Enhancing productivity of water under variable climate

Water is the most limiting factor for crop production in the semi-arid tropics (SA T) and its efficient use deserves special attention in our efforts to increase the productivity and profitably of agriculture in these areas. Rainfall in these regions is highly variable and the risk associated with such variable weather acts as a major deteffent for the farmers to invest in expensive inputs such as fertilizers and improved seeds required to achieve higher productivity. Availability of skilful forecasts from the growing understanding and advances in modeling the global climate system have opened up new opportunities for farmers to consider a number of adjustments to the management practices based on seasonal conditions predicted for the forth-coming season. Machakos district in Kenya is characterized as hot and dry with bimodal distribution of rainfall. Annual rainfall at Katumani ranged between 330 and 1260 mm with a coefficient of variation of 28%. Average seasonal rainfall is less than 300 mm with more than 40% of the seasons receiving less than 250 mm. Maize is the main food crop in the district with an average productivity of 0:8 t ha• However, average maize yields have declined by nearly 50% to 0.4 tlha during the decade 1993-2002 mainly due to the adoption of low input management techniques. Further analysis ofyield trends confirmed that the farmers' strategy is well suited for the below normal seasons but failed to capitalize on the good seasonal conditions during normal and above normal seasons. The reliability of hindcasts generated by IRI for 43 SR seasons starting from 1961 was evaluated and their potential value in reducing risk and improving productivity and profitability was assessed for Katumani situation using crop simulation model APSIM. Though the available skill in forecasts is not sufficient to predict accurately the amount of rainfall or its distribution in advance, it is possible to predict with some certainty whether the coming season is going to be below normal or not. Model simulations indicated significant gains in productivity and profitability with simple adjustments identified by the farmer such as application of recommended dose of fertilizer and high plant population in the seasons forecasted as normal to above, and low risk farmer strategies during the years forecasted as dry years. Introduction Water is the most limiting factor for crop production in the semi-arid tropics (SAT) and its efficient use deserves special attention in our efforts to increase the productivity and profitably of agriculture in these areas. Rainfall. the only source of water shows high temporal and spatial variability and the risk associated with such variable weather acts as a major deterrent for the farmers to invest in expensive inputs such as fertilizers and improved seeds required to achieve higher productivity. Farmers. particularly smallholders in developing countries, show risk-averse behavior (Binswanger, 1980) and adopt conservative management strategies that reduce negative impacts in poor years, but at the expense of reduced average productivity and profitability (Rosenzweig and Binswanger. 1993; Zimmerman and Carter. 2003). According to IPCC (2001). global changes in climate are