15 Spectral Characteristics of Water and Relevance of Remote Sensing Techniques for Hydrological Investigation

Objectives

• To understand remote sensing and their characteristics
• Spectral Characteristics of Water
• Relevance of hydrological investigation through remote sensing

     Keywords

• Remote Sensing, Electromagnetic Energy, Spectral Signature, Water Resources, Remote Sensing Satellites, Surface Water, Water Vapour, Soil Moisture.

    INTRODUCTION

Water in any forms, is the basic natural resource for sustaining the life, covers three-forth part of the earth. Earlier, it was thought that water is present in sufficient quantity on the earth because it was renewed and balanced between availability and demands, but today, the scientists around the world are alarming bells of an impending water scarcity and various issues regarding water resources. In this regards remote sending plays an important role in water resource management which includes monitoring and assessment of water resource, watershed characterisation and water resource development plans. The literal meaning of remote sensing is making observation (sensing) from a distance (remote). It a technique to acquiring information about the earth surface. There is a wide potential of remote sensing in the hydrological investigations through the interaction behavior of the electromagnetic energy with water. There are different processes and stage of water has been investigated through the spectral characteristics. Water on surface like river, lake and reservoirs; underground water; soil moisture, accessible as well as inaccessible areas, where water resources are tied up as ice and snow during the winter months, water at atmosphere, are particularly good areas to demonstrate the capabilities of remote sensing techniques.

Electromagnetic energy, Spectral regions and Spectral signature

Electromagnetic (EM) energy, refers to all energy that moves with the speed of light in a harmonic wave pattern and broadly includes light, heat and radio waves on which remote sensing is totally depends on it and propagated through the space in a sine wave with constant speed. The sun and the earth are two natural sources of EM energy and two types of energy is available for remote sensing i.e. one is reflected energy and another is emitted energy. EM energy has two fields, one is electrical and another is magnetic, consequently, called electromagnetic energy. There are three measurements are used to describe EM energy i.e. wavelength, frequency and velocity. Wavelength is a distance between successive wave crest or trough and denoted by lambda (λ) measured in metre, nanometer and micrometer. The frequency (υ) is the number of cycles of a wave passing through a fixed point on given point, measured in hertz (Hz) corresponding to one cycle per second. The velocity (c) or speed of the electromagnetic energy is equal to the speed of light i.e. 3 x 108 metre/second (3,00,000 km/second or 186000 miles/second). There is a relationship between the wavelength, frequency and energy in the electromagnetic energy  radiation.  Shorter  the  wavelength,  high  the  frequency  and  energy.

Conversely, longer the wavelength, low frequency and energy.

When EM energy is categories into on the scale of wavelength is called EM spectrum or spectral band or spectral region. These are Gamma rays and X-rays region, ultraviolet region, visible region, infrared region, microwave region and radio region. All the EM energy are not reaches to the earth surface or atmosphere is not transparent for all the EM energy to reach earth surface is known as atmospheric absorption region(2.5 – 3 m and 5 – 8 m) and selectively transmits energy of certain wavelengths known as atmospheric window, in both situation, where we are interested in remote sensing for atmosphere and earth surface (Table 1 and Figure 1).

Table 1: Spectral regions used for earth surface Remote Sensing

The spectral signature is a reflectance or emittance of EM energy from the interacting surface material of soil, vegetation and water as a function of wavelength measured at different spectral ranges. On the basis of reflecting behavior of the material at particular wavelength is determines to identify different material of the surface. The spectral response (spectral reflectance and spectral emittance) measured by the remote sensors used to assess the type and condition of features, once these responses are plotted on the scale of wavelength referred as spectral signature.

Figure 1: Atmospheric Window

SPECTRAL CHARACTERISTICS OF WATER

Water has different properties than soil and vegetation (Figure 2). Therefore, water appears distinctive on most of the satellite imagery. Basically, electromagnetic energy incident upon a waterbody is subject to absorption, reflection, or transmission by the water and further scattering by particles suspended in the water. Consequently, interaction behavior of the various wavelength of electromagnetic radiation with different water form, quantity and quality. The visible wavelength of electromagnetic radiation absorbs very little amount, which is less than 5 per cent and rest of radiation is reflected back or transmitted, but near infrared (NIR) and mid-infrared (MIR) radiation is strongly absorbs by water leaving very little radiation to be either reflected or transmitted, resulting sharp contrast between land and water resource.

