Doctoral Degrees (Soil, Crop and Climate Sciences)

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Browsing Doctoral Degrees (Soil, Crop and Climate Sciences) by Subject "Arid regions -- Sudan -- Butana"

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    Causes and impact of desertification in the Butana Area of Sudan
    (University of the Free State, 2006-11) Elhag, Muna Mohamed; Walker, S.
    English: Desertification is one of the most serious environmental and socio-economic problems of our time. Desertification describes circumstances of land degradation in arid, semi-arid and dry sub-humid regions resulting from the climate variation and human activities. The fundamental goal of this thesis was to monitor the extend and severity of the land degradation and examine climate variability and change in the Butana area of northeastern Sudan. To explore the climate variability and climate change in terms of rainfall, temperature and the aridity index for the period from 1941 to 2004, the monthly and annual time series for four weather stations (El Gadaref, Halfa, Wad Medani and Shambat) across the Butana area were analysed. The trend of the rainfall at Wad Medani and Shambat shows significant decline, while that of Halfa and El Gadaref does not show a significant decrease or increase. The Cumulative Rainfall Departure (CRD) was used to detect the periods of abrupt changes in the rainfall series. A significant decrease in the annual rainfall was observed at Shambat (p = 0.00135) and Wad Medani (p = 0.0005) from 1968 to 1987, there after the rainfall amount is close to the long-term mean. In El Gadaref there was a decline in the annual rainfall from 1971 to 1974 (p = 0.35) but it was not significant, with a recovery from 1975 to 1982 to a value higher than the long-term mean, followed by another downward turn from 1983 to 1994. In Halfa there was a significant decrease (p = 0.0304) from 1982 to 1993. The trends of maximum and minimum temperature were examined for the summer (March-May), autumn (June-October) and winter (November-February) seasons for the four weather stations. At Halfa and Shambat the trend of maximum and minimum summer and winter temperature was increasing but vi not significant, while in Wad Medani there was a significant increase for summer and winter minimum temperatures. El Gadaref station showed a significant increase in maximum and minimum temperature (p = 0.00005, p = 0.00016) respectively. The miimum autumn temperature for Halfa increased significantly, while this was the case for both the minimum and maximum autumn temperature at Shambat and Wad Medani. This significant increase in temperature, associated with autumn, is partly due to dry conditions observed during the late 1960s. The relationship between 8 km2 AVHRR/NDVI and rainfall data (1981-2003) was tested in the Butana area. The relationship was strong between the peak NDVI (end of August through the beginning of September) and cumulative July/August rainfall, but weak relationships resulted when annual rainfall and cumulative NDVI were used. The Departure Average Vegetation method showed that the area had a high percentage of departure, reaching about 40% of the long-term average during the drought years and the NDVI recovered during the following year if the rainfall was above average. There were increased trends in NDVI in the study area during the period from 1992 to 2003, despite some years during this period having higher departure although that departure was less than for the period 1981-1991. To monitor the impact of human activities on land degradation it is essential to remove the effects of rainfall on vegetation cover. Using the Residual Trend Method the differences between the observed peak NDVI and the peak NDVI predicted by the rainfall was calculated for each pixel. This method identified degraded areas that exhibit negative trends in NDVI. The human impact is more clear in the northern part. Satellite imagery provides an opportunity to undertake routine natural resource monitoring for mapping land degradation over a large area such as Butana over a long time period. This facilitates efficient decision making for resource management. Five classes of land use were achieved using unsupervised classification, whereafter an image difference technique was applied for 1987-1996 and 1987-2000. This analysis showed that the bare soil and eroded land increased by 3-7% while the vegetated area decreased by 3-6%. Also when comparing the aerial photographs (1960s and 1980s) for Shareif vii Baraket, Kamlin and El Maseid with Landsat images (2000) severe degradation of the vegetation cover was visible at all the three sites. The Moving Standard Deviation Index (MSDI) is calculated by performing a 3×3 moving standard deviation window across the band 3 Landsat images (1987, 2000). MSDI proved to be a powerful indicator of landscape condition for the study area. The MSDI increased considerably from 1987 to 2000, especially for Sufeiya, Sobagh and Banat areas, which are referred to as severely degraded sites in the literature. The Bare Soil Index (BSI) supports the finding from the MSDI. The BSI for the degraded sites Sufeiya, Sobagh and Banat increased from 0-8 in 1987 to 32-40 in 2000. The image difference of the BSI ndicated that the index increased by about 14-43 over the 13 years. A Microsoft Excel macro was used to write the algorithms for a decision support tool relating the factors that trigger and propagate desertification in arid and semi-arid areas. This was named “Tashur”. Rainfall, aridity index and NDVI were used to evaluate the condition of the landscape. If these three parameters alone were not sufficient to make a decision, then soil and human activity parameters need to be consulted for more reliable decision making. This simple and concise decision support tool is expected to provide guidelines to planners and decision makers. Different ecosystems in the Butana area are subjected to various forms of site degradation. The desertification has led to sand encroachment and to accelerated development of dunes and also increased the water erosion in the northern part of the area. The area has also been subjected to a vegetation cover transformation. Pastures have deteriorated seriously in quality and quantity, but in many parts the degradation is still reversible if land use and water point sites are organized.