Water harvesting is the act of collecting and storing rainwater for future use. This can be done in areas of low or high rainfall amounts using tanks and cisterns. Collecting rainfall can reduce the amount of run-off, flooding, and erosion in prone areas. This is especially relevant in urban areas where the land has been used to build infrastructure, and the rainwater run-off has nowhere to go which can result in flooding. In more rural areas when the water is collected, the run-off into rivers and ponds is reduced which in turn slows the contamination from surface fertilizers and pesticides. If the collected water is to be used as drinking water, it can be treated and used in environments where drinking water is scarce. Another use for the water is for non-drinking purposes such as flushing toilets. Homeowners and farmers can reduce their water bills and the use of ground water by using the collected water for irrigating crops, lawns, and trees. This benefits the environment by conserving the levels of ground water, which in many areas is precariously low.