|Biodiversity in the Western Ghats: An Information Kit (IIRR, 1994, 224 p.)|
|9. Appreciating and conserving biodiversity|
A watershed is the area drained by a single stream or river. Watersheds can be as large as the Ganges drainage basin, which covers much of northern India, to as small as the area feeding a tiny stream or tributary of a larger river.
Watershed management integrates technologies to develop and conserve land, water and plant resources within the natural boundaries of a drainage area. It aims to reduce erosion and sedimentation which can reduce soil fertility and degrade lakes, streams, rivers, estuaries, and coastal regions.
Why a watershed approach?
What happens upstream affects areas downstream. For instance, deforestation and erosion in the hills can silt rivers and cause flooding downstream. And the amount of water percolating into the soil high in a valley determines the flow of springs and rivers lower down. That makes it useful to study watersheds as a whole and implement conservation practices over the whole area rather than piecemeal.
Soil erosion is a serious problem in many areas of the Western Ghats. Removal of the topsoil by erosion lowers soil fertility and therefore the the biodiversity it is able to support. The topsoil washes downstream and is deposited in reservoirs, ponds and river beds, reducing the efficiency of irrigation systems and exacerbating floods. The suspended sediment in the water reduces the level of photosynthesis in the water and harms aquatic life.
When rain falls to the ground, part percolates into the soil,
part stays on the surface and evaporates, and part runs off the surface to swell
streams and rivers.
Some of the portion that percolates into the soil is used by plants. The rest becomes groundwater, which in turn feeds wells and springs.
The best way to conserve moisture is to control the amount of rainfall that normally runs off the land. Keeping water on the surface for a longer time allows more to percolate into the soil. This recharges the water table and water bodies and increases the amount available for agriculture and other uses.
Various methods can be used to increase percolation:
· Bunds (ridges) that run along the contour and check water flowing downslope. Plant vetiver grass on these bunds to prevent erosion.
· In situ conservation using basins, furrows, broad-based ridges and furrows, and random tie ridges.
· Check dams and structures to control the formation of gullies.
· Small ponds in individual farmers' fields to harvest runoff and make it available for farm use.
· Percolation ponds to collect excess water and recharge groundwater. The water in the percolation tank helps maintain the water table and stabilize irrigation supplies.
· Irrigation tanks to collect excess runoff for use in irrigation.
Groundwater can be used in various ways. Wells and boreholes are traditional methods. Small sub-surface dams can be constructed and groundwater pumped out for use.
The water stored and conserved in the soil should be used in the best way-for instance, by selecting crops and varieties that require little water. These will often be indigenous cultivars. Reducing water use allows the remaining soil moisture to be used for other crops such as fodder, green manure, tree and horticultural crops.
Appropriate crop management techniques and improved irrigation systems, such as sprinkler and drip, can also conserve moisture.
The ideal land use depends on numerous factors: climate, slope, soil type and elevation, as well as socio-economic considerations. Possible uses include annual cropping, agroforestry, horticulture, silvipasture, and various other combinations. A well-planned and adequately managed watershed will combine many different systems to ensure that resources are optimally used. Some possible systems and their components are briefly described below.
· About 5334 million tonnes of
productive soil are lost in India each year.
· 150 million ha suffer from erosion.
· 6.6 million ha are converted to wasteland.
· 6 million ha are affected by submergence.
· 4.5 million ha suffer from salinity.
· 4 million ha suffer from shifting cultivation
· 2.5 million ha are affected by alkalinity.
Many crops and other enterprises can accompany rice, either as intercrops or as crop sequences. For instance, a rice crop can be followed by vegetables, pulses or groundnuts. Fish can be kept in the rice fields, and trees, fodder or annual crops can be grown on bunds.
Dryland farming on slopes
Bunds and terrace risers can help check erosion on steep slopes. Such land can be planted to food crops; the terrace risers can be used to grow fodder crops such as perennial grasses and legumes. One hectare of land cropped in this way can support 1.18 livestock units. Contour bunds are constructed at vertical intervals of 0.5 to 5 m. Bench terraces can be constructed on the lower slopes, with a vertical interval of less than 1 m. Besides promoting crop diversity, such a system retains about 80 to 90% of the rainfall and reduces soil loss to below 2 tonnes/ha/year.
Livestock unit example
For horticultural production, the slope should not be more than 60%. Contour bunds at 2-m vertical intervals, half-moon terraces at the fruit tree locations and grassed waterways control runoff and erosion. Bench terraces on the lower slopes can be used to grow vegetables. Such a system retains about 70 to 80% of the rainfall and reduces soil losses to below 1 tonne/ha/year.
Farmers can choose among various livestock-based farming options. These include rice-fish culture with freshwater fish species like Rohu, Katla and Mrigal. Prawns can be grown in pens on the shallow margins of estuaries and canals. They can be cultured in both fresh and brackish water. Brackish-water culture should also integrate fish species like Chanos chanos (milk fish), Etroplus suratensis (kalunder) and mullet.
Livestock options include duck-fish culture, dairy farming, poultry and rabbit raising, and beekeeping. Biogas plants and vermiculture can provide extra income.
Trees and other perennials can be used in many ways: grown as plantations, planted as intercrops with annual crops or in pasture land. Useful perennials include coconut, cashew, banana, papaya, pineapple and pepper.
Watersheds, large and small
Catchment: A primary river is a long river draining more than 100,000 ha. It has a number of tributaries of first, second, third, fourth or even fifth order. The primary river generally discharges its load to the sea.
Sub-catchment: The secondary river is a tributary of primary river; it drains more than 40,000 ha.
Watershed: The tertiary stream is a tributary of a secondary river; it drains 4000 to 40,000 ha.
Sub-watershed: A quarternary stream is a tributary of a tertiary; it drains 2000 to 4000 ha.
Mini watershed: A pentad streamlet is a tributary of a quarternary; it drains 400 to 2000 ha.
Micro watershed: A hexed streamlet is a tributary of a pentad; it drains less than 400 ha.
Watersheds vary in many ways: size, shape, relief, drainage, geology, soil, climate, surface conditions, land use, groundwater and sociological aspects. No two places area exactly alike, so each watershed has its own distinctive characteristics or "watershed attributes". These define the potentials and problems of the watershed.
Prepared by H. R. Prabhudesai