Skip to main content

Review of the Forel–Ule Index based on in situ and remote sensing methods and application in water quality assessment


Water pollution is considered an acute worldwide environmental issue. At present, the commonly adopted method of water quality characterisation involves the retrieval of optically active water quality parameters based on remote sensing reflectance (Rrs), but this method is subject to the limitation that understanding local scatter and absorption characteristics of light is essential to precisely derive these parameters. Water colour primarily depends on water constituents and is traditionally gauged with the Forel–Ule (FU) scale. In recent years, Rrs within the visible region has been considered to determine the Forel–Ule Index (FUI) for water colour measurement. The FUI exhibits the advantages of remote sensing and does not rely on local retrieval algorithms. Therefore, this index can characterise natural waters in a simple and globally effective manner. As there exists a lack of review articles on the FUI, we present a comprehensive review of this index that may help researchers progress. First, we introduce the most recent techniques for FUI measurement, especially remote sensing–deriving methods. Then, we summarise FUI applications in water quality assessment of oceans and inland waters. Finally, FUI development trends, challenges and application perspectives are examined.

This is a preview of subscription content, access via your institution.

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9

Data availability

Not applicable.


  • Alföldi TT, Munday J (1978) Water quality analysis by digital chromaticity mapping of Landsat data. Can J Remote Sens 4(2):108-126.

    Article  Google Scholar 

  • Bardaji R, Sanchez AM, Simon C, Wernand MR, Piera J (2016) Estimating the underwater diffuse attenuation coefficient with a low-cost instrument: the KdUINO DIY Buoy. Sensors (Basel) 16(3):373.

  • Barysheva, L (1987) On the issue of intercorrespondence of color scales used in limnology. Remote Monitoring of Large Lakes:60–65.

  • Bird TJ, Bates AE, Lefcheck JS, Hill NA, Thomson RJ, Edgar GJ, Stuart-Smith RD, Wotherspoon S, Krkosek M, Stuart-Smith JF (2014) Statistical solutions for error and bias in global citizen science datasets. Biol Cons 173:144–154.

    Article  Google Scholar 

  • Bowers DG, Evans D, Thomas DN, Ellis K, PJ le B Williams, (2004) Interpreting the colour of an estuary. Estuar Coast Shelf Sci 59(1):13–20.

    Article  CAS  Google Scholar 

  • Boyce DG, Lewis MR, Worm B (2010) Global phytoplankton decline over the past century. Nature 466(7306):591–596.

    Article  CAS  Google Scholar 

  • Boyce DG, Lewis M, Worm B (2012) Integrating global chlorophyll data from 1890 to 2010. Limnol Oceanogr Methods 10(11):840–852.

    Article  Google Scholar 

  • Brewin RJW, Brewin TG, Phillips J, Rose S, Abdulaziz A, Wimmer W, Sathyendranath S, Platt T (2019) A printable device for measuring clarity and colour in lake and nearshore waters. Sensors (Basel) 19(4):936.

  • Bukata RP, Pozdnyakov DV, Jerome JH, Tanis FJ (2001) Validation of a radiometric color model applicable to optically complex water bodies. Remote Sens Environ 77(2):165–172.

    Article  Google Scholar 

  • Busch JA, Price I, Jeansou E, Zielinski O, Van der Woerd HJ (2016a) Citizens and satellites: assessment of phytoplankton dynamics in a NW Mediterranean aquaculture zone. Int J Appl Earth Obs Geoinf 47:40–49.

    Article  Google Scholar 

  • Busch JA, Bardaji R, Ceccaroni L, Friedrichs A, Piera J, Simon C, Thijsse P, Wernand MR, Van der Woerd HJ, Zielinski O (2016b) Citizen bio-optical observations from coast- and ocean and their compatibility with ocean colour satellite measurements. Remote Sens 8(11):879.

  • Carlson RE (1977) A trophic state index for lakes. Limnol Oceanogr 22(2):361–369.

    Article  CAS  Google Scholar 

  • Ceccaroni L, Piera J, Wernand MR, Zielinski O, Busch JA, Van der Woerd HJ, Bardaji R, Friedrichs A, Novoa S, Thijsse P (2020) Citclops: a next-generation sensor system for the monitoring of natural waters and a citizens’ observatory for the assessment of ecosystems’ status. PLoS One 15(3):e0230084.

