REMOTE SENS. ENVIRON. 34:207- 215 (1990)
Short Communication
Nature of Suspended Solids and IRS1A-LISSI Data: A Case Study of Tawa Reservoir ( Narmada Basin) V. tC Choubey and V. Subramanian School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, India
IRSIA-LISSI spectral digital data were analyzed to determine the feasibility of quantifying the concentration of suspended solids in the water by this sensor. For this purpose, a small tributary (Tawa) of a major river basin (Narmada Basin) in central India was studied for ground truth evaluation of digital data obtained from the Indian Remote Sensing SateUite-lA Linear Imaging Self-Scanning (IRSIA-LISSI) sensor. Tawa reservoir water samples were collected on 20 October 1988 concurrent with IRSIA overpass. The samples were analyzed to determine the concentration of total suspended matter, grain size, and mineralogy. The results indicate that, in the concentration range between lOmg / L and 50mg / L, a positive functional relationship exists between the concentration of suspended solids and the visible wavelength Bands 1, 2, and 3 (0.45-0.681~m). It has been observed that mineralogy and grain size are the main factors which influence the reflected radiance at lower concentration level (10- 50mg / L) of suspended solids. It can be concluded that as the concentration of suspended solids in the 1 0 - 50 mg / L range
Address correspondence to V. Subramanian, School of Environ. Sci., Jawaharlal Nehru Univ., New Mehraully Rd., New Delhi 110067, India. Received 10 May 1990; revised 3 October 1990. 0034-4257//90 / $3.50 ©Elsevier Science Publishing Co. Inc., 1990 655 Avenue of the Americas, New York, NY 10010
increases the spectral response increases. It can be stated that IRSIA-LISSI data provide a good foundation for further development of remote sensing as a practical tool in the estimation of suspended solids.
INTRODUCTION Rivers in India carry more than a billion tons of sediment load annually, in addition to many pollutants to various estuaries, reservoir, and adjacent marine environment (Subramanian, 1979). Suspended matter deposited in reservoir reduces their storage capacity and hence their ability to control flooding. Information on distribution patterns of sediment load during various seasons and times is a prerequisite for better reservoir management and critical in determining reservoir lifetime. Conventional methods for the measurement of suspended load and pollutants in situ and in the laboratory are expensive compared to remotely sensed data. Satellite-borne sensors have the capabilities of providing repetitive, low-cost multispectral coverage over wide areas and have the potential to monitor the water quality. The prospect of using remote sensing techniques for identification and quantification of suspended solid is promising.
207
208 Choubey and Subramanian
Several investigators have used remote sensing techniques for water quality monitoring. Landstat, Coastal Zone Color Scanner data have been used for mapping sediment distribution patterns in the reservoirs, estuaries (Khorram and Cheshire, 1985, Amos and Topliss, 1985; Ritchie and Cooper, 1988). Khorram and Cheshire (1985) suggested that shorter wavelength bands of visible region are very useful for the quantification of suspended sediments in reservoir. The shorter wavelength (0.4-0.7/zm) bands are available in Landsat TM and in IRS1A-LISSI sensors. Studies using IRS1A data have not been reported in the literature and the present study is a first attempt. Hence IRS1A-LISSI spectral data are selected in the present study. The objective of this investigation was to evaluate the relationship between suspended solids concentration and IRSIA-LISSI digital data in inland water body and to define mineralogy and grain size of suspended solids in water.
STUDY AREA For the evaluation of the relationship between suspended solids concentration and LISSI digital data, a small reservoir (Tawa) in the Narmada Basin in central India was chosen. The Tawa Reservoir is situated in central India on the Tawa River. The Tawa River is a major tributary of the Narmada River. The catchment area of the Tawa Reservoir is 5982.90 sq km. The reservoir is designed to have a storage capacity of 1.87 million acre feet (0.231m ha m) at a full reservoir level of 1166.00ft (355.397m). The water surface area is about 225 sq km. The climate of the catchment is temperate and the average rainfall is 1546.13 mm.
