Agricultural Groundwater Exploitation: An Experimental Study

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"This paper is the initial part of a larger study motivated by challenges that the current practice in groundwater exploitation poses to sustainable development of coastal zones. The growing pressure on costal groundwater resources, due to the accelerated urban and tourism industry development combined to an irrigation water demand more and more demanding on high water quality, has arisen a commons problem that could have local catastrophic consequences. Notably, the overexploitation of groundwater in coastal zones may lead to water resource degradation as consequence of salt water intrusion into the aquifer. We present the case of the Roussillon coastal plane, a zone located in the Pyrénées-Orientales department (South of France), which is a good representative case of water problems faced in southern Europe. In this region, water demand is satisfied by surface water and a two-layer local aquifer. The superficial layer has sea connections, meaning that overexploitation can generate sea water intrusion. The layers are naturally separated by an impermeable substratum. This independency can be broken by an overexploitation of the deep layer; the impermeable substratum could become locally permeable letting polluted water from the superficial layer percolate. "We concentrate our effort on the agricultural water demand that remains the second one, in quantitative terms, after the urban water demand. Farmers can generally satisfy their needs by extracting water from substitute resources, namely, surface water (from irrigation channels) and groundwater. Due to end-users demand (of crop products) and due to irrigation techniques, farmers prefer groundwater than surface, especially the deep layer. But farmers' water exploitation behavior is not well known: tube wells are not registered even if it is compulsory and there is no water meter. In order to better analyze their behavior under this configuration, we implement laboratory experiments, by studying an N-person discrete-time deterministic dynamic game of T periods fixed duration. The objective function is stage-additive and depends on a state variable, whose dynamic evolution is linked to past decisions of all players. Players have to decide whether to use a private good or, by paying a lump-sum fee, to extract on one of two imperfectly substitute Common-Pool Resources (CPRs). Two type of experiments were done: firstly, we considered only the quantitative problem; in a second step, we introduced the qualitative problem and then the possible connection between the two layers leading non only to an overexploitation of the deep layer but also to its pollution, which impact farmers' income. For each case, two treatments were done. In the affect each CPR separately. In a second treatment, the CPRs are not independent, both types of externalities are considered. The observations are confronted to three benchmark outcomes corresponding to distinct behavioural assumptions: (a) sub-game perfection, (b) joint payoff maximization, and (c) myopic behaviour."



IASC, groundwater, agriculture, sustainability, coastal resources, water resources