The water appears blue or bluish-green due to strong reflectance at shorter (visible) wavelength and darker or black in red or near infrared wavelengths. There are three major factors i.e. depth of water, surface roughness of water and suspended material in water, that effects the variability in reflectance of electromagnetic radiation. In the shallow water, the bottom of the water body is also reflecting the radiation along with water appears lighter than dark which appears in deep water. The reflectance from the surface of the water show the wave pattern or wind prevailing on water surface. The roughness of the water surface also affects the reflectance behavior of the electromagnetic radiation. In the smoother surface, the light reflected specularly, giving very high or low reflectance, depending upon the location or angle of the sensor, but in the rough surface more scattering occurs, resulting high reflectance from water surface. There is various suspended material found in the water such as sediments (non-organic) and chlorophyll. The sediments increases the reflectance in the visible energy appears lighter in colour. The water body such contains chlorophyll absorb blue and red electromagnetic energy and reflect green.

Figure 2: Spectral signature of Soil, Vegetation and Water

Conventional color photography possesses some capabilities for penetrating water surfaces. The colour of the water is due to that part of the solar radiation which penetrates the surface and is returned to the surface after selective scattering by the suspended particles or the bottom sediments. This radiation is predominately from the blue and green wavelength regions causing waterbodies to appear various shades of blue and green. So, the colour of the waterbody is determine by the volume reflection,rather than water surface, which is scattered and reflected within the waterbody.The penetration of sunlight into the waterbody is greater in blue-green region rather than blue, red and infrared region and it is influenced by two main factors such as clarity of water that absorb and scatter sunlight; and suspended particles in the water that scatter, reflect and refraction of sunlight. Rayleigh Scattering of the short wavelength (blue) by the particles in the clear deep water appearsblue or blue-green colour. Maximum transmittance of light by clear water occurs in the range between 0.44 m to 0.54 m, with the peak transmittance at 0.48 m, provides an opportunity for recording features on the bottom of the waterbody is greater. In the red region (longer wavelength), absorption of light is greater and only shallow features cane be detected. Water absorbed Near-InfraRedand Middle-InfraRed strongly leaving little radiation to be either reflected or transmitted. These results are sharp contrast between any waterbody and surrounding land surface provides to discriminate land and water only. The turbidity in the water which is suspended such as sand, silt and clay, are also change the spectral characteristics. These turbidity decreases the intensity of light and transparency of the waterbody,resultingreflectance of light in visible region is increases appears bright in the image rather than dark. When sediment concentration increases, the wavelength of peak reflectance is also shifting from blue region towards green region. The presence of larger particles means that the wavelength of maximum scattering shift toward the blue-green to green. Therefore, as sediment content increases there tends to be a simultaneous increase in brightness and a shift in peak reflectance toward longer wavelengths, and the peak itself becomes broader, so that at high levels of turbidity the colour becomes a less precise indicator of sediment content.

The wavelength outside the visible region have some advantages in the mapping of surface water. Beginning in the near infrared and continuing through the longer wavelengths a very thin layer of water will absorb most of the incident energy. The scattered radiation in these wavelengths is correspondingly less, causing clear water to appear very dark. On colour infrared imagery and Landsat FCC, water shows as dark blue to dark black. Radar images display water as completely black. All the transmitted energy is either absorbed by the water or specularly reflected by the relatively smooth surface; none of the energy is returned to the receiving unit.

RELEVANCE IN HYDROLOGICAL INVESTIGATIONS

Hydrology is the study of water on the earth, whether in the form of water vapour in atmosphere,flowing above ground as liquid, frozen as ice or snow in solid for or retained by soil as soil moisture or underground water. Remote sensing provides a synoptic view of the spatial distribution and dynamics of hydrological phenomena including water pollution, often impossible by traditional investigation methods. Microwave and Radar has brought a new dimension to hydrological studies with its passive and active sensing capabilities, allowing the near-real-time image acquisition to include extreme weather conditions or seasonal or diurnal darkness.The capabilities of remote sensing in hydrology can be explained under three heads such as water in the atmosphere, water on earth surface and water beneath the earth surface.

