Quantifying Climatic Risk In The Semiarid Tropics: ICRISAT Experience

Troll defined semiarid environments in terms of the average pcriod each year when rainfall (R) exceeded potential evaporation (PE). In the semiarid tropics (SAT), rainfall changes rapidly at the beginning and end of the monsoon so that thc pcriod whcn K > PE is insensitive to thc arbitrary way PE is dcfincd or mcasurcd. Hargreaves and Robertson used somewhat more sophisticated mcthods to spccify thc length of the growing season in terms of thc pcriod when "dependable' precipitation (i.e. the amount exceeded in at least 75% of years) was more than PE/3. This critcrion was used in early work at ICRISAT. Later, a watcr balance model (WATBAI.) was introduced to cstimatc the risk of crop failure following dry spclls at any time during the monsoon. SORGF was the first simulation met tested at ICRISAT, calibrated with a set of regional data, and then used for risk analysis. Recently, a simplcr simulation modcl (RESCRP) was developed to predict cmp responses to wcathcr and soil water. In this presentation, an even simpler version is described and uscd to estimate yicld probabilities at hvo sites with contrasting rainfall regimes as a function of niaxin~un~ available water or of light interception. Ihe analysis demonstrates thc interaction of weather, soil and plant factors in determining production risks. Many environmental constraints endured by crops and by farmers in thc SAT arc not yet amenable to modelling so that attempts to modcl risk, even with thc most con~plcx simulation models, arc somewhat unrealistic. Major problems not yct adcquatcly addressed by modellers includc germination, seedling establishmcnt and root pcnctration in difficult soils, pests and diseases, and damage caused by wind or vcry heavy rain. Achieving the right balance between productivity and risk lies at thc base of all agricultural development. The SAT is an ecologically fragile region where thc production of food is particularly hazardous bccausc rainfall is erratic, soils arc impoverished, and few fanners are able to control pests and discases cffectivcly or to apply optimal amounts of fertiliser. Because population is growing faster than food supplies in many parts of the SAT, subsistence agriculture has extended into marginal land, damaging the environment and threatening thc long-term viability of economic and social devclopmcnt. In this review, we identify salient environmental fcatures of thc SA'T and considcr briefly some of the ways in which ICRISAT scientists have bccn ablc to rcducc production risks. We then assess progress in climatic analysis and indicalc thc directions in which we believe the modelling of crop production should bc cncouragcd to dcvclop