Spatial and temporal distribution of soil water balance for a drip-irrigated almond tree

Relatively little information is available on the spatial distribution of soil water under drip irrigation, and how it is affected by root distribution, emitter placement and irrigation amounts. We hypothesize that variables such as emitter position relative to the active roots as well as irrigation amount and frequency will affect the soil water regime in general, and specifically the spatial and temporal changes in soil water content as controlled by root water uptake and leaching. A better understanding of these interrelationships will provide alternative means for proper and efficient drip irrigation water management practices. Moreover, the present study will provide an extensive database which can serve as input for analytical or numerical modelling of drip-irrigated trees. We present the results of a field study in which the soil water regime of a surface drip irrigated almond tree is investigated. The experimental site (6.6 m x 4.8 m) was intensively instrumented with tensiometers and neutron probe access tubes to infer the three-dimensional distribution of soil water and root water uptake during the irrigation season. Drainage fluxes were estimated from measured hydraulic head gradients and hydraulic conductivity data. Unsaturated hydraulic conductivity were determined from in situ measurements by the instantaneous profile method, and in the laboratory using the multi-step outflow method. The water balance results showed that the applied water was not sufficient to match the actual tree water use by evapotranspiration, causing soil water depletion around the tree as the irrigation season progressed. Moreover, soil water content data demonstrated temporal changes in the water uptake patterns. The temporal occurrence of leaching justifies the need for soil water measurements in the scheduling and design of drip irrigation systems. (C) 1997 Elsevier Science B.V.