Harvesting Structures for Ground Water Recharge
Water exists in the form of liquid (lakes, seas, rivers), solid (ice, glaciers) and gas (water vapours). Of all the fresh water in the world, only a tiny fraction is found in lakes and rivers. Most of it (98%) exists in the earth as soil moisture and ground water. Ground water thus forms the largest source of fresh water, next to glaciers and ice caps.
Replenishment of ground water takes place through rainfall, tanks, rivers and water harvesting structures and is known as ‘ground water recharging’. Over 50% of the net irrigated area in India is irrigated by ground water.
Over the past three decades, growing populations and increase in irrigation has led to excess withdrawal of ground water without commensurate recharging, resulting in a rapid fall in the water table
Development Alternatives has constructed hundreds of checkdams in Bundelkhand region of Madhya Pradesh and Uttar Pradesh to recharge the groundwater aquifers during the past 15 years.
in many parts of the country, particularly, Tamil Nadu, Andhra Pradesh, Rajasthan and Gujarat. The reason for this is that although India receives about 400 million hectare metres (mham) of rainfall each year, only 50 mham filters down to the ground water aquifer and can be pumped back. A large part (115 mham) is lost to the sea in the form of run-off and rest is evaporated.
In India, ground water holds the key for increasing irrigated crop lands. Even after harnessing all the available potential for large dams, we could at best hope to irrigate 50% of the cropped area. With limited finances, long gestation period and the adverse effect on environment and displacement of people, which has been highlighted in the recent case of Narmada Sagar and Tehri dam projects, the bulk of existing and future irrigation needs would have to be met by tapping ground water and utilizing it more efficiently. We, thus, have to initiate urgent measures to augment ground water recharge. Some of the measures which aid in ground water recharging are as under:
Increasing the vegetable cover over the land
Soil and moisture conservation measures
Proper landuse practices
Building appropriate structures such as checkdams across streams and nallahs.
A water shed is a defined area which has an undulating terrain from which the entire rainfall drains out through a single outlet. This outlet may be a nallah, stream, tributary or a large river. Thus, in terms of area, a watershed may be a large area having a number of villages, roads, hills, fields and forests within it.
Proper landuse in a micro-watershed would include:
soil conservation measures
cropping patterns based on soil characteristics and availability of irrigation, including agroforestry.
water harvesting measures
improvement in soil fertility
suitable measures for wasteland reclamation viz silviculture, silvipasture, horticulture, agroforestry etc. depending upon the soil characteristics.
Soil conservation measures for croplands
bunding of fields with suitable outlets for excess water
growing grasses and legumes in fallows
proper crop and land management
Bunding of fields
Earthen bunds of suitable height and width, depending upon the slope and size of the plot, are well tested and tried as simple measures to minimize top soil losses. The bund is provided with an outlet for excess water to flow out. This measure also enhances the ground water recharge. The outlet has been located where the slope is steepest or the flow is maximum. Legumes and grasses can be sown on these bunds to make them stronger and also provide a good quantity of forage.
This is done by spreading the open field surface with crop residue, or any other vegetation. Where residue is fed to animals, a saw-dust mulch (if available) is recommended. Mulching minimizes the splash effect of rain drops, reduces evaporation and controls the weeds on the cropland.
Growing grasses and legumes
Roots of grasses and legumes also minimize soil erosion. Inclusion of legumes in crop rotation, thus, reduces soil erosion from the cropland. Ploughing back the legume greatly improves the structure and nitrogen content of the soil.
Proper crop and land management
Planing time of crops should be selected to ensure that there is a crop cover for a maximum period to prevent erosion by wind and by splash of rain drops. Crop rotation is yet another important practice. Planting legumes with or after cereals, ensures plants feeding from different soil depths in different cropping seasons. Further, legumes enrich the soil with nitrogen. Contour ploughing on slopy lands reduces soil erosion.
Soil conservation measures for wastelands
Providing vegetal cover at the earliest
Contour trenching / stone dykes / vegetative cover
The same method as described for croplands can be followed for wastelands also.
Torrential flow of water cuts into the soil, forming rills which are gradually deepened into gullies. Gully plugs can be mechanical or vegetative barriers across the gullies to slow down the water flow and check further extension of the gully. Vegetative gully plugs are made by planting of hardy species such as chrysopogon, Vetiver or Munj grass.
Mechanical gully plugs are of the following types:
Series of small drop structures
Series of small drop structures
For a wide gully, which almost becomes a nallah, a series of small masonry drop structures could be used to check further widening of the gully and to reduce the erosive velocity of the flowing water. This also contributes to surface water storage for short periods and enhances ground water recharge.
These are gully plugs made of different brushes, bamboos, local vegetation, tree logs, etc. A barrier is erected using these materials, which slows down the erosive velocity of the water.
These are constructed by utilizing locally available boulders which are put in the shape of a stone wall across the gully. Gavion structures are boulders with a wire mesh all around them. These structures act as barriers to reduce the force of flowing water and thereby check soil erosion. As wilt fills the structure, the height of the structure needs to be increased. Silt deposited at the upstream side helps in conserving moisture for a longer period which ultimately raises the moisture regime of the area.
These are simple earthen structures being built since centuries in our country. The structure is made of locally available soil and boulders.
These are dug across the slopes of hills to catch the run-off and to reduce soil erosion. The trenches are rectangular in shape and the soil excavated from digging the trench is formed into the berm on the down side of the trench. Seeds of grasses and legumes can be sown on the berms during the rains which will not only protect the berm but also provide forage. The number of trenches of a fixed dimension per hectare area is determined by the slope/gradient of the area, average rainfall on a normal rainy day (24 hours) and the type of soil.
Stone dykes: Where the slope does not have enough soil depth to allow digging, stone walls (dykes) can be built along the contours of the slope.
Vegetative barriers: Planting of chrysopogon or Vetiver grass along the contours of the slope helps to reduce soil erosion by slowing down the erosive velocity of water.
Providing vegetal cover: This is the most important measure to check soil erosion and increase infiltration into the ground water table. Vegetal cover on barren lands may be provided either by tree plantation or through natural regeneration. q
Wasteland Development Training Manual,