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Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=thsj20 Hydrological Sciences Journal ISSN: 0262-6667 (Print) 2150-3435 (Online) Journal homepage: https://www.tandfonline.com/loi/thsj20 Global hydrological models: a review Aditya Sood & Vladimir Smakhtin To cite this article: Aditya Sood & Vladimir Smakhtin (2015) Global hydrological models: a review, Hydrological Sciences Journal, 60:4, 549-565, DOI: 10.1080/02626667.2014.950580 To link to this article: https://doi.org/10.1080/02626667.2014.950580 Accepted author version posted online: 07 Aug 2014. Published online: 10 Mar 2015. Submit your article to this journal Article views: 9130 View related articles View Crossmark data Citing articles: 77 View citing articles Global hydrological models: a review Aditya Sood and Vladimir Smakhtin International Water Management Institute (IWMI), Colombo, Sri Lanka a.sood@cgiar.org Received 20 June 2013; accepted 30 April 2014 Editor D. Koutsoyiannis Abstract Global hydrological models (GHMs) have effectively become a separate research field in the last two decades. The paper reviews and compares 12 known global modelling efforts since 1989, the year the first GHM was published. Structure, strengths and weaknesses of individual models are examined, and the objectives of model development and their initial applications are documented. Issues such as model uncertainty, data scarcity, integration with remote sensing data and spatial resolution are discussed. Key words global hydrological models; grids; remote sensing; hydrology Revue des modèles hydrologiques globaux Résumé Les modèles hydrologiques globaux (MHG) sont en fait devenus un domaine de recherche distinct au cours des deux dernières décennies. Cet article examine et compare 12 démarches de modélisation globale connues depuis 1989, année de publication du premier MHG. La structure, les forces et faiblesses de chaque modèle ont été examinées et les objectifs de développement des modèles et leurs premières applications ont été décrits. Des questions telles que l’incertitude du modèle, la rareté des données, l’intégration des données de télédétection et la résolution spatiale ont été discutées. Mots clefs modèles hydrologiques globaux ; maillages ; télédétection ; hydrologie INTRODUCTION The continuing debate on climate change (e.g. Kundzewicz and Stakhiv 2010, Huard 2011) and other global drivers of change highlights the interde- pendence of various earth systems and the need for integration of those systems into global simulation models (Wilby 2010). There is an impact on regional and, hence, global climate, from changes in soil moist- ure and terrestrial evapotranspiration (Munro et al. 1998, Koster et al. 2003, Koster et al. 2004, Seneviratne et al. 2010, Dirmeyer 2011), and hence river discharge which in turn impacts on sea character- istics (Milly et al. 2010). Land-use changes upstream affect the hydrology and water quality thousands of miles downstream (Freeman et al. 2007). Global hydrology is closely linked with the nutrient cycle (cause of eutrophication of coastal zones) (Foley et al. 2005, Rabalais et al. 2009, Fekete et al. 2010) and the carbon cycle (impacting on the climate). The changes in these global cycles eventually have social and economic implications. Due to globalization, virtual water trade, for example, has become an impor- tant factor of both the global water cycle and food security (Islam et al. 2006). While the scientific com- munity has long been aware of, and has dealt with, impacts of climate on hydrology, the feedback has only recently started to receive attention. All this leads to the notion of the ‘global water system’ (Alcamo et al. 2008, Alcamo 2009), in which the global water flow is connected to other systems through physical relationships, economics and institu- tions. This system is further complicated by the inter- ference from humans through water storage and withdrawals (Rost et al. 2008b). Models that attempt to simulate global hydrol- ogy and associated processes are similar to numerous stand-alone hydrological models and to hydrological components of the general circulation models (GCMs). However, they differ in the detail of description of processes, parameter estimation Hydrological Sciences Journal – Journal des Sciences Hydrologiques, 60 (4) 2015 http://dx.doi.org/10.1080/02626667.2014.950580 549 © 2015 IAHS approaches, time scales, and spatial resolution of input data and simulations (Haddeland et al. 2011). The stand-alone models are usually applied at the basin scale, or a smaller catchment scale, and have many parameters that need to be calibrated or esti- mated regionally. Examples of such models include the Soil and Water Assessment Tool (SWAT; Neitsch et al. 2002), the Hydrological Simulation Program- Fortran (HSPF; Bicknell et al. 1997) and the Hydrologiska Byråns Vattenbalansavdelning (HBV; Lindström et al. 1997). Some of these models can, in principle, be applied