Abstract. In this study we evaluated changes in land cover and rainfall in the Upper Gilgel Abbay catchment in the Upper Blue Nile basin and how changes affected stream flow in terms of annual flow, high flows and low flows. Land cover change assessment was through classification analysis of remote sensing based land cover data while assessments on rainfall and stream flow data are by statistical analysis. Results of the supervised land cover classification analysis indicated that 50.9 % and 16.7 % of the catchment area was covered by forest in 1973 and 2001, respectively. This significant decrease in forest cover is mainly due to expansion of agricultural land.By use of a change detection procedure, three periods were identified for which changes in rainfall and stream flow were analyzed. Rainfall was analyzed at monthly base by use of the Mann-Kendall test statistic and results indicated a statistically significant, decreasing trend for most months of the year. However, for the wet season months of June, July and August rainfall has increased. In the period 1973-2005, the annual flow of the catchment decreased by 12.1 %. Low flow and high flow at daily base were analyzed by a low flow and a high flow index that is based on a 95 % and 5 % exceedance probability. Results of the low flow index indicated decreases of 18.1 % and 66.6 % for the periods 1982-2000 and 2001-2005 respectively. Results of high flows indicated an increase of 7.6 % and 46.6 % for the same periods. In this study it is concluded that over the period 1973-2005 stream flow has changed in the Gilgel Abbay catchment by changes in land cover and changes in rainfall.

Abstract:Lake Tana's flow system is governed by four main components: the inflow from surrounding river catchments into the lake, the outflow at Bahir Dar through the Blue Nile, the direct rainfall on the lake and the direct evaporation from the lake. While recent studies applied simple pragmatic approaches to estimate runoff from ungauged catchments, here emphasis is placed on more advanced approaches based on regionalization and spatial proximity principles. In the regionalization approach, model parameters of the conceptual HBV (Hydrologiska Byråns Vattenbalansavdelning) rainfall-runoff modelling of gauged catchments are transferred to ungauged catchments to allow runoff simulation. Parameter transfer was attempted through regression, proximity procedures and catchment size. This yielded 42, 47 and 46%, respectively, of the total river inflow for the three procedures. Lake areal rainfall is estimated by interpolation of the rain gauges around the lake, open water evaporation is estimated by the Penman-combination equation while observed inflows and outflow data are directly used in the lake water balance. The water balance closure term was established by comparing the measured lake levels with the calculated levels. Results show that runoff from ungauged catchments is around 880 mm per year for the simulation period 1995-2001 with a water balance closure error of 5%. In addition, use is made of river and lake water chemistry to arrive at an estimate of the unknown inflow and outflow components through the mixing cell approach. The results obtained with this method also provide independent information with regard to the errors in the individual water balance components.

The water resource of the Blue Nile River is of key regional importance to the northeastern African countries. However, little is known about the characteristics of the rainfall in the basin. In this paper, the authors presented the space–time variability of the rainfall in the vicinity of Lake Tana, which is the source of the Blue Nile River. The analysis was based on hourly rainfall data from a network of newly installed rain gauges, and cloud temperature indices from the Meteosat Second Generation (MSG–2) Spinning Enhanced Visible and Infrared Imager (SEVIRI) satellite sensor. The spatial and temporal patterns of rainfall were examined using not only statistical techniques such as exceedance probabilities, spatial correlation structure, harmonic analysis, and fractal analysis but also marginal statistics such as mean and standard deviation. In addition, a convective index was calculated from remote sensing images to infer the spatial and temporal patterns of rainfall. Heavy rainfall is frequent at stations that are relatively close to the lake. The correlation distances for the hourly and the daily rainfall are found at about 8 and 18 km, respectively. The rainfall shows a strong spatially varying diurnal cycle. The nocturnal rainfall was found to be higher over the southern shore of Lake Tana than over the mountainous area farther to the south. The maximum convection occurs between 1600 and 1700 local standard time (LST) over the Gilgel Abbay, Ribb, and Gumara catchments, and between 2200 and 2300 LST over Lake Tana and the Megech catchments. In addition, the hourly rainfall of the station with the highest elevation is relatively closely clustered as compared to those stations at lower elevation. The study provides relevant information for understanding rainfall variation with elevation and distance from a lake. This understanding benefits climate and hydrological studies, water resources management, and energy development in the region.

Abstract:In this study, large-scale atmospheric variables are downscaled to meteorological variables at local scale for the daily time step to assess hydrological impacts by climate changes. Large-scale atmospheric modelling was by the HadCM3 General Circulation Model (GCM) while downscaling and water balance modelling was through the Statistical DownScaling Model and the HBV semi-distributed rainfall-runoff model, respectively. The area of study was the Gilgel Abay catchment that drains in Lake Tana. A selection of large-scale atmospheric variables by the HadCM3 GCM are downscaled by a multiple linear regression model, were minimum and maximum temperature and precipitation for future time horizons are calculated. Climate scenarios as developed for the A2 (medium-high emission) and B2 (medium-low emission) scenarios for a 100-year period based on the mean of 20 ensembles have been selected for this study. In addition, a synthetic incremental scenario was tested for a wide range of changes in climatic variables. Stream flow simulations by the HBV model were carried out for the 2020s (2011-2040), 2050s (2041-2070) and 2080s (2071-2099) to define hydrologic impacts. The result of downscaled precipitation reveals that precipitation does not manifest a systematic increase or decrease in all future time horizons for both A2 and B2 scenarios unlike that of minimum and maximum temperature and related evaporation. For the future horizons significant changes and variations in the seasonal and monthly flows are to be expected and for the 2080s the runoff volume in the rainy season will reduce by approximately 11Ð6 and 10Ð1% for the A2 and B2 scenarios. Results from synthetic incremental scenarios also indicate sensitivities to climate change. As much as 33% of the seasonal and annual runoff is expected to reduce when temperature increases by 2°C and when rainfall decreases by approximately 20%.

Abstract. In this study lake levels of Lake Tana are simulated at daily time step by solving the water balance for all inflow and outflow processes. Since nearly 62% of the Lake Tana basin area is ungauged a regionalisation procedure is applied to estimate lake inflows from ungauged catchments. The procedure combines automated multi-objective calibration of a simple conceptual model and multiple regression analyses to establish relations between model parameters and catchment characteristics.A relatively small number of studies are presented on Lake Tana's water balance. In most studies the water balance is solved at monthly time step and the water balance is simply closed by runoff contributions from ungauged catchments. Studies partly relied on simple ad-hoc procedures of area comparison to estimate runoff from ungauged catchments. In this study a regional model is developed that relies on principles of similarity of catchments characteristics. For runoff modelling the HBV-96 model is selected while multiobjective model calibration is by a Monte Carlo procedure. We aim to assess the closure term of Lake Tana's water balance, to assess model parameter uncertainty and to evaluate effectiveness of a multi-objective model calibration approach to make hydrological modeling results more plausible.For the gauged catchments, model performance is assessed by the Nash-Sutcliffe coefficient and Relative Volumetric Error and resulted in satisfactory to good performance for six, large catchments. The regional model is validated and indicated satisfactory to good performance in most Correspondence to: T. H. M. Rientjes (t.h.m.rientjes@utwente.nl) cases. Results show that runoff from ungauged catchments is as large as 527 mm per year for the simulation period and amounts to approximately 30% of Lake Tana stream inflow. Results of daily lake level simulation over the simulation period 1994-2003 show a water balance closure term of 85 mm per year that accounts to 2.7% of the total lake inflow. Lake level simulations are assessed by Nash Sutcliffe (0.91) and Relative Volume Error (2.71%) performance measures.