The few rivers and lakes that exist do not contain sufficient water reserves to meet the population's demand. This requires that the majority of water supplied comes from groundwater. Groundwater is a series of aquifers, springs and wells that store water below the surface. These groundwater resources take time to accumulate because they are filled by surface runoff which must seep through the pores and crevices of the soil, sands and rocks. In arid climates, its accumulation takes even longer due to the lack of precipitation each year. At the current rate of groundwater pumping, more water is pumped from aquifers and springs than is replenished naturally. With population growth expected in Tucson and around Arizona, groundwater depletion will only continue to increase.