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Gayana (Concepción)

versión impresa ISSN 0717-652Xversión On-line ISSN 0717-6538

Gayana (Concepc.) v.68 n.2 supl.TIProc Concepción  2004

http://dx.doi.org/10.4067/S0717-65382004000200004 

 

Gayana 68(2) supl. t.I. Proc. : 16-21, 2004 ISSN 0717-652X

REMOTE SENSING OF THE SEDIMENTATION PLUME OF THE RIVER SAN JUAN

 

Daniel Ballestero

Laboratorio de Oceanografía y Manejo Costero Universidad Nacional Costa Rica dab2@una.ac.cr


ABSTRACT

The River San Juan (RSJ), in the border between Nicaragua and Costa Rica, is one the major rivers in Central America and drains the largest basin in the region (38570 km2) in terms of volume. Extending from Lago Cicibolca to the Caribbean Sea, the RSJ is an important source of freshwater, sediments, nutrients and pollutants to the continental shelf. Ecosystems degradation, contamination of water bodies and overexploitation of natural resources, particularly deforestation in the southern part of the basin, modified and incremented sedimentation processes in the basin and the coastal margin during the last few decades.

Advanced Very High Resolution Radiometer and Sea-viewing Wide Field-of-view Sensor data, together with in-situ CTD, current meter, turbidity sensor and water sampling for chemical analysis data are used to study, for the first time, the spatial and temporal variability of the plume of the RSJ and the coastal zone under its influence. An atmospheric correction has been implemented in order to obtain water reflectance from the AVHRR channels 1 and 2. Results from the above satellite and in-situ observations are presented.


 

INTRODUCTION

The River San Juan, in the border between Nicaragua and Costa Rica (Figure 1), is one the major rivers in Central America and drains the largest basin in the region (38570 km2) in terms of volume. Extending from Lago Cocibolca in Nicaragua to the Caribbean Sea, the RSJ is an important source of freshwater, sediments, nutrients and pollutants to the continental shelf.

The coastal zone is characterised by persistent trade winds blowing from the E-NE with average intensities of 6-9 m s-1 all year round. Precipitation is abundant (6 m per year) with relative diminutions in April-May and October-November and discharge to the Caribbean from numerous rivers is very high. The continental shelf narrows from over 200 km in the north of Nicaragua to less than 20 km at the border with Costa Rica (Figure 2), where the slope is 1:80. Several submarine rocky formations located near the shelf edge, 20 km seaward from the coast in the limit between the countries, the Morris Shoal, serve as biological sanctuaries for several species of fish and the Green Turtle. Wave energy on the coast is 4.3 x 1010 ergs m-1 s-1 and the tidal range is less than 20 cm. The conditions above result in a typically wave-dominated delta with straight coast line and blocked and deflected distribution channels (Murray et al. 1982). The river splits in two main channels: the Río Colorado to the south, discharging in Barra del Colorado, Costa Rica, and River San Juan to the north, discharging in San Juan del Norte in Nicaragua (figure 2).

Figure. 1: The River San Juan Basin.


Figure. 2: The River San Juan delta and stations

Ecosystems degradation, contamination of water bodies and overexploitation of natural resources, particularly deforestation in the southern part of the basin, have modified and incremented sedimentation processes in the basin and the coastal margin during the last decades. The use and management of water resources generated conflictive situations among institutions and social players of Costa Rica and Nicaragua. Since 1992 both countries established mechanisms for technical and political dialogue in order to achieve sustainable development in the area.

As part of a large bilateral effort (the San Juan River Basin project) to formulate a Strategic Actions Programme for the integrated management of water resources in the San Juan River Basin, supported by the Global Environment Facility, the United Nations Environment Programme and the Organization of American States, the Universidad Nacional from Costa Rica and the Universidad Centroamericana from Nicaragua conducted a study of the sedimentation plume of the River San Juan and the coastal Caribbean zone under its influence. During 2002 until July 2003 data were obtained from polar orbiting satellites, in-situ water sampling, CTD, current meter, turbidity sensor and under water camera to describe, for the first time, the physical, chemical and biological conditions in the plume and the coastal zone. Results derived from the analysis of satellite based optical radiometers and in situ physical data are presented.

