LAND USE/LAND COVER DYNAMICS, SOIL CHARACTERIZATION, AND GIS BASED MULTI-CRITERIA DECISION METHOD FOR LAND SUITABILITY EVALUATION IN WACHACHA WATERSHED, CENTRAL HIGHLAND OF ETHIOPIA

Abstract

Land use land cover (LU/LC) analysis, soil, climate, and land resources information, and suitability evaluation are required to make land use decisions for sustainable resources use and management. Nevertheless, such information collected at a scale that enables to make site-specific land use decisions is lacking in Ethiopia in general and Wachacha area in particular. Cognizant of this research gap, a study was conducted in Wachacha area in the Central Highland of Ethiopia to investigate the spatio-temporal LU/LC dynamics and its influences on selected soil physicochemical properties, characterize and classify major soils, and conduct GIS-based multi-criteria land suitability evaluation for rainfed production of three major crops (chickpea, wheat, and teff). Aerial photograph and multi-date Landsat data of 1957, 1972, 1986, 2000, and 2014 of Wachacha watershed, and GIS techniques were used for analysis of the LU/LC dynamics, while soil samples collected from three depths (0-20, 20-40, and 40-60 cm) of five land uses were used for investigating the impacts of land use and soil depth on selected soil properties. Morphological and physicochemical properties obtained from ten pedons opened along different landscapes of the sub-watershed were used for characterization and classification of the soils. A spatial fuzzy-MCDA method was applied for crop suitability mapping along the cultivated sub-watershed area (2,718 ha) of Wachacha, Ethiopia using expert, soils, landscape and climate attributes. The LU/LC analysis identified six LU/LC types: natural forest, plantation forest, shrub and bush, grazing, cultivated, and settlement lands. Over the last six decades, the cultivated land occupied between 77 percent (2014) and 83 percent (1957) of the 406.6 Km2 of the study area, indicating that the cultivated land area declined during the study period due to its conversion into other LU/LC types. The settlement area showed a threefold increase between 1957 (2.1 percent) and 2014 (6.1 percent) due to expansion of urban areas into the sub-watershed. Unlike the case in many areas in Ethiopia, the land covered by vegetation of different types was relatively stable over the study period due mainly to protection by State. The results of study conducted to investigate effects of land use and soil depth on selected physicochemical properties of soils revealed that most of the soil properties were significantly affected by interaction effects of land use by soil depth. In general, the status of most of the soil parameters in the cultivated land soils was found to be significantly (P< 0.05) inferior to those of the other land uses. This suggests that the current agricultural practice needs to be revisited in order to restore and maintain the OM and TN soil qualities at optimum level. The results of characterization and classification of soils indicate the existence of certain variations in morphological and physicochemical properties of soils in the study area. The organic matter, total nitrogen and available phosphorous of the soils of most horizon were below the critical level; while the clay fraction, bulk density, total porosity pH, CEC, base saturation, Fe, Mn, Zn, Cu, and B were properties above the critical limit. According to FAO-WRB Soil Classification System, the soils of the study area were classified as Pellic Vertisols (28.6 percent), Mollic Nitisols (28.1 percent), Eutric Cambisols (19.4 percent), Mollic Leptosols (12.2 percent), and Calcaric Eutric Fluvisols (11.7 percent) over the study area of 4203 ha. The GIS- xvi based multi-criteria land suitability evaluation results revealed that, according to weighted sum overlay analysis; over a total study area of 2,718 ha; 97.7 percent, 88.9 percent, and 85.7 percent were suitable for chickpea, teff, and wheat production, respectively. Nevertheless, 62 ha, 301 ha, and 389 ha of the study area are not currently suitable for chickpea, teff, and wheat cultivation, respectively. The current dominant limiting factors for the crops suitability are Available phosphorous, total nitrogen, steep slope, and flooding. Moreover, CEC of the soil foro the crop (teff) associated with organic matter, organic carbon (teff), and drainage (chickpea and wheat) were also the limiting factors. Thus, making possible improvement on soil fertility (fertilizer and compost), conservation measures (terrace and mechanical drainage), and plantation of fruit trees on steep slopes of the study area which has been dominantly under cultivation over six decades of study period; while finding alternative source of fuel wood and technology to the surrounding community is vital to ease the pressure on the natural forest.