Measurement and simulation of evaporation from soil in olive orchards

Evaporation from the soil (E-s,) beneath an olive orchard was characterised in a semi-arid Mediterranean climate (Cordoba, Spain). First, the microlysimeter method was modified to measure accurately E-s, beneath tree orchards. The variability in irradiance reaching the soil beneath the orchard caused spatial variations in E, during both evaporation stages. In the first days of the drying cycle, E, was higher for high irradiance locations but the opposite occurred the subsequent days, although daily differences in E, between locations progressively declined. For the energy-limiting stage, linear relationships between E-s values and incident photosynthetically active radiation were found for different times throughout the season. The slopes of the relationships were similar, but their intercepts differed substantially, showing the importance of a variable aerodynamic component in determining E-s. A simple functional model was formulated to estimate E-s at daily time steps. During the energy-limiting stage, E-s is calculated as the sum of the equilibrium evaporation at the soil surface and an aerodynamic term, derived from the Penman equation. For the falling rate stage, Ritchie's (1972) approach is adopted for the E-s calculations. The model was successfully tested in an orchard of 6x6 m spacing, typical of intensive olive orchards, under a wide range of evaporative demand conditions. Trees covered around 36% of the soil surface. The model predicted an average seasonal E-s of 286 mm, which represents around one third of the estimated olive evapotranspiration and about 50% of the average seasonal rainfall of the area.