Abstract. The Soil and Water Assessment Tool (SWAT) isan integrated river basin model that is widely applied within the Nile basin. Up to date, more than 20 peer-reviewed papers describe the use of SWAT for a variety of problems in the upper Nile basin countries, such as erosion modelling, land use and climate change impact modelling and water resources management. The majority of the studies are focused on locations in the tropical highlands in Ethiopia and around Lake Victoria. The popularity of SWAT is attributed to the fact that the tool is freely available and that it is readily applicable through the development of geographic information system (GIS) based interfaces and its easy linkage to sensitivity, calibration and uncertainty analysis tools. The online and free availability of basic GIS data that are required for SWAT made its applicability more straightforward even in data-scarce areas. However, the easy use of SWAT may not always lead to appropriate models which is also a consequence of the quality of the available free databases in these regions. In this paper, we aim at critically reviewing the use of SWAT in the context of the modelling purpose and problem descriptions in the tropical highlands of the Nile basin countries. To evaluate the models that are described in journal papers, a number of criteria are used to evaluate the model set-up, model performances, physical representation of the model parameters, and the correctness of the hydrological model balance. On the basis of performance indicators, the majority of the SWAT models were classified as giving satisfactory to very good results. Nevertheless, the hydrological mass balances as reported in several papers contained losses that might not be justified. Several papers also reported the use of unrealistic parameter values. More worrying is that many papers lack this information. For this reason, most of the reported SWAT models have to be evaluated critically. An important gap is the lack of attention that is given to the vegetation and crop processes. None of the papers reported any adaptation to the crop parameters, or any crop-related output such as leaf area index, biomass or crop yields. A proper simulation of the land cover is important for obtaining correct runoff generation, evapotranspiration and erosion computations. It is also found that a comparison of SWAT applications on the same or similar case study but by different research teams and/or model versions resulted in very different results. It is therefore recommended to find better methods to evaluate the representativeness of the distributed processes and parameters (especially when land use studies are envisaged) or predictions of the future through environmental changes. The main recommendation is that more details on the model set-up, the parameters and outputs should be provided in the journal papers or supplementary materials in order to allow for a more stringent evaluation of these models.

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A critical discussion of recent studies that analysed the effects of climate change on the water resources of the River Nile Basin (RNB) is presented. First, current water-related issues on the RNB showing the particular vulnerability to environmental changes of this large territory are described. Second, observed trends in hydrological data (such as temperature, precipitation, river discharge) as described in the recent literature are presented. Third, recent modelling exercises to quantify the effects of climate changes on the RNB are critically analysed. The many sources of uncertainty affecting the entire modelling chain, including climate modelling, spatial and temporal downscaling, hydrological modelling and impact assessment are also discussed. In particular, two contrasting issues are discussed: the need to better recognize and characterize the uncertainty of climate change impacts on the hydrology of the RNB, and the necessity to effectively support decision-makers and propose suitable adaptation strategies and measures. The principles of a code of good practice in climate change impact studies based on the explicit handling of various sources of uncertainty are outlined. Hydrologie et climat futurs dans le bassin du Nil: une revueRésumé Une discussion critique des études récentes qui ont analysé les effets du changement climatique sur les ressources en eau du bassin du Nil (RNB) est présentée. Premièrement, les problèmes actuels liés à l'eau dans le RNB montrant la vulnérabilité particulière de ce vaste territoire aux changements environnementaux sont décrits. Deuxièmement, les tendances observées dans les données hydrologiques (comme la température, les précipitations, le débit des rivières) sont présentées, telles qu'elles sont décrites dans la littérature récente. Troisièmement, les exercices récents de modélisation quantitative des effets des changements climatiques dans le RNB sont analysés de manière critique. Les nombreuses sources d'incertitude qui affectent toute la chaîne de modélisation, incluant la modélisation du climat, la descente d'échelles spatiale et temporelle, la modélisation hydrologique, et l'évaluation des impacts sont également discutées. En particulier, deux questions contrastées sont discutées: la nécessité de mieux identifier et caractériser l'incertitude des impacts du changement climatique sur l'hydrologie du RNB, et la nécessité de soutenir efficacement les décideurs et de proposer des stratégies d'adaptation et des mesures appropriées. Les principes d'un code de bonnes pratiques dans les études d'impact du changement climatique sont décrits, qui reposent sur le traitement explicite des diverses sources d'incertitude.

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