Miro Jacob

Assessing spatio-temporal rainfall variability in a tropical mountain area Ethiopia using NOAA’s rainfall estimates

Seasonal and interannual variation in rainfall can cause massive economic loss for farmers and pastoralists, not only because of deficient total rainfall amounts but also because of long dry spells within the rainy season. The semi-arid to sub-humid mountain climate of the North Ethiopian Highlands is especially vulnerable to rainfall anomalies. In this article, spatio-temporal rainfall patterns are analysed on a regional scale in the North Ethiopian Highlands using satellite-derived rainfall estimates (RFEs). To counter the weak correlation in the dry season, only the rainy season rainfall from March till September is used, responsible for approximately 91% of the annual rainfall. Validation analysis demonstrates that the RFEs are well correlated with the meteorological station (MS) rainfall data, i.e. 85% for RFE 1.0 (1996–2000) and 80% for RFE 2.0 (2001–2006). However, discrepancies indicate that RFEs generally underestimate MS rainfall and the scatter around the trendlines indicates that the estimation by RFEs can be in gross error. A local calibration of RFE with rain gauge information is validated as a technique to improve RFEs for a regional mountainous study area. Slope gradient, slope aspect, and elevation have no added value in the calibration of the RFEs. The estimation of monthly rainfall using this calibration model improved on average by 8%. Based upon the calibration model, annual rainfall maps and an average isohyet map for the period 1996–2006 were constructed. The maps show a general northeast–southwest gradient of increasing rainfall in the study area and a sharp east–west gradient in its northern part. Slope gradient, slope aspect, elevation, easting, and northing were evaluated as explanatory factors for the spatial variability of annual rainfall in a stepwise multiple regression with the calibrated average of RFE 1.0 as dependent variable. Easting and northing are the only significant contributing variables (R2 = 0.86), of which easting has proved to be the most important factor (R2 = 0.72). The scatter around the individual trendlines of easting and northing corresponds to an increase in rainfall variability in the drier regions. Despite the remaining underestimation of rainfall in the southern part of the study area, the improved estimation of spatio-temporal rainfall variability in a mountainous region by RFEs is valuable as input to a wide range of scientific models.

Quaternary glacial and periglacial processes in the Ethiopian Highlands in relation to the current afro-alpine vegetation

Summary. The highlands of Ethiopia show a great variety in present and past climate. The environments differ in altitude, latitude and local conditions. This has an influence on vegetation and geomorphologic processes. Present knowledge of past glacial and periglacial landforms concentrates around the highest mountain ranges of Ethiopia, the Semien Mountains, the Bale Mountains and the Arsi Mountains. Many intermediate mountains stay unexplored or just briefly discussed. No present glaciated mountains exist in Ethiopia but current periglacial processes occur on the highest peaks. The climate change sensitivity of the mountain environment can be assessed which can contribute to the study of the temperature sensitive treeline and land cover changes. Afro-alpine vegetation can be influenced by the presence of relict (peri)glacial landforms, which change the growing conditions by different soil properties. Further research will need to complement the existing observations by unexplored mountains and to establish altitudinal north-south trajects in Ethiopia regarding vegetation, geomorphological processes and landforms.

Land Degradation in the Ethiopian Highlands

The high soil erosion rates in the Ethiopian highlands find their causes in the combination of erosive rains, steep slopes due to the rapid tectonic uplift during the Pliocene and Pleistocene, and human impact by deforestation, overgrazing, agricultural systems where the open field dominates, impoverishment of the farmers, and stagnation of agricultural techniques. Travelling in the Ethiopian highlands, one can see many soil and water conservation structures. Indigenous knowledge and farmers’ initiatives are integrated with these introduced technologies at various degrees. This chapter addresses the status and drivers of land degradation in northern Ethiopia, including changes over the last century.