Originally introduced for rivers, environmental ﬂows refer to the quantity of water that is necessary to maintain valued ecosystems services. This deﬁnition has been extended to groundwater dependent ecosystems (GDE) that rely to the occurrence of groundwater. When the natural drainage from an aquifer supports GDEs, increasing groundwater extraction for irrigation may threaten environmental ﬂows. However 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 resource renewability. Natural drainage should explicitly appear in the water budget to avoid the water budget myth. In doing so, the long term path of net extraction rate does not necessarily converge to the recharge rate.
In such a context, how can managers of groundwater eﬀectively take into account the maintenance of environmental ﬂows in their policies? And what are the implications in terms of hydro-economic modelling?
To answer to these questions, a stylised hydro-economic model with natural drainage is used to compare the outcome of two management strategies : the optimal control approach and the viability approach. In the optimal control approach, environmental flows are introduced as an externality in the welfare function of the water agency while in the viable approach, environmental flows are modelled as a constraint to satisfy. In both cases, the optimal and viable paths for the water table, water extraction for irrigation and environmental flows are analytically derived together with their long term values. We show how results are sensitive to some key parameters like the discount factor and the weight of the externality in the welfare function in the optimal control approach. We also show how the value of the environmental flows target in the viable approach can be derived from the optimal control approach. Numerical simulations based on the Western La Mancha aquifer illustrate the main results of the study.