REMOTE SENSING APPLICATIONS FOR WATER RESOURCES.

Whether for irrigation, power generation, drinking, manufacturing, or recreation, water is one of our most critical resources. Visual Image interpretation can be used in a variety of ways to help monitor the quality, quantity, and geographic distribution of this resource and also deciphering ground water with help from aerial photograph.

In general most of the sunlight that enters a clear body is absorbed within about 2 m of the surface. The degree of absorption is highly dependent on wavelength. Near infrared wavelengths are absorbed in only a few tenths of a meter of water, resulting in very dark image tones of even shallow water bodies on near-infrared images. Absorption in the visible portion of the spectrum varies quite dramatically with the characteristics of the water body under study. From the standpoint of the imaging of bottom details through clear water, the best light penetration is achieved between the wavelengths of 0.48 and 0.60 μm. Penetration of up to about 20 m in clear, calm ocean water has been reported in this wavelength band.

All naturally occurring water contains some impurities. Water is considered polluted when the presence of impurities is sufficient to limit its use for a given domestic and/or industrial purpose. It is rarely possible to make a positive identification of the type and concentration of a pollutant by visual image interpretation alone ( Lillesand and Kiefer,2000). However, it is possible to use visual image interpretation to identify the point at which a discharge reaches a body of water and to determine the general dispersion characteristics of its plume. In some instances, such as the case of sediment suspended in water, it is possible to make valid observations about sediment concentrations using quantitative radiometry coupled with the laboratory analysis of selective water samples. Sediment pollution is often clearly depicted on aerial and space images.

Sediment pollution is often clearly depicted on aerial and space images. Materials that form films on the water surface, such as oil films, can also be detected through the use of aerial and satellite images. Normal colours or ultraviolet aerial photography is often employed for the detection of oil films on water.

Thick oil slicks have a distinct brown or black colour. Thinner oil sheens and oil rainbows have a characteristic silvery sheen or iridescent colour banding but do not have a distinct brown or black colour.

Water pollution is any physical or chemical change in water that can adversely affect organisms. It is a global problem, affecting both the industrialized and the developing nations. It is harmful to humans, animals, to desire able aquatic life or otherwise causes significant departures from the normal activities of various living communities in or near the bodies of the water.

Not all pollutants are the result of human activity. Natural sources of pollution include such things as minerals leached from soil and decaying vegetation. When dealing with water pollution, it is appropriate to consider two types of sources: point and non point. Point sources are highly localized, such as industrial outfalls. Non-point sources, such as fertilizer and sediment runoff from agricultural fields, mining wastes, have large and dispersed source areas.

According to Verner (1977) the detection of pollutants in water is more complex because the light attenuation characteristics of water limit detection of below-surface pollutants to the visible and near-visible portions of the spectrum. Even for surface pollutants, detection is often difficult, because the characteristic scattering or reflection of sunlight by pollutants is a function of the state of surface roughness as well as the angle of incident and reflected sunlight. Also, many dissolved chemicals have no spectral signature detectable through remote analysis. On the other hand, there are classes of pollutants that may be detected when water surface conditions and sun angle permit. These are particulates, algae, petroleum products, and thermal anomalies.

Sediment pollution is a tedious problem in major rivers of the world. Sediments make the rivers, streams, channels and reservoirs to overflow. They also change the flow rates and depth of water systems( Sharma and Kaur, 1994). Sediment pollutions are clearly depicted on aerial images.

Materials that form films on the water surface, such as oil films, can also be detected through the use of aerial and satellite images. Oil enters the world´s water bodies from a variety of sources including natural seeps, municipal and industrial waste discharges, urban runoff, and refinery and shipping losses and accidents. Thick oil slicks have a distinct brown and black colour. Thinner oil sheens and oil rainbows have a characteristic silvery sheen or iridescent colour banding but do not have a distinct brown and black colour (Lillesand and Kiefer,2000).

Verner (1977) has given different criteria for detecting several common types of pollution mentioned in table below.

1. Oil leaks and seepage: Dark blue or black or rainbow colouration; difference in reflectivity from surrounding area.

2.Water outfalls : Characteristic outfall plume; colour contrast; turbulence.

3. Thermal pollution: Characteristic red colour on IR colour film; light shading on black and white IR film.

4. Eutrophication : Characteristic green colour; inability to see beneath surface.

5. General water quality: Colour, shading, or tone contrast; water opaqueness.

6. Stack effluent aerosols Haze and smog. :Characteristic stack plumes. General decrease in ground detail.

Direct human interventions over the years have lead to reduction in groundwater recharge. These include deforestation, destruction of local water systems (including traditional water systems, e.g. ponds, tanks, lakes, wetlands and so on). Deforestation also leads to change in river flow regime in the affected area that also affects the recharge in the given area.

There are larger and indirect human interventions that has also affected the groundwater recharge systems, including urbanization, concretization of more and more land, the those factors that lead to global warming also contribute in reduction in groundwater levels as evapo-transpiration needs are higher when temperatures go up, leading to more groundwater use.

Mining also leads to destruction of groundwater recharge systems in the mined areas. In fact mining areas groundwater is many times unnecessarily pumped out to the near by rivers so that mining becomes possible.

A knowledge of groundwater location is important for both water supply and pollution control analysis. Groundwater is one of the most important source of water. Almost 85% of the rural water supply in India is dependent on groundwater (Ministry of rural Development, government of India). Remote sensing plays a vital role in delineating potential areas of groundwater occurrence for detailed exploration, thus reducing the cost and time involved in groundwater exploration. Potential groundwater areas cannot be seen on satellite images directly. The clue to the groundwater search is the fact that sub-surface geological elements forming aquifers have almost invariable surface expressions, which can be detected by remote sensing techniques (Joseph,2005). Satellite data provide information about geomorphic features, structures, land uses and rock types (in a few cases) indicating the presence of groundwater. Some selected landforms and structural features that are indicators for potential groundwater zones are valley fills, palaeochannels, alluvial fans, dykes, interdunal depression etc.

Surface water forms a part of the hydrosphere which is linked without discontinuity to the groundwater and on this basis it is viewed as a direct hydrogeological index. Rivers, rivulets, lakes and temporary streams belong to this category. These are the features which are associated with recharge zones.

Groundwater is also linked with lake water, which is direct positive indicator (Nefedov and Popova,1972). In aerial photographs, fresh water lakes are deciphered from the uniform tone of the reflecting water surface. The photographic image of rivulets is analogous to that of rivers and differs only in being of smaller size. The rivulets show the presence of outlets of groundwater upstream (Nefedov and Popova,1972; ).

To facilitate recharge if a check dam is constructed across a rivulet on different intervals or stream flowing or at the meeting point of two rivers will allow sufficient water to percolate to cause effective recharge to the groundwater aquifer.

References:

Joseph, G. (2005). Fundamentals of Remote Sensing (2nd ed.). University Press, Hyderabad.

Lillesand,T.M. and Kiefer, R.W. (2000). Remote Sensing and Image Interpretation. John Wiley and Sons, Inc. New York.

Nefedov, K.E. and Popova, T.A. (1972). Deciphering of Groundwater from Aerial photographs. Amerind Publishing Co. Pvt. Ltd. New Delhi.

Pandey, S.N. (2001). Principles and applications of photogeology. New Age International Publishers.

Sharma,B.K. and Kaur, H. (1994). Water pollution. Goel publishing house, Meerut.

Verner, S.S.(1977). Environmental monitoring of mineral-related industries. In book "Remote Sensing applications for Mineral Exploration" ed. Smith, W.L. Dowden, Hutchinson and Ross, Inc. Pennsylvania. pp. 363-388.