    Article  CAS  Google Scholar 

  • Chen Q, Huang M, Tang X (2020) Eutrophication assessment of seasonal urban lakes in China Yangtze River Basin using Landsat 8-derived Forel-Ule index: a six-year (2013–2018) observation. Sci Total Environ 745:135392.

    Article  CAS  Google Scholar 

  • Chen X, Liu L, Zhang X, Li J, Wang S, Liu D, Duan H, Song K (2021) An assessment of water color for inland water in China using a Landsat 8-derived Forel-Ule Index and the Google Earth Engine platform. IEEE J Select Topics Appl Earth Observ Remote Sens 14:5773–5785.

  • Cie C (1932) Commission internationale de l’eclairage proceedings 1931. Cambridge University, Cambridge

    Google Scholar 

  • Cloern JE (1987) Turbidity as a control on phytoplankton biomass and productivity in estuaries. Cont Shelf Res 7(11–12):1367–1381.

    Article  Google Scholar 

  • Davies-Colley RJ, Smith DG (2001) Turbidity suspeni) ed sediment, and water clarity: a review. JAWRA Journal of the American Water Resources Association 37(5):1085–1101

    Article  Google Scholar 

  • Dickinson JL, Shirk J, Bonter D, Bonney R, Crain RL, Martin J, Phillips T, Purcell K (2012) The current state of citizen science as a tool for ecological research and public engagement. Front Ecol Environ 10(6):291–297.

    Article  Google Scholar 

  • Forel FA (1895) Couleur de L’eau” in Optique, Le Léman. Monographie Limnologique 2:462–487

  • Forel FA (1890) Une nouvelle forme de la gamme de couleur pour l’étude de l’eau des lacs. Archives Des Sciences Physiques Et Naturelles/societe De Physique Et D’histoire Naturelle De Geneve 6:25

    Google Scholar 

  • Garaba SP, Badewien TH, Braun A, Schulz AC, Zielinski O (2014a) Using ocean colour remote sensing products to estimate turbidity at the Wadden Sea time series station Spiekeroog. Journal of the European Optical Society: Rapid Publications 9.

  • Garaba SP, Friedrichs A, Voss D, Zielinski O (2015) Classifying natural waters with the Forel-Ule Colour Index System: results, applications, correlations and crowdsourcing. Int J Environ Res Public Health 12(12):16096–16109.

    Article  CAS  Google Scholar 

  • Garaba S, Voß D, Zielinski O (2014b) Physical, bio-optical state and correlations in North-Western European shelf seas. Remote Sensing 6(6):5042–5066.

    Article  Google Scholar 

  • Giardino C, Kõks KL, Bolpagni R, Luciani G, Candiani G, Lehmann MK, Van der Woerd HJ, Bresciani M (2019) The color of water from space: a case study for Italian lakes from Sentinel-2. In Geospatial Analyses of Earth Observation (EO) data. IntechOpen.

  • Graham JJ (1966) Secchi disc observations and extinction coefficients in the central and eastern North Pacific Ocean. Limnol Oceanogr 11(2):184–190

    Article  Google Scholar 

  • Hughes RN, Hughes DJ, Philip Smith I (2014) Citizen scientists and marine research: volunteer participants, their contributions, and projection for the future. Oceanogr Mar Biol Annu Rev 52:257–314.

    Article  Google Scholar 

  • Jafar-Sidik M, Bowers DG, Griffiths JW (2018) Remote sensing observations of ocean colour using the traditional Forel-Ule scale. Estuar Coast Shelf Sci 215:52–58.

    Article  Google Scholar 

  • Jaquet J-M, Zand B (1989) Colour analysis of inland waters using Landsat TM data. ESA SP 1102:11

    Google Scholar 

  • Jin XC, Tu QY (1990) The standard methods for observation and analysis in lake eutrophication. Chinese Environmental Science Press, Beijing, p 240

    Google Scholar 

  • Kirk JTO (1988) Optical water quality, what does it mean and how should we measure it? Journal-Water Pollution Control Federation 60(2):194–197

    Google Scholar 

  • Koenings JP, Edmundson JA (1991) Secchi disk and photometer estimates of light regimes in Alaskan lakes: effects of yellow color and turbidity. Limnol Oceanogr 36(1):91–105.

