PROMET – Large scale distributed hydrological modelling to study the impact of climate change on the water flows of mountain watersheds
Introduction
Climate change will change availability, quality and allocation of water resources on the regional scale and will force to adapt to changing future boundary conditions. Appropriate modelling tools should therefore be available to realistically describe reactions of watersheds to climate change and to identify efficient and effective adaptation strategies on the regional scale.
Regional climate models will be able to cover areas of watersheds of the order of 100,000 km2 with adequate spatial detail and an improved description of the atmospheric process within the next few years (Jacob et al., 2007). Is the hydrological modelling community prepared to make full and adequate use of these regional climate models? A mutual cooperation of hydrological and atmospheric models opens opportunities to more deeply study the impacts of climate change on the regional water cycle on the land surface in order to determine decision alternatives to adapt to the changing boundary conditions. For a recent review on land surface schemes presently used in regional climate models, which usually include land surface hydrology at different levels of complexity, see Pitman (2003). A review of hydrologic models by Singh and Woolhiser (2002) also clearly pointed out the need for a more regional to global view in future hydrologic models. Provisions should therefore be taken to allow hydrologic models to fully couple with climate models on the regional scale. Two-way coupling with regional climate models poses constraints on the architecture of hydrologic models. They should close the energy and mass balances, should be valid even under changing climate conditions and should cover large-scale watersheds. The main challenge, though, one faces when extending the catchment area to regional scales is that both complexity and heterogeneity of the watersheds generally increases considerably. With increasing watershed area different characteristic hydrologic features, like snow dominated mountain hydrology, forest hydrology at mountain foothills and agricultural hydrology in the lowlands as well as different rainfall regimes, land use patterns and geological settings simultaneously influence the overall reaction of the watershed and have to be treated in a consistent way within one model framework.
The hydrologic cycle on the land surface is complex. The processes involved couple the energy and water cycle and incorporate a large range of compartments of and physical interactions within the Earth system. On the regional and local scale the hydrologic cycle is increasingly influenced by humans, who make decisions on land use and runoff conditions including the construction of hydraulic structures. This highly complex system of interactions cannot be treated adequately with black- or grey-box input–output relations especially when future states of the system should be simulated under changing boundary conditions.
Spatial heterogeneity of the land surface on all considered scales adds an additional dimension of complexity. The factors dominating heterogeneity though depend on the size of the watershed. Large watersheds stretch over different climatic zones. For small watersheds the hydrologic regime can be assumed constant. Large-scale watersheds as defined in the context of this paper are influenced by different hydrologic regimes within one climate zone. This especially applies to mountainous watersheds and their lowlands. An excellent overview of the challenges of modelling mountainous watersheds is given by Klemes (1990).
Numerous hydrologic model studies exist on different levels of complexity which investigate water fluxes in small to large-scale watersheds. They range from attempts to fully physically describe all water flows in small catchments within, e.g. the multi-purpose model SHE (Bathurst and O’Connell, 1992) to simplified, empirical approaches with no physical foundation like the unit-hydrograph approach for flood forecasting (Sherman, 1932). Most models concentrate on the simulation of river runoff with an hourly to daily temporal resolution. Hydrologic model approaches can further be divided into lumped and spatially distributed, where the first category converts rainfall into runoff by using one (constant or dynamic) runoff coefficient for the considered watershed. It is not the intention of this paper to give a full overview of hydrologic models. Nevertheless a more detailed analysis of the second category of approaches, which consider spatial heterogeneity, will be conducted here.
Usually models of this category follow the assumption, that the land surface is composed of hydrologically homogeneous areas of arbitrary shape and extension of a few square kilometres (Kunstmann et al., 2006, Leavesley et al., 1996, Lindström et al., 1997, Moussa et al., 2007, Wagner et al., 2006, Shrestha et al., 2007, Smithers and Schulze, 1995, Todini, 1996, Yu, 2000). They can be differentiated in hydrotopes or hydrological response units (HRUs) (Becker and Braun, 1999, Leavesley et al., 1983) or representative elementary areas (REA, Wood et al., 1988). Watersheds are composed of a connected set of HRUs, for which a more or less complex hydrologic model is solved. In many cases lumped parameters are used to describe the hydrologic properties of the HRUs and classical channel routing schemes are applied based on linear elements. In a recent publication Moussa et al. (2007) have used flow directions derived from a digital terrain model to delineate HRUs, which were then connected through a river network. A more flexible treatment of spatial heterogeneity by introducing variable source areas was provided with TOPMODEL (Beven and Kirkby, 1979, Beven et al., 1995). For a review of rainfall runoff models including variable source area models see Beven, 2001.
Another class of models assumes that the land surface heterogeneity can be represented through spatial grids (Bates and De Roo, 2000, De Roo et al., 2000, Fortin et al., 2000, Horritt and Bates, 2001, Jasper et al., 2002, Lee et al., 2005, Ludwig and Mauser, 2000, Mauser and Schädlich, 1998b, Niehoff et al., 2002, Refsgaard and Storm, 1995, Schulla and Jasper, 2007, Strasser and Mauser, 2001). A grid element is similar to a HRU, in the sense that it represents a defined area on the land surface, which is assumed to be hydrologically homogeneous. As soon as the area covered by each grid element becomes small enough, raster-grids allow to treat spatial (evapotranspiration, infiltration, water flow in unsaturated media, etc.) and linear hydrologic processes (channel network, etc.) with one consistent data set. At this point the raster- and HRU-concept converge. Much research has been carried out to adequately parameterize raster representations of watersheds and to identify the right process models and interfaces between them to best describe water flows on different scales. The question which solution is best suited for different raster resolutions and watershed sizes as well as purposes is still open.
