Remote detection of water leaving radiance is quantitatively related to three marine constituents, which exist in the water column – chlorophyll (CHL); suspended matter (SM) such as sediments; and colored dissolved organic matter (CDOM), also known as Gelbstoff. The variation of the volume reflectance of these constituents is rather complicated.
Volume reflectance of water with minimum CHL concentrations is relatively high at the blue spectral region (400-500 nm) and minimal at the red (600-700 nm). Conversely, high CHL concentrations lower the volume reflectance at the blue and heighten it at the red. The null point of these vertical shifts is located at around 500 nm. Increasing concentrations of CDOM result in a significant decrease of the volume reflectance at the blue with almost no effect at the red. As for the SM, increasing concentrations produce a considerable increase of the volume reflectance at the red with minimal effect in the blue.
Therefore, detecting CHL separately from SM is a challenging image processing task since the developed algorithms are based on volume reflectance variations at 555 nm. Any increase in the volume reflectance in this spectral band may indicate increasing concentration in either CHL, or SM, or both. Such a complex situation characterizes outlets of many big rivers around the world where the SM is the dominant constituent, and in particular the in vicinity of the Nile delta.
The Nile River, the longest river in the world, starts in central Africa and flows north towards the Mediterranean. Its discharge is 84 billion m3 per year. Prior to the construction of the High Dam in Aswan in Upper Egypt in 1964, the Nile carried between 120 and 140 million tons of sediments per year and the delta front progressed seaward at an average rate of 15 m/yr. It used to be the main source of sediments in the circulation cell of the southeastern Mediterranean. After the completion of the dam, most of the sediments have been trapped by. Consequently, the transported sediments have decreased significantly to an amount of about 2 million tons per year. Also from that time, the delta front has undergone major erosion of about 200 m/yr. The dominant direction of winds in this region is northwest. These winds force water and sediments to move from the delta region eastwards. Additionally, the geostrophic circum-Mediterranean anti-clockwise gyre is an effective mechanism in the eastward transport of the Nile sediments.
The estimated longshore sediments transport ranges between 2 million tons per year east of the delta to less than 80,000 tons per year in northern Israel. As a result of the sediment depositions east of the Nile delta, the continental self of Sinai is relatively wide – 42 to 50 km and off Israel is between 10 and 26 km.
An experimental cruise was undertaken off the coast of the southern part of Israel, in July
1992, along a 25 km transect perpendicular to the coastline.
Laboratory results from the water sampled during this experiment provide evidence that the study area is characterized by relatively high concentrations of inorganic fine-grained SD (between 9 mg/l near the shore to about 2.5 mg/l off shore), very low concentrations of CHL (between 0.36 mg/l near the shore to 0.028 mg/l off shore). The CDOM was found to be negligible.
Before masking the sediments
After masking the sediments
The model is implemented by using a null point at 497 nm (corresponding to SeaWiFS band 4 centered at 510 nm) where the level of reflectance is not affected by change in chlorophyll concentration. Any shift of reflectance above the null point indicates suspended matter present in the water. Masking of pixels with values above the threshold results in elimination of the turbidity. The 510 nm null point can also be used for calculating suspended matter concentrations in the area, since it defines the effects of suspended matter on the spectrum.
Department of Geosciences
University of Nebraska – Lincoln
Jacob Blaustein Institute for Desert Research
Ben-Gurion University of the Negev
Sede-Boker campus, Israel