WETS Irrigation Technician Practice Exam

ET-based irrigation scheduling estimates how much water the crop will need by tying evapotranspiration to the crop's characteristics and the environment. The central calculation is ETc, the crop’s actual evapotranspiration, which comes from multiplying the reference evapotranspiration (ETo) by the crop coefficient (Kc): ETc = ETo × Kc. ETo is derived from weather data—temperature, humidity, wind, and solar radiation—so the weather inputs capture atmospheric demand. The crop coefficient adjusts for the specific crop and its growth stage, reflecting how different plants use water.

Including soil moisture status helps you know how much water is already available in the root zone, so you irrigate only when the soil water declines to the allowable depletion and you apply the right amount to meet crop demand without waste. Plant type matters because different crops (and their growth stages) have different Kc values, altering ETc and thus the irrigation scheduling.

All together, these inputs—ETc from ETo and Kc, weather data, soil moisture status, and plant type—provide a framework to decide when and how much to irrigate.

Why other inputs don’t fit as the basis for ET-based scheduling: rainfall data alone doesn’t quantify the crop’s evaporative demand; soil texture and pH describe soil properties but not the plant’s water use; water price and scheduling frequency affect decisions but don’t determine the actual crop water needs.