Dagnachew Legesse

Environmental changes in a tropical lake (Lake Abiyata, Ethiopia) during recent centuries

Lake Abiyata is a small, closed, saline–alkaline lake located in the central part of the Ethiopian Rift Valley, East Africa. A multi-proxy study of a sediment core, 116 cm long and with undisturbed mud–water interface, was performed to test the sensitivity of the lake system and of different proxies to the changes in climate and human activities that occurred in the catchment during the past few centuries. The 210Pb analyses suggest that the upper 80 cm of the core represent the past 200 years. This study complements millennial-scale environmental records available in the region. The main freshwater-climatic and biological features of the modern lake system and their variations over the past decades, as known from observations, are first summarised. Results derived from individual proxies analysed along the core are then presented (successively, major physical and chemical properties of bulk sediments, diatoms and pollen). Uncertainties on the chronological framework are discussed. Major limnological stages are finally identified based on the multi-proxy interpretation of our record. Our record shows large variations in the lake water and salt balances, in the sediment sources, and in the vegetation distribution in the basin. Using our 210Pb chronology, major changes observed in the core are tentatively compared with environmental events known from instrumental and historical records. The upper 41 cm of the core (210Pb age: 1940–1998 AD) reveal several fluctuations in diatom-inferred water depth and salinity which seem to be consistent with known changes in water level. Human impact on vegetation clearly appears since about 30 years. The interval 85–41 cm suggests a period of overall water deficit. Lake Abiyata experienced episodes shallower and more saline than over the past decades, especially around 68–66 cm, 210Pb dated at ca. 1890 AD. This level may coincide with one of the worst droughts known in the Ethiopian history during 1888–1892. The lower part of the core includes a stage (108–85 cm) of lake level much higher than today and which ended before 1800 AD. Although its base is undated so far, this stage suggests that conditions much wetter than today have prevailed in the region during at least part of the 18th century. Lake Abiyata appears to be a suitable site for a detailed environmental reconstruction over the recent past, although further work is needed to reduce the uncertainties on our record, as discussed in the conclusions.

Flood Hazard and Risk Assessment Using GIS and Remote Sensing in Fogera Woreda, Northwest Ethiopia

Flood is a natural disaster. However human activities in many circumstances change flood behavior. The objective of this study was to assess flood hazard and risk of Fogera woreda (district), which is one of the most severely flood affected areas in Ethiopia in general and Ribb–Gumara Catchment in particular, using Geographic information system (GIS) and remote sensing techniques. Land use/land cover change detection was done for the catchment using the 1973, 1985 and 1999 Landsat images and the general trend showed that vegetative and grassland areas were mainly changed to agricultural lands. Comparison between long year (1974–2006) annual maximum daily rainfall and annual maximum daily gauge levels (1971–2005) data of Ribb and Gumara Rivers showed that rainfall slightly decreases while gauge level increases, and this can be attributed to land cover conversion especially in the upper catchment. Flood frequency analysis was done using Ribb and Gumara Rivers annual maximum daily gauge levels by Gumbel’s, and the likely flood levels in different return periods were found. Digital elevation models (DEM) and the 100 year return period base-flood were combined in the GIS environment in order to produce flood inundation maps. More over, flood causative factors were developed in the GIS and remote sensing environment and weighted and overlaid in the principle of pair-wise comparison and Multicriteria Evaluation (MCE) technique in order to arrive at flood hazard and risk mapping. The major findings of the study from both the two methods revealed that most of the areas in the downstream part of the catchment and the different land uses in these areas were within high to very high flood hazard and risk level. The presence of risk assessment mapping will help the concerned authorities to formulate their development strategies according to the available risk to the area.