Water in the atmosphere

Water vapour:Detection of water vapour in the atmosphere is also important for the water resources because water vapour play a key role in hydrological cycle. It is possible to detect with remote sensing technology from water vapor absorption band particularly around 6.7 m wavelength. METEOSAT 1, a geostationary satellite launched in 1977 by ESA (European Space Agency) with MVIRI sensor (METEOSAT Visible and Infra-Red Imager). This sensor has water vapour band (5.7 – 7.1 m). METEOSAT 8 (MSG 1- Meteosat Second Generation) satellite with SEVIRI (Spinning Enhanced Visible & Infrared Imager) sensor has channel 5 water vapour 6.2 (5.35 – 7.17 m) and 6 water vapour 7.3 (6.85 – 7.85 m). it provides continuity of the Meteosat first generation broadband water vapour channels to measure mid-atmospheric water vapour and to produce tracers for atmospheric winds. GOES (Geostationary Operational Environmental System satellite) and MODIS (Moderate-resolution Imaging Spectro-radiometer) is also providing water vapour information. GOES provide information at 6.7 m region but MODIS provides the water vapour information in the near-infrared region of the spectrum including band 17 (0.89 – 0.92 m), 18 (0.931 – 0.941 m) and 19 (0.915 – 0.965 m). It is a region where the solar radiation is strong and interference from other atmospheric gases is weak. The span of water vapour wavelength region is 0.89 – 0.99 m. There are three major water vapour absorption bands such as, strong absorption band centered near 0.942 m and weaker absorption bands centered around 0.906 and 0.977 m.

Cloud:The visible and thermal infrared spectral region is used for cloud information. In daylight, cloud appears bright in visible region satellite imagery than the land and water features on earth surface. GOES (USA-NASA) and METEOSAT (ESA) geostationary satellites provide information in visible and infrared region imagery. They used to monitor frontal system, intense thunderstorms such as hurricanes and tornados. In general, cloud do not reflect solar radiation equally well in all directions. Therefore, a single measurement of reflectivity from a single direction make it difficult to determine the total light reflected by cloud (albedo) relative to incident energy. Solution is to obtain multiple images of a cloud from different vantage points. The EOS (Earth Observation Satellite) Terra MISR collects stereoscopic cloud information by viewing each cloud from nine angles. MISR data can distinguish different types of clouds, aerosol particles and surfaces as well as analysed to yield three-dimensional quantitative information about cloud height, structure, thickness, shape and roughness of cloud top.

