The climate and rainfall of the area varies considerably across the region. Encircled by hills and plateaus, rainfall varies even more than temperatures. The average annual rainfall reaches a peak of 13,390 mm in the Cherrapunji-Mawsynram region. But areas that fall in the rainshadow region of the Meghalaya plateau need irrigation. While the northern slopes of the Brahmaputra valley receive an annual average rainfall of 2,500 mm, the area south of the valley and the northern part of Meghalaya receive an annual rainfall of about 2,000 mm.
Distribution of the population in the Northeast is also very uneven. Within the plains there are pockets of very high population density, such as the Manipur plains (400 persons/sq km) and the Nowgong plains (302 persons/sq km). The vast hill tracts, however, have a low population density.
The water resources potential of the region is the largest in the entire country. Given its heavy rainfall, it also has abundant groundwater resources. But only a small part of the region has been studied to estimate the groundwater potential. The maximum scope for development of groundwater exists in Assam, Tripura and Arunachal Pradesh. The available surface water resources have hardly been tapped because of the rugged nature of the terrain. Hence, cultivation in the region is largely rainfed and jhum cultivation (shifting cultivation) has been widely adopted.2
Nonetheless, there are documented instances of some indigenous rainwater harvesting systems used for cultivation, of which some are ingenious. Settled agriculture is practised in the form of irrigated terrace cultivation in parts of Nagaland and a few villages of Meghalaya. Channels are dug to irrigate these fields. The other chief indigenous source of irrigation is the bamboo irrigation system found in parts of Meghalaya, and in some villages in the Mokokchung district of Nagaland
