Good irrigation scheduling means applying the right amount of water at the right time. Scheduling maximizes irrigation efficiency by minimizing runoff and percolation losses. It also results in lower energy and water use, maximizes the effectiveness of fertilizers, and produces optimum crop yields

Ideally, irrigation should be scheduled by continuously monitoring the soil moisture, and starting irrigation when measurements indicate it is needed. Because soil moisture monitoring requires equipment, this approach is not for everyone. Instead, soil moisture is usually measured infrequently, and estimated between measurements to produce an irrigation schedule.

One of the simplest forms of irrigation scheduling is called the “checkbook method.” Instead of balancing dollars in and dollars out, we are balancing water in the soil with plant water use. Evapotranspiration, or “ET” for short, accounts for the withdrawals from the soil moisture account, and irrigation or rainfall are the deposits to the account.

How to Determine Scheduling

• Step 1. Estimate the plant-available water of your soil.

  If you begin your “checkbook” in the spring, you can assume your soils are at field capacity. In this case you can use the plant available water for your soil type from the county Soil Survey published by the USDA Natural Resources Conservation Service, or online at http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm.

  Table 1 lists some average ranges of plant-available water for common soil types.

• Step 2. Find the percent Management Allowable Depletion and the Effective Root Zone for the crop at its current growth stage.

  The percent Management Allowable Depletion or ‘% MAD’ is the percent of water in the root zone that plants can utilize before experiencing stress or yield loss. See the table for examples for our area. The effective root zone is depth in which crops get about 70% of their water. Remember that this depth will change as the crop grows.

  You can monitor the depth of your crop’s roots during the growing season, or Table 2 will give you some average values for mature plants. Multiply the plant available water for your soil from step one by the effective root depth from step 2 to get the available water capacity for your crop. If the crop is newly planted, reduce the effective root depth. As the crop matures increase the effective root depth.

  Plant-available Water (in/ft) × Effective Root Zone (feet) = Available Water Capacity

  Now, to get the Management Allowable Depletion (MAD) in inches for your irrigation checkbook multiply the available water capacity you just calculated by the % Allowable Depletion for your crop.

  Available Water Capacity × % Allowable Depletion = Management Allowable Depletion (MAD)

  Let’s look at an example of this calculation:
  You want to irrigate mature blueberry bushes on a silt loam soil. Plant available water for a silt loam is 2 inches per foot of soil (see table 3.1). The effective root zone for blueberries is 3 feet and the %MAD is 50% (see table 6.2).

  Available Water Capacity = 2 in/ft × 3 feet = 6 inches available water in the effective root zone at field capacity

  Management Allowable Depletion (MAD) = 6 inches × 0.50 = 3 inches available water

  Enter this value as your beginning balance.

• Step 3.

  In the Pacific Northwest, we are fortunate to have the AgriMet system which provides this information on a daily basis in ‘Crop Water Use Charts.’ Learn more and check out the charts here.

  First, look up the code for your crop under the ‘identify chart crop codes’ link. Next, read the information on how to use the chart under the ‘More about the Charts’ link. Then click on the name of the nearest weather station, look for your crop code in the left column, find the crop water use value for the day in the next column, and enter it in your checkbook. Agrimet also provides rainfall amounts at the weather stations (see http://www.usbr.gov/pn/agrimet/wxdata.html ), or you can use a rain gauge to measure rainfall on your property. If less than 0.05 inches falls, ignore this amount.

• Steps 4 and beyond – download the Rural Landowner Handbook!

  Download the Rural Living Handbook and check out the Irrigation section starting on page 19. All of this information, useful tables and examples of how to use the checkbook method are included!