I would like to know how to interpret the Chlorophyll concentration range of Ullyssys Water Quality Viewer (UWQV) visualization image. I understand that the image is color based and green to red represents high concentration of Chlorophyll. But, is there any way to understand the Chlorophyll concentration in mg/m3 for this particular high bucket (red for example).
Your inputs will be highly appreciated.
so far Ulyssys Water Quality Viewer has not been calibrated against actual water quality measurements. Our choice of Reflectance Line Height algorithms was based on Palmer et al 2015 (https://www.sciencedirect.com/science/article/pii/S0034425714002739), but applying calibration coefficients was avoided on purpose - globally valid calibration would be way out of scope for such a simple algorithm. From anecdotal evidence, it seems that the red colour corresponds to chlorophyll observations somewhere above 100 ug/l, but do really take this with a critical approach. The script can act as an early warning system or help you identify locations for sampling - but if you have the chance to compare against water sample measurements, please do let us know.
Thank you Andras for your inputs and reference. I could go through the paper and found that from Fig. 6, the Chl-a concentration mapping used the range (0 - 50 mg per m3). And I see that green to red the range is coming as 20 - 45. If I need a similar range for Chl-a from UWQV image (green to red) , what the range of Chl-a would be based on your suggestion?
you will see in the script that the chlMin and chlMax parameters can be tuned, and they determine what colour corresponds to green and red. When releasing the script we tuned them on imagery of Lake Balaton from the bloom of September 2019. You can find a paper describing in situ chlorophyll measurements from the same place and time here: https://onlinelibrary.wiley.com/doi/full/10.1111/fwb.13903
From Fig.1 of this paper, I would guess that the limit for red is higher than I wrote in my previous answer, somewhere around 200, but still, take this with caution - we only know that FLH/MCI is one of the more accurate algorithms for these cases, but we can’t be sure how stable it is across lakes globally.
I suggest you find a few dates where you do have reference data, tune your chlMin and chlMax parameters based on that, and then create a custom configuration to use those Min/Max values permanently: Customising your Copernicus Browser Experience | Copernicus Data Space Ecosystem