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4/18/2014

How to Justify Filling a Pool in a Drought

Pool Design article by David J. Peterson, P.E., SWD Master

While the jet stream has shifted expected wet and cold weather patterns north this year, it has left California and other southern states in drought conditions. Meanwhile, northern states have record snow and cold temps. In some cases the drought has caused local jurisdictions to enact moratoriums on pool construction in an attempt to save water. This is the case in Santa Barbara/Montecito where a pool project was struggling to get through permitting. This is how the problem was solved.

First, it was acknowledged that the pool was replacing a lawn. The lawn required a certain amount of irrigation to overcome its evapotranspiration which is the combined soil and plant surface evaporation plus the plant tissue transpiration. The pool would have evaporation, so the question was: will the pool area require more water than the lawn or less water? If the government is going to place restrictions on pool construction then let’s start with their own data to justify the build.

The California Irrigation Management Information System (CIMIS) is part of the Department of Water Resources Office of Water Use Efficiency. Starting with the http://wwwcimis.water.ca.gov website, you can determine the evapotranspiration (ETo) for a standardized grass surface which is “well-watered actively growing closely clipped grass that is completely shading the soil”. This reference crop closely matches a residential lawn and the CIMIS site provides data in inches of water use for about 120 stations in California. Farmers use that data and specific crop coefficients to determine water use for other types of crops.

For example, the ETo for my zip code 92130 (station 184) is 2.07 inches for January, 2.42 inches for February, etc. The annual use is 46.50”. For an uncovered standard pool I would estimate ¼” per day of evaporation. That’s 0.25” x 365 = 91.25” or double the water use of the lawn that would be removed and replaced with a pool. Add a pool cover and evaporation is reduced by at least 70% which would reduce annual evaporation to 30% of 91.25” = 27”. This is a 41% reduction in water use that a lawn would have used. Conclusion: save water by building pools with covers where your lawn is location.

Note that ¼” per day of evaporation is an average use for normal pools but it may be higher with water features, vanishing edges, high-wind, dry air, etc. This also does not take into account the initial filling of the pool but that volume could be worked out or the pool could be filled from a source outside of the jurisdiction that is trying to block the construction. Don’t let the authorities prevent you from making a living and your clients from enjoying their pools – figure out a way for everyone to win!


David Peterson P.E., SWD Master

David Peterson P.E.

Watershape Consulting, Inc.
140 Lomas Santa Fe Dr Ste 202
Solana Beach, CA 92075-1252
T (858) 720-1001
F (858) 720-9001


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