Analysing the effect of global change on the historical trends of water resources in the headwaters of the Llobregat and Ter river basins (Catalonia, Spain)

The headwaters of the Llobregat and Ter Rivers are the main sources of water resources for the metropolitan areas of Barcelona and Girona. The historical discharge records of the headwater basins of these rivers during the period 1940–2000 have been analysed in relation with the changes in climate forcing and land cover occurring in these South Eastern Pyrenees catchments. Flow records showed high inter-annual variability, with a succession of decadal wet and dry spells, which made the detection of long-term trends in most of the gauging stations difficult. A comparison of the measured flows with those simulated by two rainfall–runoff models from climatic records showed some clear divergences. The trends of the measured flows for the study periods were negative whereas those explained by climate forcing had practically no trend. Analyses of other data periods may show negative and positive trends. Differences found between observed and simulated flows showed negative trends that were attributed to land use changes. Indeed, middle mountains in the study catchments have undergone an important increase of forest cover in the last decades. The main change observed in the area was an improvement of forest from sparse to dense coverage, and secondarily the change of land use from agriculture to pasture and forestry. The foreseeable effect of this land cover change is an increase of land evaporation caused by the increase in forest cover, not taken into account by the models used. The conclusion is that, although stream flows showed great decadal variability and significant trends driven by climate during some decadal-scale periods, water resources modelled from climate data for the whole period 1940–2000 did not show significant trends. In contrast, actual water resources experienced an annual decrease of about 0.25% of the mean annual resources during the same time span, due to an increase in forest cover in the headwaters.