Calculating the Current Watering Advice

Mark Peterson

Ever wonder where those watering advice numbers come from? It’s all very scientific. Weekly watering recommendations are calculated based on weather conditions, soil makeup and plant types.

Throughout the year, we get numerous inquiries about how we calculate the weekly watering recommendations for the current watering advice. The process has been used for more than a half-century and is used in agriculture and landscaping.

Texas A&M University’s Irrigation Technology Center provides us with an estimated water requirement for the San Antonio area called the baseline evapotranspiration, or ETo. Another name is potential evapotranspiration, or PET. This is a theoretical estimate using accepted scientific models and 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, it’s tall fescue, or more specifically, “4 inches of tall fescue growing under well watered conditions without stress.”

To adjust from the theoretical to the actual world, we apply a species coefficient. For most of Texas, this is a warm season grass with a coefficient of .6. In other words, the baseline ETo is multiplied by .6 or a reduction of 40 percent. Then, we apply a seasonal coefficient that represents a normal appearance. Again, the seasonal coefficient is .6. During the summertime, when plant needs are higher, we raise the coefficient to .8. For most of the year, the ETo has been reduced by a factor of 64 percent (.6 x .6 = .36). Finally, we subtract any effective rainfall, that is, a culmination of all rainfall between ¼ inch and 1 inch, which may have occurred during the survey period.

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

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