The increasing global demand for food and other agricultural products calls for urgent measures to increase water use efficiency which is, with plant nutrient availability, one of the two main limiting factors in crop production. Although only 20% of all cultivated land in the world is under irrigation, it provides 35-40% of all crop production. Because of higher yields under irrigated agriculture, investments for irrigation are usually a top priority. However, it has become a matter of serious concern in recent years that, despite their high co~ts, the performance of many irrigation projects...