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Disasters
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Soil Erosion Mapping with Universal Soil Loss equation and GIS
C.Mongkolsawat, P. Thirangoon,S. Sriwongsa
Computer Centre, Kho Kaen University, Khon Kaen 4002, Thailand
Abstract
The purpose of this study is to establish spatial information of soil erosion using the Universal Soil Loss Equation (USLE) and GIS. The study area, Huai Sua Ten watersheds, covers an area of approximately 41 thousand hectares and located in Khon Kean and Udon Thani provinces, Northeast Thailand. Landscape of the study area is gently undulating with sparse dipterocarp tress and isolated patches of forest remnants.
A set of factors as identified in the USLE were studied and reviewed. These include rainfall erosivity factor (R-factor), soil erodibility factor (K-factor), slope and slope length factor (LS-factor), and vegetative cover factor (C-factor) and conservation practice factor (P-factor). Each factor which consists of a set of logically related geographic features and attributes is used as data input for analysis. The factor layers were collected from existing information and extracted from Landsat MT imagery.
Analysis of daily rainfall data of past 11 years gives the R-factor. Spatial K-factor was formulated from detailed reconnaissance soil ma of the Land Development Department (LDD) of Thailand. Digital elevation model (DEM), interpolated from elevation contours, was employed to generate the slope and LS-factor. Spatial vegetative cover, extracted from Landsat TM imagery, was used to determine the spatial C-factor and consequently P-factor, values of which are based on experimental results.
Each of the above mentioned USLE factors, with associated attribute data, was digitally encoded in a GIS database to eventually create five thematic layers. Simultaneous overlay operation on these five layers produces a resultant polygonal layer, each polygon of which is a homogeneous area with respect to each of the five factors.
USLE model calculation applied to the resultant polygonal layer gave values of soil loss in tons/ha/year. These are then aggregated into 8 classes of 0-10, 10-20, 20-30, 30-40, 40-50, 50-100, 100-150 and > 150 tons/ha/year. The study indicates that low erosion classes are located in areas of land is used for field where slope gradient is very low. High erosion classes concentrate in areas where land is used for field crops with no conservation practice.
To evaluate its reliability, the resultant soil erosion map, produced using GIS, was checked against existing soil erosion maps and data of field surveys. It was found to be satisfactory. This study confirms that the use of GIC and remotely sensed data can greatly enhance spatial modelling of soil erosion.
1. Introduction
For the past two decades, encroachment of agricultural activities on forest reserves has become an acute problem in Thailand. Not only does encroachment account for forest loss, but soil fertility depletion as well. Official report shows an annual encroachment rate of 4564 sq.kms. for the period 1961-1991. (Charuppat, 1992). This corresponds to a 53 percent reduction of forestland for the same period. As a result, the 1991 statistics shows only 26 percent of the country is forestland (Charuppat, 1992). Clearing of forests has had serious consequences on surface water hydrology and accelerated the processes of soil erosion, particularly on steeply sloping land. According to the Land Development Department (LDD), about 17 million sq. kms., or one third of the country, is subject to moderate to severe erosion hazards. (Omakupt, 1991).
For years, LDD has been using the Universal Soil Loss Equation (USLE) to predict soil losses based only on field experiments on a number of landaus patterns. No information on spatially distributed soil erosion rate is provided.
It was only recently that an effort to map soil erosion distribution in the Northeast of Thailand was introduced, with the establishment of a manual method by LDD (Wichaidit et al 1992). Given the limited capacity of the manual method, there is a growing need to systematically map soil erosion, using GIS and related technologies for speed and accuracy.
The objectives of this study are:-
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to establish GIS method for spatial of soil erosion, based on the USLE.
- to evaluate the utility of GIS with regard to soil erosion mapping.
2. The Study Area
The study area is the Huai Sua Ten watersheds, as area of about 41,080 ha. It is located in the Northeast of Thailand, covering part of Khon Kaen and Udon Thani provinces.
Geologically, the area is underlain by a thick sequence of Mesozoic rock of the Maha Sarakam Formation. This formation was deeply weathered in the Tertiary period. It contains sandstone's, silstones and sandy shells with interbeded evaporites. The landscape is gently undulating with sparse trees, and is of about 180-200 m. above mean sea level. Annual rainfall averages 1200 m.m.
Rice is grown in the depression and field crops, mainly cassava and sugar cane, are restricted to the upland. Isolated forest remnants typify the remaining forests in the area. Soil is inherently low in fertility and have sandy texture, with low cation exchange capacity.
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