An Agroclimatological Characterization of the Indo-Gangetic Plains

Summary The climate of the Indo-Gangetic Plains (IGP) is dominated by the Asian summer monsoon. The cool, dry winter is followed by a warming trend with daytime temperatures reaching as high as 45°C in June or July. The temperature rise is broken by the onset of the monsoon rains, when the daytime maximum temperature will immediately drop 5°C or more with the first rains. Summer temperatures are generally higher in the northwest part of the IGP, corresponding to later onset of the rainy season. In most of the IGP proper, winter temperatures is mild allowing production of wheat, potatoes and other cool season crops where irrigation is possible. Annual precipitation varies from less than 400 mm in western Pakistan to over 1600 mm in eastern India and in Bangladesh. Based on a critical value of 0.5 for the ratio of precipitation to potential evapotran-spiration, most of the IGP have abundant water for summer rainfed crops. This is reflected in the widespread cultivation of lowland rice. Variation in rainfall over years heavily influences risk in agricultural production in the IGP. Over a 90-year period, the mean annual rainfall at Ludhiana, India was 710 mm, but in 1899, it received only 240 mm as compared to over 1400 mm in 1988. Failures of the monsoon rains cause crop failures and in the past have contributed to famines. Flooding due to excess rain can also be destructive, particularly in the eastern IGP. Attempts to predict the time of onset and amount of monsoon rains based on the El Niño-Southern Oscillation (ENSO) process and other information show promise but are not yet reliable enough for routine use in guiding agricultural management.