The main advantages of raster-based hydrologic models against HRU-based models are, that they can more easily be coupled with atmospheric or groundwater models, which are usually also raster-based. Their spatial architecture also coincides with the organisation of spatial data fields derived from remote sensing data and therefore facilitates their use in hydrology.
None of the cited hydrological models explicitly states that they simultaneously obey two fundamental physical principles, conservation of mass and energy, throughout the whole modelling chain from rainfall to river runoff at the outlet gauge. Most conserve mass throughout the hydraulic description of the water flows in single sub-models (e.g. when treating channel flows) but they often at the same time violate conservation of energy when modelling, e.g. evapotranspiration.
The impacts of climate change on the hydrological cycle can be severe especially in mountainous watersheds with extended lowlands. In these regions the hydrologic regime of the whole watershed may shift considerably through temperature changes and changes in the amounts of rainfall and its spatial and temporal pattern. The consequences of climate change may also be large reductions in forest cover through management or burning, the expansion of agriculture, the introduction of irrigation, the reduction of snow cover as well as vanishing glaciers. These impacts may cause that any present calibration of a model may become invalid, which means that future states of the watersheds will virtually correspond to those of ungauged basins. Assuming that regional climate change will not introduce new hydrologic processes, which are not yet covered, models that are prepared for this situation should therefore at least demonstrate that they can describe the present hydrologic situation well even without calibration.
We conclude that the combination of the usual calibration procedures using measured streamflow, simplified process representations, lumped model parameters (especially in HRUs) and the fact that the simultaneous conservation of mass and energy is not guaranteed, makes it difficult and potentially risky to use these approaches to predict future states of regional hydrologic systems under changing boundary conditions with respect to climate. For the same reasons they can hardly be coupled directly with regional climate models, which are the physically most profound source of information on how regional climate will change in the future.
Therefore, the aim of this paper is to introduce the grid-based based large-scale hydrologic model PROMET, which is based on physical principles, a strict conservation of mass and energy and no calibration using measured streamflow records, as well as to use it to investigate the possible impact of climate changes on the future low-flow conditions in a complex large-scale mountainous watershed.
Section snippets
Model approach and principles
Following the boundary conditions introduced above, the distributed, physically based hydrologic model PROMET (Processes of Radiation, Mass and Energy Transfer) was specifically developed to study the impact of climate change on the water cycle of large scale, complex watersheds influenced by different hydrologic regimes. It targets at coupling with regional climate models. In order to proof its applicability it was built up and tested in the Upper Danube catchment in Central Europe, which due
The test catchment
In order to validate PROMET it is applied to the Upper Danube catchment in Central Europe. The Upper Danube catchment has an area of 76,653 km2 and covers parts of Southern Germany, Austria, Switzerland and Italy (Fig. 4). The catchment is characterized by its Alpine topography, the relief stretching from altitudes of 287 m a.s.l. at the discharge gauge Achleiten up to 4049 m a.s.l. at Piz Bernina in its Alpine headwaters. The Upper Danube catchment shows strong meteorological gradients with
Model validation
PROMET was run without calibration for the Upper Danube catchment for the period from 1.1.1970 to 31.12.2003. This simulation period was deliberately extended beyond the standard climate period from 1971 to 2000 to be able to give the model time to spin up and to include the extremely warm Central European Summer of 2003 into the analysis. PROMET was continuously run over the whole simulation period using the input data set described above and a modelling time interval of 1 h. Analysis of the
Climate change impact example: low flows
The validation of PROMET has proven that the historical low-flow conditions in the Upper-Danube watershed are well captured by the model. In order to exemplify the future use of PROMET for regional climate impact studies a possible change in annual low-flow is simulated for the upcoming 50 years from 2011 to 2060. Two scenarios are chosen for the future development of climate in the watershed:
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It is assumed that climate change will come to a stop in 2011 and climate will not change during the
Conclusions and outlook
The study shows, that the consideration of the guiding principles of “Introduction” during the design and implementation of PROMET results in a spatially distributed physically based hydrologic model, which describes well and in detail the variability of the annual water balance and the daily water fluxes in the complex, large scale, mountainous Upper-Danube watershed for an extended, climatologic period of 33 years. The water balance, daily discharges and return periods of peak and low-flow
Acknowledgements
The provision of meteorological and hydrological data by the German Weather Service DWD and the Austrian Weather Service and the Environmental Agency of the Free State of Bavaria is gratefully acknowledged. Our thanks go to all members of the GLOWA-Danube projects for many fruitful discussions, which considerably improved the PROMET modelling approach. We thank Markus Weber from the Glaciology Division of the Bavarian Academy of Science for valuable inputs on snow dynamics. Special thanks go to
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