Abstract In the spanish humid zone, the irregular within-year distribution of rainfall often leads to a need for supplementary irrigation during the summer. In the Terra Chd region of Galicia (northwest Spain), this is the case despite the hydromorphic characteristics of the soil. During 1990, we used a neutron probe to monitor variation in soil water content under rainfed grassland. Soil water balance over the study period was also simulated with an empirical model, ISAREG. In general, there was good agreement between measured and predicted soil water storage; cumulative water use was also satisfactorily predicted. The model was then used to predict the probability distribution of annual net irrigation requirements for pasture in this area, on the basis of 24 years' climatic data. Interannual variation was very high, with modelled requirement ranging from 0 to 232 mm. In view of this variability it is not possible to reliably predict the optimum irrigation schedule at the start of each growing season. Finally, we discuss various aspects of irrigation management in this area, taking into account soil available water and climate.
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