Helgi Arst

A hyperspectral model for interpretation of passive optical remote sensing data from turbid lakes

A hyperspectral model was developed for the interpretation of remote sensing data collected above inland waters. Specific absorption and scattering coefficients proposed by other authors were not suitable for modelling of the irradiance reflectance in 12 studied lakes. Therefore, special studies were carried out to estimate absorption and scattering coefficients as well as backscattering probability of suspended matter in turbid waters. AC-9 and Li-1800UW results were used for these purposes. The algorithms obtained were used to improve the model, which was then tested in forward and inverse modes.

Preliminary optical classification of lakes and coastal waters in Estonia and south Finland

A preliminary optical classification of lakes in Estonia and south Finland which can also be used for small bays of the Baltic Sea is elaborated. The classification is based on the optical properties of water (diffuse attenuation coefficient, diffuse reflectance) and parameters that are routinely monitored in water bodies (Secchi depth, concentration of chlorophyll-a, total suspended matter and yellow substance). The data complex used for our classification covers different types of water ecosystems (ranging from oligotrophic to hypertrophic) and the variability of water constituent concentrations in the ice-free period in Estonia and south Finland. Using cluster analysis, we found 5 optical classes of waters: clear (C), moderate (M), turbid (T), very turbid (V) and brown (B). There is satisfactory correspondence between class of water, shape of diffuse attenuation coefficient and diffuse reflectance spectra and trophic state of the lakes.

Relation between underwater irradiance and quantum irradiance in dependence on water transparency at different depths in the water bodies

The ratio between underwater quantum irradiance (q) and irradiance (E) for Jerlovs [1976] oceanic and coastal water types and for 11 Estonian and Finnish lakes was studied. This ratio was found to depend on the depth in the water body and the transparency of the water. The ratio q/E for the photosynthetically active radiation (PAR) region of the spectrum may differ from its value in air by up to 24%. The results of the present paper can be used to convert the underwater radiation data from units of W m−2 to μmol s−1 m−2 (or vice versa) and to estimate the errors caused by applying the “air” value of q/E instead of its real value in the water.

Remote sensing reflectance model of optically active components of turbid waters

A mathematical model that simulates the spectral curves of remote sensing reflectance is developed. The model is compared to measurements obtained from research vessel or boat in the Baltic Sea and Estonian lakes. The model simulates the effects of light backscattering from water and suspended matter, and the effects of its absorption due to water, phytoplankton, suspended matter and yellow substance. Measured by remote sensing spectral curves are compared by multiple of spectra obtained from model calculations to find the theoretical spectrum which is closest to experimental. It is assumed that in case of coincidence of the spectral curves concentrations of optically active substances in the model correspond to real ones. Preliminary testing of the model demonstrates that this model is useful for estimation of concentration of optically active substances in the waters of the Baltic Sea and Estonian lakes.

Informative water layer, determined by attenuation depth, in water bodies of different turbidity

By using values of vertical diffuse attenuation coefficient at certain visible wavelength (or its mean value for some spectral region) we can estimate the depth of informative to remote sensing water layer sometimes called penetration depth, but more often attenuation depth. This value is an important parameter because about ninety percent of the information detected by optical remote sensing instruments is related to the sea layer with this depth.

Optical inverse problem in turbid waters

Two methods have been used for solving the inverse problem of remote sensing in case of turbid coastal an inland waters. Color indices are elaborated for estimation of Secchi depth, chlorophyll, yellow substance, suspended matter and total phosphorus concentrations and mean beam attenuation coefficient using passive optical remote sensing data. The second method is spectral modeling. A simple mathematical model is elaborated for simulating the diffuse reflectance spectra beneath and just above the water surface.

Evaluation of remote sensing algorithms for the retrieval of optically-active components in turbid natural waters

A simple mathematical model is elaborated for estimation of chlorophyll, yellow substance and suspended matter concentrations in turbid coastal and inland waters from diffuse reflectance spectra of the water body measured using remote or underwater spectrometers. Measurements were carried out on the Baltic Sea, Estonian and Finnish lakes. Correlation between measured and estimated by means of model calculations concentrations of optically active components were calculated.