Although, being two separate entities, surface- and groundwater are part of an interrelated system, namely the global water cycle (hydrological cycle). When surface water seeps through the soil it becomes groundwater and conversely, surface water sources can also be fed by groundwater.
Serving most of life's needs, surface water makes up only around 1.2% of the earth's total freshwater amount (being only 2.5% of all earth's water). Groundwater makes up around 30.1% of all freshwater. The following figure (left on the page) illustrates the composition of the total global water in percentages.
FIGURE The Earth's Water (accessed 2013-11-06)
Surface water is the name given to any type of natural water on ground level that is, compared to groundwater, naturally open to the atmosphere such as: rivers, lakes, seas, wetlands, streams, and oceans.
Sources of surface water are:
Losses can be:
| evaporation (vaporization from the surface)|
| recruitment of groundwater
| absorption by plants|
| seepage into the ground|
| abstraction by mankind for e.g. agriculture, industry, living|
Global distribution of surface water
Throughout the globe surface water is naturally distributed in varying amounts since it is affected by precipitation, evaporation and runoff.
FIGUREThe World's Surface Water,accessed 2013-11-06)
Precipitation is any product of the condensation of atmospheric water vapour that falls under gravity. This can be drizzle, rain, sleet, snow, graupel and hail, as main forms.
Evaporationis the vaporation of water from land or from the water surface.
When the soil is infiltrated to full capacity and excess, the water from rain, meltwater or other sources flows over the land, which is then calledrunoff.
Surface water pollution
Based on their origin, sources of surface water pollution are generally grouped into two categories – point sources and non-point sources. If contaminants enter a waterway from a single, indentifiable source (e.g. pipe, ditch, discharge from a sewage treatment plant) it is called a point source. It is defined in section 502(14) of the U.S. Clean Water Act.
A non-point source is any source of water pollution that does not meet the legal definition of "point source" in section 502(14) of the Clean Water Act. Examples for non-point sources are:
- Excess fertilizers, herbicides and insecticides from agricultural lands and residential areas
- Oil, grease and toxic chemicals from urban runoff and energy production
- Sediment from improperly managed construction sites, crop and forest lands, and eroding stream banks
- Salt from irrigation practices and acid drainage from abandoned mines
- Bacteria and nutrients from livestock, pet wastes and faulty septic systems
- Atmospheric deposition and hydromodification
Contaminated water poses a high risk to health. Common water and sanitation-related diseases are diarrhoea, arsenicosis, cholera, flourosis, guinea worm disease, HIV/AIDS, intestinal worms, malaria, schisosomiasis, trachoma or typhoid.
Groundwater represents one of the most important resources for drinking water for human consumption. Worldwide 2.5 billion people are solely dependend on groundwater to satisfy their daily needs for water. With withdrawl rates of currently 982 km3/year (estimated) groundwater is the most extracted raw material in the whole world. Groundwater can be seen as a natural water storage and can be a buffer against shortages of surface water, such as during times of drought.  About 38 % of the worldwide irrigated lands are irrigated with groundwater.
If surface water seeps into the soil it becomes soil moisture or groundwater. In the graphic groundwater flow, two types of aquifers (confined and unconfined) and three types of wells (artesian; flowing artesian and a water table well in an unconfined aquifer) are shown. The figure illustrates how groundwater is recharged and how it circulates through the aquifers.
FIGURE Groundwater: aquifers, wells and circulation(accessed 2013-12-12)
An aquifer is a layer of porous substrate that contains and transmits groundwater.
An aquifer is confined when overlain by a relatively impermeable layer of rock or substrate such as an aquiclude (substrate with porosity that is so low it is virtually impermeable to groundwater) or aquitard (substrate with low porosity).
An aquifer is unconfined when water can flow directly between the surface and the saturated zone of an aquifer.
An artesian well can be created when groundwater becomes pressurized as it flows, this happens when a confined aquifer follows a downward grade from its recharge zone.
Groundwater may flow through the substrate at different rates. This can vary from 50 feet per year or 50 inches per century, depending on the permeability of the substrate. Substrates may be any rock types of any grain size such assandstone, conglomerate, fractured limestone and unconsolidated sand and gravelbut they must be permeable and porous.Being fractured, even rocks like granit and shist can be aquifers. The amount of water storaged in an aquifer can vary between the seasons and from year to year. 
Global distribution of groundwater
Due to climatic conditions and geology the global distribution of groundwater varies considerably. The figure illustrates the amounts of groundwater held by different regions of the world. Asia and Africa provide the greatest groundwater resources, followed by North America and South America. Europe and Australia possess the smallest groundwater resources.
