The El Dorado Weather 7 Day Irrigation Index is meant as a guideline for how much you might need to water your turf or lawn in Placerville California and the surrounding Motherlode area. It should also be fairly accurate for those living in the Sacramento and San Joaquin Valley's since we have very similar summertime weather conditions in many valley locations.
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 El Dorado Weather's live Weather Station in Placerville, California.
If the 7 Day Irrigation Index is negative or zero, then you should not need to water your lawn at all, as more rain has fallen than ET has removed from the soil.
If the irrigation index is positive, then this is the value on how much irrigation water you might need to deliver over the next 7 days.
How to determine your sprinkler system's water 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 shall know how many inches per hour they deliver. You can also set out a rain gauge for water measurements if you have one.
For example, 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 sprinkler circuit.
Figure out how long to water each week:
To decide how long to run your sprinklers in a week, divide the current 7 Day Irrigation Index amount by the delivery rate you just calculated.
For example, if the current 7 Day Irrigation Index measurement was 0.6 inches, then:
0.6 inches / 1.5 inches per hour = 0.4 hours.
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
Some more background on the Irrigation Index:
The 7 Day Irrigation Index 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 quite sufficient, especially during times when there is little change in the day to day weather pattern.
Our area is currently in a drought, and we have a limited supply of water, It was developed to help get an idea of how much water your grass might need based on current and recent weather conditions.
The formula may need to be adjusted when we get sufficient rain to really get the soil wet.
Evapotranspiration 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. The evapotranspiration model El Dorado Weather uses is for turfgrasses.
The watering & irrigation index, as well as the above text is based on the SLOweather.com watering indexes and documentation.