Separation of Evapotranspiration and Deep Percolation

Deep percolation losses of water from the root zone under a growing crop continue for a considerable time. Soil moisture depletion methods of determining evapotranspiration include these deep percolation losses. By assuming that deep percolation rate is a function of soil moisture content for a given soil profile, soil moisture depletion data can be separated into components of evapotranspiration and deep percolation. Data from two crops and two soils when separated also show that evapotranspiration rates decrease as soils become drier. Measurements of soil moisture depletions from covered areas and adjacent cropped areas are needed for the analysis. On a sandy loam planted to alfalfa, one-third of the soil moisture depletion for an 18-day period was due to deep percolation. Frequent irrigations that completely wet the soil profile increase evapotranspiration and deep percolation.