FIGURE Global groundwater resources (accessed 2013-12-16)
Only limited quantities of groundwater are provided by local and shallow aquifers. Areas in green symbolize more complex basins which might comsist of multiple aquifers separated by impermeable rock or include layers of saltwater as well as freshwater. Major groundwater basins (in blue) provide abundant, relatively easily extractable groundwater resources.
FIGURE Global groundwater resources, aquifers(accessed 2013-12-16)
The replenishment of groundwater can occur through rain, snow and surface waters. The recharge rate varies tremendously from place to placedue to a differentgeology and climate of all the countries.Depending of aquifer size and type, the recharge rate determines the extent to which groundwater withdrawl happens in a sustainable scale.The biggest areas with the highestrecharge rates occur in Indonesia, in Papua New Guinea, in the Democratic Rebublic of the Congo and in the Amazonas region.Regions with the lowest recharge rates are the areas of Nevada, the Sahara, Yemen, Oman, parts of Saudi Arabia, and parts of East China.
FIGURE Groundwater recharge in the world(accessed 2013-12-16)
Goundwater is traditionally considered as the safest drinking water source. The quality of groundwater is most commonly affected by waste disposal but also from agricultural activities, groundwater development, mining, spills, leakage from underground pipes and tanks, and road salting. Due to the slow residence time (turnover) of groundwater, effects of pollution may remain in the aquifers for years, decades, or centuries.
Groundwater and agriculture
Agriculture is responsible for up to 60% of withdrawn groundwater whereas the remaining 40 % can be almost equally divided between the industrial and domestic sectors. Poor drainage of soil can lead to rising water tables which come to the surface in low-lying areas. Those can result in major land degradation problems of water logging and soil salinity and increasing levels of salt in surface waters. Consequently, this causes major damage to economies and environments of the places where this occurs. The following figure shows that, due to agricultural use, about 20% of the world's aquifers are being overpumped.
FIGURE Groundwater stress in the world (accessed 2013-12-16)
- ↑ "Precipitation". Glossary of Meteorology. American Meteorological Society. 2009. Retrieved 2009-01-02
- ↑ H. Häckel: Meteorologie. UTB 1338. Ulmer Verlag, Stuttgart 1999 (4. Aufl.)
- ↑ Keith Beven, Robert E. Horton's perceptual model of infiltration processes, Hydrological Processes, Wiley Intersciences DOI 10:1002 hyp 5740 (2004)
- ↑ http://water.epa.gov/polwaste/nps/whatis.cfm [accessed 1.12.2013]
- ↑ UNICEF (2003) Water, Sanitation and Hygiene. URL: http://www.unicef.org/wash/index_wes_related.html, accessed 2013-12-12
- ↑ UNESCO (2012) News 31.05.2012 URL: http://www.unesco.org/new/en/natural-sciences/environment/water/single-view-fresh-water/news/worlds_groundwater_resources_are_suffering_from_poor_governance_experts_say/#.UqjsvyeotF4 [accessed 11.12.2013]
- ↑ Margat, J., and J. van der Gun (2013) Groundwater around the World, CRC Press/Balkema
- ↑ Anrew J. Elmore, Sara J. Manning, John F. Mustard, Joseph M. Craine (2006) Decline in alkali meadow vegetation cover in California: the effects of groundwater extraction and drought. Journal of Applied Ecology 43 (4), 770–779.
- ↑ Siebert, S., et al. (2010) Groundwater use for irrigation. Hydrology and Earth Systems Science, 14, 1863–1880. www.hydrol-earth-syst-sci.net/14/1863/2010/doi:10.5194/hess-14-1863-2010
- ↑ http://imnh.isu.edu/digitalatlas/hydr/concepts/gwater/aquifer.htm [accessed 2013-12-11]
- ↑ Hrubec J. (1995) Water Pollution - Dinking Water and Drinking Water Treatment. The Handbook of Environmental Chemistry 5/5B. ISBN: 978-3-540-48468-4 (Online)
- ↑ A. Zaporozec Dr. (1981) Ground-Water pollution and its sources. GeoJournal 5(5); 457-471
- ↑ Vrba, J., and J. van der Gun. 2004. The World’s Groundwater Resources, http://www.un-igrac.org/dynamics/modules/SFIL0100/view.php?fil_Id=126
- ↑ Agricultural Drainage Criteria. Oosterbaan, On website www.waterlog.info, Chapter 17 in: H.P.Ritzema (Ed.), Drainage Principles and Applications. International Institute for Land Reclamation and Improvement ( ILRI), Publication 16, second revised edition, 1994, Wageningen, The Netherlands. ISBN 90 70754 3 39 [accessed 2013-12-12]
- ↑ Ludwig, D.; Hilborn, R.; Walters, C. (1993). "Uncertainty, Resource Exploitation, and Conservation: Lessons from History". Science 260 (5104): 17–36.