Rainwater Harvesting

Credits: Biovision-Infonet

Introduction

Rainwater running off land surfaces can be harvested, stored and utilized using a technique called rainwater harvesting instead of being wasted in rivers, lakes and the sea.  

Rainwater harvesting consists of 5 components: 

1.  Rainfall 
2.  Catchment areas, also called watersheds, onto which the rainwater falls.
3.  Gutters, or conveying channels, to bring rainwater from a catchment area to storage reservoir. 
4.   Storage reservoirs can be tanks, ponds, dams and in situ storage in sand and soil.
5.  Retrieval water is extracted from reservoirs either by gravity or by pumps and lifts.

A rural homestead should preferably have the following variety of structures for harvesting rainwater to avoid water shortages during dry seasons: 

1) A roof catchment system for clean domestic water that consists of gutters fixed to the roof which drain the rainwater into a storage tank. The size of a storage tank depends mainly on the financial capacity of the owner and to a lesser degree on the size of the roof and the volume of rainfall. However, the ability to supply sufficient water during years with drought depends on the size of the roof and the tank.

2) A pond or an earth dam for watering livestock and garden irrigation can be excavated by hand or animal drawn implements at a low place in the farmland where rainwater flows, or accumulates, during rainy seasons. Ponds and dams can initially be built small and enlarged during the following dry seasons until they might supply water throughout the years. 

3) A hand dug well may supply water if sunk into shallow ground water, e.g. downstream of an earth dam or near evergreen trees growing on riverbanks. 

In situ harvesting of rainwater in farmland increases the yield of crops and will often determine whether there will be anything to harvest at all. Most farmers know and apply some of the techniques of soil conservation that make rainwater percolate into the soil instead of eroding the farmland. Among several techniques, the following are being used by many farmers:

1. Contour planting in horizontal ploughing lines.
2. Contour trash lines with grass and farm waste.
3. Contour ridges that develop into terraces. 
4. Contour bunds of stones that develop into terraces.
5. Bench terraces that develop slowly from contour ridges.
6. Micro catchments with U and V shaped soil bunds for growing grass and crops and trees.
7. Trapezoidal bunds with a farming area of 1,350 m2 (1/3 acre) for grass and crops.
8. Cut-off drains to discharge surplus rainwater run-off into gullies with check dams and streams.
9.  Check dams of stones and vegetation to heal eroded gullies.
10.  Check dams of stones and vegetation in valleys.