Irrigation scheduling is the process of determining when and how much to irrigate. Growers monitor the soil, and the trees, and calculate the water used so that the timing and amounts of water applied are in sync with the requirements of the fruit trees and soil water holding capacity. The WSU irrigation scheduler mobile app is a useful tool to help plan timing.
Evapotranspiration (ET) is the loss of water through evaporation and transpiration. Transpiration is the movement of water through a plant from the soil to roots, up the stem, and out through the leaves. AgWeatherNet’s weather stations located around Washington State measure the weather parameters (solar radiation, air temperature, humidity and wind speed) needed to calculate ET. You can calculate ET using the Penman plant water use calculator here.
Field capacity Water will drain from soil over time to a point where the soil will hold a certain amount of water indefinitely against the downward pull of gravity. This is call field capacity.
Permanent wilting point occurs when the soil dries out to a point where the suction or pull of the soil on the water exceeds the plant’s ability to absorb water. Without subsequent irrigation, plants under this soil moisture condition will wilt and die.
Soil water tension is how hard a dry soil is pulling on soil water, measured using vacuum or pressure units (psi, cbars). For more information on soil water tension, see Irrigation monitoring using soil water tension, C. C. Shock, et. al., Oregon State University Extension publication EM8900, 2013.