Ayele Almaw Fenta

Comprehensive assessment of soil erosion risk for better land use planning in river basins : case study of the Upper Blue Nile River

In the drought-prone Upper Blue Nile River (UBNR) basin of Ethiopia, soil erosion by water results in significant consequences that also affect downstream countries. However, there have been limited comprehensive studies of this and other basins with diverse agroecologies. We analyzed the variability of gross soil loss and sediment yield rates under present and expected future conditions using a newly devised methodological framework. The results showed that the basin generates an average soil loss rate of 27.5tha-1yr-1 and a gross soil loss of ca. 473Mtyr-1, of which, at least 10% comes from gully erosion and 26.7% leaves Ethiopia. In a factor analysis, variation in agroecology (average factor score=1.32) and slope (1.28) were the two factors most responsible for this high spatial variability. About 39% of the basin area is experiencing severe to very severe (>30tha-1yr-1) soil erosion risk, which is strongly linked to population density. Severe or very severe soil erosion affects the largest proportion of land in three subbasins of the UBNR basin: Blue Nile 4 (53.9%), Blue Nile 3 (45.1%), and Jema Shet (42.5%). If appropriate soil and water conservation practices targeted ca. 77.3% of the area with moderate to severe erosion (>15tha-1yr-1), the total soil loss from the basin could be reduced by ca. 52%. Our methodological framework identified the potential risk for soil erosion in large-scale zones, and with a more sophisticated model and input data of higher spatial and temporal resolution, results could be specified locally within these risk zones. Accurate assessment of soil erosion in the UBNR basin would support sustainable use of the basins land resources and possibly open up prospects for cooperation in the Eastern Nile region.

Diurnal rainfall variability over the Upper Blue Nile Basin: a remote sensing based approach

In this study we aim to assess the diurnal cycle of rainfall across the Upper Blue Nile (UBN) basin using satellite observations from Tropical Rainfall Measuring Mission (TRMM). Seven years (2002-2008) of Precipitation Radar (PR) and TRMM Microwave Imager (TMI) data are used and analyses are based on GIS operations and simple statistical techniques. Observations from PR and TMI reveal that over most parts of the basin area, the rainfall occurrence and conditional mean rain rate are highest between midand late-afternoon (15:00-18:00 LST). Exceptions to this are the south-west and south-eastern parts of the basin area and the Lake Tana basin where midnight and early morning maxima are observed. Along the Blue Nile River gorge the rainfall occurrence and the conditional mean rain rate are highest during the night (20:00-23:00 LST). Orographic effects by large scale variation of topography, elevation and the presence of the UBN river gorge were assessed taking two transects across the basin. Along transects from north to south and from east to west results indicate increased rainfall with increase of elevation whereas areas on the windward side of the high mountain ranges receive higher amount of rainfall than areas on the leeward side. As such, mountain ranges and elevation affect the rainfall distribution resulting in rain shadow effect in the north-eastern parts of Choke-mountain and the ridges in the north-east of the basin. Moreover, a direct relation between rainfall occurrence and elevation is observed specifically for 17:00-18:00 LST. Further, results indicate that the rainfall distribution in the deeply incised and wide river gorge is affected with relatively low rainfall occurrence and low mean rainfall rates in the gorge areas. Seasonal mean rainfall depth is highest in the south-west area and central highlands of the basin while areas in the north, north-east and along the Blue Nile gorge receive the least amount of rainfall. Statistical results of this work show that the diurnal cycle of rainfall occurrence from TRMM estimates show significant correlation with the ground observations at 95% confidence level. In the UBN basin, the PR conditional mean rain rate estimates are closer to the ground observations than the TMI. Analysis on mean wet season rainfall amount indicates that PR generally underestimates and TMI overestimates the ground observed rainfall.

Dynamics of Soil Erosion as Influenced by Watershed Management Practices: A Case Study of the Agula Watershed in the Semi-Arid Highlands of Northern Ethiopia.

