Figure 18. If you have a source of evapotranspiration (ET) data, you can use a water budget to estimate the interval between irrigations. You can also use pan evaporation and the crop water use coefficient (see Fig. 19 and text) to develop your own ET figures. Keep a running total starting the day after irrigation. When the total approaches the allowable depletion, check soil moisture in the field to make a final decision on when to irrigate.

Illustration from Integrated Pest Management for Potatoes in the Western United States, 2nd edition. UC ANR Publication 3316.