Moreover, this reallocation stimulus was simple, and its transactions costs low. In fact, managing an effective mechanism to monitor compliance by the many farmers in the Zhang He basin (who own only a small fraction of the total cultivated land) would have been impossible. One of their greatest advantages is that they reduce dependence on direct abstraction of freshwater supplies and are therefore especially fit for areas where significant political, economic and environmental barriers to procuring new sources of water supplies exist (see Box 4.4 for an illustration in Southern California). The area is under great water stress and is vulnerable to severe droughts.