ICES 1987

Summary.

Using NOAA-AVHRR imagery in assessing water

quality parameters

by

G.J. Prangsma and J.N. Roozekrans

Royal Netherlands Meteorological Institute (KNMI)

P.O. Box 201,3730 AE De Bilt, Netherlands.

C.M. 1987/C:41

Ref. MEQC

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The AVHRR instrument, carried by the NOAA-TIHOS/N series of operational

meteorological satellites can provide on a routine basis observational data,

which allow interpretation in terms of parameters related to water quality.

In principle, some - though not all of the algorithms applied to CZCS

data can be transformed for use with AVHRR observations. In combination with

the operational character of the NOAA satelli tes this opens up the way to

applications in monitoring of open sea and inland waters.

Results for some examples of such potential applications are presented.

Introduction.

Over the past decade, the Coastal Zone Color Scanner (CZCS) carried by

the Nimbus-7 satellite has proven the potential of satellite-borne ocean

colour instrumentation for a wide - and still widening - range of marine

applications. For inland waters the same applied for the Landsat Multi

Spectral Scanner (MSS) and Thematic Mapper (TM) instruments, be it on

different space and time scales.

Overviews of what can be done in this respect can be found in the

literature (see e.g. refs. 1,2,3). More specific applications are described in

e.g. refs. 4-8.

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Several applications discussed in these references have natures to a

state where routine use for e.g. monitoring, fisheries guidance etc. is

possible and even under way at some locations. Such routine use, however,

presupposesthe regular availability of observational data. With the lifetime

of the CZCS instrument ended - remarkably °beyond its design duration - the

only remaining data sources are the Landsat and - since early 1986 - SPOT

satellites, which have a severe handicap in that their swath width and orbital

configuration are such as to observe a given location only once about every

fortnight. (The pointability feature of the SPOT instrument might somewhat

enhance this repetition rate in practice).

Such observational frequency is far from adequate, due to recurring cloud

cover more often than not over many ocean areas of interest •

Thus it seemed an interesting experiment to explore the potential of data

originating from the AVHRR (Advanced Very Hight Resolution Radiometer) carried

by the NOAA-TIROS/N series of operational meteorological satellites.

The swath width (2700 km) and pixel size (1.1 km) for AVHRR are largely

compatible with the CZCS characteristics (1650 km, resp. 0.8 km) with daily

daytime observations. On the other hand the spectral resolution is much less:

2 channels in the visible and near-infrared bands for AVHRR versus 5 for CZCS.

In the course of our study on a variety of quantitative land- and sea­

applications of AVHRR data, we have therefore used AVHRR derived quant i ties

for open sea and inland waters to investigate such parameters as total

suspended matter, plankton distribution and - for inland waters - floating

blue algae patches •

Available data.

Archived AVHRR data have been obtained from the University of Dundee, for

a number of case studies.

All data tapes were treated with the standard algorithm package at KNMI,

which is based on the APOLLO (AVHRR .!:rocessing Qver Land cLoud and Ocean)

packageO made °available by the British Meteorological Office Research Unit in

Oxford, wi th additions and enhancements developed in house. Some of these

additions apply to cloud-clearing which are reported elsewhere (ref. 9),

others are transcriptions of known CZCS- and/or TM algorithms adapted to the

different spectral resolution of the AVHRR instrument.

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The sca-surface temperature (SST) algorithm has been compared separately

with in-situ data for the North Sea and is described in ref. 10.

Total suspended matter.

The algorithm for the calculation of total suspended matter (TSM) closely

follows the analysis by Viollier et al. (ref. 11) and is based on the

assumption that the sea surface is a pure black body radiator in the near­

infrared band (AVHRR channel 2), and that the radiation in the visible band

(AVHRR channel 1) is partly due to reflectance of sea water laden with

particulate matter.

Since the Rayleigh and aerosole scattering in the atmosphere in both

~ spectral bands are strongly related, the observed radiation in channel 1 can

be corrected for atmospheric effects, giving a value R(l) for the true sea

surface reflectance for each pixel. Correction for sun- and scan- angle

effects is built in the applied atmospheric model. This reflectance can be

related to TSM values by:

log TSM = a log R(l) + b

(D. Spitzer, private communication)

(1)

•where a and bare adjustable parameters varying in time; i.e., a and b have to

be computed from a regression between in-situ TSM observations and co-located

satellite measurements.

A typical example for the result of such regression is shown in fig. 1,

in which in-situ measurements along the Dutch coast for the period October, 28

until November, 7, 1984 are compared with satellite-derived data of October,

31 until November, 2, 1984. Correlation coefficients depending on whether we

take nearest dates, or average values, vary between 0.84 and 0.89.