MATERIALS AND METHODS

Thematic Mapper

Two cloud free images were identified in the archive of the LANDSAT series for dates 02/02/1986 (Landsat-5) and 15/06/2001 (Landsat-7). Only the image of 1986 shows useful information for the southern part of the plume and the coastal zone.

SeaWiFS

Data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) radiometer collected during the study at the ground station operated in the Laboratorio de Oceanografía y Manejo Costero (LAOCOS) were used to produce 57 1 km resolution maps of water-leaving radiance at 670 nm and chlorophyll-a concentration using the NASA algorithms as implemented by the Seadas software. 29 images had enough cloud-free areas to provide useful information

AVHRR

The data from the Advanced Very High Resolution Radiometer (AVHRR) acquired at the LAOCOS ground station constitute the main body of remote sensing information for this study. Over 300 night and day passes were identified with cloud free areas and 83 were selected to produce 1 km resolution maps of the area of interest. The higher frequency of sampling provided the AVHRR, together with the extension of the historical data archive available at LAOCOS, proved a valuable tool for this study.

The Multichannel sea Surface Temperature algorithm (McClain et al. 1985), using the infrared channels 4 and 5 of the AVHRR, was implemented to produce sea surface temperature (SST) images where the river-borne water was traced using its thermal contrast with the Caribbean resident water.

Channels 1 and 2 of the AVHRR were used to trace the plume where high concentrations of suspended sediments increase water reflectance. Although channels 1 (visible, 580-680 nm) and 2 (visible-near infrared, 725-1100 nm) of the AVHRR were not designed for marine applications, a procedure was implemented to remove the atmospheric component of the radiance measured by the radiometer as in Stumpf (1987). The visible channels of the AVHRR have been successfully used to study turbid coastal waters (eg. Stumpf 1987, 1992, Stumpf and Pennock 1989, Walker 1995), cocolithophorid blooms (Groom and Holligan 1987) and cyanobacteria blooms in the Baltic Sea.

Atmospheric effects are removed in two steps. In the first step the atmospheric Rayleigh (molecular) radiance arriving at the sensor is removed and the reflectance measured by the radiometer for each channel is calculated as

(1)

where l is the wavelength (630 nm for channel 1 and 912 nm for channel 2), A is the albedo measured by the AVHRR, E0 is the Solar Constant with spectral values taken from Fröhlich (1986), r is the normalised distance between the Earth and the Sun, q0 is the solar zenith angle, T0 is the transmittance of the atmosphere in the Earth-Sun direction, T1 is the atmospheric transmittance between the satellite and the point observed on the surface and Lr is the atmospheric Rayleigh radiance arriving at the satellite calculated as

exp( -tg(λ)/ cosθ0)

(P(ψ) + 0.052P(ψ+ )) (2)

where P(ψ±) is the phase function for Rayleigh dispersion (Gordon et al.1983) and tg and tr are the optical depths for gaseous absorption and Rayleigh dispersion respectively.

In the second step the radiance arriving at the satellite originated in atmospheric aerosols dispersion (Mie dispersion) is removed following a scheme similar to Gordon et al (1983). After removing the Rayleigh dispersion component, it is assumed that all the radiance measured over the ocean arriving to the satellite in the band corresponding to channel 2 is originated in atmospheric aerosols dispersion. The reflectance calculated for channel 2, R2, is therefore identified as the contribution from aerosols which has to be subtracted from the reflectance calculated for channel 1, R1. The quantity obtained as R1-R2 is the water reflectance where atmospheric contributions have been removed. Figure 3 illustrates the procedure applied to the AVHRR data from the NOAA 12 satellite on April 10th 2003 at 21:54 GMT (15:54 local time). The white band in figure 3(b) corresponds to the satellite track.

