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IRRIGATION SCHEDULING

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Using the WWD Irrigation Scheduling Charts

The usefulness of the water-budget equation is to allow a prediction of the next irrigation. The District has developed Irrigation Scheduling Charts that may suit your operations better than trying to do very detailed calculations. These Charts provide an estimate of the next scheduled irrigation. They require you to . . .

  • Know what type of soil you have. District or private consultants can help identify them.
  • Know the effective root zone of the crop. Experience, private consultants, or just probing in the field can provide estimates.
  • Know the current soil moisture depletion. There are several ways to measure soil moisture, the "feel" method is the cheapest, fastest, and least expensive method
  • Know the estimated daily evapotranspiration of the crop, ETc. These are provided by the District Irrigation Guide.
  • Know the allowable depletion percent.

Figure IS-3 is an example Scheduling Chart for some of the lighter soils in the District, in this case a Wasco Sandy Loam (upper right corner). To demonstrate the use of the Scheduling Chart, assume the following :

  • The effective root zone is 4 feet.
  • The current soil moisture depletion is 30 percent (note the bottom section of the chart has representative descriptions of the feel of a soil sample at different depletions).
  • The allowable depletion is 60 percent.
  • The current crop ETc is about .16 inches/day.       
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Now, enter the chart at lower left with a vertical line up from the Current Depletion (30 percent). Where the vertical line intersects the correct Allowable Depletion line (say, 60 percent), draw a horizontal line going right. Where the horizontal line intersects the current crop ETc rate (.16 inches/day), draw a vertical line down. Now picking the row of numbers corresponding to the effective root zone (row 4, a four-foot root zone), read the approximate Days to Next Irrigation.

Using the Chart provides an estimate of scheduling the next irrigation 7 days from now. This may or may not be the optimum date of irrigation. You should be looking at the crop closely starting four days from now and planning the irrigation. The above example used a constant root zone of four feet and a constant ETc of .16 inches/day. If you are growing an annual crop and the root zone and/or crop ETc is increasing rapidly, a different approach is taken.

Estimate the effective root zone 5 days from now. Using the last estimated ETc from the Crop Water Use guide, go through the process. If the Days to Irrigation is more than 5-8 days, estimate the effective root zone 10 days from now. Also increase the ETc slightly. If the Days to Irrigation is more than 10 then come back to the field in 5 days or so. Then make another estimate of the soil moisture depletion in the root zone and go through the process again.

Separate high resolution, pocket and full sized (Coarse, Medium, Fine, Very Fine soil texture) Irrigation Scheduling Charts for most of the important soils in the District are available for download in PDF format(200K-275K). Also there are Crop Water Management Guides that will provide information on allowable depletions and rooting depths.

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FIGURE IS-3 Irrigation Scheduling Chart

    

Last updated September 2000