Common pool resources (CPR) are characterised by free access (non, or limited, excludability) and rivalry in the use of the resource. CPR have the particularity of placing individual and collective interests in apparent opposition as well as creating tension in the choice of action, which is commonly referred to as a social dilemma. (Dubois et al., 2021). Rivalry in extractive CPR like groundwater implies that agents may think they should consume as much of the good as possible, fearing that the others leave nothing. This behaviour, known as ‘free riding’, is conducive to a phenomenon called ‘tragedy of the commons’ by Hardin (1968). Users depleting a CPR typically face several types of appropriation externalities (Gardner et al., 1997). In the real life, two types of externalities are faced by extractors: a static one (i.e.: the extraction cost at t is determined by the total extraction at t), and a dynamic one (i.e.: the extraction cost at t grows with the depth of the water table, depending on the total extraction at t-1). In such a non-cooperative game, the Nash equilibrium is reached when each individual maximizes his/her utility, whilst the Social optimum is reached when the sum of individual utilities are maximized.
Groundwater is a very important source of irrigation water, which alone represents more than 70% of the total water uses on earth (FAO, 2022). In North Africa half of current groundwater water withdrawals exceed natural rates of water recharge (Mayaux et al., 2022). Maghreb Countries depend on groundwater for their development, and public policies in the last decades have triggered radical changes in newly irrigated areas (extension areas) and in traditional oases (Kadiri et al., 2022). This resulted in a quick intensification of local agriculture, like in Tunisia, where oasis are currently facing sustainability concerns due to “uncontrolled expansion of irrigated areas, over-exploitation of groundwater resources, and soil degradation” (Ghazouani et al., 2009; Mekki et al., 2013).
In Tunisia, the Complex terminal Aquifer in Kebili is characterised by an important phenomenon of groundwater over-exploitation, resulting in 1 meter of lowering of the watertable/year. This is due to the almost irrelevant level of water recharge compared with the high water use (Trigui et al., 2021). In Tunisian oases, traditional farmers organised in water users associations, or GDA (French acronym for Agricultural Development Groups), coexist with newly settled extension farmers. Many GDA farmers have also plots in the extensions, where, like the extension farmers they dig illicit private boreholes that are not declared to the local authority for water management (CRDA) (Farolfi et al., 2022). Water extractions in the extensions are considered by many authors as the main cause of groundwater over-exploitation (Mekki et al., 2013), which will lead to the collapse of the system in the short or medium term if drastic measures are not take rapidly (Petit et al., 2017).
Current policies to match the problem are limited to the attempt by the CRDA to reduce access to water by limiting the number of boreholes. However, the overwhelming presence of illicit boreholes in Tunisian newly irrigated areas shows clearly the ineffectiveness of the measure. Alternative governance tools are necessary in order to face groundwater over-exploitation in the Tunisian oases.
To this end, like Gardner et al. (1997), we designed laboratory experiments to assess the performance of various groundwater governance policies and the applicability of game theory to behaviour in such system. We adapted the groundwater extraction dynamic model by Gardner et al., 1997 by introducing in the model the types of players (farmers), as found in a survey in the Jemna oasis, Governorate of Kebili in Southern Tunisia (Farolfi et al., 2022). The model implies a CPR recharge rate = 0, which is adapted to the local situation, as indicated by Trigui et al. (2021). The model is used to run experiments in the lab (France and Tunisia) and in the field, with Jemna oasis farmers, in order to test the performance of various groundwater governance policies in the studied frame.
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