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How We Calculate Watering Recommendations

Throughout the year, we get numerous inquiries on how we calculate the weekly watering recommendations. Although our process is based on established science, it is actually quite simple.

Texas A&M University’s Irrigation Technology Center provides us with an estimated water requirement for the San Antonio area called Potential Evapotranspiration, or PET. This is a theoretical estimate based on accepted scientific models and is derived from sunlight, relative humidity, temperature and wind.

Since we can’t measure the water requirement for every plant or every blade of grass, we use a standard representative plant and then make adjustments accordingly. In some parts of the country, the representative plant is Kentucky bluegrass. In the South, including San Antonio, the representative plant is fescue. More specifically, “4 inches of tall fescue (Festuca arundinacea) growing under well-watered conditions without stress.”

To adjust from the theoretical to the actual world, we apply a species factor. In our case, a warm season grass factor of .6 is used. Then, we apply a seasonal factor that represents a normal appearance. Finally, we subtract any effective rainfall which may have occurred during the survey period.

From time to time, we may use our professional intuition to reduce the recommendations because of weather patterns or plant dormancy, but for much of the year we rely on the principles of weather, soil and plants.

Mark Peterson
Mark Peterson
Mark A. Peterson is a conservation project coordinator for San Antonio Water System. With over 30 years of experience as an urban forester and arborist, Mark is probably the only person you know who actually prunes trees for fun. When not expounding on the benefits of trees and limited lawns, you’re likely to find him hiking San Antonio’s wilderness parks or expounding on the virtues of geography and history to his friends.
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