It is widely believed that partial root drying (PRD) reduces water losses by transpiration without affecting yield. However, experimental work carried out to date does not always support this hypothesis. In many cases a PRD treatment has been compared to a full irrigated treatment, so doubt remains on whether the observed benefits correspond to the switching of irrigation or just to PRD being a deficit irrigation treatment. In addition, not always a PRD treatment has been found advantageous as compared to a companion regulated deficit irrigation (RDI) treatment. In this work we have compared the response of mature 'Manzanilla' olive trees to a PRD and an RDI treatment in which about 50% of the crop evapotranspiration (ET c ) was supplied daily by localised irrigation. We alternated irrigation in the PRD treatment every 2 weeks in 2003 and every 3 weeks in 2004. Measurements of stem water potential (Y stem ), stomatal conductance (g s ) and net CO 2 assimilation rate (A) were made in trees of both treatments, as well as in trees irrigated to 100% of ET c (Control trees) and in Rain-fed trees. Sap flow was also measured in different conductive organs of trees under both PRD and RDI treatments, to evaluate the influence of alternating irrigation on root water uptake and tree water consumption. We found small and random differences in Y stem , g s and A, which gave no evidence of PRD causing a positive effect on the olive tree performance, as compared to RDI. Stomatal conductance decreased in PRD trees as compared to Control trees, but a similar decrease in g s was also recorded in the RDI trees. Sap flow measurements, which reflected water use throughout the irrigation period, also showed no evidence of g s being more reduced in PRD than in RDI trees. Daily water consumption was also similar in the trees of the deficit irrigation treatments, for most days, throughout the irrigation period. Alternating irrigation in PRD trees did not cause a change in either water taken up by main roots at each side of the trees, or in the sap flow of both trunk locations and main branches of each side. Results from this work, and from previous work conducted in this orchard, suggest that transpiration is restricted in trees under deficit irrigation, in which roots are left in drying soil when water is applied by localised irrigation, and that there is no need to alternate irrigation for achieving this effect.

A model was developed that predicted evaporation based on combined information on the physiology of overstorey and substrate, micrometeorological conditions and spatial distribution of plants. Predicted evapotranspiration was veri®ed at stand level for Retama sphaerocarpa shrubs, and model parameters were tested to determine their importance in controlling evaporation. Model predictions were compared with evapotranspiration, measured by a Bowen Ratio Energy Balance system (BREB), and transpiration, measured by sap¯ow of the stems of the shrubs in south eastern Spain. Modi®cations made to the original two source clumped model in the tested model signi®cantly improved agreement between predicted surface evapotranspiration rates and rates measured by the Bowen Ratio method. The modi®cations made to the model were improved parameterisation of soil surface conductance, a more detailed description of the radiation balance, and improved parameterisation of the soil aerodynamic conductance terms. Improvements in the soil surface conductance estimates made the most signi®cant change to model predictions, the second two modi®cations showed no signi®cant improvement in prediction of evapotranspiration. A sensitivity analysis indicated that relatively large variations of leaf area index or albedo caused little variation in evapotranspiration during the period measured, whereas variations in soil water content caused large changes in predicted evapotranspiration. Transpiration rates of shrubs (measured and modelled) indicated an independence from surface soil moisture (0±25 cm) supporting the view that R. sphaerocarpa had access to reserves of water deep in the soil which enabled it to survive and grow vigorously in this type of semiarid environment. Thus, it was concluded that land use changes which affect redistribution of water resources (overland and subsurface¯ow) may threaten the stability survival of R. sphaerocarpa stands. #

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