Variable Rate Irrigation Calculator

The soil AWC, root depth, and depletion inputs represent an average or representative zone, and the calculator uses that single set of values to compute total AWC, the MAD trigger point, and days until the next irrigation event. It then divides field acreage evenly across your zone count to size average zone volume and total run time. To see how a specific dry or wet zone behaves differently, rerun the calculator with that zone's own AWC and depletion numbers rather than the field average.

It's computed as 8% plus 15% times how far your soil AWC deviates proportionally from a 1.8 in/ft baseline, capped at 25% — so a field with soils close to that baseline shows savings near the 8% floor, while a field with widely varying soil (very high or very low AWC) shows more. This is a modeled heuristic reflecting the 10-15% range commonly cited in extension literature for variable-rate versus uniform application, not a measurement of your actual field's soil variability or your specific zone map's accuracy, so treat the number as directional rather than exact.

It's the remaining inches of water between your current depletion and the MAD threshold, divided by your daily crop ET rate — in other words, how many days of crop water use the soil can supply before you hit the point where irrigation is recommended, assuming no rainfall in the meantime. Because it's a simple linear projection at a constant ET rate, an approaching heat wave or a change in crop growth stage will shorten or lengthen that runway in ways this single-day ET input doesn't capture.

Gross application depth is the net water deficit (current depletion in inches) divided by system efficiency as a fraction, so a higher efficiency means less water has to be pumped to deliver the same net amount to the root zone — a 100% efficient system would apply exactly the net deficit, while an 85% efficient system has to apply about 18% more to make up for wind drift, evaporation, and runoff losses. That gross figure is what then gets converted to gallons and run time, so efficiency assumptions directly drive your estimated system run time.