Advanced ET weather control for PRO-C Family controllers Using highly advanced technology, the Hunter ET System measures key climatic conditions to calculate local evapotranspiration (ET) factors. ET is a measurement of the water lost from the soil surface by evaporation and the moisture lost from the plants by transpiration. The ET System will calculate irrigation schedules to replenish only the water that is actually needed for plant and soil conditions. The Hunter WiltGard technology can intervene to trigger protective watering when extreme conditions threaten vegetation. The result is a dramatic savings in water, healthier root zones, and longer plant life.