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Water degradation Land degradation, particularly soil and vegetation degradation, has resulted in a deterioration in the quantity and quality of both surface and groundwater resources over much of the Asia Pacific region. With less vegetative cover to protect against the impact of raindrops causing surface sealing, a decline in pore spaces resulting from loss of organic matter and loss of structural stability following cultivation, less rain infiltrates the soil. Runoff increases, stream flows fluctuate more than before (in particular stream flow storm hydrographs are likely to have sharper and greater peaks), flooding becomes more frequent and extensive. Groundwater recharge decreases, streams and springs may cease and the water table is likely to drop so that wells and boreholes may dry up. Increased runoff encourages upland erosion while an increase in severity of flash flooding encourages stream bank erosion. As a result sediment loads in rivers are increasing. The storage capacity of dams and weirs is reduced by siltation, lowland irrigation schemes are affected by silted up canals and sediment deposited in the fields, hydro-electric schemes are damaged, navigable waterways are blocked and water quality is deteriorating. High silt loads reduce the fish catch not only in inland waters but also where silt-laden rivers discharge into coastal waters. In Thailand measurement of the suspended sediment and bedload transported by the Ping, Wang, Yom and Nan rivers suggests that each year some 27.5 million tons of soil are removed from a total catchment area of about 70,000 km2. The Harbour Authority of Thailand reportedly dredges over 19 million m3 of sediment every year out of the last 18 km of the Chao Phya River at a total cost of 424 million baht in order to keep the channel open and to control floods. The implications of water degradation for sustainable agriculture are serious. With less water entering the soil and stored for use during dry periods, crop yields are falling. In the drier agroclimatic zones of the Asia Pacific region this may mean the difference between success or failure in producing a worthwhile crop. In Asia a significant amount of the staple food crop production (rice and wheat) depends on irrigation, with water being supplied from dams, run of the river offtakes and boreholes. In these areas widespread catchment degradation has affected both surface and groundwater resources. With less water available for irrigation, less land can be irrigated and less water used in individual fields with the consequence that crop yields and total production is declining. Many farm households are unable to meet their subsistence consumption needs from the production of their lowland irrigated plots. Frequently they grow a mixture of dryland crops, and/or gather forest products for sale in adjacent upland areas, to supplement their lowland farm production. The result is further degradation of the upper catchment areas contributing to yet further decline in the quantity and quality of the water resource. A recent environmental study (van Gils and Baig 1992) has identified groundwater depletion as the most ominous component of the land degradation process in Baluchistan Province, Pakistan. This is because the vast majority of both the rural and urban population of the province depend on groundwater, for which no economic alternative is available. Current levels of groundwater pumping are unsustainable with the level of the watertable dropping in some places by as much as 3 metres per year. New tubewells are continuing to be sunk leading to increased exploitation of a declining resource. If this present situation continues the study concludes that in the next 10-20 years loss of the groundwater will lead to a decline of the province's largest economic sector, irrigated agriculture, as well as the collapse of the Quetta City water supply. In some areas the rising use of fertilisers and pesticides has led to a deterioration in water quality due to increasing contamination from a range of agrochemicals. This, combined with bacterial and chemical contamination from industrial, urban and rural (humans and livestock) sources has affected drinking water supplies. Ill health arising from water borne diseases will seriously affect the agricultural productivity of farm households. |