Gap filling and homogenization of climatological datasets in the headwater region of the Upper Blue Nile Basin, Ethiopia

Abstract

Long‐term time series of climatological data measured at meteorological stations provide one of the most authentic annals of climate in the past. However, there are a host of factors that affect measurements of climate parameters and make the data unsuitable for direct use and analysis. Serial completion and homogenization thus have to be undertaken in order to draw valid conclusions about the climate, or to apply the time series in impact studies. This study reports on gap filling and homogenization results of climatological elements in the headwater region of the Upper Blue Nile Basin from 1980 to 2013. Firstly, approaches for reconstruction of the time series from neighboring stations using different techniques were compared and selected. Neighboring stations were selected based on horizontal distance and coefficient of correlation. Secondly, the reconstructed time series were homogenized using Multiple Analysis of Series for Homogenization (MASH). The results show improved spatial coherence of the final data series. Higher spatial coherence was revealed for maximum temperature than for minimum temperature and rainfall. For rainfall and minimum and maximum temperatures, the coefficient of correlation weighting method outperforms other candidate methods, such as the normal ratio method (NRM), the modified NRM, and the inverse distance weighting method. Rainfall series for half of the stations considered were found to be homogeneous, and thus inhomogeneity corrections were not applied. Inhomogeneity in the remaining stations either underestimates or overestimates annual rainfall series. All stations revealed inhomogeneity in the mean annual maximum and minimum temperature series. These serially complete and homogenized data on rainfall, and minimum and maximum temperatures of the present study can be used for climate change and hydrological studies in the basin.

Analysis of combined and isolated effects of land-use and land-cover changes and climate change on the upper Blue Nile River basin's streamflow

“…The selection and number of adjacent stations for interpolation are important for the accuracy of interpolated values. As mentioned by Woldesenbet et al (2017a), different authors used different criteria to select neighbouring stations. Because of the relatively low number of network stations, a geographic distance of 100 km was considered for most stations when selecting neighbouring stations.…”

“…Haile et al, 2017) and changes in LULC (e.g. Woldesenbet et al, 2017b) have affected the magnitude of streamflow. Effective planning, management, and the regulation of water resource development is therefore required to avert conflicts between the competing water users.…”

“…Only the understanding of the hydrological processes and sources impacting water quantity, such as LULC change and climate change, can achieve this, as they are the key driving forces that can modify the watershed's hydrology and water availability (Oki and Kanae, 2006;Woldesenbet et al, 2017b;Yin et al, 2017). LULC change can modify the rainfall path to generate basin runoff by altering critical waterbalance components, such as groundwater recharge, infiltration, interception, and evaporation.…”

“…Although progress has been made in assessing the impacts of LULC and climate change on the UBNRB's hydrology, only a few studies have endeavoured to assess the attribution of changes in the water balance to LULC change and climate change. Woldesenbet et al (2017b), used partialleast-squares regression (PLSR) and a modelling approach based on the Soil and Water Assessment Tool (SWAT) to quantify the contributions of changes in individual LULC classes to changes in hydrological components in the Lake Tana and Beles sub-basins. Woldesenbet et al (2017b) reported that increases in cultivated land area and decreases in woody shrub and woodland appear to be major environmental stressors affecting local water resources through, for example, increasing surface runoff and decreasing ground water contribution in both watersheds; however, the impacts of climate change were not considered.…”

“…Woldesenbet et al (2017b), used partialleast-squares regression (PLSR) and a modelling approach based on the Soil and Water Assessment Tool (SWAT) to quantify the contributions of changes in individual LULC classes to changes in hydrological components in the Lake Tana and Beles sub-basins. Woldesenbet et al (2017b) reported that increases in cultivated land area and decreases in woody shrub and woodland appear to be major environmental stressors affecting local water resources through, for example, increasing surface runoff and decreasing ground water contribution in both watersheds; however, the impacts of climate change were not considered. Nonetheless, proper water resource management requires an in-depth understanding of the aggregated and disaggregated effects of LULC and climate change on streamflow, and water-balance components such as the interaction between LULC, climate characteristics, and the underlying hydrological processes are complex and dynamic (Yin et al, 2017).…”