Since the past two decades, watershed management practices such as construction of stone bunds and establishment of exclosures have been widely implemented in the semi-arid highlands of northern Ethiopia to curb land degradation by soil erosion. This study assessed changes in soil erosion for the years 1990, 2000 and 2012 as a result of such watershed management practices in Agula watershed using the Revised Universal Soil Loss Equation. The Revised Universal Soil Loss Equation factors were computed in a geographic information system for 30 × 30 m raster layers using spatial data obtained from different sources. The results revealed significant reduction in soil loss rates by about 55 % from about 28 to 12 t ha−1 per year in 1990–2000 and an overall 64 % reduction from 28 to 10 t ha−1 per year in 1990–2012. This change in soil loss is attributed to improvement in surface cover and stone bund practices, which resulted in the decrease in mean C and P-factors, respectively, by about 19 % and 34 % in 1990–2000 and an overall decrease in C-factor by 29 % in 1990–2012. Considerable reductions in soil loss were observed from bare land (89 %), followed by cultivated land (56 %) and shrub land (49 %). Furthermore, the reduction in soil loss was more pronounced in steeper slopes where very steep slope and steep slope classes experienced over 70 % reduction. Validation of soil erosion estimations using field observed points showed an overall accuracy of 69 %, which is fairly satisfactory. This study demonstrated the potential of watershed management efforts to bring remarkable restoration of degraded semi-arid lands that could serve as a basis for sustainable planning of future developments of areas experiencing severe land degradation due to water erosion.

Response of streamflow to climate variability and changes in human activities in the semiarid highlands of northern Ethiopia

Climate variability and human activities are two major drivers influencing changes in streamflow response of a watershed, and thus assessing their relative effect is essential for developing sustainable water resources planning and management strategies at watershed-scale. In this study, a runoff model driven by rainfall and potential evapotranspiration was established to estimate the effect of climate variability on the changes in annual streamflow of Agula watershed in northern Ethiopia. Significant decreasing trends were observed for annual and wet season streamflow between 1992 and 2012, while dry season streamflow showed an increasing trend. Analyses of seasonal and annual rainfall records showed no significant trends. The change-point test revealed that an abrupt change in annual streamflow occurred in 2000. In the period 2000–2012, the mean annual and wet season streamflow decreased by 36 and 49%, respectively compared with 1992–1999, while dry season streamflow increased by 57%. Climate variability was estimated to account for 22% of the total reduction in mean annual streamflow, whereas human activities (e.g., proper watershed management practices and associated changes in land use/land cover among other factors) were responsible for 78%; indicating that human activities were the major drivers of changes in the streamflow response. The results of this study point to the potential that reduced wet season flow and improved dry season water availability can be achieved by proper planning and implementation of appropriate watershed management practices.

The dynamics of urban expansion and land use/land cover changes using remote sensing and spatial metrics: the case of Mekelle City of northern Ethiopia

ABSTRACT Information on the rate and pattern of urban expansion is required by urban planners to devise proper urban planning and management policy directions. This study evaluated the dynamics and spatial pattern of Mekelle City’s expansion in the past three decades (1984–2014). Multi-temporal Landsat images and Maximum Likelihood Classifier were used to produce decadal land use/land cover (LULC) maps. Changes in LULC and spatial pattern of urban expansion were analysed by post-classification change detection and spatial metrics, respectively. The results showed that in the periods 1984–1994, 1994–2004, and 2004–2014, the built-up area increased annually by 10%, 9%, and 8%, respectively; with an average annual increment of 19% (100 ha year−1), from 531 ha in 1984 to 3524 ha in 2014. Between 1984 and 2014, about 88% of the gain in built-up area was from conversion of agricultural lands, which decreased by 39%. Extension of existing urban areas was the dominant growth type, which accounted for 54%, 75%, and 81% of the total new development during 1984–1994, 1994–2004, and 2004–2014, respectively. The spatial metrics analyses revealed urban sprawl, with increased heterogeneity and gradual dispersion in the outskirts of the city. The per capita land consumption rate (ha per person) increased from 0.009 in 1984 to 0.014 in 2014, indicating low density urban growth. Based on the prediction result, the current (2014) built-up area will double by 2035, and this is likely to have multiple socioeconomic and environmental consequences unless sustainable urban planning and development policies are devised.