    Article  Google Scholar 

  • Kondratyev KY, Pozdnyakov DV, Pettersson LH (1998) Water quality remote sensing in the visible spectrum. Int J Remote Sens 19(5):957–979.

    Article  Google Scholar 

  • Kowalczuk P, Stoń-Egiert J, Cooper WJ, Whitehead RF, Durako MJ (2005) Characterization of chromophoric dissolved organic matter (CDOM) in the Baltic Sea by excitation emission matrix fluorescence spectroscopy. Mar Chem 96(3–4):273–292.

    Article  CAS  Google Scholar 

  • Krümmel O (1893) Die Farbe der Meere. Geophysikalische Beobachtungen der PlanktonExpedi tion. Ergebnisse der Plankton-Expedition der Humboldt-Stiftung, Bd. I-C. Kiel und Leipzig: Verlag von Lipsius & Tischer. Tafel II: 89-110

  • Kulk G, George G, Abdulaziz A, Menon N, Theenathayalan V, Jayaram C, Brewin RJW, Sathyendranath S (2021) Effect of reduced anthropogenic activities on water quality in Lake Vembanad, India. Remote Sens 13(9):1631.

  • Leeuw T, Boss E (2018) The hydrocolor app: above water measurements of remote sensing reflectance and turbidity using a smartphone camera. Sensors (Basel) 18(1):256.

  • Leeuw T (2014) Crowdsourcing water quality data using the iPhone camera.

  • Lehmann MK, Nguyen U, Allan M, Van der Woerd HJ (2018) Colour classification of 1486 lakes across a wide range of optical water types. Remote Sens 10(8):1273.

  • Li J, Wang S, Yanhong Wu, Zhang B, Chen X, Zhang F, Shen Q, Peng D, Tian L (2016) MODIS observations of water color of the largest 10 lakes in China between 2000 and 2012. Int J Digit Earth 9(8):788–805.

  • Li M, Sun Y, Li X, Cui M, Huang C (2021) An improved eutrophication assessment algorithm of estuaries and coastal waters in Liaodong Bay. Remote Sensing 13(19):3867.

    Article  Google Scholar 

  • Lovell S, Hamer M, Slotow R, Herbert D (2009) An assessment of the use of volunteers for terrestrial invertebrate biodiversity surveys. Biodivers Conserv 18(12):3295–3307.

    Article  Google Scholar 

  • Luksch J (1901) Expeditionen S.M. Schiff "Pola" im Mittelländischen, Ägäischen und Rothen Meere in den Jahren 1890-1898. Wissenschaftliche Ergebnisse XIX. Untersuchungen über die Transparenz und Farbe de Seewassers. Berichte der Commission für Oceanographische Foeschungen. Collectiv-Ausgabe aus dem LXIX Bande der Denkschriften Kaiserlichen Akademie der Wissenschafte. A. Forschungen im Rothen Meere. B. Forschungen im Östlichen Mittelmeere: 400-485

  • Malthus TJ, Ohmsen R, Van der Woerd HJ (2020) An Evaluation of citizen science smartphone apps for inland water quality assessment. Remote Sens 12(10):1578.

  • Megard RO, Settles JC, Boyer HA, Combs WS Jr (1980) Light, Secchi disks, and trophic states. Limnol Oceanogr 25(2):373–377.

    Article  CAS  Google Scholar 

  • Menon N, George G, Ranith R, Sajin V, Murali S, Abdulaziz A, Brewin RJW, Sathyendranath S (2021) Citizen science tools reveal changes in estuarine water quality following demolition of buildings. Remote Sens 13(9):1683.

  • Mobley, Curtis D, and Lydia K Sundman (2008) HYDROLIGHT 5 ECOLIGHT 5. Sequoia Scientific Inc.

  • Moore GK (1980) Satellite remote sensing of water turbidity/Sonde de télémesure par satellite de la turbidité de l’eau. Hydrol Sci J 25(4):407–421.

    Article  Google Scholar 

  • Morel A, Prieur L (1977) Analysis of variations in ocean color. Limnol Oceanogr 22(4):709–722.

    Article  Google Scholar 

  • Nelson NB, Siegel DA (2013) The global distribution and dynamics of chromophoric dissolved organic matter. Ann Rev Mar Sci 5:447–476.