METHODOLOGY Field Work The Tawa Reservoir was sampled by a motor boat all along the reservoir at predetermined sampling locations. The water samples were immediately taken to the laboratory for further analysis. Location of sampling points are given in Figure 1. To reduce the time lag between IRS1A overpass and sampling, the samplings were carried out between 7 a.m. and 6 p.m. on the day of overpass,
i.e., 20 October 1988. In order to obtain the desired depth (water clarity) of the sample point, the Secchi disc was used to measure the depth at which the disc becomes invisible. Water samples were collected with a depth sampler similar to a Punjab type bottle sampler (Ramasesha et al., 1985).
Laboratory Methodology Water samples were filtered through membrane filter papers (millipore 0.45/zm) to estimate the total suspended matter in m g / L . Suspended sedinmnts of five water samples representing various reaches of the reservoir were selected for bulk mineralogy. Slides of suspended sediments were prepared by a drop on slide technique (Gibbs, 1967) and glycolated. The mineral compositions were determined using a Philips X-ray diffractometer with Cu-K radiance and Ni filter. The mineral identification and estimation of abundance were carried out following the method of Carrol (1970). Thirteen water samples representing various reaches of the reservoir were chosen for the particular grain size analysis. The size analysis of suspended solids for reservoir water was done by the Fritsch Laser Particle Sizer (Model ANALYSETTE-22).
Satellite Data IRSIA-LISSI geometrically corrected computer compatible tapes (CCT) for the study area pathrow, 27-52, for 20 October 1988 were purchased from the National Remote Sensing Agency, Hyderabad, India. The acquired IRS1A-LISSI spectral digital data was in four spectral bands with spatial resolution of 72.5 m. The time of overpass was 10.25 a.m. (IST) and sun elevation 47.2320 °. At the time of overpass the sky was clear with dry weather (temperature 25°C), and no wave action was observed on the surface water of the reservoir. The details of the LISSI bands and their wavelength range are given below (IRS, 1986):
LISSI Bands Band Band Band Band
1 2 3 4
Wavelength Range (~m) 0.45-0.52 0.52-0.59 0.62-0.68 0.77-0.86
visible visible visible near infi'ared
Remote Sensing of Suspended Solids 209
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The digital data were analyzed using a VAX 11/780 computer. The land and water interface was identified and was masked (Lodwick and Harrington, 1985) so that only water would be analyzed. Histogram of the response levels for all four LISSI bands were produced and shown in Figure 2. Digital data for each spectral band were extracted for the 3 x 3 array encompassing each of the 44 sampling locations. The mean pixel values were calculated for the pixel block of each array for all four bands.
RESULTS AND DISCUSSIONS The Tawa Reservoir can be divided into three hydrodynamic water quality zones: Tawa and Denwa River inflow, transition zones, and the main body of the reservoir (Fig. 1). The inflow area is characterized by high concentration of sus-
pended solids. The main body and transition zone shows relatively less suspended solids concentration. The measured values of suspended solids and mean pixel values for all the LISSI four bands are given in Table 1. A review of Table 1 indicates that in general, the Tawa and Denwa inflow areas carried between 2 2 - 3 0 m g / L and 4 0 - 4 5 m g / L and relatively low concentration between 11-20 m g / L and 3 2 - 4 0 m g / L of suspended solids in transition zone, respectively. Between 2 0 m g / L and 2 8 m g / L concentration of suspended solids was observed in the main body of the reservoir. This may be due to the inflow from the river joining the reservoir and the mixed influence of both river waters. The histogram of grain size distribution of suspended solids is given in Figure 3. It was observed that 0 . 5 - 1 5 / ~ m size particles are dominant in the surface water of the reservoir main
