Reducing peak supplemental irrigation demand by extending sowing dates

Supplemental irrigation (SI) is a common practice in the dry environments a nd aims at improving and stabilizing rainfed crops by adding small amounts of water to rainfed crops during times when rainfall fails to provide suffi cient moisture for normal plant growth. Results from long-term research in experimental stations and farmer fields showed substantial increases in rai nfed crop yields and water use efficiency in response to SI. Nevertheless, SI comes at a cost. The date of sowing winter wheat in a rainfed Mediterranean-type environment depends upon the onset of rainfall. The optimal date for achieving highest yield under rainfed conditions is around mid-November. However, farmers te nd to sow wheat later than this date because of the delay and/or unreliabil ity of initial rains. With SI, early sowing and crop establishment can be e nsured. However, early sowing of all the fields' results in higher water de mand during a very short period in spring because all the fields will be at the peak use rate. Spreading out dates of sowing allows peak water demand to occur over a longer period, thus reducing the discharge and the size of irrigation system needed, and hence improves the economics of this practice . In this paper, the impact of adopting a multi-sowing date strategy on far m water demand and crop production is considered. A simplified optimization model solved by linear programming is presented. Four-years' data (1992-19 96) from field experimental research conducted on bread wheat in northern S yria have been used in the analysis. We showed that a multi-sowing date strategy has reduced the peak farm water demand rate by more than 20%, thus potentially reducing irrigation system capacity and/or size. Alternatively, the water demand rate of a larger area can be met with the same water supply. However, optimal sowing dates that minimize farm water demand rate do not always maximize total farm productio n. The outcome depends on crop water requirements and yield for each sowing date. Furthermore, this selection is greatly influenced by the level of wa ter scarcity. The approach used can help in reducing the cost of irrigation and improving the efficiency of water use in SI. (C) 2001 Elsevier Science B.V. All rights reserved.