Inherent Optical Properties, Apparent Optical Properties, and Water Quality Attributes
The Inherent Optical Properties (IOPs) of water describe the interaction of light with water and in-water constituents, independent of illumination conditions (e.g., clouds or time of day). These metrics fall into two main categories: absorption (a) and scattering (b) including backscattering (bb) (Mobley 1999; Werdell et al., 2018). IOPs functionally separate interactions of sunlight with dissolved and particulate matter in water into absorption and scattering effects. Sunlight can be absorbed by constituents in the water, or scattered in different directions. The simplest IOPs are the bulk absorption and scattering coefficients of all the constituents in the water column. The beam attenuation coefficient, which is a measure of how sunlight dissipates in the water column, is the sum of the bulk absorption and scattering. Absorption and scattering coefficients can be further broken down into operationally defined specific groups of in-water constituents, such as chromophoric dissolved organic matter (CDOM), pigmented algal particles, and non-algal particles (NAP, e.g., sediment & detritus).
The Apparent Optical Properties (AOPs) of water depend upon the presence of in-water constituents present, and theas well as illumination conditions. These properties are used to gain insights into the state of a waterbody (Mobley 2001). Commonly measured AOPs include reflectance and diffuse attenuation coefficients (K-functions), which describe the depth of light penetration in the water column. Reflectance is a measure of the fraction of the incoming sunlight (downwelling irradiance (Ed)) relative to the amount that is reflected (water-leaving, or upwelling, radiance (Lw) from within the water column and can be based on upwelling irradiances or radiances. Remote sensing reflectance (Rrs) is often used in aquatic remote sensing. It is derived through normalizing water-leaving radiance (Lw) to the incoming sunlight reaching the water surface (Ed). It can be measured in the field by using radiometric sensors that measure upwelling radiance and downwelling irradiance and is often used for comparisons with equivalent satellite data products.
Diffuse attenuation coefficients measure the amount of incoming sunlight dissipating when passing through the water column and can be derived from the same radiometric variables as reflectance. The diffuse attenuation coefficient of downwelling irradiance (Kd), and more specifically of photosynthetically active radiation (KPAR), is a standard measure in environmental assessments, such as the U.S. EPA National Coastal Condition Assessment (NCCA). Optical water quality attributes including Secchi disk depth, turbidity, suspended particulate matter (SPM), chlorophyll-a (Chl a), accessory algal pigments, such as phycocyanin (PC) and phycoerythrin (PE) , CDOM, among others with extra calibration (Mouw et al., 2015; Topp et al., 2020; Gholizadeh et al., 2016) are commonly derived from satellite remote sensing reflectance using retrieval models.
References
Mobley, C.D., 1999. Estimation of the remote-sensing reflectance from above-surface measurements. Appl. Opt. 38, 7442. https://doi.org/10.1364/AO.38.007442
Werdell, P.J., McKinna, L.I.W., Boss, E., Ackleson, S.G., Craig, S.E., Gregg, W.W., Lee, Z., Maritorena, S., Roesler, C.S., Rousseaux, C.S., Stramski, D., Sullivan, J.M., Twardowski, M.S., Tzortziou, M., Zhang, X., 2018. An overview of approaches and challenges for retrieving marine inherent optical properties from ocean color remote sensing. Progress in Oceanography 160, 186–212. https://doi.org/10.1016/j.pocean.2018.01.001
Mouw, C.B., Greb, S., Aurin, D., DiGiacomo, P.M., Lee, Z., Twardowski, M., Binding, C., Hu, C., Ma, R., Moore, T., Moses, W., Craig, S.E., 2015. Aquatic color radiometry remote sensing of coastal and inland waters: Challenges and recommendations for future satellite missions. Remote Sensing of Environment 160, 15–30. https://doi.org/10.1016/j.rse.2015.02.001
Topp, S.N., Pavelsky, T.M., Jensen, D., Simard, M., Ross, M.R.V., 2020. Research Trends in the Use of Remote Sensing for Inland Water Quality Science: Moving Towards Multidisciplinary Applications. Water 12, 169. 1507 https://doi.org/10.3390/w12010169
Gholizadeh, M., Melesse, A., Reddi, L., 2016. A Comprehensive Review on Water Quality Parameters Estimation Using Remote Sensing Techniques. Sensors 16, 1298. https://doi.org/10.3390/s16081298