GEF

Unit 2  |  Water Science and the Water Cycle 51 space than darker land surfaces or sea water. Amajor concern of climate scientists is that feedback loops will develop in which higher temperatures melt ice or snow cover, further raising global temperatures as the Earth absorbs more energy from the sun. Mountain glaciers are vital fresh water sources for many regions of the world, causing concern as they become smaller due to climate change. The length of time water spends in any part of the cycle is called its residence time . The various locations in which water can be stored as it moves through the water cycle are called reservoirs . A water molecule spends an average of only eight days in the atmosphere. However, in some long-term reser- voirs such as oceans, glaciers, large lakes, and groundwater, residence times can be hundreds or even thousands of years. Table 1.1 shows typical residence times for water in reservoirs other than oceans. Rapid use of water can deplete the resource by removing water more quickly than it can be replaced. For instance, a large and growing percentage of agriculture depends on withdrawing groundwater for irrigation, a demand which is frequently unsustainable. table 1.1  Typical Residence Times of Water in Different Reservoirs RESERVOIR AVERAGE RESIDENCE TIME Glaciers 20 to 100 years Seasonal snow cover 2 to 6 months Soil moisture 1 to 2 months Groundwater: shallow 100 to 200 years Groundwater: deep (aquifers) 10,000 years Lakes 50 to 100 years Rivers 2 to 6 months FEEDBACK LOOP In biology, a process in which the level of one substance influences the level of another substance. RESIDENCE TIME The length of time that water spends in sections of the water cycle. RESERVOIR A location that is part of the water cycle in which water is held, such as the oceans or groundwater.