Secchi depth estimated using three algorithms

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    Secchi depth

    This video shows secchi depth estimates in three different ways. Secchi depth is an estimate of the water clarity. It is traditionally measured by lowering a secchi disk into the water until the disk can not be seen. The BioGeoChemical model estimates secchi in several different ways. The first way is an estimate based on the modelled light attenuation at 490 nm. The model estimates the optical properties of the water based on the sediment and nutrients in the water. These all affect the scattering and absorption of light as it passes through the water allowing the model to estimate the attenuation of the light through the water. The light attenutation rate is an estimate of the percentage light that is lost for each metre of water that the light passes through.

    Secchi depth - In situ light attenuation (1.7/Kd 490 model)

    The secchi depth can be estimated from the modelled light attenutation rate using the empirical relationship Kd * SD = 1.7, where Kd is the light attenutation and SD is the secchi depth. This imperical relationship was established by Poole and Atkins (1929) and is used world wide. This represents the model's best estimate of secchi depth if it were compared to in situ measurements.

    Secchi depth - Simulated MODIS remote sensing standard algorithm (1.7/Kd 488 rsr)

    To better understand the skill of the model two additional estimates of secchi depth were developed so that the model could be compared directly with secchi estimates from remote sensing satellites such as the MODIS satellite operated by NASA.

    How secchi depth is estimated from remote sensing

    For secchi depth to be estimated from remote sensing the satellite image is first corrected for atmospheric effects to estimate the light reflected off the ocean. The light attenuation (Kd 490) is then estimated based on a non-linear scaling of the ratio of the relative reflectance of the blue and green colour bands (the parameters in the scaling formula vary based on the satellite). These parameters have been optimised to for open ocean conditions. The secchi depth is then estimated from the light attenuation. 

    How the eReefs model simulates remote sensing and the secchi depth

    To allow the model to be compared directly with remote sensing results a simulation of remote sensing was generated from the BGC model. The BGC simulates the optical properties of the water to create an estimate of what the reflected surface of the water would look light from a satellite. In particular it simulates the spectral bands matching those of the NASA MODIS satellites. These simulated remote sensing images are then passed through the same algorithms that are typically used on the MODIS image to estimate light attenuation and then secchi depth. 

    Secchi depth - simulated MODIS remote sensing Weeks 2012 algorithm

    The standard estimate of light attenuation (and secchi depth) from MODIS imagery is optimsed for open ocean conditions and is not tailored to inshore areas such as the Great Barrier Reef. Weeks et al. (2012) developed a regional empirical optimisation of the secchi estimate using in-situ secchi measurement from the region. This measure of secchi depth from the model is a simulation of the same process as used by Weeks, using the simulated remote sensing outputs from the model. 

    References

    Poole HH, Atkins WR (1929) Photo-electronic measurements of submarine ilumination throughout the year. J Mar Biol Assoc 16:297–394 Google Scholar

    Weeks S, Werdell PJ., Schaffelke B, Canto M, Zhongping L, Wilding JG. Feldman G. (2012) Satellite-Derived Photic Depth on the Great Barrier Reef: Spatio-Temporal Patterns of Water Clarity, Remote Sensing 2012, 4(12), 3781-3795; doi:10.3390/rs4123781