at the global scale, but, due to severe information constraints, this never happens in practice. The hydrological models that are a part of GCMs are usually land surface schemes (LSSs) that simulate the energy balance at soil, atmosphere and vegetation interfaces at finer time scales (often hours), and do not have a flow routing component. Examples of such models include the Biosphere– Atmosphere Transfer Scheme (BATS; Dickinson et al. 1986), the Simple Biosphere Model (SiB; Sellers et al. 1986) and the Joint UK Land Environment Simulator (JULES; http://www.jchmr. org/jules/index.html). The actual global hydrological models (GHMs) have few calibratable parameters and are calibrated either at eco-region, climatic-region or large river basin scales (Vörösmarty et al. 1989, Döll et al. 2003, Widén-Nilsson et al. 2007). Some models, such as the Water Balance Model – Water Transport Model (WBM-WTM; Vörösmarty et al. 1998), are not calibrated per se but have an adjustment factor to tune them. The model Water – Global Analysis and Prognosis (WaterGAP; Alcamo et al. 2003, Döll et al. 2003) is a combination of calibration and tuning. It is first calibrated with a single parameter against streamflow. The basins that underestimate or overesti- mate flows are then tuned by two adjustment factors, runoff and discharge correction (Döll et al. 2003). The spatial resolution of GHMs is defined by the resolu- tion of available global climate input data. GHMs are relatively new and have emerged in the last two dec- ades. Lately, there has been increasing activity in this field and a concerted effort is evolving (Lawford et al. 2004, Döll et al. 2008). Also, GHMs are becom- ing more complex and resolute as more functionality is added to them and finer global spatial datasets are becoming available. Issues of the sensitivity of models to varying spatial and temporal scales of input and output data, measuring modelling uncertainty and cou- pling GHMs with other models have become promi- nent (Döll et al. 2008, Voß et al. 2008). The explosion of global data availability from satellites in the last two decades (Tang et al. 2009) has had an influence on the development of GHMs. Although some GHMs have been applied at a range of scales, they are, as a rule, built for global-scale studies. They would not be the preferred choice in basin-scale applications, due to the coarser resolution of GHMs at present, and the fact that there is a large family of hydrological models that have been designed for this purpose. However, GHMs may provide valuable spatial and temporal estimates of global water resources, and help to analyse possible projections/scenarios of changes of those estimates; GHMs have been built effectively for this purpose. Global estimates obtained through GHMs could be an improvement over those simply based on the statistical analysis of ground-based observed data, which, at a global scale, remain limited and, hence, contain a lot of uncertainty (Rodda 1995). An even greater use of GHMs is revealed when they are linked with other models, i.e. describing global econ- omy, ecology, trade, biodiversity, energy balance, land-use change, climate change, crop growth and other development issues/components related to water (Vörösmarty et al. 2000, Islam et al. 2006, Alcamo 2009). The requirements from the GHMs depend upon the demands of such associated models. To the best of the authors’ knowledge, a single source/compilation of GHMs does not exist. Recently, Trambauer et al. (2013) examined GHMs in the context of drought forecasting in Africa. This paper extends the discussion to the broader context of developing simulation tools for global food trade, agriculture and economy, and examines constraints and development trends of GHMs, including those of uncertainties, input data quality and integration with remote sensing data. EXISTING GLOBAL HYDROLOGICAL MODELS—DESCRIPTION Over the last few decades, many isolated efforts were made to simulate the global hydrological cycle. A review of the existing literature suggests that there are at least 12 GHMs (or tools that can be interpreted as such) at present. Table 1 lists these GHMs in chronological order of their development and includes details of the main developers, their objec- tives for developing the model and some example applications. Table 2 summarizes the technical details of these models. 550 Aditya Sood and Vladimir Smakhtin Table 1 Origin and known applications of existing global hydrological models. No. Model Developed/maintained by Year Objective(s) Applications 1 HDTM 1.0 (HydroDynamic Model) / WBMplus / WBM-WTM (Water Balance Model - Water Transport Model) University of New Hampshire, USA 1989 To study global biogeochemical cycles. Linked to Terrestrial Ecosystem Model and Trace Gas Model through soil moisture and evapotranspiration 1. Comparison of PET methods on US watersheds—WBM (Vörösmarty et al. 1998) 2. Using GHM uploads/Geographie/ global-hydrological-models-a-review.pdf