The chief features of spatial variability of annual rainfall are deduced by analysing expansion and contraction of moisture regions. The mean annual isohyets of 560 mm, 1040 mm, 1420 mm, 1630 mm and 2450 mm can be used to demarcate six Thornthwaite type climatic moisture regions – arid with 15.32 per cent, semiarid with 33.79 per cent, dry subhumid with 24.3 per cent, moist subhumid with 8.86 per cent, humid with 12.38 per cent and perhumid with 5.34 per cent of India’a geographical area. The different moisture regions display intrinsically large spatial variability, expansion/contraction, from one year to another. The areas of the country with mean annual rainfall (MAR) 320 mm or less experience arid conditions with probability 90 per cent and greater, 472 mm or less with probability 70 per cent and greater, and 582 mm or less with probability 50 per cent and greater. The aridity can spread up to mean annual isohyets of 692 mm thrice in 10 years, up to 870 mm once in 10 years and up to 1203 mm once in 100 years.
Based on topography, landforms, climate, soil, drainage and distribution of flora and fauna the country is divided into 15 major physiographic divisions and 48 subdivisions or provinces. Chief features of annual, seasonal and monthly rainfall fluctuations over physiographic regions are described as they provide useful information for development and management of water bodies — rivers and canals; wet lands; reservoirs; tanks and ponds; bells; oxbow lakes and derelict water; and brackish water. The longest possible rainfall series (1813-2006) has been developed for each of the 15 divisions and 48 subdivisions. Established statistical analysis has been carried out to infer recent trends in different rainfall series.
The Indo-Gangetic Plains River channel changes through tectonic disturbances, hydrologic phenomena depending upon ground slope, rate and depth of flow, width and size of the flood plain, sediment transport etc., and meteorologic factors — spatial shift in rainfall/climatic belt and strength and direction of surface winds over the Indo-Gangetic Plains are natural hazards of serious concern.
During cooler northern hemisphere atmosphere, monsoon circulation will be weaker and low level westerlies over the Indo-Gangetic Plains stronger, consequent upon which rainfall activities and river courses would shift eastward. While opposite will be true with warmer atmosphere, monsoon circulation and low-level easterlies over the Indo-Gangetic Plains will be stronger, consequent upon which rainfall activities and river courses would shift westward. However, in the recent complex situation with warmer lower troposphere (+2.2°C/100-year) and cooler upper troposphere and lower stratosphere (-2.2°C/100-year) the monsoon is likely to be highly variable. In the random occurrence if the entire depth of troposphere is warmer the monsoon will be intense over northwest India and the river channels of the Indo-Gangetic Plains would shift westward, and if the full depth of troposphere is cooler the monsoon will be weaker and the rivers will shift eastward. Meteorologic factor appears to be the main cause of recent (18th August 2008) eastward shift of the Kosi River.
The spatial-temporal variability of rainfall over India is complex. During the same year contrasting conditions, increasing/decreasing, expansion/contraction, droughts/floods, can be seen in different parts and some times, even congruous parts of the same region. Some effects of rainfall variability are described above, however there could be many more. A comprehensive monitoring scheme for rainfall variations over and across the country is being undertaken at IITM, Pune by using the longest available, highly quality-controlled rainfall data from an elaborate network of 316 stations. The multifaceted GIS enabled analyses have pointed towards a spread of dry provinces and a shrinking of the wet provinces in the recent years. The integrated effect is large scale drier conditions. Earliest possible action may be initiated to store the available surplus rainfall to the maximum possible extent in order to have stable food grain production and to meet drinking water supply and hydroelectric power generation.