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. Accurate quantification of the amount and spatial variation of evapotranspiration is important in a wide range of disciplines. Remote sensing based surface energy balance models have been developed to estimate turbulent surface energy fluxes at different scales. The objective of this study is to evaluate the Surface Energy Balance System (SEBS) model on a landscape scale, using tower-based flux measurements at different land cover units during an overpass of the ASTER sensor over the SPARC 2004 experimental site in Barrax (Spain). A sensitivity analysis has been performed in order to investigate to which variable the sensible heat flux is most sensitive. Taking into account their estimation errors, the aerodynamic parameters (h c , z 0M and d 0 ) can cause large deviations in the modelling of sensible heat flux. The effect of replacement of empirical derivation of these aerodynamic parameters in the model by field estimates or literature values is investigated by testing two scenarios: the Empirical Scenario in which empirical equations are used to derive aerodynamic parameters and the Field Scenario in which values from field measurements or literature are used to replace the empirical calculations of the Empirical Scenario. In the case of a homogeneous land cover in the footprints of the measurements, the Field Scenario only resulted in a small improvement, compared to the Empirical Scenario. The Field Scenario can even worsen the result in the case of heterogeneous footprints, by creating sharp borders related to the land cover map. In both scenarios modelled fluxes correspond Correspondence to: J. van der Kwast (hans.vanderkwast@vito.be) better with flux measurements over uniform land cover compared to cases where different land covers are mixed in the measurement footprint. Furthermore SEBS underestimates sensible heat flux especially over dry and sparsely vegetated areas, which is common in single-source models.
, within which numerous crops are grown, on both irrigated and dry land, alongside fields of bare soil. The campaigns were carried out in the framework of the Earth Observation Envelope Programme of the European Space Agency (ESA) with the aim of supporting geophysical algorithm development, calibration/validation and the simulation of future spaceborne Earth Observation missions. Both campaigns were also contributions to the EU 6FP EAGLE Project. The emphasis of this contribution is on the in situ measurements of landatmosphere exchanges of water, energy and CO 2 as well as the thermal dynamic states of the atmosphere, the soil and the vegetation. Preliminary analysis and interpretation of the measurements are presented. These two data sets are open to the scientific community for collaborative investigations.
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