It’s about to be springtime and the start of irrigation season! Crop evapotranspiration (ET) is low in the spring compared to what it is in the middle of the summer. Can I shut my irrigation system off for a large portion of the time in the spring? If so, how long before I need to turn it back on?
A new tool was recently developed and is available for free online at http://irrigation.wsu.edu/Content/Calculators/Historic/SimpleSched.php that helps answer these questions (Figure 1). It uses 30-year historical average reference evapotranspiration data for 233 different weather stations throughout Washington State and historical mean crop coefficients for 75 different crops to make estimates of the mean historical crop water use for that crop in a typical growing season. This is used, along with some defaults for typical rooting depths for that crop, soil water holding capacities, and an estimate of the irrigation system’s designed capacity to estimate when the irrigation system might typically need to be started, and when it typically might be shut down for a few days to save labor, pumping costs, and water (Table 1 and Figure 2).
Table 1. Example percent run table for the sample irrigation system.
| Month | Avg Crop Water Use (in/day) | Avg Crop Water Use (in/week) | % of Time Running Req’d to replace ET | OR Hrs ON/Day Req’d | OR Hrs OFF/week |
| Apr | 0.081 | 0.57 | 24% | 5.7 | 127.9 |
| May | 0197 | 1.38 | 58% | 13.9 | 70.4 |
| Jun | 0.31 | 2.17 | 91% | 21.9 | 14.4 |
| Jul | 0.356 | 2.49 | 105% | 24 | 0 |
| Aug | 0.323 | 2.26 | 95% | 22.9 | 7.9 |
| Sep | 0.193 | 1.35 | 57% | 13.7 | 72.4 |
Figure 2: Sample table of the mean soil water content in the field over time compared to the water holding capacity in the root zone and the irrigation system’s design capacity.
Because it uses long-term historical averages, this is something that can be run once and printed for future reference. For more advanced irrigation scheduling related to this year’s current weather conditions that allow modifications to the underlying crop, season, and soil assumptions, please use the Irrigation Scheduler Mobile app.
Revised estimates of the historical average water needs for these different crops and stations are also included here.
Send questions and feedback to Troy Peters (troy_peters@wsu.edu).
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