Case studies       The use of cisterns has become very widespread and has been useful in collecting water all over the globe. One place where cisterns are used regularly is in the United States Virgin Islands. Here, harvesting of rainwater, as well as storage of runoff, is required by law. The high standards of living demand potable water and require that the rain harvesting systems are designed, constructed, operated and maintained here. The amount of rainfall in the USVI averages about 44 inches annually. There is little ground water, and the ground water needs careful management to avoid contamination from seawater. In some cases this resource is increasingly threatened by bacterial contamination from septic tanks and chemical pollution from commercial and industrial sources. High evapotranspiration rates return much of the rainfall to the atmosphere and create unfavorable conditions for surface water. Existing surface water, ground water, and desalinated water are not options available to individual residences. The law implemented in the USVI requires that typical one-story houses provide ten gallons of storage capacity for each square foot of roof area. A cistern of approximately 16,000 gallons is needed to contain the water. Here a typical residential rainwater harvesting system consists of a rooftop water-harvesting surface, a conveyance system for the harvested water, a cistern for storage of the water, and a means of distributing the water to the point of use.