Sustainable management of a natural resource: the case of the Gironde aquifer system.

Summary The purpose of this thesis is to provide, from the economic theory, elements of reflection for an effective management of the aquifer system of Gironde. In the first chapter, a spatial model for the management of a coastal aquifer subject to saline intrusion is presented. This model integrates the displacement of the interface between the salty water of the sea and the fresh water of the aquifer. The analysis, conducted at stationary equilibrium, allows us to identify the cost externalities that users exert on each other in space. In chapter 2, we show that a city located upstream can, through high withdrawals, deny a city downstream direct access to the aquifer resource. The social costs then diverge from the private costs. The introduction of spatially differentiated abstraction charges leads city utility managers to make optimal decisions. In Chapter 3, we consider a joint exploitation of the aquifer and a substitute resource. We show that the optimal supply from both water resources depends on the location of the users. Here again, the upstream users' withdrawals from the aquifer resource must take into account the scarcity of the latter. In the second part of this thesis, we estimate water demand functions for the Gironde department. The estimation of the water demand function of the Gironde domestic users, presented in chapter 5, is carried out on a sample of communes observed from 1990 to 1994. The results show a real, but moderate, sensitivity of domestic users to the price of water. In chapter 6, we estimate the water demand functions of the industrialists of Gironde by a dual multiproduct approach of the representation of their technology. The results of the estimations finally reflect a greater sensitivity to the price of water for industrial users than for domestic users.