    Article  Google Scholar 

  • Nie Y, Guo J, Sun B, Lv X (2020) An evaluation of apparent color of seawater based on the in-situ and satellite-derived Forel-Ule color scale. Estuar Coast Shelf Sci 246:107032.

  • Novoa S, Wernand MR, Van der Woerd HJ (2013) The Forel-Ule scale revisited spectrally: preparation protocol, transmission measurements and chromaticity. J Eur Opt Soc Rapid Publ 8.

  • Novoa S, Wernand MR, Van der Woerd HJ (2014) The modern Forel-Ule scale: a ‘do-it-yourself’ colour comparator for water monitoring. J Eur Opt Soc Rapid Publ 9.

  • Novoa S, Wernand MR, Van der Woerd HJ (2016) WACODI: a generic algorithm to derive the intrinsic color of natural waters from digital images. Limnol Oceanogr Methods 13(12):697–711.

    Article  Google Scholar 

  • Ouma YO, Waga J, Okech M, Lavisa O, Mbuthia D (2018) Estimation of reservoir bio-optical water quality parameters using smartphone sensor apps and Landsat ETM+: review and comparative experimental results. Journal of Sensors 2018:1–32.

    Article  CAS  Google Scholar 

  • Pahlevan N, Smith B, Binding C, Gurlin D, Li L, Bresciani M, Giardino C (2021) Hyperspectral retrievals of phytoplankton absorption and chlorophyll-a in inland and nearshore coastal waters. Remote Sens Environ 253:112200.

    Article  Google Scholar 

  • Pitarch J (2017) Biases in ocean color over a Secchi disk. Opt Express 25(24):A1124–A1131.

    Article  Google Scholar 

  • Pitarch J, Bellacicco M, Marullo S (2021a) hue-FU-secchi. Zenodo.

  • Pitarch J, Bellacicco M, Marullo S, Van der Woerd HJ (2021b) Global maps of Forel-Ule index, hue angle and Secchi disk depth derived from 21 years of monthly ESA Ocean Colour Climate Change Initiative data. Earth Syst Sci Data 13(2):481–490.

  • Pitarch J, Van der Woerd HJ, Brewin RJW, Zielinski O (2019a) Twenty years of monthly global maps of hue angle. Forel-Ule and Secchi Disk Depth, Based on ESA-OC-CCI Data, PANGAEA.

  • Pitarch J, Van der Woerd HJ, Brewin RJW, Zielinski O (2019b) Optical properties of Forel-Ule water types deduced from 15 years of global satellite ocean color observations. Remote Sens Environ 231:111249.

  • Prieur L, Sathyendranath S (1981) An optical classification of coastal and oceanic waters based on the specific spectral absorption curves of phytoplankton pigments, dissolved organic matter, and other particulate materials. Limnol Oceanogr 26(4):671–689.

    Article  Google Scholar 

  • Tang Q (2019) Global change hydrology: terrestrial water cycle and global change. Sci China Earth Sci 63(3):459–462.

    Article  Google Scholar 

  • Bukata RP, Bruton JE, Jerome JH (1983) Use of chromaticity in remote measurements of water quality. Remote Sens Environ 13(2):161–177

    Article  Google Scholar 

  • Rodhe W (1969) Crystallization of eutrophication concepts in northern Europe.

  • Rosen MTh (1930) Die Skala von Forel-Ule zur Bestimmung der Farbe des Wassers. Nachrichten Des Zentralbüro Für Hydrometeorologie 9:20

    Google Scholar 

  • Sathyendranath S, Prieur L, Morel A (1989) A three-component model of ocean colour and its application to remote sensing of phytoplankton pigments in coastal waters. Int J Remote Sens 10(8):1373–1394.

    Article  Google Scholar 

  • Schott G (1902) Oceanographie und Maritime Meteorologie. Wissenschaftliche Ergebnisse der Deutschen Tiefsee-Expedition auf dem Dampfer "Valdivia" 1898-1899 republished by Karl Chun. Jena: Verlag von Gustav Fischer: 198-207

  • Secchi PA (1864) Relazione delle esperienze fatte a bordo della pontificia pirocorvetta Imacolata Concezione per determinare la trasparenza del mare. Memoria del PA Secchi. Il Nuovo Cimento Giornale de Fisica, Chimica e Storia Naturale, Ottobre 1864, Published 1865 20:205–237.

