If you have a recent model irrigation controller with a watering budget or "percentage" adjustment, you can use the El Dorado Weather Watering Index to easily make regular adjustments to your watering times.
The Index represents the El Dorado Weather's recommended percentage setting for the watering budget or "precentage" adjustment feature. During the irrigation season, the Watering Index is recalculated using our local weather data to reflect the landscape's changing need for water. The controller's percentage adjust feature should be adjusted to match the current Index value.
Your controller should be set so that it waters at 100% during the hottest, driest weeks of the year. Around here, those are in July and August. Then, check the El Dorado Weather Watering Index once a week and set the percentage adjustment to match. This will then adjust the run times of your controller accordingly.
Why is weekly adjustment important?
Weather conditions change constantly and watering times should be changed to compensate. You can save a considerable amount of water by adjusting your controller via the Watering Index. Reducing your water use will reduce your water bill and help us conserve limited water resources.
How is the Watering Index calculated?
The Watering Index is based on evapotranspiration data. Evapotranspiration estimates the loss of water evaporated from the soil and transpired by plants. We calculate evapotranspiration from solar radiation, air temperature, relative humidity, and wind velocity. Evapotranspiration values are primarily dependent on the amount of solar radiation and wind. Your watering schedule should be designed to replace the amount of water lost via evapotranspiration.
El Dorado Weather's implementation of the Watering Index compares the current running 7 day evapotranspiration total from the highest weekly average evapotranspiration value recorded over the last 10 years. The historical high occurs in July and equates to 100 percent on the Index. The concept of the Watering Index was developed by Mr. John Wynn, of the State of California Department of Water Resources.
We also enhance the original DWR Watering Index by calculating it normally, and then subtracting the last 7 days running total of local rainfall from it.
We recalculate the Watering Index daily at about 1:00 AM, and that it will change slightly from day to day. However, you should only need to adjust your controller once every week or two, depending on the weather and how fast the index changes.
What if my controller is set at the suggested Index value and my plants look stressed?
The Watering Index is a guide. It's calculated from data gathered at the El Dorado Weather site at about the 1,700 foot level in the Sierra Nevada Foothills, but should be useful at many different elevations. The Index provides you with data on the percentage adjustments for your controller. However you must pay attention to the health of your landscape and the operation of your irrigation system (i.e. broken pipes or irrigation heads). If you have confirmed that your irrigation system is operating correctly and have set your percentage adjust feature at the weekly Index value and your plants still look stressed, increase the base runtimes (the "100%" value) for the individual stations that look stressed by 5 minutes.
Common signs of plant stress are droopy leaves, or a grayish blue tinge to your grass, or grass that stays flat after being stepped on. After making the adjustment, wait several days to see if the stress disappears. If signs of stress remain, increase the adjust feature up another five minutes. Be aware that plants suffering from overwatering may show similar signs as under watered plants – leaves will wilt and yellow and sometimes fall off.
Will the El Dorado Weather Watering Index ever be greater than 100 percent?
Sure. If there are exceptionally hot and dry periods, or periods of high winds during the year, the Watering Index may exceed 100 percent. Your percentage adjust feature can be set for values greater than 100 percent .
The watering & irrigation index, as well as the above text is based on the SLOweather.com watering indexes and documentation.