Characterisation of stable isotopes to identify residence times and runoff components in two meso-scale catchments in the Abay/Upper Blue Nile basin, Ethiopia

摘要

Measurements of the stable isotopes oxygen-18 (18O) and deuterium(2H) were carried out in two meso-scale catchments, Chemoga(358 km2) and Jedeb (296 km2) south of Lake Tana, Abay/Upper BlueNile basin, Ethiopia. The region is of paramount importance for the waterresources in the Nile basin, as more than 70% of total Nile water floworiginates from the Ethiopian highlands. Stable isotope compositions inprecipitation, spring water and streamflow were analysed (i) to characterise the spatial and temporal variations ofwater fluxes; (ii) to estimate the mean residence time of water using a sinewave regression approach; and (iii) to identify runoff components usingclassical two-component hydrograph separations on a seasonal timescale.The results show that the isotopic composition of precipitation exhibitsmarked seasonal variations, which suggests different sources of moisturegeneration for the rainfall in the study area. The Atlantic–Indian Ocean,Congo basin, Upper White Nile and the Sudd swamps are the potential moisturesource areas during the main rainy (summer) season, while the Indian–Arabianand Mediterranean Sea moisture source areas during little rain (spring) anddry (winter) seasons. The spatial variation in the isotopic composition isinfluenced by the amount effect as depicted by moderate coefficients ofdetermination on a monthly timescale (R2 varies from 0.38 to 0.68) andweak regression coefficients (R2 varies from 0.18 to 0.58) for thealtitude and temperature effects. A mean altitude effect accounting for?0.12‰/100 m for 18O and ?0.58‰/100 m for2H was discernible in precipitation isotope composition.Results from the hydrograph separation on a seasonal timescale indicate thedominance of event water, with an average of 71 and 64% of the totalrunoff during the wet season in the Chemoga and Jedeb catchments,respectively.Moreover, the stable isotope compositions of streamflow samples were dampedcompared to the input function of precipitation for both catchments. Thisdamping was used to estimate mean residence times of stream water of 4.1 and6.0 months at the Chemoga and Jedeb catchment outlets, respectively. Shortmean residence times and high fractions of event water components recommendcatchment management measures aiming at reduction of overland flow/soilerosion and increasing of soil water retention and recharge to enablesustainable development in these agriculturally dominated catchments.