The protection of environmental flows is a main challenge pursued by water regulating agencies in their groundwater management policies. A stylised hydro-economic model with natural drainage is used to compare the outcome of the optimal control approach in which environmental flows are introduced as an externality with the viable approach in which environmental flows are modelled as a constraint to satisfy. 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 monetary value of the externality in the optimal control approach. We 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.