Precipitation:Remote sensing is also useful in estimation of precipitation. In the initial stage, indirect methods were used by the visible and infrared region such as cloud reflectance, cloud-top temperature, etc. The reflectance from the cloud is appear bright in the imagery and assumed that brightness of cloud likelihood of bearing rain but all bright cloud could not precipitate. Similarly, temperature of the cloud-top was used to determine the precipitation. Basically, cooler the cloud-top likelihood of precipitation but all cold cloud-top not precipitate. The direct method of precipitation estimation by the use of active and passive microwave sensor, which is sensitive to detect ice and water particles within the cloud. Conceptually, precipitation-size ice particles and raindrop significantly reduce the emissivity of the cloud and thus reduce its brightness temperature to below normal background level for computation of rainfall estimate. The Special Sensor Microwave Imager (SSMI) has been carried onboard Defense Meteorological Satellite Program (DMSP) near-polar orbiting satellites since 1987. This sensor included a high frequency channel at 85.5 GHz. band with Vertical and Horizontal (VH) polarization, spatial resolution is 15 Km X 13 Km could distinguish rainfall over land. Tropical Rainfall Measuring Mission (TRMM) satellite, launched on November 27, 1997 carries TRMM Microwave Imager (TMI) passive sensor among five onboard sensor (Visible Infra-Red Scanner (VIRS), Lightning Imaging Sensor (LIS), Precipitation Radar (PR) and Clouds and Earth’s Radiant Energy System (CERES), which is best suited for rainfall estimation over oceans at tropical region. The Precipitation radar provide the information about the rainfall actually reaches to the earth surface operating whereas, passive sensor (TMI) have difficulty to measure rain over earth surface. It also provides three-dimensional map of storm structure. These measurements yield invaluable information on the intensity and distribution of the rain, rain type, storm depth and height at which snow melts into rain. After 17 years of TRMM service was end. Further, Global Precipitation Mission (GPM), is a Core Observatory satellite launched by a consortium of GPM partners in the United States, Japan, France, India and Europe for rain and snow measuring. The GPM constellation of eight or more satellites can observe precipitation over the entire globe every 2-3 hours. The GPM Core satellite has two main payloads such as GPM Microwave Imager (GMI), an advanced, large aperture, 13 different channel passive Microwave rain radiometers to observe energy from the different types of precipitation through clouds for estimating heavy to light rainfall and snowfall. Dual-frequency (Ka/Ku-band) Precipitation Radar (DPR), provides three-dimension information about precipitation specially size of precipitation particles with in cloud system. The Ka-band scans across a region of 125 kilometre and is nested within the wider scan of the Ku-band of 254 kilometre.

Water on the earth surface

Surface water:Remote sensing is widely applied to discriminate the land and water features. The near-infrared and mid-infrared energy is absorbed by the clear water resulting dark colour appears on the imagery. Thus, identification and distribution of waterbody on surface in any form such as river, lake, pond, ocean, etc. is very easy and first step towards hydrological investigations.The monitoring and assessment of surface water resource either seasonal or yearly can be done directly with the help of temporal characteristic of the satellite.

Oceanography and marine resources:The measurement including sea ice, temperature, current, suspended sediments and water colour. Only satellite remote sensors can determine currents synoptically over extensive ocean and coastal regions. Satellite altimetry is one of the essential remote-sensing techniques for monitoring dynamic ocean conditions, including surface currents, local wind speed, and significant wave height. Satellite altimetry measures sea surface heights, providing data on geostrophic circulation, including major ocean currents. Ocean currents can also be determined by satellite synthetic aperture radar (SAR) or tracking the movement of thermal and color features in the ocean. The flow patterns of currents like the Gulf Stream are being mapped with satellite infrared scanners.

Water surface Temperature:Remote sensing is only way to get information about the water surface temperature, it may be inland water surface temperature or sea surface temperature (SST) collected during day-light and night-time using the thermal infrared energy.Heating and cooling behavior of the water is very slow than the land surface as well as temperature difference between day-night is very less in waterbody. Resulting, waterbodies appears warmer and cooler than surrounding land surface during night and day time respectively. The remote sensing satellites NOAA-AVHRR (National Oceanic and Atmospheric Administration-Advanced Very High Resolution Radiometer), and NOAA-GOES (Geostationary Operational Environmental Satellites) has thermal infrared instruments used to measure sea-surface temperature.Landsat thermal band is used to map small waterbodies such as reservoirs, pond and lakes.

Ground water:The assessment of subsurface or ground water is not possible directly with remote sensing, but it is an economical source of reliable and real-time data required for various studies involving various land surface feature such as land use/cover, drainage network, hydro-geomorphological mapping (geomorphology, lithology, structures) which are important parameters to derive groundwater potential zones. The hydrogeological analysis from the satellite imagery provides the information about the groundwater resource as applied in Rajiv Gandhi National Drinking Water Mission (RGNDWM). The integration of various thematic information such as lithology, structure includes lineaments, folds, bedding, geomorphology and hydrology includes rivers, ponds, reservoirs, canal, spring, rainfall,irrigated area, wells, and water recharge structures for the assessment of ground water resource.

