SOIL-WATER REGIME UNDER PASTURE IN THE HUMID ZONE OF SPAIN - VALIDATION OF AN EMPIRICAL-MODEL AND PREDICTION OF IRRIGATION REQUIREMENTS

In the spanish humid zone, the irregular within-year distribution of r ainfall often leads to a need for supplementary irrigation during the summer. In the Terra Cha 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 conte nt under rainfed grassland. Soil water balance over the study period w as also simulated with an empirical model, ISAREG. In general, there w as good agreement between measured and predicted soil water storage; c umulative water use was also satisfactorily predicted. The model was t hen used to predict the probability distribution of annual net irrigat ion requirements for pasture in this area, on the basis of 24 years' c limatic data. Interannual variation was very high, with modelled requi rement ranging from 0 to 232 mm. In view of this variability it is not possible to reliably predict the optimum irrigation schedule at the s tart of each growing season. Finally, we discuss various aspects of ir rigation management in this area, taking into account soil available w ater and climate.