Abstract
Urban watershed is a discrete and complex system where a diverse number of factors govern its quality and health. Soil erosion by water is the most dominant factor that determines a watershed quality, and considered as one of the most significant forms of land degradation that affects sustained productivity of land use. The principal aim of this paper is to utilise spatial-based soil erosion information to assess land suitability at a watershed level. The specific aim is three-fold: (i) to develop techniques based on a GIS in the parameterisation of a soil erosion model, which is designed for use at a large scale assessment; (ii) to assess and map the spatial distribution of average annual rate of soil losses in; (iii) to employ such related concept as soil loss tolerance to determine land suitability at a watershed level. An analytical procedure is used to analyse an urban watershed of Tallo River, in South Sulawesi, Indonesia, with a total area of 43,422 ha. The procedure is executed using RUSLE (Revised Universal Soil Loss Equation), in a GIS environment, utilising available information in the region (including climate, soil, slope, and land use and land conservation practices), and with the assistance of ground surveys. The results indicate that around 56.5% of the area experience annual soil loos of less than 1 ton/ha/year, while erosion rate of more than 25 ton/ha/year covers a total area of 8.9%. Due to a good ground cover in forested land, most of the slopping areas have actual soil losses of 1-5 ton/ha/year. This study reveals that areas categorized as high risk, where only forest cover allowed consist of 9.4%, and those with very low risk cover a total area of 5.4%. Most of the study region (around 84%) experience moderate and low erosion risk, and suitable for cropping with special management practices (CS) + perennial crops (PC) + grass (GR) + and forest (FR). This study suggests that the outputs of this modeling procedure can be used for the identification of land management units based on degradation levels, as well as the most suitable land use to be practiced on individual land units on a sustainable basis.
Highlights
Urban watershed is a complex system where many different land use practices take place
The result of this study shows that the Tallo River Watershed at the present time generally undergoes a low erosion rate, as more than 80% of the area tested have a soil loss rate of less than 5 ton/ha/year
The results of land suitability analysis presented here indicate the safest types of land use/land cover that should occur on every cell of the area of interest, and serve as a guide to land managers for selecting alternative conservation practices suited to the land use management conditions
Summary
Urban watershed is a complex system where many different land use practices take place. A long time practice of a particular land use type will give a different effect on, one to another, urban land degradation, especially erosion and sedimentation. As noted by Boyd et al (1993) an urban area is basically made up of impervious, pervious, and semi-impervious surface. It can be categorised into three types: (i) impervious areas which are directly connected to the drainage system; (ii) additional impervious areas which are not directly connected, runoff from which flows over pervious surfaces before reaching the drainage system; (iii) the remainder, pervious or semi-pervious area consisting of lawns, gardens and parklands, forested land, agriculture area, etc. The location of such land use and facilities will determine the rate of soil erosion and sedimentation (see von der Thannen et al, 2012)
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