How to Calculate the Weekly Normal Amount:
A “normal amount” of weekly irrigation must be established in order to use this Model. This is best accomplished by calculating the total amount of water applied to the vineyard from first irrigation to final irrigation during past seasons. The total should then be divided by the number of weeks from first irrigation to final irrigation during that season. This will give you the “normal amount” of irrigation water per week for your vineyard. If possible, keep track of your “normal amount” for several seasons and calculate an average.
Formula: total # of gallons used last year / weeks of irrigating = gallons applied per week (“normal amount”)
For example, last season a total of 5000 gallons of water was applied between first irrigation and final irrigation. If the first irrigation occurred around April 15 and final irrigation was on September 9, it was a 21 week season. By dividing 5000 by 21 you get a weekly normal amount of irrigation of 238 gallons. When the Model tells you to “Irrigate Today” at 1X normal you should apply 238 gallons. If the Model tells you to irrigate at 1.5X normal, you would irrigate 357 gallons, etc.
To obtain the number of hours to irrigate you must know or calculate your water system flow rate. The water system flow rate is equal to the number of drip emitters times their individual flow rates. By dividing the system flow rate into the number of gallons to be applied, you obtain the time to apply 238 gallons.
Formula: # of gallons per week applied / (number of emitters x flow rate of emitter) = normal irrigation hours per week (or “normal amount” of time)
Assume in the example that there are 1000 emitters and each has a flow rate of .5 gallons per hour. Therefore, our system flow rate is .5 X 1000 = 500 gallons per hour. Now divide the total gallons to be applied (238) by the system flow rate (500) and you come up with .48 hours or 29 minutes is required (60 min. x .48 = 28.8). If the Model instructs you to water 1.5X the normal amount you should water your vineyard for 43 minutes, and 2X the normal amount would be 57 minutes.
If irrigation history is not available, a grower may estimate the “normal amount” by using the average number of hours per week irrigated. For example, in past years the grower normally irrigated 4 hours approximately every 2 weeks. That would be an average of two hours per week (4 hours divided by 2 weeks). The “normal amount” for that vineyard would be 2 hours.
“Normal Amount” Ratio = hours irrigated: 1 week
Calculating your “normal amount” is critical to the success of this Model. If you need help or have further questions please contact the Calaveras Winegrape Alliance at [email protected] or through the website www.calaveraswines.org.