks, etc.


If you want to skip the details, go right to the summary by clicking: Conclusions and Recommendations.



Tucson Water Service Area


The TW Service Area is shown on the map below. You will note that the water service area goes well beyond the Tucson City boundary (red).


In Marana (blue boundary), TW provides water service to Dove Mountain and the Silverbell Area. However, in Dove Mountain for example, the TW Service Area is limited to specific boundaries (e.g. Dove Mountain Specific Plan) related to older water service agreements with developers.


TW cannot expand water service in the northeast sector of Marana (Tortolita Fan including the Tortolita Preserve and surrounding lands) per the current Tucson Water Service Area Policy adopted by the City of Tucson Mayor and Council and the current Intergovernmental Agreement with the Town of Marana. In addition, this area is not currently in Marana Water's Intended Service Area.




Methodology


The basic process that TW used for establishing future Water Demands (Usage) was to first develop population projections (high-medium-low) in terms of Housing and Employment out to the year 2100 and then apply unit water usage factors to the these projections to establish future water use.


The following presents the details of the water use projection methodology. This analysis will only cover the medium or average projections.


Population Projections

  • 2018-2045

  • Used Pima Association of Governments (PAG) projections for Housing Units (residential) and Employment (non-residential)

  • 2018 Housing Units = 311,434 : 2045 Housing Units = 350,000 (+38,566)

  • 2018 Employees = 290,000 : 2045 Employees = 376,000 (+86,000)

  • Population = Housing Units x 2.46 persons/unit x 0.95 (vacancy factor)

  • 2018 Population = 727,821 : 2045 Population = 818,000 (+90,179)

  • 2045-2100

  • Estimate Housing and Employment for specific potential development areas and infill projects in the Tucson Water Service Area and some minor annexations

  • Add these projections to the PAG 2018-2045 projections

  • 2100 Housing = 405,393 (+93,959 or 1,146/year for 82 years)

  • 2100 Population = 947,403 (+219,582 or 2,678/year [0.4%/year] for 82 years)

  • The Population Projections are shown in Figure 22 (below) and you will note that the projections are about 272,000 less persons than the previous Water Plan 2000-2050.



Water Usage Projections


The Housing projections (residential) and Employee Projections (non-residential) were applied to unit water consumption factors and adjusted for; (1) water conservation (reduction in usage), (2) climate change (increase in usage) and (3) system water loss (non-revenue water). The assumptions utilized are:

  • Household Consumption - 208 gallons per day (gpd). Assuming 2.46 persons/household yields 85 gallons per capita per day.

  • Employee Consumption-59 gpd

  • Water Conservation System-wide (average)-1,050,000 gpd

  • Climate Change-1.5% increase from 2045 to 2100

  • Non-Revenue Water-increase projection by 8.7%

The resulting Water Demand formula:


Water Demand=[(Housing Units x 208 gpd/unit) + (Employees x 59 gpd/employee)-(Water Conservation Savings)] x 1.015 (climate change) x 1.087 (non-revenue water).


The results are shown below in Figure 13 and summarized as follows:

  • 2050 Average Water Demands-125,000 AFY which is 35,000 AFY less than the Water Plan 200-2050 projection.

  • 2100 Average Water Demands-133,000 AFY



Water Supply



Tucson Water's Designated Assured Water Supply (DAWS) totals 182,852 AFY (see chart). It was last approved by the Arizona Department of Water Resources (ADWR) in 2014. Remember, DAWS is a demonstration that sufficient supplies of adequate quality water are physically, continuously and legally available to meet the estimated demands of the community for a 100-year period.


You can learn more about how TW DAWS at Know Your H2O-Part X-Marana Water & Tucson Water.


TW utilizes only a portion of its DAWS for planning purposes. TW's "firm" water supplies (TW utilizes the term "wet water" which I believe leads to confusion) are shown in Table 11. TW's CAP Supply (Colorado River Water) has increased since 2014 to 144,191 AFY and represents 96% of its water supply. This is significant as the CAP Supply is considered a permanent, renewable water supply. Adding TW's Groundwater resources (6,371 AFY)(4%) yields a total firm water supply of 150,562 AFY.



TW does have other water supplies available that are listed in the Table below. TW has stored surplus CAP Supply and Effluent as Long Term Storage Credits (LTSC) totaling 508,300 AF in the aquifer(s) for use when the firm water shown in Table 11 is not available.


TW also has water rights in the Avra Valley area for 1.2 million AF of groundwater which are also reserved for future use.


The LTSC and Avra Valley Groundwater Rights are specific volumes of water and not annual amounts. To get an idea of the extent of these supplies, they have been converted to Years of Supply by dividing the volume by the current potable water demand of 85,000 AFY to get 6.0 years and 14.1 years, respectively.




