Subwatershed Prioritization for Watershed Management
using Remote Sensing and GIS
Bhuwneshwar Prasad. Sah Kiyoshi Honda & Shunji Murai
Star/Serd, Asian Institute of Technology,
P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand,
Tel. : +66-2-524-6148, Fax.: +66-2-524-6147
E-mail : sah@aith.ac.th,
Honda@ait.ac.th,
murai@ait.ach.th
Abstract
Watershed degradation is the global problem, which is more serious in developing countries like Nepal. As Nepal is mountainous country, a large number of watersheds can be delineated with different aerial coverage and most of them are in degraded condition. While considering the watershed conservation work, it is not feasible to take whole area at once. This calls to divide the watershed in small units, that is subwatershed, by considering its drainage system. As the condition of subwatersheds may not similar, they can be prioritized for conservation work. In this study, subwatersheds of of Trijuga river watershed in eastern Nepal were prioritized by considering their degradation condition and land sensitivity. Here land sensitivity is defined as the locational relationship between forest loss and soil loss. Universal soil Loss Equation in conjunction with Remote Sensing and GIS had been utilized for estimating soil loss and land cover change. Two time series data, that area aerial photographs of year 1978 and Landsat TM data, that of year 1991, were used to make the analysis of soil loss and land cover change. The rate of forest degradation of the study area was 0.57 per cent per year, which is too high while considering the sustainability, along with 0.42 mm/yr increment in soil erosion rate between 1978 to 1991. The new concept of conservation prioritization has been proposed by considering subwatershed degradation speed, sensitivity index and present rate of soil erosion. Based on conservation prioritization, subwatershed conservation activities proposed.
1.Introduction
watershed deterioration is the commen phenomena in most parts of the word . among several causes for this , improper and unwise utilization of watershed resources without any conservation work is the prime one which on which is more severe in developing countries (FAO, 1985). The watersheds of Nepal are good example of such deterioration (DSCWM, 1983). The deterioration occurs generally in terms of forest loss and land degradation by soil erosion. Among several factors, the major one is deforestation followed by unsuitable agricultural practices. Therefore, it is necessary to develop a sustainable land management system that does not cause the degradation of such valuable resources. This is especially important in a predominantly mountainour country like Nepal (Thapa & Weber, 1990). For this, effective monitoring of land cover/land cover change is the basic and essential step. Further analysis, like land degradation, subwatershed prioritization etc., has to be done so that protective measures can be better planned and implemented.
Remote sensing (RS) is very useful in assessing the land cover of an area at a particular time and monitoring the change over a given period (Lillesand & Kiefer, 1987). Furthermore, the land cover/land cover being spatial in nature, Geographic Information System (GIS) can be employed as a powerful tool in such monitoring. Besides, with the creation of digital terrain model, it is possible to make digital representation of the topography of the area, which is very useful in making soil erosion and other analysis (Burrough, 1986; Aronoff, 1991). Under this context, the present study was carried out with the following objectives:
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To estimating the forest and soil loss of the watershed,
- To prioritize the subwatershed based on its present condition, extent of degradation and sensitivity and
- To recommend the subwatershed conservation activities.
2. The Study Area
Geopraphically the study area lies between 26o 42' and 26o 59' N latitude and 86o 33' 46" and 86o 59' 48" E longitude in the Eastern Region of Nepal (Map 1). There are several river and rivulets in the watershed, which drain in the main river, called as Trijuga River. The name of watershed, that is Trijuga watershed, is derived form this river. Total area of the watershed is 732 sq. km and altitude varies from 75 m to 1942 m. The tropical climate of the low-lying valley gradually passes into the sub-tropical towards higher elevation; north. The average annual temperature 20oC (WEC, 1942) with 1942 mm rainfall. More than 70 percent rainfall is concentrated from May to October. The forest cover is
nearly 58 percent which is dominated by tropical Sal (shorea robusta) forest, followed by 24 percent area of agriculture. Agriculture along with livestock is the important source of income and livelihood of the people.
Map 1. Location Map of the Trijuga Watershed, Nepal
3 Methodology
An integrated approach of digital image processing of satellite data and visual interpretation of aerial photograph combined with GIS and USLE was carried out for land cover change and soil loss estimation.
3.1. Collection of Data
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Physiographic survey: 35 soil samples were taken for soil texture and permeability analysis.
- Satellite data: Landsat TM of path/row 140/041, acquired 21 Dec. 1991.
- Aerial photograph: Scale 1:50,000, Nov., 1978
- Reconnaissance maps: 1:50,000, 1982, Land System map: 1:50,000, 1984, and Political map: 1:250,000, 1987.
