NORTHEAST India is endowed with a bounty of water resources, accounting for about 40% of the total water resources in the country, i.e. about 60 million hectaremetre. The region is in the highest rainfall zone of the country and enjoys typical monsoon climate, with conditions ranging from tropical to temperate. The rapid changes in topography result in climatic changes within short distances. For example, the average annual rainfall reaches a peak of 13,390 mm in the Cherrapunji–Mawsynram region in the Meghalaya plateau, while the northern and adjoining central areas in the same plateau that fall in the rain-shadow region need irrigation during most part of the year. The pre-monsoon showers (March–May) accounts for 25% of annual rainfall, while bulk of the rainfall (67%) occurs during June–September, which constitutes the monsoon season. The monsoon withdraws from the Northeast almost abruptly by the last week of September; post-monsoon rainfall (October–December) and winter season rainfall are scanty, limiting the scope for agricultural activities during the rabi season.

 Delay in pre-monsoon showers and slow onset of monsoon along with skewed distribution of rainfall not only lead to serious dislocations, but also cause damage to the crops and also severe water shortage. On the other hand, excessive precipitation causes rapid run-off on steep slopes, resulting in heavy soil loss as well as siltation of riverbeds. It may also lead to catastrophic flood hazards in the plains, excessive leaching losses and also dangerous landslides. All this only underscores the need for a scientific and technical approach towards water management, with focus on harvesting and multiple uses of water. 

 In this context, it has been observed that rainwater harvesting can be implemented as a viable alternative to conventional water-supply schemes in the region, considering the fact that any land anywhere can be used to harvest rainwater.

 Rainwater harvesting besides helping meet the everincreasing demand for water, helps reduce run-off which is choking storm drains, reduce flood hazards, augment the groundwater storage and control the decline in the water level, improve quality of groundwater and reduce soil erosion. This is considered to be an ideal solution for water problem, where there is inadequate groundwater supply or where surface resources are either not available or insufficient. Rainwater is bacteriologically pure, free from organic matter and soft in nature. The suggested structures for harvesting rainwater are simple, economical and eco-friendly. Previous studies have shown that subsistence agriculture in the hilly northeastern region could be successfully transformed into a profit-earning enterprise by tapping and utilizing rainwater in limited quantities.

 Rainwater harvesting, irrespective of the technology used, essentially means harvesting and storing rainwater in days of abundance, for use during the lean days. Storing of rainwater can be done in two ways: (i) in an artificial storage and (ii) in the soil media as groundwater. 

 A demand–supply analysis is required while designing water-collection tanks. Factors such as amount and frequency of rainfall, run-off coefficient of the collecting surface, number of users, daily requirement and dearth period are important for calculating the size and capacity of the storage tank.