Past hydrological trends on the Pyrenees: towards a higher spatial heterogeneity

The Pyrenees are a fundamental source of water resources for the territories surrounding this mountain range and beyond, and like other mountainous areas they are very vulnerable under global change. The CLIMPY project (Interreg-POCTEFA) calculated an increase of 1.5 ℃ on average temperature for this region in the last 60 years.

One of the aims of the PIRAGUA project (Interreg-POCTEFA) is to make a regional and temporal characterisation of the water resources of the Pyrenees. To achieve that objective, a common standardized and homogenized database was created for the first time in this transboundary region with streamflow data measured by the different water agencies operating in the area (1956-2015).

To avoid human impacted gauging stations (e.g. upstream reservoirs and large irrigation withdrawals), and to analyse only those with a reasonable quality, only a number of the initially obtained streamflow series were considered. A set of indicators was calculated from the selected daily streamflow series concerning mean, high and low flows at annual and monthly scales for different time periods ending in 2015. 

Results show that median discharge decreased an average of 30% in all gauging stations between 1956 through to 2015. High and low streamflow also decreased during the same period. On average, the number of days below the first quartile increased 10 days per decade, and the number of days above the third quartile decreased 6 days per decade. The interquartile range decreased 4% per decade on average showing that streamflow suffered a generalised reduction between 1956 and 2015. Regarding monthly streamflow, trends for median streamflow and the first quartile are similar to the annual scale. The most significant decrease is observed during spring (12-15% on average), and the lowest decrease occurs in the autumn (6-9% on average). Since 1986, trends change and streamflow increases are observed at some gauging stations with results that are spatially very heterogeneous. This inflection may be an effect of a more spatially heterogeneous climate in the recent past or of land use changes that are not regionally homogeneous, or a combination of both.