TR(BR)-6/97-98 : Feasibility study of rainwater harvesting in semi arid region

Abstract

Rain water harvesting in hot arid and semi-arid regions should be based on the study of overland flow on porous surface, effect of surface soil treatment on quantity and quality of runoff produced, extent of land treatment to get required volume of water to be conserved. In view of the short duration rainfall and high infiltration and evaporation rates in the arid regions, the time steps used for computations should be much less than the duration of rainfall. Runoff produced from short duration rainfall may be lost during transmission to ephemeral stream. Therefore the runoff should be collected before it is lost during transmission. The runoff should be stored in underground storage tanks so that the evaporation losses are minimum.

From the study of overland flow on porous surface it is found that the catchment should be treated such that the hydraulic conductivity of the treated surface soil should be less than one third of the rainfall intensity. If the in-situ soil has conductivity less than one third of rainfall intensity, overland flow occurs which can be harvested.

Drinking water can, be supplied at a rate of about Rs. 42 per 1000 litres in an area receiving an annual rainfall of 300 mm and having potential evaporation loss of 1800 mm. For an area where the annual rainfall is 250 mm and potential evaporation loss is 3000 mm, water can be harvested at a rate Rs. 50 per cubic meter (i.e paisa 5 per litre) to a population of 1000 with water requirement of 125 litres per capita per day. Detailed hydrological investigations are necessary before implementation of the water harvesting project.