esm223_13_Reading_Gw_Systems_Flow_and_Storage_Alley_et_al -...

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REVIEW: HYDROLOGY Flow and Storage in Groundwater Systems William M. Alley, 1 * Richard W. Healy, 2 James W. LaBaugh, 1 Thomas E. Reilly 1 The dynamic nature of groundwater is not readily apparent, except where discharge is focused at springs or where recharge enters sinkholes. Yet groundwater flow and storage are continually changing in response to human and climatic stresses. Wise development of groundwater resources requires a more complete understanding of these changes in flow and storage and of their effects on the terrestrial environment and on numerous surface-water features and their biota. G roundwater is a crucial source of fresh water throughout the world. More than 1.5 billion people worldwide ( 1 ) and more than 50% of the population of the United States ( 2 ) rely on groundwater for their primary source of drinking water. Groundwater is an essential part of the hy- drologic cycle (Fig. 1) and is important in sustaining streams, lakes, wetlands, and aquatic communities. During the past 50 years, groundwater depletion has spread from isolated pockets to large areas in many countries throughout the world. Prominent examples include the High Plains of the central United States, where more than half the groundwater in storage has been depleted in some areas, and the North China Plain, where depletion of shallow aqui- fers is forcing development of deep, slowly replenished aquifers with wells now reaching more than 1000 m ( 3 ). Groundwater deple- tion may be the single largest threat to irri- gated agriculture, exceeding even the buildup of salts in soil ( 3 ). In arid regions, much of the groundwater removed from storage today was recharged during wetter conditions in the last ice age, causing further concerns about present withdrawal rates. Global groundwater depletion has been appreciable enough to contribute to sea-level rise during the past century as a result of water pumped from wells that returns to the sea either by runoff or by evapotranspiration followed by precip- itation ( 4 ). Many unfamiliar with its dynamic nature view groundwater as a static reservoir. Even specialists may overlook its linkages across the biosphere and consider it an isolated part of the environment ( 5 ). Yet, as discussed below in general terms and through exam- ples, the dynamic aspects of groundwater flow systems, their recharge, and interactions with surface water and the land surface are numerous and extend over many different time scales. Dynamics of Groundwater Flow Systems A groundwater system comprises the subsurface water, the geologic media containing the water, flow boundaries, and sources (such as recharge) and sinks (such as springs, interaquifer flow, or wells). Water flows through and is stored within the system. Under natural conditions, the travel time of water from areas of recharge to areas of discharge can range from less than a day to more than a million years ( 6 ). Water stored within the system can range in age ( 7 ) from recent precip- itation to water trapped in the sediments as they were deposited in geologic time.
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esm223_13_Reading_Gw_Systems_Flow_and_Storage_Alley_et_al -...

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