Soil moisture:Air occupy the space between the soil particles during dry condition of the soil called interstitial air space,but due to the groundwater rise or due to precipitation/irrigation, infiltration process occurs and that space between the soil particles are occupies by water. So,the soil particles are surrounded by thin layer of water known as soil moisture.Again, the amount of moisture is depending on the texture of soil. Fine soil texture has more capable than coarse soil texture to maintain water as soil moisture. Resulting, higher moisture content in fine soil and absorb more radiant energy and less reflected energy. Themeasurement of soil moisture by remote sensing produce the spatial coverage rather than local as conventional methods.In the visible region (0.4 m – 0.7 m), increasing spectral response by fine and coarse texture soil.But increases in the soil moisture decreases the reflectance form the soil. In the infrared region, water absorption bands at 1.4 m, 1.9 m and 2.7 m exist and absorbed the incident energy produces dips in the spectral graphs in this region.

Water quality: Water quality describes whether or not water is suitable for a specific use or whether or not the surrounding environment may be endangered by pollutants in water. The water pollution can be divided into two sources, one is point source pollution where effluent discharged through pipes and open channels from human habitat areas and industrial waste water, another is non-point source pollution or diffuse pollution which normally occurs from rainwater runoff. Monitoring and assessing the quality of water are critical for managing and improving the water quality. In situ measurements and laboratory analysis of water sample are accurate for a point of time and space but they do not give either the spatial or temporal view of water quality needed for water management purposes. Remote sensing plays an important role by its characteristics such as synoptic coverage, multi-spectral and temporal, near real time information for water quality assessment and monitoring particularly for non-point source pollution. Visible spectral region can be used for total suspended solid detection. Reflectance increases with the increase in sediment concentration. Water transparency, colour, chlorophyll, algal blooms, aquatic vegetation of eutrophication can be monitored through use of colour infrared wavelength region. The oil slick create films on the water surface can also detect from satellite data. It can be detectable in visible/near infrared wavelength band and radar imagery.The thick oil slicks appear brown or black colour in imagery but thin slicks appear silvery or rainbow colour on the water surface.

Irrigation water management:The optimum allocation and economic use of a crucial and scarce input like irrigation water is lifeblood of agriculture. The remote sensing technology significantly provide information of irrigation command area for various crops during different season has become very easy, accurate as well as cost-and-time effective promising technology provides information of irrigated land, identification of crop types, their extent, crop condition and production estimation. Inventory of different sources of irrigation such as canals and their leakage detection, tanks and ponds with dry/perennial conditioncan be mapped with remotely sensed imagery.Injudicious irrigation leading to ineffective water management resulting the soil problem like salinity, alkaline and waterlogging have been mapped and monitored with this technology. Irrigation scheduling is also important for water management and it can be optimized by information about the soil moisture and crop condition over the wide area. The conventional irrigation scheduling uses crop, soil and meteorological parameters resulting limitations in rapid assessment of crop-water stress. The conjunction of remote sensing and conventional method can overcome this problem. The measurement of temperature by infrared thermometry provides the measurement of crop canopy radiation temperature which is directly indicator or crop-water stress. Conceptually, when the plant water potential is high, the transpiration is more, which mean plant temperature is higher than that of ambient air and vice-versa.

River morphology:River morphology is a science of form produced by the action of flowing water due by the process of erosion, transportation and deposition. The synoptic coverage and temporal resolution of remotely sensed data is helpful in river pattern (braiding and straight), delineation of channel condition, bank erosion and deposition, river meandering, changes in channel flow, extent of floods and their damage, waterlogged area, flood plain features anddelta. The major changes in river form such as migration of sandbars, changes in bank lines, reconstruction of paleo-channel can be monitored as well as quantitative evaluation such as river length and width can be done with temporal remotely sensed imagery.

Geo-engineering investigation: Identification of suitable construction site for engineering structures such as dams and barrage across river valley for the irrigation and electric power generation is another avenue of remote sensing application in hydrology. The information about different perspective of terrain (3D modelling) and physical characteristics such as geology (lithology, lineament, unconsolidated material), geomorphology, surface drainage, landslide, sedimentation, watershed characteristics and land use is valuable information for site determination. These information is analyzed carefully from aerial photographs or satellite imagery.

