Short notes on the ecological role of mangrove forests and on sustainable irrigation methods

The question asks for any two of the four notes. Notes (i) and (ii) are answered.

(i) Ecological role of mangrove forests

What mangroves are. Mangroves are salt-tolerant trees and shrubs of sheltered tidal shores, at their richest in river deltas, where the water is brackish because river and sea mix; Bharucha places the largest mangrove swamps in the Sunderbans, in the Ganges delta.

Their ecological role

• Keeping silt near the shore. Standing mangroves keep river silt from being swept out to sea. Bharucha notes that when mangroves beside a reef are cut down, the silt is carried offshore, settles on the coral and kills it, so a healthy mangrove belt also protects the reefs next to it.
• A living sea wall. Their dense stems and tangled roots take the force out of waves and storm surges and hold the shoreline against erosion, so coastal villages behind an intact belt suffer less when a cyclone strikes.
• Breeding ground for sea fish. Mangroves provide habitat that marine fish need in order to breed; the young fish shelter among the roots before moving out to open water.
• A base for coastal food webs. In biomass produced, the brackish deltas that mangroves cover rank among the most productive ecosystems in the world. Fallen leaves feed crabs and small fish, which in turn feed larger fish and wading birds.
• Support for fishing villages. Because these waters stock the coastal fishery, the loss of a mangrove belt is felt in the catch of nearby fishing families as well.

The main pressures on mangroves are felling for firewood and the digging of prawn ponds.

(ii) Sustainable irrigation methods

Why irrigation must be sustainable. Too much water can ruin farmland as surely as too little. Fields flooded season after season turn waterlogged and salty, and salty soil is very costly, often impossible, to bring back into use; thirsty cash crops such as sugarcane make the problem worse. Sustainable irrigation meets a crop's real need without draining the source or spoiling the soil.

Methods, from adding to the supply to cutting the demand

• Catching the rain. Farm ponds, small percolation tanks and harvested rainwater give the farm a store of its own and let water soak down to the groundwater below.
• Drip irrigation. Thin tubes laid along the rows let water fall drop by drop at each plant's roots, so almost nothing is lost to runoff or to wetting bare ground.
• Sprinkler irrigation. Water is sprayed over the crop like light rain; it suits uneven land and sandy soils, where flooding would run off or sink away.
• Losing less on the way. Watering in the cool hours, covering the soil with crop residue and lining field channels all cut evaporation and seepage.
• Choosing crops that fit the water. In dry districts, growing millets and pulses rather than a very thirsty crop such as sugarcane lowers the call on wells and canals.

Used together, these methods stretch the same water over more land while keeping the soil productive.