The Réal Collobrier river basin, a special opportunity to observe Mediterranean hydrology

A result of a varied climate and landscape, the hydrological behavior of the Mediterranean region has always been particularly temperamental: limited water resources, dry summers, episodes of sudden and intense rain, etc. A clear understanding of the hydrological processes unique to the region is also important to improve management of water resources and the risks linked to flooding, requiring monitoring over particularly fine time and space scales.

A unique observational river basin in its field

It is in this context that the Réal Collobrier observational river basin,[1] managed by Irstea Aix-en-Provence for the past 50 years, becomes particularly relevant. Located in the Massif des Maures (Var), the site is equipped with many hydro-climate measurement tools, specifically rain gauges and water level recorders, distributed over an area of 70 km2, which have provided continuous rain and water level readings for the Réal Collobrier river since 1967.

"In contrast to most other observational river basins, the Réal Collobrier has experienced very little direct human impact (excluding fires). Its context - rural and forest - provides a better view of natural, climatological and hydrological processes. Very little development, such as water withdrawal, hydraulic structures or riverbank houses, has affected the transfer of water during flooding or periods of low-water,"[2] notes Nathalie Folton, an Irstea research engineer. Another unique aspect is that the measuring instruments actually cover seven interconnected river sub-basins of various sizes. The density of measurement points and their strategic location across all the basins have allowed scientists to gather data on both a large scale (70 km2) and a very small scale (two river basins of 1.5 km2). This has provided a particularly in-depth and rarely studied understanding of Mediterranean hydrological processes and their significant variation across one single river basin.

The result is a wealth of information with which to follow developments and create field tools.

Although there are over 3,000 flow measurement stations in France, there are few outside the Réal Collobrier that have over 50 years of data. This long history of data collected from the Réal Collobrier has provided a remarkable database that now makes it possible to analyze the evolution of hydrological parameters for all time scales (yearly, monthly and daily) and to extract trends reliably (see text box). These trends have been particularly relevant for climate change impact studies of water resources.

Furthermore, given the amount of spatial and time data available, the data collected at Réal Collobrier has been used to develop mathematical models capable of simulating the extreme variability of hydrological behaviors that are typical in Mediterranean regions, both during flooding and low-water periods. 

For example, scientists have perfected models that reproduce the various responses of the river basin to flooding, by incorporating influencing factors such as soil type and plant type. These models, which transform rainfall parameters into river flows, have been used to develop flood management tools currently in use today. One such example is the AIGA model, a warning device specifically adapted to fast flood prediction, that has been deployed in both Mediterranean and mountain regions. Another example is the VigicrueFlash tool, used by flood prediction services (SPC), that predicts extreme floods by combining the flow rate and its rarity. Two specific examples among many that are testament to the usefulness of this exceptional observation site.

Analyzing 50 years of data from Réal Collobrier: remarkable trends
  • Rain

? of number of days without rain (increase linked to a drop in the number of light rain)

 Low ? of yearly rain accumulation, although marked ? of monthly rain in February and March

  • Temperature

? of temperature extremes for the hottest months

? of evapotranspiration[3], from which ? water feeding into the river, from which ? flow

  • Flow

? light annual flows

? spring flows (March and April)

2017: an exceptionally poor year for rain and flow

  • Lowest recorded yearly rain accumulation since 1967
  • Record number of days without rain since 1967
  • Longest recorded period without flow for Réal Collobrier since 1967: 6 months (compared to 1 to 3 months usually)

Read the full study report

 

Read the monitoring data for the Réal Collobrier river basin

For more information

[1] A river basin is an area limited by the highest topographical lines, which drains each drop of water it receives towards an outlet, river or the sea.

[2] Lowest water level for a river.

[3] Water evaporated from the ground and bodies of water, and transpired by plants.