What It Tells You
The drought-stress index is a number from 0 to 100, banded low, moderate or high. It sets the water the air will draw from the field against the water the soil holds. Reference evapotranspiration (ETo) is the demand side: how much a well-watered grass surface would lose in the next 24 hours at the forecast temperature. The probe’s soil moisture and the rain ahead are the supply side. A high band at a stage where water still drives yield is the signal to look at the crop. The index is a stress signal. It is not a soil water balance and does not say how much to irrigate. Every index carries a confidence of low or moderate. The index is computed by the Water Stress model.How It Works
Every six hours the model computes ETo with the Hargreaves equation from the field’s forecast temperature and latitude. It sets ETo against the latest soil moisture reading and the three-day forecast rain. Band edges are 30 and 60. A high band at a sensitive growth stage raises an advisory at most once per field every three days.
The sun draws water from the crop while the gauge sits in the moderate band and rain is on the way.