(a)

(b)

(c)

Figure. 3: AVHRR albedo at the 630 nm channel 1 (a), Rayleigh reflectance at 630 nm (b) and water reflectance obtained by subtracting the Rayleigh corrected channels 1 minus 2.

Field observations

Four oceanographic campaigns were conducted during one year spanning all conditions from dry, very low river discharge to very high discharge in the peak of the rainy season. Vertical profiles of CTD and acoustic current meter (Falmouth Scientific 2D) together with turbidity sensor (SeaPoint) water sampling for suspended sediments data were obtained at stations along five lines perpendicular to the shore as shown in figure 2, between 10 km south of Barra del Colorado in Costa Rica to 10 km north of San Juan del Norte in Nicaragua. Electronic averaging of the 2 Hz velocity data and further filtering were applied to eliminate orbital wave motions velocities.

RESULTS

Analysis of the AVHRR and SeaWiFS data shows a coastal boundary layer extending up to 25 km seaward covering the whole shelf in the area of discharge of the river San Juan. The maximum seaward extension of the front of turbidity, however, is 8 km during high discharge. The water within the turbidity front is identified as the plume itself and is characterised by temperatures lower than in the shelf water. Figure 4 shows a SST image documenting an event of high discharge on 18/12/2002, where the coastal boundary layer is rapidly warmed up by solar heating and extends up to the shelf edge. 40 % of the SeaWiFS and AVHRR reflectance images show turbid water over the Morris Shoal formations. Most images suggest that the plume of fresh water moves to the south confined to a coastal layer within the shelf.

Figure. 4: AVHRR derived SST (Celsius) on 18/12/2002.

Satellite observations were confirmed by field data and a consistent picture of the behaviour of the plume and the coastal boundary layer has emerged. Figure 5 shows vertical contours of salinity in psu (a), offshore velocity in cm s-1 (b, offshore direction positive) and alongshore direction in cm s-1 (c, northward direction positive) obtained in August 02, 2002, along a transect seaward from Barra del Colorado (stations 1a to 4a in figure 2) during high river discharge conditions.

The fresh water plume confined to the upper 5 m of the water column, whit turbidity levels = 3 FTU (turbidity data not shown in figure 5), spreads over a permanent, stratified in salinity, coastal boundary layer up to a distance of 7 km from the coast in figure 5 (the turbidity front was visually identified during two high river discharge conditions, out of four campaigns, between 7 and 8 km off-shore). The coastal boundary layer is the result of mixing of continental fresh water with shelf water of salinity in the range 36-37 psu.

The whole water column in the inner part of the coastal boundary layer (including the plume itself) has a well defined, rapid movement seaward of 45 cm s-1 and alongshore toward the south of 55 cm s-1 (figures 5b and 5c respectively) and the external part of the layer moves on-shore at 55 cm s-1, with a slow long-shore counter flow to the north of 5 cm s-1. Resuspension of bottom sediments was found during all campaigns within a band of 5 km along the coastline. During low river discharge conditions and fair weather the currents were very weak. During S-SE wind events the circulation shows a seaward component, particularly in the southern part where wind reversals from the south are more frequent.

(a)

(b)

(c)

Figure. 5: Salinity in psu (a), offshore velocity (b) and alongshore velocity(c) in cm s-1, 02/08/2002.

Alongshore suspended sediments transport was evaluated for all stations within a littoral band of 6 km. Net transport to the south was found over 60 % of the time and 30 % of the time it was directed to the north from the northernmost stations to Barra del Colorado. In the southern stations southward transport was found only 50 % of the time and 37 % of the time net transport was directed to the north. A net transport of 1.8 x 106 Tons year-1 to the south of the system was calculated for the 6 km band, but this estimate does not include the important littoral transport which is probably directed to the south according to visual observations of the wave field.