210 Choubey and Subramanian
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body. However, in the inflow to transition zone of both the rivers (Tawa and Denwa) 0.5-50/xm size particles are dominant. The suspended solids are moderately to poorly sorted in all the hydrodynamic zones of both rivers. The average skewness of the suspended solids varies from coarse skewed to fine skewed (Table 2). The suspended solids in the Tawa Reservoir are platykurtic to laptokurtic (Cronan, 1972), which suggests that the suspended sediment of the inflow area of both the rivers are better sorted than transition zones of the reservoir. Particle size is the dominant factor relating suspended sediment concentration to reflectance (Hoyler, 1978). The Tawa inflow-transition zones contain a higher percentage of fine grained matter compared with Denwa zone. The abundance of relatively coarse-grained quartz, feldspar, and montmorillonite clay in the Denwa inflow zone is a controlling factor for higher spectral response than the Tawa zone. However, further investigation to determine the influence of grain size and
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mineralogy individually over incident solar energy is needed. The mineralogy of suspended solids of the reservoir water are given in Tables 3a and 3b. These tables indicate that the Tawa River discharges suspended load into the reservoir, which carries about 55% clay minerals (illite 43% and montmorillonite 9%) and about 45% nonclay mineral (feldspar 45%), whereas the Denwa River discharges about 62% quartz and feldspar and about 20% montmorillonite and 8% kaolinite. The mixed influence of both the rivers was observed in the reservoir main body, where illite followed by montmorillonite is dominant in clay minerals and feldspar is abundant in nonclay minerals. The dominance of quartz, feldspar, and montmorillonite in the Denwa inflow and transition zone is one of the important factors which influence the spectral response. A plot of the concentration of suspended solids against the mean pixel value for each of the LISSI bands (Fig. 4) shows that, as the concentration of
Remote Sensing of Suspended Solids
Table 1. Field Measured Values of Suspended Solids Concentration and 20 Oct. 1988 LISSI Mean Pixel Values in all Four Bands Sample No. a
Suspended Solids in PPM
Band l
Band2
Band3
Band4
17.6 16.3 17.3 17.3 17.3 17.3 17.7 18.2 16.9 16.8 16.8 16.8 16.1 15.7 16.1 16.2 15.4
23.6 19.0 15.3 16.8 16.4 16.4 16.2 16.3 17.1 14.1 13.1 13.1 13.7 13.0 13.0 12.9 13.5
15.4 15.5 15.2 14.3 14.3 15.5 15.2
12.5 12.2 12.0 11.9 11.0 13.0 17.2
16.6 17.0 17.3 20.4 20.4 19.0 19.1 19.9 19.0
12.0 12.0 12.0 12.6 12.6 12.0 12.0 12.0 13.0
23.1 23.1 22.6 23.4 19.9 22.9 23.0 23.2 23.2 26.0 23.1 26.1 25.7 25.2
19.0 14.4 16.2 14.0 17.7 15.8 14.9 14.9 15.0 16.5 20.8 17.5 17.0 17.3
Tawa Inflow Zone 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
30 30 22 24 22 22 23 23 22 15 20 20 21 14 16 21 13
18 19 20 21 22 23 24
11 13 12 12 12 11 14
25 26 27 28 29 30 31 32 33
20 22 22 24 27 24 23 28 23
34 35 36 37 38 39 40 41 42 43 44 45 46 47
32 40 32 39 24 35 38 37 38 43 27 44 43 46
36.3 37.1 36.2 36.7 36.7 36.7 37.4 37.1 36.6 36.0 36.2 36.1 36.2 36.0 35.9 37.0 36.1
20.2 20.0 20.1 20.3 20.1 20.1 21.0 20.3 20.0 19.2 19.0 19.0 19.1 18.5 19.0 19.3 13.9
Tawa Transition Zone 35.9 36.1 35.8 35.8 35.9 35.3 36.3
18.4 18.1 18.3 18.2 18.5 18.3 19.3
Main Body 38.7 39.2 39.6 41.2 42.2 41.3 41.2 42.8 41.1
21.0 21.3 21.4 24.0 24.3 24.0 23.9 25.0 24.0
Denwa Inflow Zone 43.1 43.3 41.1 43.4 39.8 41.8 42.7 42.4 42.6 44.0 40.0 43.6 43.6 43.2
25.7 26.6 24.8 26.4 22.9 25.7 26.0 26.0 26.0 27.2 24.4 26.9 26.9 26.9
aSample locations are shown in Figure 1.
