Water quality remote sensing: scientific report
Authors:   Moniteq Ltd.
Published: August 2023

The development of analytical techniques is required for deducing subsurface water parameters such as the particulate concentration and the chlorophyll a concentration using remotely sensed water colour data obtained from high altitude airborne or satellite
platforms. Mathematical models are needed for the removal of the effects due to absorption and scattering of the intervening atmosphere, sunglint and sky reflectance so that the inherent radiance of the target can be determined. Also needed is the interpretive capability to analyze the deduced
optical reflectance data in terms of useful water quality parameters through the use of radiative transfer models. In order to determine the validity of these mathematical models developed at MONITEQ Ltd., and to achieve their refinement and verification, the Canada Centre for Remote Sensing (CCRS),
the Canada Centre for Inland Waters (CCIW), and MONITEQ Ltd. conducted a field experiment to acquire simultaneous satellite, airborne and in-situ ship data. Simultaneous measurements of atmospheric parameters were also made. A multi-stage experiment was conducted over Lake Ontario during the
surnrner season of 1979 in conjunction with Nimbus 7 CZCS overpasses. Data collection was planned to occur during 5 different 1 week intervals where aircraft, ship and ground based measurements were simultaneously available. Data was acquired on May 8, June6, July 13, August 11 and September 24.
Airborne data acquisition was done from a Canada Centre for Remote Sensing DC-3 aircraft equipped with optical sensors and flying a line across Lake Ontario. The flight line was chosen in the direction and on the occasion of Nimbus 7 overpasses. Flights were conducted at 2900 m and 150 m above water
level from a point at the centre of the Niagara River to the east end of Toronto. This line was selected to assure a broad range of water quality including the more heavily sedimented river discharge plume, the shoreline water near Toronto and the relatively clear mid-lake water. Ground based
measurements of the atmosphere were made with two solar radiometers, one at each end of the flight line. One device was capable of extinction measurements only, and was used on some occasions. The second device, with a 1 degree field of view, contained a filter at 823 nm for estimation of
atmospheric water vapour. The second radiometer was also used to make sky radiance and downwelling irradiance measurements from the ground locations. In-situ measurements were made by CCIW at the 20 sampling stations. A variety of optical insitu measurements were made including spectral
transmissometer and spectral i r radiance depth profiles and volume reflectance at a depth of one meter. Temperature profile measurements were also taken. Water samples were collected for subsequent laboratory analysis of water quality. The collected satellite and aircraft data were analyzed and
processed for radiometric and atmospheric correction of the imagery, and the sunglint and water-air interface reflectance correction. The comparison of these corrected data were made with the simultaneously acquired ship based and low altitude aircraft data for confirmation of the atmospheric
scattering and water-air interface models. The microscopie water reflectances were used to compute chlorophyll and sediment concentrations in Lake Ontario and Lake Erie. These derived data sets and water quality maps thus produced, upon their comparison with the in-situ water quality measurements,
show that quantitative mapping of these parameters in the Great Lakes can be done on a regular operational basis. Errors involved in comparison of the data were of a random nature attributable to different pixel averages for in-situ and airborne measurements and to temporal and spatial
inconsistencies of the compared data sets.