CPRs are defined as natural or man-made structures characterized by costly exclusion and subtractability of units (Ostrom and Ostrom, 1977). Examples include surface irrigation systems, groundwater basins, fisheries, forests and grazing lands. Both exclusion and subtractability present challenges for governing CPRs sustainably. Exclusion involves defining who may enter a resource and who may not - making such a determination is rarely a straightforward process. Ideally, exclusion should occur in a manner that limits access to the number of users whose use will not threaten the resource. Physical, institutional and social issues often confound such efforts (Ostrom et a/., 1994). The sheer size of some resources makes enforcing access limitations in any meaningful or cost-effective manner virtually impossible. In other instances, national or state constitutions forbid denying citizens access to natural resources. In other settings, there may be political or economic reasons for avoiding strict access controls. For instance, a number of surface irrigation systems have been described as long and lean - the goal being to provide at least some water to as much land as is possible. Rationales range from equity concerns, i.e. assisting many people, to cost-benefit analysis issues, i.e. the more land included in a scheme, the better the cost/benefit ratios. In either case, too much land can be included within a system with some farmers experiencing chronic water shortages.
Exclusion is critical for sustainability, but also for governance. Resource users are much less likely to undertake costly and time-consuming efforts to manage CPRs if they cannot capture many of the benefits resulting from good management. Why design a water allocation scheme that conserves water if the additional water supplies may be captured and used by someone else? Why invest in groundwater recharge projects if others can pump the recharged water? Inadequate exclusion promotes free riding, and free riding discourages collective action (Dietz et a/., 2002).
Even if exclusion is adequately addressed in relation to a CPR, sustainability is not ensured because of substractability. Subtractability means that each 'unit' harvested from a CPR is not available for other users to harvest. The groundwater that a well owner pumps and uses to water his crops is not available for other well owners to pump. Since each resource user gains the value of each unit harvested but imposes some of the costs of harvesting on all resource users, resource users are likely to harvest more than is economically or ecologically desirable (Gordon, 1954; Scott, 1955; Dietz et a/., 2002). Or, as Ostrom et a/. (1994, p. 10) explain: '[I]ncreased water withdrawal by one pumper reduces the water other pumpers obtain from a given level of investment in pumping inputs'. The problem of exclusion may be adequately addressed but the CPR may still be overused because of the harvesting actions of the resource users. Consequently, if CPRs are to be governed sustainably, the challenges posed by difficult and costly exclusion and substractability must together be addressed.
Considerable attention has been devoted to the problem of overuse. The earliest formal models of resource use, such as those developed for fisheries (Gordon, 1954; Scott, 1955), focused on it, and many models since then have followed suit (e.g. Hardin, 1968; Clark, 1980; Norman, 1984). While overuse is problematic, resource users are likely to confront a host of CPR dilemmas (Ostrom et al., 1994). Ostrom et al. (1994) define CPR dilemmas as suboptimal outcomes produced by the actions of resource users and the existence of feasible institutional alternatives, which, if adopted, would lead to better outcomes (Ostrom et a/., 1994, p. 16).
In addition to overuse, resource users may engage in a variety of actions that produce suboptimal outcomes in their use of a CPR. For instance, well owners may place their wells too close together, interfering with one another's pumping; a farmer may install a deep tube well near another farmer's shallow well, drying it up; farmers may fail to maintain a tank that would otherwise serve to capture rainwater and recharge it into the underground aquifer.
Ostrom et al. (1994) relax the implicit assumptions underlying formal models focused on overuse to develop a typology of CPR dilemmas. Most models assume a uniformly distributed resource. By relaxing that assumption and allowing resources to be patchy, so that some areas of a resource are more productive than others, assignment problems may emerge. Assignment problems involve resource users competing over productive areas and interfering with one another's harvesting (Ostrom et a/., 1994, p. 11). Furthermore, most formal models assume identical harvesting technologies among resource users. By relaxing that assumption and allowing diverse technology utilization, technological externalities may emerge among resource users. Technologies used by harvesters interfere with one another causing conflicts among resource users. For instance, a high-capacity well may dry up a shallow tube well (Ostrom et a/., 1994, p. 12). Thus, in addition to overuse, or what Ostrom et al. (1994) term appropriation externalities, resource users may experience assignment problems and technological externalities.
As Ostrom et al. (1994) note, appropriation problems stemming from when, where, how and how much to harvest are not the only problems resource users are likely to experience. Another class of dilemmas - provision problems - is also likely to emerge in many CPR settings. Provision problems relate to developing, maintaining and/or enhancing the productive capacity of the CPR. For instance, adequately functioning surface irrigation systems require that diversion structures, headworks, canals and outlets be regularly repaired and maintained. The productivity of an aquifer may be enhanced by capturing water during wet seasons and directing that water underground to be used during dry seasons. Provision problems are distinctly different from appropriation problems. Appropriation problems require resource users to coordinate their harvesting activities; provision problems require resource users to cooperate and contribute to the production of public goods.
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