211
212 Choubey and Subramanian
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suspended solids increased, there was an increase in pixel value. This indicates that, as the concentration of suspended solids in the surface water (at secchi depth) increases, more solar radiation is reflected back to the atmosphere (Ritchie et al., 1987). Table 1 also indicates that there is a correlation between the increasing amplitude of the
reflected radiance and increasing suspended load from the Tawa and Denwa Rivers of the reservoir. In general, Bands 1, 2, and 3 (0.45-0.68/xm) of the visible region exhibit substantial grey level variation across a reservoir surface. This grey level variation which is related to reflected energy detected by LISSI is highly correlated with the
Table 2. Statistical Parameters of Suspended Solids (Units in ~b)a Sample Nos. Mean Std. deviation Skewness Kurtosis
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Remote Sensing of Suspended Solids
213
Table 3a. Mineralogy of Suspended Sediment for 20 Oct.
Table 3b. Percentage of Clay Minerals in Suspended
1988 Reservoir Water Samples (in %)a
Sediment for 20 Oct. 1988 Reservoir Water Samples (in %)a
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43 10 14 2 12
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15 31 40 42 46
2 7 10 2 7
80 16 23 5 20
17 51 48 50 50
2 20 7 20 10
2 3 5 4 1
0 10 16 21 19
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aSample no. 15 from Tawa inflow zone• Sample no. 31 from main body. Samples nos. 40 42 and 46 from Denwa inflow zone. Sample locatmns are shown m Figure 1.
s u s p e n d e d solids p a t t e r n in the reservoir surface water. This high correlation is d u e to scattering from s u s p e n d e d s e d i m e n t (Jerlov, 1968). Bands 1 a n d 2 s h o w e d p o o r correlation b e y o n d 40 m g / L (total s u s p e n d e d matter), w h i c h indicates that t h e y are useful for relatively clear water. At h i g h e r ( 4 0 m g / L ) concentration, points scattered a n d s h o w fiat response. H o w e v e r , Band 3
( 0 . 6 2 - 0 . 6 8 / x m ) exhibits good correlation ( r = 0.93) over the entire range o f s u s p e n d e d solids c o n c e n tration ( 1 0 - 5 0 m g / L ) . Band 4 ( 0 . 7 7 - 0 . 8 6 /zm) reflection levels are very low and show very poor correlation ( r = 0.48) with increasing concentration o f s u s p e n d e d solids. It can be stated that solar radiation reflected from w a t e r surface varied with a m o u n t of sus-
Figure 4. Relationship between suspended solids concentration and LISSI mean pixel values in all four bands.
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Choubey and Subramanian
pended solids and wavelength (Ritchie et al., 1976). The lower wavelength band range, 0.45-0.59/zm (Bands 1 and 2), does not show linearity with suspended solids beyond 4 0 m g / L . However, as the concentration increased, the response value also increased. As the wavelength range advanced from 0.45 to 0.68kcm, the change in response value with change in suspended solids exhibited a more and more regular pattern. In general, the mineralogy and grain size varied in inflow, transition, and main body of the reservoir. The montmorillonite in combination with quartz, feldspar, and relatively coarse particles controls the spectral response in the Denwa inflow zones. The fine-grained illite and montmorillonite in conjunction with feldspar influences the spectral response in Tawa. These observations indicate that in more or less the same concentrations of suspended solids the Denwa inflow zones reflect more energy than the Tawa. Therefore, it can be stated that mineralogy and grain size are probably the prime factors which influence the reflectance at lower concentration levels ( 1 0 - 5 0 m g / L ) of suspended solids. This observation also confirms the laboratory findings with respect to black and brown sediments in suspension (Choubey and Subramanian, 1990). It has been observed that, for low concentrations, shorter wavelengths are more useful than longer wavelengths (near infrared) in quantifying the concentrations of suspended solids. Previous research showed that color of the sediment is also a factor which influences the radiance for turbid water (Choubey, 1990), but this observation is proved to be not important under the field conditions we observed. This may be due to intense mixing of different river water in the reservoir producing uniformity in color.