Snow:Assessment of snow covered area and their depth is important parameter to determine the runoff after snow melting. Standard snow survey provides limited sample while remote sensing systems can provide comprehensive data in computer compatible format for rapid analysis.Snowwas first observed by satellite in eastern Canada from the TIROS-1 satellite in April 1960. Since then, the potential for operational satellite based snow cover mapping has been improved by the development of higher temporal frequency satellites such as GOES (Geostationary Operational Environmental Satellite), Landsat, SPOT and IRS series sensors such as LISS I, II, III, IV and WiFS have been extensively used for snow line determination and also snow cover estimation in the basins. NOAA AVHRR, NIMBUS-SMMR and DMSP SSM/I (low resolution satellites) is also widely used for snow cover estiamtion. The IRS series Snow is very distinctive feature that can be detected readily with a variety of remote sensing techniques. Various wavelength bands are sensitives to differing snow properties. Snow has a very high albedo in the visible region of EMR. In visible band (0.4-0.7 m) strong reflectivity contrast between snow and non-snows area exists, which facilitate the mapping of area covered by snow/non-snow. However, in many case, snow and cloud cannot be discriminated in visible region because both has very bright reflectance. The near infrared region (0.8-1.1 m) do not posses such strong snow reflectivity but, can be traced to the occurrence of surface melting, which is important for runoff or break up forecasting. Though in visible and near-infrared region, snow and cloud reflect about equal amount of radiant flux. In middle-infrared region, particularly the 1.55-1.75 and 2.10-2.35 m wavelengths ranges are very useful in discrimination of snow and clouds.At this range snow has very low reflectance, while the reflectance of clouds remains high. Therefore, both cirrus and optically thick clouds can be directly classified and distinguished from snow at the 1.6 m (Warren, 1982). Thermal observation of the snow-pack in the thermal band can be used to delineate the area covered by snow because of the temperature contrast with the snow free area. Temperature of the snowpack can never rise above 0 ⁰C. this can be used to identify when the snowpack surface reaches 0 ⁰C could possibly by melting.

Flood mapping and their monitoring:Every year many rivers and streams spill over their banks, inundating cities and agricultural field, causing crore of rupees of damage to public and private property. Here, remote sensing can be used to monitor the extent of flooding and providing data for streamflow routing. It provides information on flood inundated area for different magnitudes of floods so that the extent of flood can be related to the flood magnitude. Duration of flooding can be estimated by the multiple coverage of same area within 2-3 days by satellites. High resolution satellite data provides the information on floodplain and details of flood control works. By using the contour information on the flood plain can be used for flood risk zone mapping. The microwave sensing provides the vital information during flood because visible and infrared sensing cannot provide ground information during rainy season due to cloud and microwave sensing can be used effectively since, it has penetration capability through the clouds.

Drought:Remote sensing of water and vegetation on the earth’s surface and combination of weather parameter can provide vital information about the drought disaster. Vegetation is the only land category known to strongly absorb visible light and highly reflects and transmits in the near-infrared reflectance, popularly known as vegetation index, which helps in monitoring the photosynthetically active vegetation. Numerous vegetation canopy parameters such as Leaf Area Index (LAI), biomass, absorbed photosynthesis are related with vegetation index. This physical principle is used in rapid crop condition assessment from satellite observation for drought monitoring, early warning and damage assessment.

Conclusion

Remote sensing, though it characteristics of synoptic coverage, near-real-time information capability and different resolutions such as spectral, radiometric, spatial and temporal provides an important information to detect water resources at different forms. In the initial stage of remote sensing technology, it was only applied for the surface form of water detection and discrimination of land and water feature only. But with passage of time it is utilized in almost every component of hydrological cycle such as water in the atmosphere, surface, sub-surface and ocean. It can also helpful in various kind of hydrological investigations such as watershed characteristics, groundwater modelling, river morphology, floods, droughts, soil moisture, water vapour, rainfall, etc.

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Web link

• https://trmm.gsfc.nasa.gov/
• https://www.nohrsc.noaa.gov/technology/avhrr3a/avhrr3a.html