Spatial distribution and temporal trends of rainfall and erosivity in the Eastern Africa region

Soil erosion by water is one of the main environmental concerns in the drought-prone Eastern Africa region. Understanding factors such as rainfall and erosivity is therefore of utmost importance for soil erosion risk assessment and soil and water conservation planning. In this study, we evaluated the spatial distribution and temporal trends of rainfall and erosivity for the Eastern Africa region during the period 1981–2016. The Precipitation Concentration Index (PCI), Seasonality Index (SI) and Modified Fournier Index (MFI) have been analysed using 5×5 km resolution multi-source rainfall product (Climate Hazards Group InfraRed Precipitation with Stations, CHIRPS). The mean annual rainfall of the region was 810 mm ranging from less than 300 mm in the lowland areas to over 1200 mm in the highlands being influenced by orography of the Eastern Africa region. The PCI and SI revealed a spatial pattern of rainfall seasonality dependent on latitude, with a more pronounced seasonality as we go far from the equator. The MFI showed high spatial variability with about 55% of the region subject to high to very high rainfall erosivity. The mean annual R-factor in the study region was calculated at 3246 ±1895 MJ mm ha-1 h-1 yr-1, implying a potentially high water erosion risk in the region. Moreover, increasing and decreasing trends of annual rainfall and erosivity were observed, but with substantial spatial variability for both. This study offers useful information for better soil erosion prediction as well as can support policy development to achieve sustainable regional environmental planning and management of soil and water resources.

Satellite rainfall products and their reliability in the Blue Nile Basin

In the Upper Blue Nile (UBN) basin, there is very sparse and uneven distribution of ground-based meteorological stations which constrain assessments on rainfall distributions and representation. To assess the diurnal cycle of rainfall across the UBN basin, satellite observations from Tropical Rainfall Measuring Mission (TRMM) were used in this study. Data of 7 years (2002–2008) of Precipitation Radar (PR) and TRMM Microwave Imager (TMI) were processed, with analyses based on geographic information system (GIS) operations, statistical techniques, and harmonic analysis. Diurnal cycle patterns of rainfall occurrence and rain rate from three in-situ weather stations are well represented by the satellite observations. Harmonic analysis depicts large differences in the mean of the diurnal cycle, amplitude, and time of the amplitude across the study area. Diurnal cycle of rainfall occurrence has a single peak in Lake Tana, Gilgel Abbay, and Jemma subbasins and double peaks in Belles, Dabus, and Muger subbasins. Maximum rain rate occurs in the morning (Gilgel Abbay, Dabus, and Jemma), afternoon (Belles, Beshilo, and Muger), and evening (Lake Tana and along the river gorges). Results of this study indicate that satellite observations provide an alternative source of data to characterize diurnal cycle of rainfall in data-scarce regions. We noticed, however, that there are a number of constraints to the use of satellite observations. For more accurate assessments, satellite products require validation by a network of well-distributed ground stations. Also, we advocate bias correction.

Changes in future climate indices using Statistical Downscaling Model in the upper Baro basin of Ethiopia

Climate indices are the diagnostic tools used to define the state of climate system. The main objective of this study was to investigate the climate index change in future time periods in the upper Baro basin of Ethiopia. The daily precipitation and maximum and minimum temperature data were downscaled using Statistical Downscaling Model (SDSM). The precipitation and temperature data were estimated according to UK Hadley Centre Coupled Model version 3 (HadCM3) global circulation model with medium-high (A2) and medium-low emission (B2) scenarios in three future time interval periods. The De Martonne Aridity Index and Pinna Combinative Index change of the future time periods centered at 2020s, 2050s, and 2080s was computed. The analysis was based on percentage change between the baseline and three future time periods. The monthly De Martonne Aridity Index result showed that there are months in the dry season classified as semi-dry with value of less than 20 and the land needs irrigation in these months. The Pinna Combinative Index value also showed the same trend like that of the De Martonne Aridity Index and a high correlation coefficient was noticed, verifying similar trend of the two indices for the three future time period changes. Overall, humidity is expected to decrease in most of the months in the three future time periods for both A2 and B2 emission scenarios because of the increment of temperature in the future.