SUMMARY AND CONCLUSIONS
In general, the Tawa Reservoir spectral response values obtained from LISSI data show maximum response values in the inflow zones and decrease toward the transition zone and the main body of the reservoir. However, the Denwa flank of the reservoir shows a higher response value than the Tawa due to variation in grain size and mineralogy. The mixed influence of sediment load from both rivers was observed in the main body of the
reservoir. Here illite and montmorillonite, feldspar, and carbonate percentage are significantly high, which are mainly contributed by the Denwa River. This may be a reason for the higher spectral response values in the main body of reservoir and to some extent to inflow of sediment load from rivers joining the main body. In general, Bands 1, 2, and 3 (0.45-0.68/xm) of the visible region show substantial gray level variation across reservoir surface water. This appear to be due to the suspended solids. The result of the calibration of IRSIA-LISSI spectral digital data to concentration suspended solids are encouraging. On the basis of results obtained from present study it can be concluded that: 1. In the concentration range between 1 0 m g / L and 5 0 m g / L a statistical significant relationship exists between concentration of suspended solids and visible wavelength Bands 1, 2, and 3 (0.45-0.86/xm). As the concentration of suspended solids increases, the spectral response values increase in the range of 10-50 mg/L. 2. IRSIA-LISSI spectral data can be effectively used to quantify suspended solids concentration. 3. Visible wavelength bands of LISSI are more useful than the near infrared band (Band 4), especially Band 3 (0.62-0.68/zm) for the quantification of concentration of suspended solids. Bands 1 and 2 show poor correlation beyond 40 m g / L concentration. However, Band 3 exhibits good correlation across a wider range in suspended solids concentration (10-50 m g / L ) . 4. The concentration of suspended solids should be constant over the depth at which the secehi disk disappeared. The secchi disk depth should be less than the water depth to avoid bottom noise. 5. The results obtained tkom the study may not be applicable to other reservoirs due to variation in the catchment characteristics and climate.
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Remote Sensing of Suspended Solids
215
the Nimbus 7 coastal zone color scanner, Can. J. Ren~te Sens. 11:85.
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Carrol, D. (1970), Clay minerals: a guide to X ray diffraction, Geol. Soc. Am. Spec. Pap. 125:85.
Lodwick, G. D., and Harrington, (1985). Deriving sediment information of lake Athabasca using Principal component analysis of Landsat data, Can. J. Remote Sens. 11:1283-1289.
Choubey, V. K., (1990), Modelling sediment and dissolved load of Tawa reservoir and river (M. P) by remote sensing techniques, Ph.D. thesis, Jawaharlal Nehru University, New Delhi, India. Choubey, V. K., and Subramanian, V. (1990), Spectral response of suspended sediments in water under controlled conditions, J. Hydrol., forthcoming. Cronan, D. S. (1972), Skewness and kurtosis in polymodal sediment from Irish Sea, J. Sed. Petrol. 42:102-106. Gibbs, R. J. (1967), Quantitative X ray diffraction analysis using clay mineral standard extracted from the samples to be analysed, Clay Min. 7:79-90. Hoyler, R. J. (1978), Towards universal multispectral suspended sediment algorithms, Remote Sens. Environ. 7:323. IRS (1986), IRS Data User Hand Book, Document No. 1RS/NRA/NDC/HB-01/86, NRSA, Hyderabad. Jerlov, N. G. (1968), Optical Oceanography, Elsevier, New York, Vol. 5, p. 194. Khorram, S., and Cheshire, H. M. (1985), Remote sensing of
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