Environmental impacts of some scenarios of land-use Change: a gis assisted assessment of soil erosion hazard in The Mutirithia-Kariunga area of Laikipia District, Kenya.
Characterizing soil erosion hazard and its spatial variability is critical for maintaining user confidence in planning soil- and water- conservation programmes and general land-use management. Predicting the average rates of soil erosion for a combination of specific soil and land-use types is vital. This is because such predictions form a basis for providing guidelines for effective erosion control. The purpose of this investigation was to examine the erosion effects of alternative land-use practices in the Mutirithia-Kariunga area of Laikipia District, Kenya. Four different resource management systems or land-use scenarios were investigated. These were the base scenario representing the current state of land use in the study area, the maize scenario which represented an annual crop production situation, the maize-management scenario representing land use in the form of annual maize cropping in combination with a mulching management option and finally a perennial cropping scenario. The effect of each land-use scenario on annualised erosion rates was then examined through simulation modelling on all fields of the study area. The study then evaluated the effects of the latter three land-use scenarios i.e. the maize scenario, maize-management scenario and the perennial scenario against each other and against the current land use; the base scenario. The Universal Soil Loss Equation of Wischmeier and Smith (1978) was employed to estimate annual erosion rates. The Arc Info & IDRISI Geographical Information Systems (GIS) assisted with the determination and mapping of the spatial distribution of impacts due to soil erosion from the various land-use scenarios. Soil loss was then calculated for the study area in cells or pixels with a true ground-size of 50 meters by 50 meters. Results showed that the optimal resource management scenario which minimized net annual erosion rates and maximized returns to land, labour and management consisted of a scenario maintaining a perennial crop cover, i.e., the perennial scenario. This , land-use scenario decreased total area erosion of regions experiencing an annualised soil loss of over 10 tlha yr', by 145 times relative to a representative annual crop scenario - the maize scenario. A soil loss tolerance of 10 tlha yr', is the standard established for conservation compliance though this value may vary from place to place depending on the depth of different soils. The perennial scenario also decreased total area erosion by 3 times in comparison with a representative x management modified annual crop scenario, the maize - management scenario and 13 times relative to the base scenario. The worst-case scenario was the maize scenario in which soil loss of over 10 tlha yr' was experienced on over 70% of the study area. Apart from a standard flat tillage and a previous incorporation of crop residue treatment for all the cases, this maizescenario was without extra management. On introducing land management in form of residue mulch at a rate of 2 tonslha to this maize scenario, the resultant maize - management scenario experienced a substantial reduction in areal extent with respect to places experiencing soil loss of over 10 tlha yr', Such areas reduced in extent to cover only 3% of the study area - a reduction of over 65%. This improvement is also evidenced by a comparative "mere" 3 times that the soil loss in the maizemanagement scenario was over the best-case perennial scenario. The planting of a low bushy perennial crop reduced the annual soil loss from over 97% of the study area to less than 5 tlha yr', Furthermore, only three percent of the study area experienced soil losses above the five tonnes per hectare value in this perennial scenario. Vital implications for achieving and maintaining sound soil management practices arise from the results of this study. The conclusions drawn from the study results indicate that land-use with greatest negative impact on the study area with respect to soil erosion is annual cropping with no soil conservation. However, investment in land management techniques such as mulching substantially reduced the soil loss from cropping land. A vegetative cover of perennials on the other hand also greatly reduced erosion. The reduction of soil erosion rate therefore absolutely requires improved management on both perennial and annual croplands. Erosion will only be eventually reduced through improved levels of technology and increased technical knowledge of the farmers. It is recommended that planners move in rapidly to prioritise erosion protection measures on those areas that have been identified to suffer extremely higher erosion and take remedial action before they are rendered completely useless. Specifically this would entail, improved land use management, strengthening of the Ministry of Agriculture extension activities, revision of district wide development plans policies to reflect current and anticipated land use trends and concerns of study area. This should be followed by educating the farmers of this area on appropriate land management techniques using ongoing participatory planning Xl processes and developing public education and land use advocacy programmes, each prescription being area specific due to differential rates of susceptibility to erosion. On the other hand, the use of GIS tools to characterise annual soil loss potentially offers great promise to expand quantitative databases. These tools provide capabilities to provide vast information and knowledge to resource planners and land managers in concise and informative map formats. The combined modelling and GIS approaches allowed for rapid assessment of erosion hazard of the entire study area with minimal data. This methodology was also flexible enough to rapidly simulate different scenarios of land use, a feat that would take many years to achieve under fieldwork conditions. The results of the study do not necessarily reflect the exact amount of erosion that is occurring at specific sites of the study area but it does provide planners with a guide showing areas that would require most attention from a land management compliance standpoint. Much more confidence in the results would only be achieved if consistent ground-truthing of the study results is undertaken. It is recommended to scholars that validation of the model to this specific area is necessary and would go a long way in developing models that can be used to confidently and rapidly predict long-term soil loss relevant to this region. It is specifically recommended that further research be conducted within an expanded realm to encompass whole watersheds which will allow an understanding of region level dynamics, that further spatial inventories of land and ecological resources be undertaken and that much more accurate prediction models be built for this region and be integrated with GIS.