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ATEAM final report Section 5 and 6 (2001-2004) 30 indicates that forest management may be at least as important a driver as climate change and land use in Europe (see section Forestry). Being a forest model, the EFISCEN cannot give results for the total terrestrial carbon balance of Europe, including crop- and grassland. The LPJ model gives an approximation of the total terrestrial carbon balance, however, the exact current and future total terrestrial European carbon balance is still unknown. The influence of human management and nutrition changes need further attention, as do the process of soil carbon mineralisation, plant product harvesting and the life time of the harvested products. The LPJ consortium is currently working to further improve the representation of forest and agricultural management in the LPJ model. A number of papers have already been published and others are in preparation for peer-reviewed journals (see Annex 2). Flux data benchmarking Principal investigators: Pablo Morales, Pete Smith, Martin T. Sykes, Ben Smith, Colin Prentice, Harald Bugmann, Pierre Friedlingstein, Bärbel Zierl, Anabel Sánchez, Santi Sabaté, Eduard Pla, Carlos A. Gracia, Sönke Zaehle The evaluation and comparison exercise of four process-based vegetation models (providing monthly output) that are part of the ATEAM project using a monthly dataset of carbon and water fluxes over fifteen EUROFLUX sites was completed. A paper on the comparison is to be submitted shortly to Global Change Biology (see Annex 2). Temperature dependency of soil respiration Principal investigators: Jo House, Colin Prentice (with Wolfgang Knorr, Max Planck Institute for Biogeochemistry Jena and Beth Holland, NCAR, USA) In an ongoing examination of the controversial issue regarding the effects of temperature on soil respiration, modelling studies were completed and a paper prepared in 2003. The paper has been revised and submitted to Nature as a letter (see Annex 2). We found that the controversial results could be accounted for providing a multiple-pool approach was used. Since this is the approach used by LPJ and ROTH-C, we concluded there was no need to alter the models used in the ATEAM Project. 6.2.2.4 Water Principal investigators: Nigel Arnell and David Wilson This part of the ATEAM project used a hydrological model to simulate the potential changes in streamflow and indicators of water resources across Europe following defined climate and land cover changes. The project applied an established macro-scale hydrological model to simulate runoff across Europe at a resolution of 10’x10’. In general terms, the model (Arnell 1999; 2003) calculates the evolution of the components of the water balance at a daily time step. Model parameters are not calibrated from site data, but are determined from spatial data bases. A validation exercise showed that the model simulated reasonably well the magnitude and variation in runoff across Europe. The model is run using the time series of monthly precipitation from 1961 to 1990 to simulate a sequence of 30 years of monthly streamflow in each grid cell. Different sequences of random numbers used to generate daily precipitation and temperature produce slightly different streamflow sequences, and in order to reduce the effect of this random variation each time series is simulated six times. Although the model is implemented at a scale of 10x10’, for most of the analyses runoff is aggregated to the 0.5°x0.5° scale. Döll and Lehner’s (2002) drainage direction map is used to link the 0.5°x0.5° cells together and enable the accumulation of flows along the river network. The following hydrological and water resources indicators have been calculated under current and future conditions: − runoff by grid cell
ATEAM final report Section 5 and 6 (2001-2004) 31 − − − − − − runoff by major European basin water resources per capita by major European basin summer runoff volumes by grid cell annual runoff that is exceeded in nine years out of ten (“drought runoff”) maximum monthly runoff as indicator of change in flood regime (“flood runoff”) mean monthly regimes at a number of identified locations along major rivers Ideally the calculations at the basin scale would use the major river basins defined for the implementation of the European Framework Water Directive, but unfortunately these have not yet all been precisely defined (European Commission, 2002). A total of 94 major river basins have therefore been identified, based on currently-proposed river basins and major topographic boundaries. Basin areas range from just over 10,000 km 2 to 373,000 km 2 . Changes in hydrological regime Figure 21 shows the change in average annual runoff, at the 10x10’ grid scale, by 2021-2050 under the A2 emissions scenarios. Whilst there is some broad consistency in the pattern of change – reductions in runoff in southern Europe and increases in northern Europe – the magnitudes and precise geographic patterns vary between climate models. Reductions in 30-year mean runoff in parts of southern Europe can be as great as 30%; percentage increases in northern Europe are smaller. Separate model runs keeping land cover constant have shown that changes in climate have a much greater effect on changes in runoff than alterations to catchment land cover. Figure 21 also shows change in “drought” runoff (the annual runoff exceeded in nine years out of ten) and “flood” runoff (the mean maximum monthly runoff). Note that neither have been routed, so represent drought and flood characteristics in small catchments, not characteristics along the major rivers in Europe. In general terms, the patterns of change are the same as for annual runoff, but there are some differences. Percentage reductions in drought runoff tend to be larger than percentage changes in annual runoff and, under HadCM3 at least, in some areas of northwestern Europe climate change may produce simultaneously more severe droughts and more extreme floods. Much of the large reduction in maximum flows in eastern Europe is due to the shift in timing of flows from spring to winter, as a greater proportion of precipitation falls as rain rather than snow. Figure 22 summarises changes in mean monthly flow regimes at three locations along major European rivers – the Rhine, Rhone and Danube – under the HadCM3 scenario for 2021-2050. In each case there is a change in the timing of river flows, largely due to the reduction in the amount of snowfall and hence spring snowmelt, which will affect both navigation and run-of-river hydropower potential. Implications for indicators of water resources A key indicator of water resources pressures in a basin is the amount of water available per capita. Two key thresholds are values of 1700 m 3 capita -1 year -1 – below which there are likely to be water-related resources problems – and 1000 m 3 capita -1 year -1 – below which there is real water scarcity at times of the year. Figure 23 summarises water resources pressures across Europe in 2051 under the A2 world. The left-most panel shows the distribution of resources per capita (by major basin) with an A2 population in the absence of climate change. The central panels show the change in basin average annual runoff under the four climate models, and the right-most panels show the distribution of resources per capita with climate change. River basins with the lowest available water resources per capita, in the absence of climate change, are mostly in highly-populated western-central Europe. Note that the indicator used does not take into account actual abstractions of water: abstractions per capita are higher in southern Europe due to irrigation. Climate change tends to increase the numbers of basins in southern Europe with water scarcity. The spatial pattern of change is very similar with the other emissions scenarios and future populations, except that with the lower population increases under the A1, B1 and B2 scenarios the lower Danube basin has greater water resources than 1700 m 3 capita -1 year -1 in the absence of climate
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