Irrigation has been practised in the Indus plain since 8th century AD and the history is full of descriptions of various artificial canals constructed for ameliorating conditions of people and increase revenues. Land along riverbanks used to be irrigated through a network of inundated canals, which could divert water only during high-flow season. As needs grew, human ingenuity devised methods to extend irrigation facilities to higher land. The process was totally transformed with the construction of Sukkur barrage (1932), Kotri barrage (1955) and Guddu barrage (1962), to control irrigation of canals.
In Sindh, out of total geographical area of 14104 square km, 5.7 million hector land is cultivated, which is rapidly degrading leading to decline in agriculture product and seriously affecting the livelihood of rural communities.
Water Logging: Because of the flat nature of Indus plain, natural sub-surface drainage through down-valley movement of ground water is restricted. Persistent seepage over the years from unlined canals and large network of distributary channels and irrigation surpluses from the field have caused the water table to rise close to land surface creating water-logged condition.
The area with ground water level less than 1.5 m is considered by Wapda as disaster area and is a primary candidate for drainage treatment. In pre-Sukkur barrage days, the ground water level in Sindh was below 30 feet. Intensive use of Indus water for irrigation has altered the hydrological balance of basin-seepage losses and deep percolation has resulted in gradual rise in the water table resulting in water-logging and salinity in many areas. Indus basin has flat topography, porous soil, and semi-arid climate with high evaporation rate. In these conditions, it was required to develop proper drainage system in parallel with an irrigation system to control water logging but that was not done.
Salinity and sodicity: The two major causes of salinity, are (1) application to land of irrigation water containing high dissolved solids (TDS). Evaporation of this applied water leaves the salt behind. 2) With the rise in water left on surface and the capillary action moves water to ground surface, if not transpired by plants, this water evaporate leaving salt behind.
About 10 tons per year of salt is brought in by Indus river in Sindh and use of ground water mobilize about 4 ton per year of salt by tube wells. The water from tube wells contain high amount of TDS, and it’s injudicious use has contributed significantly to salinity. Whereas the irrigation is required to supplement the natural rainfall to remove the soil moisture deficiency, drainage is necessary to ensure a satisfactory balance between moisture and salt concentration in root-zone. It ensures that root-zone contains sufficient moistures to allow crops to extract water required for transpiration and at the same time does not remain saturated for extended period. For zone to remain relatively salt free net-water movement should be downward to prevent salt build up.
In Sindh, the construction of drainage system started in 1970, with the initiation of small-scale project, that is Salinity Control and Reclamation Project (SCARP). The move culminated in launching two major drainage systems on both sides of Indus that is the LBOD and RBOD in 1986 and 1995, respectively.
The sodic soil containing high amount of exchangeable sodium can also be reclaimed by replacements of exchangeable sodium with calcium that will improve soil permeability and leaching. Gypsum is normally used as chemical amendments to reclaim sodic soil. In the biological technique, salt tolerant plants, like Kaller grass, Jentar and rice are grown. These are tolerant to saline soil conditions.
Soil mining: The agricultural soil contain large amount of nutrition reserve that are made available to plants by slow process. The organic matter helps soil to retain moisture and nutrients by absorbing water readily and releasing it slowly.
It also helps in maintaining soil structure facilitating both root aeration and nitrogen fixation. Organic matter also helps maintaining population of micro-organism that converts mineral nutrients into soluble form. The soil in Sindh is low in organic matter. Over harvesting results in removals of nutrients at a rate that exceed natural rate of input. For better production, farmers in Sindh use massive nitrogen fertilizer as result the soil is depleted of micronutrients especially phosphorous, zinc, iron and boron.
The organic matter and other nutrients can be supplied to soil by proper management practices as incorporating agricultural residues green manure and applying farmyard manure. Unfortunately the in-situ burning of agricultural residue by the farmer results in loss of organic matter.
Deforestation: Trees help controlling soil erosion, check run-off, reduce desiccation of crops, and add favourable nutrients to soil. They also improve physical and chemical properties of soil and enhance rate of biological processes.
In Sindh, forests occupy only 2.5 per cent of the total land area. They entirely depend on monsoon floods in riverine tract and canal water in mainland area. According to recent reports, trees have been decreasing with the shortage of irrigation water, arid climatic condition and illegal cutting. The riverine forests are rapidly deteriorating because of the reduced flow of water. The forest particularly affected are those below Kotri barrage. Many areas are entirely devoid of vegetation and have virtually become unproductive. The inland forest located outside river dikes was introduced over 100 years ago to provide fuel wood for railways.
The unchecked cutting of trees for grazing, firewood and furniture has aggravated the problem. Some time back, following growth in activities of dacoits, many forest areas were cut by law enforcement agencies as part of operation against dacoits. The impacts of deforestation include soil depletion loss of soil fertility, reduction in recharge of aquifer, enhanced sedimentation, lowering of water table, etc.
The Sindh forest department has designated the mangrove forest along coastline as protected forest. The satellite images indicated that above 260,000 hectares of delta are covered with mangroves. The mangrove is tolerant to seawater but depend on steady supply of fresh water to keep the salinity level within certain limits. The mangrove provides excellent nursery area to young fish and shrimp. The coastal villager uses mangrove for fuel wood and fodder for domestic animals.
Another important benefit of mangrove ecosystem is that it protects the coast from wind and ocean current. Over the past fifty years the Indus has been diverted for irrigation, power generation, domestic and industrial water supply. Of 150 million acre feet (MAF), the flow reaches only 20 MAF. For nine months of a year, fresh water flow out at all and there are proposals to reduce the flow to 10 MAF. The reduced flow means that already high saline level in creek water will become still high.
This tends to stunt the growth of both trees and animals. This will have significant impact upon mangrove forest. There are sign of stresses already in seemingly aging population of trees with lack of substantial recruitment in some areas. There is pressure from overgrazing and looping for fuel wood and fodder, in some areas. Within vicinity of Karachi, mangrove forest is facing stresses from steady growth of this industrial city. The untreated domestic sewage and significant discharge from major industries end up into creeks. This has affected the efficiency of breakdown of mangrove leaf litter and changed the soil characteristics. Sea Water intrusion: Before dams and barrages, due to large quantity of freshwater and silt brought up by the river Indus, the delta land was richest in agriculture. With progressive reduction of Indus river outfall to sea, the silt flow, which was 200 million tones/year in 1947, has become reduced to only 30 million tones/year. This has resulted in erosion and seawater intrusion in delta areas. With the disappearance of fresh water, seawater moving to delta channel, agriculture in coastal belt is no longer possible. The people in the coastal region have lost bulk of their most fertile land. It is estimated that about 1.22 million acres of agriculture land in Thatta and Badin districts has been lost and people of these areas have migrated to other areas in search of livelihood.
The desertification, which has occurred in canal-irrigated area, riverian area and delta region during recent past is due to unrealistic policies and unprofessional management. The poverty in rural areas has reached about 70 per cent of defined income level. The unemployment has reached unto 35 per cent. To save the land of Sindh from further degradation the policies are required to be reviewed and water use of Indus system should be regulated and and planned in a professional manner.