In fig. 2 the corresponding spatial distributions are presented for 3

successive days with reasonably cloud-free satellite data.

In fig. 3 the rcsults for May, 2, 1986 are presented.

The results for October/November 1984 (fig. 2) show the TSM distribution

to be fairly conservati vc an a day-to-day basis, al though details may vary

depending on e.g. meteorological conditions.

The patterns are generally in line with what is commonly known about the

area, but do carry much detail hitherto unthought (D. Spitzer, private

communication) .

Note also the di fferences in the separate compartments of the inland

waters: Markermeer (SW-ly part) carries a lot more suspended matter on all

occasions than the IJsselmeer (N-ly basin), in line with in-situ measurements

(G. Stokman, private communication). Research as to whether the same algorithm

and coefficients (eq. (1» can be used .for fresh inland water and for

seawater, is underway.

Plankton distributions, chlorophyll content.

If R1 and R2 represent the radiances at - suitable chosen - wavelengths

Al and A2

(Al < A 2) corrected for atmospheric influences, the commonly used

algor i thm .for converting CZCS data into chlorophyll concentrations (ChI) is

given by:

log (ChI) (2)

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where a and bare adjustable coefficients. If Al and A2 are sufficient close

together as are R1 and R2 eq. (2) can be trans.formed to read:

d log Rlog (ChI) = a - b ~A dA

For land (agricultural) applications, the so-called Normalized Difference

Vegetation Index (NDVI) is commonly used:

NDVI (4 )

Under the same assumptions as used in the derivation of eq. (3) from eq. (2),

we can transform eq. (4) to read:

NDVI ~A d log R2 dA (5 )

In this case, how~ver. the wavelengths Al and A2 differ from those used

in the chlorophyll algorithm. Still it is clear that for weIl-chosen

wavelengths. a correspondence might be found between the NDVI and chlorophyll

COncentrations.

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In other words, the NDVI algorithm when applied to sea-pixels, might show

patterns which - to say the least - fear some relationship to chlorophyll

distributions.

Due to the choice of the wavelength bands of the AVHRR instrument and the

mathematical similarity between the TSM and chlorophyll/NDVI algorithms (see

eqs. (1) and (2» both TSM and NDVI distributions might well show strong

similaritiesj Le., it will be difficult to distingguish unambiguously TSM

from plankton population, based on AVHRR data alone. Still we feel that

observed NDVI distributions (fig. 4 and fig. 5) are encouraging and worthy

further investigation, especially if we note the correspondance between a

plankton- and sea surface temperature (SST) front, following the 40 m line

south of Doggerbank (fig. 5) •

Application to inland water; floating blue algae.

Fresh water lakes in the Netherlands during the summer months show large

areas of floating blue algae. The biological and physical behaviour of these

layers are the subject of various studies, one of which is adressing the

quest ion of regular and synoptic observations of their occurrence and extent.

This led us to investigate the potential of daily observations using

AVHRR imagery.

In the previous section we have demonstrated the sensitivity of the NDVI

to plankton concentrations. Applying the same algorithm (eq. (4» to AVHRR

data to the IJsselmeer area we found amazing results, shown in fig. 6, taken

from a longer series (ref. 12) •

From fig 6 we observe that positive ND VI values seem to be indicative of

the presence of blue algae at the surface. Normally, since water is nearly a

black body for the near-infrared, very little reflectance is seen in AVHRR

channel 2, the NDVI going negative fOr water surface normally. With blue algae

floating at the very surface, the near-IR reflectance is increased

dramatically due to the abundant presence of chlorophyll.

The surface temperature distribution (fig. 7) confirms the strong surface

heating (several degrees) due to the presence of these biologically acti ve

absorbers.

When the wind picks up during August, 20, enhanced vertical mixing causes

the distribution of the blue algae through a larger part of the water column

which is also seen in the disappearance of the hot surface layer.

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Discussion.

The results presented above indicate that NOAA-AVHRR data have

significant potential in deriving quantitative information for parameters at

and below the sea (water) surface, given an adequate transcription of known

CZCS or TM based algorithms, or newly developped ones.

Strong point in favour of the AVHRR is the daily coverage (even twice

daily if we use both the morning and afternoon satellites), giving access to

the area of interest whenever we have a cloud-free day, as opposed to the

Landsat overpasses every 16 days only. Also the costs and the potential real­

time use of AVHRR-data favour the AVHRR over the expensive Landsat and SPOT­

data, which troublesome accesibility drives many user to despair.

A weak point is the redueed spectral resolution espeeially in the visible

wavelengths. An the other hand the radiometrie resolution of the AVHRR (10

bits) is mueh better than the resolution of most other 8-bit satellite

sensors.

