With its abundance and its properties as a solvent, water has long been used for cleaning and for dissolving waste products. When we return water to nature in this state, we are relying on natural processes to cleanse it. These wastes may be toxic to these other living things. The more concentrated these wastes become, the more toxic we can expect them to be. Water treatment facilities must be used to remove these wastes before the water is returned to nature.
Humans rely on water to sustain their agricultural crops .
Usually, the natural precipitation will be sufficient to water these crops. In other situations, artificial watering systems, usually referred to as irrigation, are used. These irrigation schemes involve modification of the natural water distribution patterns. Sometimes, extra water is drawn from natural groundwater reservoirs far below the soil surface. Other times, rivers are dammed and their waters are diverted through canals to be used on agricultural fields.