This paper shows that the protection of environmental flows as a management objective for a regulating agency needs to be consistent with the aquifer water balance and the degree of renewability. A stylized hydro-economic model is used where natural discharge, which sustains environmental flows, is considered both in the aquifer water budget and in the welfare function as an ecosystem damage. Groundwater recharge and the associated natural drainage may be neglected for fossil aquifers where groundwater is mined. However, when dealing with a renewable aquifer for which recharge is not negligible, natural drainage should explicitly appear in the water budget. Doing so, the optimum path of net extraction rate does not necessarily converges to the recharge rate, but depends on the costs associated with ecosystems damages. The optimal paths and equilibrium values for the water volume and water extraction are analytically derived and numerical simulations based on the Western La Mancha aquifer illustrate the theoretical results of the paper.

Keywords: aquifer; optimal control, environmental flows, ecosystem damages, water budget myth.