The report gives an account of a feasibility study of rainwater harvesting in semi-arid regions. A review of water harvesting systems is followed by a section wherein numerical solutions for surface flow equations are explained.
The report states that rainwater 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 & quality of runoff produced and extent of land treatment to arrive at the 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.
The study of overland flow on porous surface indicates that the catchment should be treated so that the hydraulic conductivity of the treated surface soil is less than one-third of the rainfall intensity. If the in-situ soil has conductivity less than one-third of the rainfall intensity, overland flow occurs, which can be harvested.
The report presents a section on economical design of a storage tank in which the variation of cost of unit volume of water with depth of the storage tank follows a parabolic equation. The cost of the harvested water depends upon the annual rainfall, the life and efficiency of treatment, evaporation losses and cost of concreting of storage structures. The FORTRAN program for calculation of cost of water along with the test data is presented in an appendix. It is seen that -
- 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 a 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 of Rs 50 per cubic meter for a population of 1000 with water requirement of 125 litres per capita per day.
The report concludes by saying that detailed hydrological investigations are necessary prior to the implementation of water harvesting projects.
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