  • Smith RC, Baker KS (1981) Optical properties of the clearest natural waters (200–800 nm). Appl Opt 20(2):177–184.

    Article  CAS  Google Scholar 

  • Stiles WS (1982) Color science: concepts and method, quantitative data and formulae.

  • Sündermann, J (1993) Suspended particulate matter in the North Sea: field observations and model simulations. Philos Trans Royal Soc London Series A Phys Eng Sci 343 (1669):423–430.

  • Topp SN, Pavelsky TM, Dugan HA, Yang X, Gardner J, Ross MR (2021) Shifting patterns of summer lake color phenology in over 26,000 US lakes. Water Resour Res:e2020WR029123.

  • Tulloch AIT, Possingham HP, Joseph LN, Szabo J, Martin TG (2013) Realising the full potential of citizen science monitoring programs. Biol Cons 165:128–138.

    Article  Google Scholar 

  • Tyler JE (1968) The secchi disc. Limnol Oceanogr 13(1):1–6.

    Article  Google Scholar 

  • Ule Willi (1892) Die bestimmung der Wasserfarbe in den Seen. Kleinere Mittheilungen. Dr. A. Petermanns Mittheilungen aus Justus Perthes geographischer Anstalt:70–71.

  • Van der Woerd HJ, Wernand MR, Peters M, Bala M, Brochmann C (2016) True color analysis of natural waters with SeaWiFS, MODIS, MERIS and OLCI by SNAP. Proceedings of the Ocean Optics XXIII, Victoria, BC, Canada, pp 23–28

  • Van der Woerd HJ, Wernand MR (2018) Hue-angle product for low to medium spatial resolution optical satellite sensors. Remote Sens 10(2):180.

  • Visser, MP (1967) Secchi disch and sea colour observations in the North Atlantic Ocean during the Navado III cruise, 1964–1965, Aboard H. Neth. MS “Snellius”(royal Netherlands navy). Neth J Sea Res 3(4):553–563.

  • Wagh P, Sojan JM, Babu SJ, Valsala R, Bhatia S, Srivastav R (2021) Indicative lake water quality assessment using remote sensing images – effect of covid-19 lockdown. Water 13(1):73.

    Article  CAS  Google Scholar 

  • Wang S, Lee Z, Shang S, Li J, Zhang B, Lin G (2019) Deriving inherent optical properties from classical water color measurements: Forel-Ule index and Secchi disk depth. Opt Express 27(5):7642–7655.

    Article  CAS  Google Scholar 

  • Wang S (2018) Large-scale and long-time water quality remote sensing monitoring over lakes based on water color index. University of Chinese Academy of Sciences.

  • Wang S, Li J, Shen Q, Zhang B, Zhang F, Zhaoyi Lu (2015) MODIS-based radiometric color extraction and classification of inland water with the Forel-Ule scale: a case study of Lake Taihu. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 8(2):907–918.

    Article  CAS  Google Scholar 

  • Wang S, Li J, Zhang B, Lee Z, Spyrakos E, Feng L, Liu C et al (2020) Changes of water clarity in large lakes and reservoirs across China observed from long-term MODIS. Remote Sens Environ 247:111947.

  • Wang S, Li J, Zhang B, Spyrakos E, Tyler AN, Shen Q, Zhang F et al (2018) Trophic state assessment of global inland waters using a MODIS-derived Forel-Ule Index. Remote Sens Environ 217:444–460.

    Article  Google Scholar 

  • Wang S, Li J, Zhang W, Cao C, Zhang F, Shen Q, Zhang X, Zhang B (2021) A dataset of remote-sensed Forel-Ule Index for global inland waters during 2000–2018. Scientific Data 8(1):1–10.

    Article  Google Scholar 

  • Wernand MR, Hommersom A, Van der Woerd HJ (2013a) MERIS-based ocean colour classification with the discrete Forel-Ule scale. Ocean Sci 9(3):477–487.

    Article  Google Scholar 

  • Wernand MR, Van der Woerd HJ, Gieskes WW (2013b) Trends in ocean colour and chlorophyll concentration from 1889 to 2000, worldwide. PLoS ONE 8(6):e63766.

