Rainfall redistribution beneath an olive orchard

Rainfall redistribution beneath an orchard can be substantial because of localized runoff and runon induced by the spatial variability of infiltration rate, rainfall interception by the tree and surface roughness. We present the results of a rainfall simulation experiment on two runoff plots, one tilled and the other with a 3 m wide cover crop strip between the tree rows. Infiltration was measured over the whole plot by measuring rainfall and runoff, and also calculated from the change in soil water storage measured with a neutron probe at eleven locations within each plot, before and after simulated rainfall. Data were obtained for eight simulations conducted during the summer of 2003. The results show higher cumulative infiltration beneath the olive trees as compared to the bare soil in the lanes between tree rows. Cumulative infiltration beneath trees was greater than the rainfall because of incoming runoff from the bare soil in the lane upslope. Cumulative infiltration in the cover crop strip was intermediate between that observed beneath the trees and bare lanes. Spatial variations in cumulative infiltration were explained by differences in saturated hydraulic conductivity of the soil. The runoff generation model of Gómez et al. [Gómez, J.A., Orgaz, F., Villalobos, F.J., Fereres, E., 2002. Analysis of the effect of soil management on runoff generation in olive orchards using a physically based model. Soil Use Manage. 18, 191–198] calibrated for the experimental conditions reproduced the observed differences in cumulative infiltration among the different areas. A summary model was used to estimate the long-term effect of the spatial variability in annual infiltration within an orchard. For Cordoba, Spain, with an average annual rainfall of 552 mm, the model predicted annual cumulative infiltration differences ranging from 44 to 102 mm between the area beneath the tree and the bare lane, depending on annual rainfall and soil management practices. Such differences may be critical for augmenting the tree water supply in dry years and should be considered when designing soil and water conservation strategies for rain fed olive orchards.