Water Quality Parameters

ACOLITE can derive various water quality parameters from surface reflectance. Parameters are computed in L2W processing and specified using the l2w_parameters setting.

Specifying Parameters

List parameters as comma-separated values:

l2w_parameters=chl_oc3,t_nechad2016,spm_nechad2016

Or in Python:

settings = {'l2w_parameters': 'chl_oc3,t_nechad2016'}

Chlorophyll-a Algorithms

Ocean Color (OC) Algorithms

Band-ratio algorithms for chlorophyll-a concentration.

Parameter	Description
`chl_oc2`	OC2 algorithm (2-band ratio)
`chl_oc3`	OC3 algorithm (3-band ratio, recommended)
`chl_oc4`	OC4 algorithm (4-band ratio)
`chl_oci`	OCI algorithm (hybrid OC/CI)

Example output: Chlorophyll-a concentration in mg/m³

Applicable waters: Clear to moderately turbid waters (Case 1)

Red Edge Algorithms

Algorithms using red edge bands for turbid/productive waters.

Parameter	Description
`chl_re_gons`	Gons red edge algorithm
`chl_re_moses3b`	MOSES 3-band algorithm
`chl_re_mishra`	Mishra NDCI algorithm

Example output: Chlorophyll-a concentration in mg/m³

Applicable waters: Turbid and productive waters (Case 2)

Other Chlorophyll Algorithms

Parameter	Description
`chl_lins`	Linear regression algorithm
`chl_chl2`	Carder-type 2-band algorithm

Turbidity Algorithms

Nechad Algorithms

Semi-analytical turbidity algorithms from Nechad et al.

Parameter	Description
`t_nechad`	Nechad turbidity (generic)
`t_nechad2009`	Nechad 2009 calibration
`t_nechad2009_ave`	Nechad 2009 average
`t_nechad2016`	Nechad 2016 calibration (recommended)

Example output: Turbidity in FNU

Settings:

nechad_range=600,900

Dogliotti Algorithm

Parameter	Description
`t_dogliotti`	Dogliotti turbidity algorithm

Example output: Turbidity in FNU

Applicable waters: Wide turbidity range with automatic band switching

Suspended Particulate Matter (SPM/TSM)

Nechad SPM

Parameter	Description
`spm_nechad`	Nechad SPM (generic)
`spm_nechad2016`	Nechad 2016 SPM (recommended)

Example output: SPM concentration in g/m³

Jiang TSS

Parameter	Description
`tss_jiang`	Jiang TSS algorithm

Example output: Total suspended solids in mg/L

QAA-Based Products

Quasi-Analytical Algorithm products.

Parameter	Description
`qaa`	Full QAA output
`qaa_a_*`	Total absorption at wavelength
`qaa_bb_*`	Backscattering at wavelength
`qaa_aph_*`	Phytoplankton absorption
`qaa_adg_*`	CDOM + detritus absorption

Spectral Products

Remote Sensing Reflectance

Parameter	Description
`rrs_*`	Remote sensing reflectance at band wavelength
`Rrs_*`	Same as rrs_* (alternative notation)

Example: rrs_443, rrs_560, rrs_665

Output: Rrs in sr⁻¹

Water-Leaving Reflectance

Parameter	Description
`rhow_*`	Water-leaving reflectance (ρw = π × Rrs)

Indices

Vegetation and Water Indices

Parameter	Description
`ndvi`	Normalized Difference Vegetation Index
`ndwi`	Normalized Difference Water Index
`ndci`	Normalized Difference Chlorophyll Index
`fai`	Floating Algae Index
`fai_rhot`	FAI from TOA reflectance

NDVI settings:

ndvi_wave=660,865
ndvi_wave_diff=40,100

Hue Angle

Parameter	Description
`hue_angle`	Van der Woerd hue angle
`forel_ule`	Forel-Ule water color index

Other Algorithms

Castagna Algorithms

Parameter	Description
`castagna`	Castagna contraband detection
`castagna_chl`	Castagna chlorophyll

Pitarch Algorithms

Parameter	Description
`pitarch`	Pitarch QAA variant

Van der Woerd

Parameter	Description
`vanderwoerd`	Van der Woerd color algorithms

Thermal Products

Available when TACT is enabled (tact_run=True):

Parameter	Description
`st`	Surface temperature (K)
`lst`	Land surface temperature (K)
`sst`	Sea surface temperature (K)

Custom Parameters

ACOLITE supports custom parameter algorithms. Place custom algorithm files in acolite/custom/ following the existing module structure.

Parameter Masking

Water parameters are automatically masked based on:

l2w_mask=True
l2w_mask_water_parameters=True
l2w_mask_wave=1600
l2w_mask_threshold=0.0215

Disable masking for all pixels:

l2w_mask=False

Output Units

Parameter Type	Units	Notes
Chlorophyll	mg/m³	Milligrams per cubic meter
Turbidity	FNU	Formazin Nephelometric Units
SPM/TSM	g/m³	Grams per cubic meter
Rrs	sr⁻¹	Per steradian
Temperature	K or °C	Kelvin or Celsius

Example Processing

Coastal water quality:

l2w_parameters=chl_oc3,chl_re_gons,t_nechad2016,spm_nechad2016

Inland turbid waters:

l2w_parameters=chl_re_gons,t_dogliotti,spm_nechad2016,ndci

Ocean color (clear waters):

l2w_parameters=chl_oc3,rrs_443,rrs_490,rrs_560

Full bio-optical suite:

l2w_parameters=chl_oc3,chl_re_gons,t_nechad2016,spm_nechad2016,qaa,hue_angle