The water requirements of other crops are determined from PET by utilizing research tested crop coefficients. Each crop grown on the High Plains has a specific coefficient. As a crop develops from seedling to reproduction to maturity, it advances through several different water-requirement levels. Rainfall is a major contributor to the process of determining soil-available water and must be accounted for when using PET to schedule irrigation. Precipitation and applied irrigation must be subtracted from ET in order to calculate an exact ET water use amount.
Plant growth is dependant on sunlight for photosynthesis, hence development.
A plant must receive a specified amount of heat units to initiate it into the next stage of development. As an example, cotton must have received around 1000 heat units to place it into the blooming cycle. An ongoing accumulation of heat units received by a crop determines its particular developmental stage at a point in time. With computer-aided crop modeling, heat units are used to advance the crop through the coefficient. The application of water at the precise time it is needed results in many benefits.