Summary

Among the elements which enable us to evaluate historical environmental changes, river flows, examined within various time frames (between years, annual, seasonal, lowest water levels, flood events), provide integrated data, a pertinent indicator of climatic dynamics : how changes in rainfall, temperature and hours of sunshine act on snow storage capacities, evapo-transpiration losses, levels of tree cover.The fact that hydrometric data have been recorded for so long allows us to detect and analyse modifications to relevant elements of the hydrological regime, an approach which takes on a special dimension, even a forecasting role, in the context of the climatic changes and water shortages which have been predicted. It is with this type of investigation in mind that the authors have tried to detect changes in the levels of watercourses flowing down from the French side of the Pyrenees.

The first point to notice is the low overall level of change. If systematic hydrometrics had existed during the 18th and 19th centuries, the results would perhaps have been different. The reduction in levels over 80 to 100 years is generalised - but not systematic - and often lies between 5% and 10%. This range may certainly correspond to the margin of metrological error or to anthropic disturbance ; but the trend affects a sufficiently large number of sites to be credible.

At shorter timescales, we have identified reversible changes, almost everywhere and at the same period, which makes the idea of metrological problems relatively unlikely : phases of abundance alternate with phases of relative shortage. Another indisputable fact is the lower springtime water levels in watercourses descending from medium-altitude or high-altitude mountains, clearly linked to a decline in snowfall and in the retention of snow. For the same reasons, winter flows show an increase.