assessment of soil erosion using usle model and gis for integrated ...

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the sea (Eyles, 1987), bringing very abundant sedimentary contributions that induce changesin the profile and degrade coastal reefs fringing (Dumas, 2004). To assess the sedimentdeposition into the lagoon, it is necessary to characterize the erosion process on thewatersheds and to highlight the areas most affected by erosion.Many models for soil erosion loss estimation have been developed (Nearing et al., 1989,Bathurst and O.Connell, 1992, Arnorld and Williams, 1995, Adinarayana et al., 1999, Neistchet al., 2002, Shen et al., 2003). However most of these models often apply to geographicallylimited areas and require many complex field measurements which are not available in thePacific countries. These types of models are thus unsuitable in the undeveloped countries likemost Pacific island countries where financial supports in research are limited. For thesereasons we chose to apply the Universal Soil Loss Equation (USLE), a deliberately simplisticmodel. USLE, one of the most widely use throughout the world, is an empirical quantitativemodel designed for the evaluation of the annual soil loss rates on a long-term basis. With thismodel, one of the specific objectives is to create risks maps, identified, located and prioritizedthe watersheds most pollutant. This type of document could be useful particularly fordecisionmakers. It’s one of the objective of the GERSA project (a French acronym for «Watershed and Coastal Reef Zones Integrated Management: from Satellite to Stakeholders »)implemented in the South Pacific region particularly on the west coast of New Caledonia, onthe North western coast of Viti Levu (in Fiji), in Moreea and Tahiti in French Polynesia andin the island of Efate in Vanuatu (Dumas and Fossey, 2009). This program aims at creatingtools designed to better understand the interactions between watersheds and coastal reef zonesin order to optimize coastal zones and protected marine areas management. The project is ledby the French Institute for Development (IRD) and the University of New-Caledonia, is thewatershed component of the CRISP program (Coral Reefs Initiatives for the Pacific).Thiscommunication presents some results about the application of the USLE equation on theNorth western coast of New Caledonia.II/ STUDY AREANew Caledonia is a French territory located in the subregion of Melanesia in the southwestPacific, at 1200 kilometers at east of Australia. The population was estimated in 2009 to be245 000. It comprises a main island (“Grande Terre”), the Loyalty Islands, and severalsmaller islands. Approximately half the size of Taiwan, it has a land area of 18 575 squarekilometers. The “Grande Terre”, with 450 kilometers in length and 50 to 70 kilometers wide,is the largest and the only mountainous island (16 300 square kilometers). From North toSouth a mountain range, the “Central Chain”, runs the length of the island, with five peaksover 1500 meters. New Caledonia lieing astride the Tropic of Capricorn, between 19° and 23°south latitude, has a tropical climate. During the warm season (mid-November to mid-April),frequent tropical depressions and cyclones produce heavy rainfall which is the main drivingfactor for soil erosion. In the main island many areas are made up of deeply weatheredgeological materials as very erodible limonite layers (laterites: highly vulnerable to erosionwhen stripped). These soils are therefore the first target of erosion in a cyclonic regime ofprecipitations. Particularly, the effect of runoff increases when degradations of protectiveplant cover appear after bush fires or nickel openpit mining. All of these factors constitutecatalysts to the erosion process. Runoff mobilized soils particles from high terranes passquickly through creeks and rivers, stay in the cultural plain when inundations occur and stoptheir way in the closed lagoon after just fewer 20 to 30 kilometers since soil removal. Thencoral reef and lagoon life is disrupting by sporadic episodes of hypersedimentation. All2
human activities chain that depend on natural resources are regularly affected by soil erosionand consequences of this phenomenon. It’s also important to note that UNESCO listed NewCaledonia Barrier Reef on the World Heritage List under the name The Lagoons of NewCaledonia: Reef Diversity and Associated Ecosystems on July 2008.The study area is located in the north of New Caledonia on west coast. This region includesthe communes of Koné, Pouembout and Voh in the North Province (Fig. 1) and 23watersheds, covering an area of 1 350 square kilometres. The study area presents a dichotomybetween a coastal plain and a mountain zone and hills covering three quarters of the region. Interms of pressures that exists in New Caledonia, this region is representative with naturalpressures due to the amount and the intensity of rainfall and the steepness of slopes and withhuman pressures to bushfires, agriculture, land-clearing by fire and mining activities. Indeed,there are several old mines (closed before 1975) in this area, not revegetated, that are stillcontributing to soil losses, as well as a new mining project on the Koniambo Mountain witchwill be the largest nickel mines in the world.Fig. 1: Location, digital elevation model and map of the study area (communes of Koné, Pouembout and Voh)III/ MATERIALS AND METHODSThe Universal Soil Loss Equation, later revised as the RUSLE (Renard et al., 1997), wasdeveloped by Wischmeier and Smith (1978). It’s a mathematical and an empirical model usedto describe soil erosion processes and based on many factors such as soil, topography,climate, land cover and human activities. The USLE predicts potential erosion and is widelyused in the world. Potential erosion, representing by soil losses, has units of weight per unitarea per year (t/ha/yr). This equation is the product of six factors:A = R x K x LX S x C x PWhere A is the average soil loss due to water erosion (t/ha/yr), R the rainfall-runoff erosivityfactor; K the soil erodibility factor, L, the slope length, S the slope steepness, C the land covermanagement factor and P the soil conservation practice factor. This model is derived frommore than 10,000 plot-years of data collected on natural runoff plots and an estimatedequivalent of 2,000 plot-years of data from rainfall simulators. Numerical values for each ofthe six factors of the equation were derived from analyses of the assembled research data andfrom National Weather Service precipitation records in USA. USLE only predicts the amountof soil loss that results from sheet or rill erosion on a single slope and does not account foradditional soil losses that might occur from gully, wind or tillage erosion. Although originallydeveloped for agricultural purpose, its use has been extended to watershed with other landuses.3
Page 1: ASSESSMENT OF SOIL EROSION USING US
Page 5 and 6: Fig.2: Localisation of the weather
Page 7 and 8: under the GIS software. The LS fact
Page 9 and 10: Fig. 9: Potential soil loss erosion

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