To survive over time, a population must live within the carrying capacity of its ecosystem, which represents a form of natural capital. One of the most important elements of the ecosystem is potable water. Without water there can be no life. As populations grow water is needed for drinking, bathing, to support irrigated agriculture and industry.
Unlike other natural resources or raw materials, groundwater is present throughout the world. Possibilities for its abstraction vary greatly from place to place, owing to rainfall conditions and the distribution of aquifers (rock and sand layers in whose pore spaces the groundwater sits). Generally, groundwater is renewed only during a part of each year through precipitation, but can be abstracted year-round. Provided that there is adequate replenishment, and that the source is protected from pollution, groundwater can be abstracted indefinitely.
Groundwater forms the invisible, subsurface part of the natural water cycle, in which evaporation, precipitation, seepage and discharge are the main components. The “visible” components are all strongly affected by weather and climate, and although they can be contaminated quickly, they generally recover quickly too. By contrast, the subsurface processes of groundwater are much slower and longer lasting, ranging from years to millennia. However, with careful management, these different timescales can be used to create an integrated system of water supply that is robust in the face of drought.
The groundwater cycle in humid and arid regions differ fundamentally from each other. In humid climates, with high rainfall, large volumes of water seep into the groundwater, which contributes actively to the water cycle feeding streams, springs and wetlands during periods when the rainfall is lower. In semi-arid and arid climates, there is by contrast practically no exchange between the surface water and groundwater because the small volume of seepage from the occasional rainfall only rarely penetrates the thick and dry (unsaturated) soils. The groundwater is much deeper and isolated from surface contact. In these areas groundwater resources are only minimally recharged. Our understanding of the complete water cycle is only rudimentary.
Any attempt to accurately model the groundwater component of the water cycle requires adequate measurements and observations over decades. The computer models in common use in the United States only address the shallower groundwater and surface water interactions; GSFLOW (USGS) and ArcHydro (ESRI) are two commonly used models. This has not yet been done, instead rules of thumb and common knowledge assumptions are utilized instead of facts. Robert Bisson of Earth Water Global believes that there is much more water below the Earth’s surface than commonly believed and that the majority of the earth’s water passes beneath the measured surface and groundwater zones undetected.
We do know that groundwater availability varies by location. Precipitation and soil type determines how much the shallower groundwater is recharged annually. However the volume of water that can be stored is controlled by the reservoir characteristics of the subsurface rocks. Groundwater may be present today even in places with very dry climates because of the nature of the local geology and the historic climate cycles that have occurred through time. In the north-eastern Sahara, the Nubian Sandstone Aquifer System underlies an area of more than 750,000 square miles in Chad, Egypt, Libya and Sudan, and contains huge amounts of fresh groundwater. Giant groundwater deposits of comparable size and limited recharge are thought to exist on nearly all continents, but the amount of groundwater that can be pumped out is unknown. Water resources can be used sustainably only if their volume and variation through time are understood. However such information is often lacking, even in so-called developed regions. Hydrology as a science is very young and so little is known. Is it possible that water in these arid regions is finite and non-renewable because of changes in the earth’s climate over the millenia? According to Victor Ponce of San Diego State University, deep percolation represents 2% of the precipitation in California. He believes that the shallow groundwater belongs to surface waters. Any pumping of shallow water (especially for irrigation) effectively shortcuts the natural process, returning to the land surface groundwater that was going to return to the surface waters anyway.
Groundwater is usually cleaner than surface water. Groundwater is typically protected against contamination from the surface by the soils and rock layers covering the aquifer. This is the only available clean drinking water in many parts of the world. However, rising world population, changes in land use and rapid industrialization increasingly place groundwater in jeopardy. Once contaminated, groundwater is very difficult to clean and often after removal of contaminated plumes only long term abandonment of use to allow for natural attenuation is the only possible course of action. As droughts and water shortages appear the value of groundwater has begun to be more fully appreciated. Precious groundwater resources increasingly need to be protected and well managed to allow for sustainable long-term use.
The demand for water is rising as population, economic activity and agricultural irrigation grow. However, worldwide resources of accessible water are decreasing, due to overuse or pollution. The balance between demand (consumption) and supply (resource) is becoming unstable. More than 30 countries suffer from serious chronic water shortage, and groundwater is increasingly being used to cover the demand. According to UNESCO Water for People between 700 and 800 billion US tons of ground water are pumped each year. This is two hundred times the annual consumption of oil and coal used each year.