DISCUSSION

The atmospheric corrections applied to channels 1 and 2 of the AVHRR proved useful to monitor the turbid waters in the Caribbean coastal zone of Costa Rica and Nicaragua and to compare water reflectance measured in different days under different conditions. The 10 years archive of AVHRR data stored in LAOCOS can now be used to study inter annual variability and changes in sedimentation patterns. Furthermore, if suspended sediments data can be obtained to calibrate AVHRR reflectance, an extremely valuable tool will be ready to estimate sediment inputs and transport in the coastal zone. Such calibration was one of the goals of the study but no contemporary suspended sediments and satellite data could be obtained due to persistent cloud cover in the area. The limitation above can only stress the utility of the AVHRR sensor, with a long time of service and high sampling rate, together with SeaWiFS, MODIS and other ocean colour radiometers.

The analysis of figure 5 reveals the main forces at play. A barotropic pressure gradient associated to the sloping sea surface toward the coast, combined with the coriolis force, result in the observed movement off-shore and along shore toward the south in the inner part of the coastal boundary layer. The sloping of the sea surface is the result of the abundant river discharge and is reinforced by the sustained on-shore piling up of water by the E-NE wind stress and wave action. The on-shore and along shore to the north movement of the outer part of the coastal boundary layer is associated with the baroclinic pressure gradient resulting from the sloping isohalines (figure 5a) and the E-NE wind stress.

As a consequence of the dynamics of the coastal zone the turbid water discharged by the San Juan and other rivers in the area tend to be confined to a coastal band a few kilometres width and the net alongshore transport is directed to the south. This has important consequences for the faith of sediments and pollutants introduced into the continental shelf and should be taken into account in coastal management plans. Southern wind reversals favour seaward circulation and provide conditions to export pollutants far off-shore. The southern part of the coastal zone studied shows more frequently wind forcing from the south and circulation with seaward and northward components.

REFERENCES

Fröhlich, C., 1986: Extraterrestrial Solar Radiation. In Physical Climatology for Solar and Wind Energy, Rodolfo Guzzi and Carl Gerald Justus eds., W orld Scientific Publishing Co., Singapur, pp. 38-54. [         [ Links ]1]

Gordon, H.R., Clark, D.K., Brown, J.W., Brown, O.B., Evans, R.H. & Broenkow, W.W., 1983: Phytoplankton pigment concentrations in the Middle Atlantic Boght: comparison of ship determinations and CZCS estimates, Applied Optics 22, 1, 20- 36. [         [ Links ]2]

Groom, S. & Holligan, P., 1987: Remote sensing of coccolithopore blooms, Adv. Space. Res., 7, 2, 73-78. [         [ Links ]3]

McClain, E. P., Pichel, W. G. & Walton, C. C., 1985: Comparative performance of AVHRR-based multichannel sea surface temperatures, Journal of Geophysical Research, 90, C6, 11587-11601. [         [ Links ]4]

Murray, S., Hsu, S., Roberts, H., Owens, E. & Crout, R.1982: Physical processes and sedimentation on a broad, shallow bank, Estuarine, Coastal and Shelf Science, 14, 135-157. [         [ Links ]5]

Stumpf, 1987: Remote sensing of suspended sediments in estuaries using atmospheric and compositional corrections to AVHRR data. In Proceedings of the 21st International Symposium on Remote Sensing of Environment, Ann Arbor, Michigan, October 26-30, 1987. [         [ Links ]6]

Stumpf, R.P., 1992: Remote sensing of water clarity and suspended sediments in coastal waters, en Proceedings of the First Thematic Conference on Remote Sensing for Marine and Coastal Environments, New Orleans, LA, 15-17 June, SPIE 1930, Environmental Research Institute of Michigan, Ann Arbor, MI, pp. 293-305. [         [ Links ]7]

Stumpf, R.P. & Pennock, J.R., 1989: Calibration of a general optical equation for remote sensing of suspended sediments in a moderately turbid estuary, Journal of Geophysical Research, 94, C10, 14363-14371. [         [ Links ]8]

Walker, N.D., 1995: Satellite assessment of Mississippi river plume variability: causes and predictability, Remote Sensing of the Environment, 58, 21-35. [         [ Links ]9]

 

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