    Article  CAS  Google Scholar 

  • Wernand MR, Van der Woerd HJ (2010) Spectral analysis of the Forel-Ule ocean colour comparator scale. J Eur Opt Soc Rapid Publ 5.

  • Van der Woerd HJ, Wernand MR (2015) True colour classification of natural waters with medium-spectral resolution satellites: SeaWiFS, MODIS. MERIS and OLCI Sensors (basel) 15(10):25663–25680.

    Article  Google Scholar 

  • Van der Woerd HJ (2013) Citclops - Key scientific aspects of quality control. Zenodo.

  • Buytaert W, Zulkafli Z, Grainger S, Acosta L, Alemie TC, Bastiaensen J, Bièvre BD, Bhusal J, Clark J, Dewulf A (2014) Citizen science in hydrology and water resources: opportunities for knowledge generation, ecosystem service management, and sustainable development. Front Earth Sci 2:26.

  • Woźniak SB, Meler J (2020) Modelling water colour characteristics in an optically complex nearshore environment in the Baltic Sea; quantitative interpretation of the Forel-Ule scale and algorithms for the remote estimation of seawater composition. Remote Sens 12(17):2852.

  • Wyszecki G, Stiles WS (1982) Color science. Vol. 8: Wiley, New York

  • Xu Y, Feng L, Hou X, Wang J, Tang J (2021) Four-decade dynamics of the water color in 61 large lakes on the Yangtze Plain and the impacts of reclaimed aquaculture zones. Sci Total Environ 781:146688.

    Article  CAS  Google Scholar 

  • Xu Y, Wang Y, Lu J, Feng L (2020) Study on water color of Tonle Sap Lake in Cambodia based on FUI model. J Cent China Norm Univ (Nat.Sci.) 54(3):454–462.

  • Yang Y, Cowen LLE, Costa M (2018) Is ocean reflectance acquired by citizen scientists robust for science applications? Remote Sens 10(6):835.

  • Zhan J, Zhang D, Zhou G, Zhang G, Cao L, Guo Q (2021) MODIS-based research on Secchi disk depth using an improved semianalytical algorithm in the Yellow Sea. IEEE J Sel Top Appl Earth Observ Remote Sens 14:5964–5972.

  • Zhao Y, Wang S, Zhang F, Shen Q, Li J, Yang F (2021a) Remote sensing-based analysis of spatial and temporal water colour variations in Baiyangdian Lake after the establishment of the Xiong’an new area. Remote Sens 13(9):1729.

  • Zhao Y, Wang S, Zhang F, Shen Q, Li J (2021b) Retrieval and spatio-temporal variations analysis of Yangtze River water clarity from 2017 to 2020 based on Sentinel-2 images. Remote Sens 13(12):2260.

  • Zhou Y, He B, Congju Fu, Giardino C, Bresciani M, Liu H, Feng Qi, Xiao F, Zhou X, Liang S (2021) Assessments of trophic state in lakes and reservoirs of Wuhan using Sentinel-2 satellite data. Eur J Remote Sens 54(1):461–475.

Download references


The authors wish to thank the National Key R&D Program of China (2017YFC0406006, 2017YFC0406004), the Science Foundation of the Beijing Municipal Education Commission (SQKM201710028013) and the Beijing Outstanding Young Scientist Program (BJJWZYJH01201910028032) for the provided financial support. And thank the two anonymous reviewers for their constructive feedback.


This research was jointly supported by the National Key R&D Program of China (2017YFC0406004, 2017YFC0406006), the Science Foundation of Beijing Municipal Education Commission (SQKM201710028013) and the Beijing Outstanding Young Scientist Program (BJJWZYJH01201910028032).

Author information

Authors and Affiliations



Miao Ye contributed to reference collection and manuscript writing. Dr. Yonghua Sun contributed to the paper outline (choice of topics, structure and references) and text editing. Both authors have read and agreed to the published version of the manuscript.

Corresponding author

Correspondence to Yonghua Sun.

Ethics declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Additional information

Responsible editor: Xianliang Yi

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Ye, M., Sun, Y. Review of the Forel–Ule Index based on in situ and remote sensing methods and application in water quality assessment. Environ Sci Pollut Res 29, 13024–13041 (2022).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:


  • Water colour
  • Forel–Ule Index
  • Remote sensing
  • Water quality
  • Forel–Ule scale
  • Hue angle