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Comparison of Evaporation Reduction Effectiveness of Various Swimming Pool Covers


California is in the midst of one of the most severe droughts the state has ever experienced. To cope with the drought, Governor Brown issued an Executive Order in April 2015 mandating a 25 percent reduction in water use for all urban water users ( Water conservation is considered the cheapest, quickest, and reliable way to reduce water use and meet the mandate. Accordingly, this study plans to examine water use reduction that could be realized if outdoor swimming pools are covered by different types of market available covers. The number of residential swimming pools in California is estimated as 1.18 million. Even with conservative assumptions including average swimming pool surface area of 400 ft2 and average annual evaporation rate of 40 inches in California, a 30 percent reduction in evaporation from outdoor swimming pools would save more than 10,833 acre-foot of water per year which is enough to supply a city of about hundred thousand people. Given the severity of drought California is confronted with, this potential water saving is substantial.

Cal Poly Evaporation Study

As such, the objective of this study is to reasonably determine the comparative evaporation reducing effectiveness of different types of market available swimming pool covers. The research will be conducted at the National Pool Industry Research Center (NPIRC) located on California Polytechnic State University (Cal Poly) campus. The study will involve daily water level measurements and weekly water chemistry readings for nine pools at the NPIRC for sixty days. Climate data including air temperature, wind speed, and rainfall collected at a weather station at Cal Poly will also be used for the study. Project deliverable will be a report that consists of description of the research methodology, the raw data collected during the study period, daily evaporations observed for the pools tested, comparison of the evaporation reductions achieved by the cover types tested, and conclusions.

Evaporation Estimation Method

The rate of evaporation from open waters such as a swimming pool varies depending on climate variables including net solar radiation, wind speed, temperature, and humidity. While evaporation can be estimated using indirect methods such as pan evaporation or equations, the water balance approach will be used for this project because of its accuracy, ease, and suitability for the study. According to the water balance method, evaporation from a swimming pool, for example, in a day, can be described as,

Evaporation = Initial Water Level + Rainfall + Water Added to the Pool - Water Lost via Leakage and Splash - Final Water Level

Therefore, if no water is added to the pool and if there is no leakage or splash, evaporation can be determined by measuring rainfall and change in water level in the pool. For this study, rainfall is measured at a climate station near the NPIRC and water levels will be measured in the pools to be used for the study. Water level measurements will be taken once a day between 6 am and 9 am when wind is expected to be calm so that water surfaces in the pools would be fairly even or undisturbed. To further minimize error in water level measurements, four water level readings per pool (one reading at each side) will be taken daily.

Some of the pools could be refilled once or twice during the study period depending on the extent of evaporation. If depth of water in a pool drops by more than 2 ft then the pool will be refilled to lessen the effect of volume of water in the pools on evaporation. The amount of water added to a pool during a refilling process will be recorded.

Pools to be Monitored

Nine pools will be used for the study. Two pools will be used as controls (i.e., will not be covered) to determine evaporation from uncovered pool. The remaining seven pools will be covered (one cover type per pool) by the following market available cover types.

1. Solid track pool cover

2. Plastic bubble cover

3. Solar rings

4. Solar squares

5. Foam cover

6. Liquid cover A

7. Liquid cover B

All nine pools have identical shape, size and exposure to the sun and wind. To ensure consistency on reflectivity (i.e., albedo), medium blue color will be used for all cover types. This implies that since the environmental variables that affect evaporation are fairly identical for all pools, any difference in evaporation rate observed among the pools would be attributed to the respective covers.


Figure 1. Pools and Spas at the National Pool Industry Research Center at Cal Poly

Getting the Pools Ready for the Research

The pools to be used for the study will be cleaned and water proofed by an NPC member, and will be tested for leakage over a period of two weeks. If leakage is observed, the leaking pools will either be fixed by an NPC member or will be replaced by other non-leaking pool(s) for the study.

Water Chemistry and Pool Cleaning

Water chemistry including chlorine, pH, alkalinity, and cyanuric acid will be tested weekly during the study period. Local IPSSA members will train a Cal Poly graduate student on how to use a test kit to measure the water chemistry. The pools will also be cleaned weekly. Local IPSSA members will conduct weekly test and add necessary chemicals to the pools to ensure that the water chemistry is maintained at desirable levels. Test kit will be donated by an NPC member.


Dr. Misgana Muleta, Associate Professor of Civil and Environmental Engineering at Cal Poly, will be the principal investigator. Dr. Muleta will hire a graduate student who will help with water level measurement, monitoring water chemistry, and cleaning the pools.

Project Deliverable and Time Line

Deliverable will be a report that consists of description of the research methodology, the raw data collected, daily evaporations observed for all the pools tested, comparison of the evaporation reductions achieved by the cover types tested, and general conclusions. The National Plasterers Council (NPC) Research Committee will also prepare project summary based on findings of the study. Specific tasks and estimated time lines are shown in the table below.

Tasks 3 and 4 are expected to begin around July 6 and will be completed by the first week of September. Task 5 will be completed by the first week of October.








1. Pool cleaning and plastering






2. Testing for leakage






3. Evaporation testing






4. Water chemistry monitoring






5. Data analysis and report writing







 Click here for the full study.

We would like to extend our gratitude to the many NPC Members who have made personal financial contributions and volunteered many hours of their time to make this project come to fruition. 
Additionally we would like to thank the following corporate donors for their support:

New Image Pool Interiors


Pool Electrical Products (PEP)


Cover Pro

Halo Source

Cover Pools, Inc

Alternative Pool Covers

Aquamatic Pool Covers

California Pool Covers

Cover Star

Summers Pool Equipment

Gardner Plastering


Universal Cement

Pebble Tec


Wet Edge

Pool Corp

T-Star Enterprises