Supply Versus Demand


Figure 18 presents the projected water supply versus the projected water demand. In 2100, the average water demand is 133,000 AFY and the available supply is 150,562 AFY. Therefore, there is an adequate existing supply of water to meet the needs of current and future TW customers for 80-years.



TA has recently sent a letter to the United States Bureau of Reclamation (USBR) recommending a permanent 20% cut across the board of Colorado River water delivery contracts because the Colorado River is over-allocated and aridification of the southwest will mostly cause the Colorado River water supply to diminish in the future.


The graph below is Figure 18 annotated to show the 20% Colorado River water cut which brings TW's CAP Supply allotment down to 115,000 AFY. Under this scenario, there is enough CAP Supply to meet average demands out to 2035 but would have to be supplemented with groundwater after that. In 2100, about 18,000 AFY of groundwater would be needed to supplement CAP Supply to meet average demands. This groundwater could come from the firm incidental recharge and groundwater allowance plus the LTSCs. Therefore, TW has enough water resources to meet the water demands for its Water Service Area with a 20% cut to its CAP Supply out to 2100.





Conclusions


  • TW Service Area Population is projected to increase from 727,821 (2018) to 947,403 (2100) [+219,582 or 2,678/year (0.4%/year) for 82 years]

  • TW per capita potable water usage is 85 gallons per day which is the lowest of major water utilities in the western US

  • TW non-revenue water is 8.7% which is considered high and should be cut to 5.0% or less

  • TW Potable Water Demand (Usage) is projected to increase from 85,000 AFY (2018) to 133,000 AFY (2100) [+48,000 AFY or 585 AFY/year (0.7%/year) for 82 years]

  • Population and Water Use Projections are much lower than the 2000-2050 Water Plan which probably is due to a more realistic, sustainable and refined approach utilized for this plan

  • Current firm water supply of 150,562 acre-feet/year (AFY) which consists of 144,191 AFY (96%) of CAP Supply and 6,371 AFY (4%) of Groundwater

  • Current water supply is adequate to meet projected demands to 2100

  • Current water supply with 20% cut to CAP Supply is adequate to meet water demands to 2100

  • Additional Groundwater-LTSCs totaling 508,300 AF (6-year supply at current demands) are available in addition to the firm water

  • Additional Groundwater Avra Valley Water Rights totaling 1.2 million AF (14.1-year supply at current demands) is available in addition to the firm water

  • TW cannot expand water service in the northeast sector of Marana (Tortolita Fan including the Tortolita Preserve and surrounding lands)

  • TW has done an excellent job securing a large, renewable, permanent water supply, i.e. CAP Supply (Colorado River Water)

  • TW has done an excellent job developing a recycled water system that treats and returns 90% of Pima County Wastewater effluent to large recreational areas and aquifer replenishment

  • TW has done an excellent job with water conservation, i.e. 85 gallons per capita per day

  • The One Water 2100 is an excellent long-term, comprehensive analysis and plan to ensure a sustainable water supply for the TW Service Area for the next 80 years


Recommendations


  • TW is located in the Tucson Active Management Area (TAMA) and TW should meet TAMA's goal to achieve and maintain aquifer safe yield

  • From 1985 through 2020, TAMA has a cumulative overdraft of 1.8 million acre-feet (ADWR TAMA Webpage). This means that over this 35-year period, 1.8 million acre-feet more water has been pumped from the aquifer than replenished. TW should pledge a portion or all of its LTSC and/or Avra Valley Groundwater Rights to payback the cumulative overdraft.

  • TW must take the lead and thoroughly model and analyze the impacts of PFAS compound pollution in the TAMA aquifers

  • TW must take the lead and analyze any adverse water quality and aquifer alluvium impacts due to many years of recharging these aquifers with non-native Colorado River water

  • Water supply planning must extend beyond the local system to include regional and multi-state systems. TW must continue its excellent comprehensive water supply planning to ensure that TW, TAMA and the entire southwest have adequate water supplies well beyond 2100.

  • TW must continue to implement water conservation initiatives, e.g. rebates for weather-based irrigation controllers, hot water heater recirculation pumps, etc.

  • Other TAMA public water agencies should adopt the same planning period and methodologies as One Water 2100 to ensure water supply planning consistency in the region

  • TA and other organizations and residents must be vigilant and ensure that the Tucson Water Service Area Policy and the Intergovernmental Agreement (Marana) are not amended to proliferate unsustainable growth



Next Up:

Comprehensive Review of Marana's Designated Assured Water Supply (DAWS) Application