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Where Is SLO’s Future Water Coming From? One Word – Nacimiento

May 29,2016-

The City of San Luis Obispo’s Planning Commission took up a a draft water management plan for the future at its May 25 meeting. The City assumes it needs to plan for a population of 57,200 within its urban reserve until 2035, up from 48,802 as of 2015.

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To supply the water needs of that population, they are counting on surface water shipped from Nacimiento Lake near the Monterey County border fed by pipeline. With little prospect of groundwater sources and only a small increase in recycled water – in March 2016, the City Council approved the addition of 2,102 acre feet per year from Nacimiento Reservoir to the City’s secondary water supply.

The City says the secondary water supplies are used to meet short term losses to the City’s water supply due to events such as drought, pipeline maintenance, and repair of infrastructure. With uncertainty of future climatic conditions, regulation and aging infrastructure, the additional supply of Nacimiento water to the City’s portfolio reduces pressure on use of water supplies in the Whale Rock and Salinas reservoirs. It would serve to extend these stored supplies during critical water shortage periods

That takes the annual supply of potable water from Nacimiento to 5482 AF added to its other reservoir water sources – assuming a loss of some supply from siltation at Whale Rock Reservoir.

The upshot is to meet the additional need for water from growth in the city – Nacimiento Lake is the go-to source although other existing reservoirs offer a base supply.

The San Luis Obispo County Flood Control and Water Conservation District (County) has held an entitlement to 17,500 acre-feet of water from Nacimiento Lake since 1959.

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The lake has some 125,000 acre-feet stored as of this week.The reservoir has a storage capacity of 377,900 acre-feet and serves the purpose of abating seawater intrusion in the groundwater aquifers of the Salinas River Valley.The County began construction in 2007 on a 45-mile pipeline project to deliver water from the Nacimiento Reservoir to participating agencies and cities.

Despite  what they call a “reliable” water portfolio for the future totaling 12,109AF the City is expected to need – demand some 6035 acre-feet as of 2015 and increase demand to 7779 AF by 2035 even after 3 dry years.

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As one can see from this chart the City’s share of water from Salinas Reservoir (Santa Margarita Lake) and Whale Rock combine enough to supply our annual use for many years with Nacimiento being the insurance  policy if things got worse.

In a second report presented the same day the planning commission  heard their analysis of residential and non residential growth expected.

The report offers some interesting insights.Although the residential population here is 48,000 – the city’s daily population is over 70,000.

CalPoly is the top employer with 3055 jobs.As the top employer, Cal Poly maintains a student enrollment between 18,260 and 19,780 students with a projected enrollment at 20,912 students by the year 2020. While Cal Poly has its own water supply source, the City treats and distributes water to the University which is located just outside of City limits.The report notes that  Cal Poly is out of session at this time, thus reducing the overall daily population served by the City during the summer months. In 2015, with the update to its Master Plan underway, Cal Poly indicated that it is considering the idea of year-round instruction by expanding summer classes.

Adding the numbers regards new residential growth the City’s plan assumes an additional 5000 new units to be added by 2035.The analysis  indicates that nonresidential growth will remain below the City’s one percent average annual growth rate throughout the general plan buildout period to 2035.

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