7-Day Watering Index
The 7-Day Watering Index (7DWI) is a guideline for how much you might need to be watering your lawn in the Tyler, Texas area. It is a running total of the last 7 days of rainfall subtracted from the running total of the last 7 days evapotranspiration or ET, as measured and calculated by the TylerTexasWeather.com weather station. (The current index value is located in the right sidebar.) If the 7DWI is negative or zero, then you should not need to water your lawn at all since more rain has fallen than ET has removed from the soil. However, if it is positive, then that value is how much water you might need to deliver over the next 7 days.
How to determine your sprinkler system's output
You can measure how much water your sprinklers deliver over a period of time by setting out a few clean, shallow cans, such as tuna or pet food cans, and measuring the depth of the water in them after an irrigation cycle. Divide the average inches of water by the time in decimals of an hour your sprinklers ran, and you'll know how many inches per hour they'll deliver. For instance, if you measured a half inch of water on a 20 minute cycle (20 minutes = 0.33 hours), then 0.5 inch / 0.33 hours = 1.52 inches per hour. Your sprinklers delivery rate is about 1.5 inches of water per hour. If you have more than one sprinkler circuit, repeat the above for each one.
Figure out how long to water each week
To decide how long to try running your sprinklers in a week, divide the current 7DWI by the delivery rate you just calculated. If the current 7DWI was 0.6 inches, then 0.6 inches / 1.5 inches per hour = 0.4 hours and 0.4 hours x 60 minutes per hour = 24 minutes per week. If you water 3 times per week, then 24 minutes / 3 days = 8 minutes per day.
The 7DWI is not intended to be referred to every day. Using it to adjust your irrigation timer once a week or even every 2 or 3 weeks should be sufficient, especially during times when there is little change in day-to-day weather pattern.
Evapotranspiration, or ET, is a calculation based on temperature, wind, relative humidity and solar insolation. It estimates the amount of water lost from the soil from both EVAPOration and plant TRANSPIRATION.