A Tool to Determine Annual Ground-Water Allocations in the Tarn-et-Garonne Alluvial Aquifer (France)

Abstract

The Tarn-et-Garonne department is crossed by three main rivers (Garonne, Tarn and Aveyron) whose alluvial plain covers an area of almost 1000 km2. Since 1996, the “Direction Départementale des Territoires” (Gov. Administration at county level), with the technical help of the French geological survey (BRGM), initiated the development of a groundwater model and a decision support tool to define annual groundwater abstraction allocations. As the field data and the computing capacities increased and the law evolved, three versions of the groundwater model were successively developed to better assess the Maximum Permissible Volumes (MPV) of groundwater abstraction on a yearly basis. The last transient state version takes into account the annual fluctuations in groundwater recharge and the water exchanges between the aquifer and the rivers. The MPVs are calculated each year in 21 management zones outside of the previously defined riverside aquifer. These zones are now managed by five agricultural users’ associations, known as collective management agencies (or OUGCs). Further improvement should lead to the decision support tool being available online, to encourage OUGCs and farmers to be more proactive in managing the groundwater resource.