The total suspended matter (TSM) algorithm has matured to the stage that

routine use i3 feasible, given regular in-situ data for updating the

coefficients. Plankton concentration, however, can not yet be distinguished

unambiguously from TSM, but qualitative information ean be ontained.

Quantitative results ean only be obtained if further research towards

algorithms is done.

Present satellite techniques allow more detailed studies into the day-to­

day dynamics of floating layers of blue algae in inland waters, at the same

~ time being a usefull monitoring tool.

In summary we eonelude that increasing knowledge of the spectral

eharaeteristics of components in the aquatic ecosystem ean be translated into

algorithms for routine quantitative assesment of such eomponents for which the

speetral texture is compatible with the AVHRR operational instrument.

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References.

1. G. Serie and P. Cornillon, Remote sensing, a tool for managing the marine

environment: eight case studies.

Ocean Engineering, Uni versi ty of Rhode Island, Mar ine Technical Report

77,1981.

2. Anonymous, Ocean colour. The potential for commercial applications.

Report prepared by Oxford Computer Services Ltd., (OCS July 1986) for the

British National Space Centre.

3. P. Cornillon, Satellite oceanography: a new tool for marine poliey

makers.

Marine Poliey, January 1986, pp 57-60.

4. M.-C. Mouchot and E. Lambert, Les pecheurs auront un oeil dans l'espace,

GEOS 1986, No. 3, pp 19-21.

5. J.W. Campbell and J.E. O'Reilly, Role of satellites in estimating primary

productivity on the Northwest Atlantic continental shelf,

Preprint 1986 (Bigelow Contrib. Nr. 86033).

•6. J.J. Simpson, C.J. Koblinsky, J. Pelaez, L.R. Haury and D. Wiesenhahn,

Temperature -Plantpigment - optical relations in a recurrent offshore

mesoscale eddy near Point Conception, California, J. Geophys. Res. 91

(1986) pp 12919-12936.

7. U. Horstmann, H. van der Piepen and K.W. Barrot, The influence of river

water on the Southeastern Baltic Sea as ofserverd by Nimbus-7/CZCS

imagery.

Ambio 15 (1986), No. 5, pp 286-289.

8. R.J. Charlson, J.E. Lovelock, M.O. Andreae and S.G. Warren, Oeeanic

plankton, atmospheric sulphur, cloud albeda and elimate.

Nature 326 (1987) pp 655-661.

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9. J.N. Roozekrans and G.J. Prangsma, Cloud clearing algori thms wi thout

AVHRR-channel 3.Summary Proc. of 2nd AVHRR Users Meeting, 15-16 April 1986. Rutherford

Appleton Lab., Chilton, Didcot, Uni ted Kingdom, pp 19-21.

10. G.J. Prangsma and J.N. Roozekrans, Processing of raw digital NOAA-AVHRR

data for sea- and land applications.thProc. 7 Intern. Symp. on Remote Sensing for Resources Development and

Environmental Management (ISPRS Commission VII), Enschede, 25129 August

1986, pp 63-66.

11. M. Vioiiier. D. Tanre and P.Y. Deschamps, An aigorithm for remote sensing

o~ water color from space.

Boundary-Layer Meteor. 18 (1980) pp 247-267.

12. J.N. Roozekrans, G.J. Prangsma and G. Stokman. The observation of

occurrence and extent of biue aigae float layers in the IJsselmeer area.

(In preparation).

TSM IN·SITU(mg I!)

10

o AVHRR 31/10/84 14.41 \lml

A 1/11/1414.31gml

• 2/11/ " 14.25 \lml

• AVERAGE VAlUES

iig. 1: Comparison of in-situ TSM values with satellitederived values (eg.(1))

•0.1+-----­

0.1,10 tOO

----+TSM AVHRR (mg l,tl

•C10UD or no AVHRH-data

Fig. 2: Distribution of TSM for 3 successive days.

LAND

0-4

4 - 12

12 - 25

25 >

mg/l

11

"

Fig. 3: Distribution of TSM (for legend see fig. 2).

-- --- - ~--- ... ~

oHIGH VALUl;S

LOI-I VALUES

•Fig. 4: NDVI pattern far May, 2, 19~6 •

NJJVI

Fig. 5: Results for November 2, 1984.(images not geometrie eorreeted)

SST

HIGH VA.LLJES LO"; VALUES

NDVI:

18 - 8 - 83 19 - 8 - 83 20 - 8 - 83

NDVI: <: O. 0.25 > LAND

Fig. 6: NDVI distribution for 3 successive days in August 1983.For areas with NDVI higher than 0, blue algae float layerscan be expected.

SUHFACE TEMPERATURE:

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18 - ö - 83 19 - 8 - 83 20 - 8 - 83

551:: < 17 19 21 23 c> LAND

Fig. 7: Surface temperature distribution for the same days in August 1983.