GROWTH AND YIELD OF RAIN-FED WHEAT ON THE SEASONALLY DRY AEGEAN ISLANDS

The effects of water regime on the rate of growth, the growing period and the yield of a winter wheat crop in the summer-dry climate of Aege an Islands are examined. It is shown that wheat growing period is sign ificantly restricted by either, unfavourable weather conditions at pla nting (coinciding with the start of rains), or by early soil moisture depletion at the end of the wet season. The probability of a successfu l early planting, which is conditional on a considerable pre-planting rainfall not being following by a long (10-day) dry spell, is estimate d by recurrence relationships. Farmers on the driest (south) islands w ill have a 25% risk for unsuccessful planting before November 14. Evap otranspiration rates, estimated by the Penman-Monteith equation, are o ptimum for crop growth for about two months after wintering. The growi ng season on average comes to an end by the end of spring (soon after anthesis), when the available soil moisture. (estimated by a simple wa ter balance equation) drops to zero. The water shortage, especially du ring the grains-filling period, may reduce yields by up to 75%, depend ing on the length and severity of the soil moisture deficit at the sit e. Reliability and distribution of rainfall suggest that the risks of water deficits in rainfed cropping vary across the region. In order to minimise yield losses from crop failures; farmers should adjust areas sown each year according to the date when the wet season starts.