ADB_book_18 April.qxp - Himalayan Document Centre - icimod

Table 3.4: Estimated Soil Erosion Rates at Selected Sites in NepalSiwalik RangeMiddleMountainha = hectareSource: CBS (2004) Table 4.36.Location and CharacteristicsLand UseErosion Rate(tons/ha/year )Eastern Nepal: south aspect, sandstone foot Forest to grazinghills7.8–36.8Far West Nepal: south aspect , sandstone, Degraded forest 20.0foot hills of SurkhetGully land 40.0Degraded, heavily grazed gully land 200.0Central Nepal: Mahabharat Lek, steep slope, Degraded forest and agricultural land 31.5–40.0metamorphic and sedimentary rocks Gully land 63.0–420.0Degraded forest and shrub land 27.0–45.0Kathmandu Valley: northern foothills Overgrazed shrub land 43.0Severe gully land 125.0–570.0Kathmandu Valley: south Dense forest (75%) 8.0Pokhara Valley: Phewa Tal wate rshed Protected pasture 9.2Overgrazed grass land 22.0–347.0Gully, overgrazed grass land 29.08.7 tons/ha/year, which is less than the 2.5–16.4tons/ha/year over cultivated terraces. Likewise therate of land loss in open degraded areas is 25–40tons/ha/year compared with 3–25 tons/ha/year fromcultivated outward sloping terraces (ICIMOD 1998).However, erosion rates in excess of 200 tons/ha/yearare common in grazing lands below 1,000 maslwhere there is overgrazing, which causes gullyerosion (Carson 1992).Owing to weak geological formations such asshallow and coarse lands and loosely compactedrocks, the Churia hills are more vulnerable to rainfallthan the mountain areas. As a result, the bare slopesof the Churia hills are readily exposed to degradationevery rainy season. The meteorological data showthat the Churia hills receive much more intenserainfall than the northern high mountains. IntenseGlacial Lake Outburst Flood EventsThe periodic or occasional release of large amounts ofstored water in a catastrophic outburst flood is referredto as a glacial lake outburst flood (GLOF). GLOF events aresevere geomorphological hazards and their floodwaterscan wreak havoc on all human structures located in theirpath. Much of the damage created during GLOF events isassociated with the large amount of debris thataccompanies the floodwaters. Damage to settlements andfarmland can take place at very great distances from theoutburst source. In Nepal, GLOFs have caused extensivedamage to major infrastructure like roads, bridges,trekking trials, and villages, as well as incurring loss ofhuman life (WECS 1996). The government hasundertaken some mitigation steps to minimise the riskfrom one lake by establishing a telemetric early warningsystem in Tsho Rolpa and lower areas that could beaffected. The open canal constructed to lower the lakelevel of Tsho Rolpa Glacial Lake has been operating sinceJune 2000.Source: Mool et al. (2001)soil loss from the Churia hills takes place during thepre-monsoon season when there is less vegetativecover. About 60–80% of all annual soil loss occursduring the pre-monsoon season (DSCWM 1999).The mountain terrain is rugged andcharacterized by unstable, steep slopes, making itvulnerable to exogenous factors. The torrentialmonsoon rainfall that occurs within a short span oftime is an important cause of soil erosion. Differentforms of mass wasting such as landslides, slumps,rock falls, and river cuttings also contribute tosedimentation in the valleys, plains, and river basinsand cause degradation of soil fertility.The Himalayas (covering 15% of Nepal’s area)are also susceptible to land degradation from glaciallake outburst floods (GLOF). Since the second half ofthe 20th century, the high mountains have beenexperiencing melting of their large glaciers, resultingin the formation of a large number of glacial lakesbehind the unstable “dams” formed from the nowexposed end moraines. A slight disturbance canbreak the balance of the dam, resulting in an abruptrelease of a great amount of water and generatingfloods that can cause serious damage toinfrastructure, houses, and the environmentdownstream. A recent study identified 27 potentiallydangerous lakes in the Nepal Himalayas, and foundthat GLOF events had occurred in 10 of them in pastyears (Mool et al. 2001). Nepal experienced its mostcatastrophic glacial lake outburst from the Dig Tshoin 1985.Anthropogenic activity is equally significant forland degradation. Deforestation, overgrazing,inappropriate use of agro-chemicals, productionintensification, shifting cultivation with a shortenedcycle, development work, maldistribution oflandholdings, industrial waste, and others are allimportant causes of land degradation.34 Environment Assessment of Nepal : Emerging Issues and Challenges