Technical, economic and social rehabilitation of old canals to cope with global change: the case of the Neste Canal (France)

Many canals were built during the 19th century to satisfy multiple uses. The priorities among them reflected the societal needs of the time: navigation, drinking water, irrigation, hydroelectricity, minimum flow in rivers for sanitation goals, etc. Today, multiple changes affect these priorities: the decline of available resources makes it difficult to satisfy all uses. In addition to climate change, these water infrastructures are subject to controversies as a result of changes in agriculture and irrigation, claims for minimum flows for aquatic ecosystems, the urban sprawl, and the need for hydropower. Everything is questioned: the hydraulic management and indicators, the economic model for recovering the sustainable cost, the social contract that defines priority uses, and the fair distribution of powers and duties between the stakeholders. To summarize, the role of these old canals in global change adaptation strategies is being questioned, as dam projects. Our hypothesis is that these old hydraulic works can help the territories to adapt, if reforms of their hydraulic, economic and institutional management are carried out at the same time.

The recent history of the Neste Canal illustrates this complex ongoing transformation. Since 1962, it has diverted yearly 220 Mm³ from the Neste River, a tributary of the Garonne River in the foothills of the French Pyrenees, to supply 17 rivers in Gascony. Artificial lakes have progressively filled out this complex interconnected hydraulic system. This "Neste system" covers a territory of 8,400 km², irrigates about 60,000 ha, and supplies 350,000 inhabitants with drinking water (see map). In this paper, we describe the evolution of the multiple uses as well as the decrease in the flows derived over the last 70 years, and then the structural imbalance of its economic model. Its future depends on i) the political recognition of its contribution to the minimum water flows of the rivers of Gascony, ii) the introduction of a payment for this ecological function, and iii) changes in hydraulic regulation system to satisfy this last function previously managed as a hydraulic constraint.