Spectral measurements of underwater downwelling radiance of inland water bodies

Figures & data

Fig. 1 Location of (a) Alqueva and Monte Novo reservoirs and (b) Thau lagoon.

Fig. 2 (a) Second version of the frame for underwater measurements of spectral downwelling zenith radiance and (b) the tip in detail.

Table 1. Field campaign details

Campaign date	Place	Sun zenith angle (°)	Wind speed near surface (m s^−1)	Turbidity (NTU)	Chlorophyll α (µg L^−1)
24 August 2011	Thau	52.4	3.1	0.5	–
22 June 2012	Pool	86.6	4.3	–	–
9 July 2012	Pool	42.9	2.7	–	–
12 July 2012	Monte Novo	26.6	1.2	14.8	50.3
31 August 2012	Alqueva	37.8	8.4	13.8	33.2
6 September 2012	Alqueva	41.5	2.6	14.9	41.7
13 September 2012	Pool	78.5	3.2	–	–

Fig. 3 Underwater environment in a 5-m deep pool from the municipal swimming complex of Évora.

Fig. 4 Measurements of spectral downwelling zenith radiance at several levels deep in a 5-m pool with clean water for two different zenith angles: (a) 42.9° on 9 July 2012 and (b) 86.6° on 22 June 2012.

Fig. 5 Measurements of spectral downwelling zenith radiance at several levels deep in Monte Novo reservoir on 12 July 2012, for a sun zenith angle of 26.6°. In the lower panel the average radiance profiles for the blue (400–500 nm), green (500–600 nm) and red (600–700 nm) parts of the spectrum.

Fig. 6 Measurements of spectral downwelling zenith radiance at several levels deep in Alqueva reservoir on (a) 31 August 2012 with a wind speed of 8.4 m s⁻¹ and sun zenith angle of 37.8° and on (b) 6 September 2012 with a wind speed of 2.6 m s⁻¹ and sun zenith angle of 41.5° (right panel). In the lower panels the average radiance profiles for the blue (400–500 nm), green (500–600 nm) and red (600–700 nm) parts of the spectrum.

Fig. 7 Measurement of spectral downwelling zenith radiance at several levels deep in Thau lagoon on 24 August 2011. In the lower panel the average radiance profiles for the blue (400–500 nm), green (500–600 nm) and red (600–700 nm) parts of the spectrum.

Fig. 8 Spectral attenuation coefficient for pure water obtained from Smith and Baker (1981) and for five cases of the campaigns during the summers of 2011 and 2012, derived from eq. (5).

Table 2. PAR attenuation coefficients calculated using eq. (5) for the field campaigns

Campaign date	Place	K(θ,φ,PAR) (m^−1)	(m^−1)	σ (m^−1)	N
24 August 2011	Thau	0.37	N/A	N/A	1
22 June 2012	Pool	0.21	N/A	N/A	1
9 July 2012	Pool	N/A	N/A	N/A	1
12 July 2012	Monte Novo	1.65	1.41	0.37	6
31 August 2012	Alqueva	1.28	1.31	0.23	10
6 September 2012	Alqueva	1.16	1.26	0.22	8
13 September 2012	Pool	0.20	0.17	0.04	2

K(θ,φ,PAR) represents the PAR attenuation coefficient of the most significant profile, represents the average value of PAR attenuation coefficient of the campaign, σ represents the standard deviation of the attenuation coefficient of number of profiles (N).

Table 3. Relationships between attenuation coefficient (K) and turbidity (T) for several authors and for this work

Authors	Year	Country	Relationship	R ^2	N
Walmsley et al.	1980	South Africa	K=0.10(T) + 0.44	0.71	43
Grobler et al.	1983	South Africa	K=0.13(T) + 0.55	1.00	5
Roos and Pieterse	1994	South Africa	K=0.06(T) + 2.35	0.94	39
Oliver et al.	1999	Australia	K=0.04(T) + 0.73	0.98	16
Giblin et al.	2010	United States	K=0.69(T) + 0.53	0.82	360
This work	2012	Portugal	K=0.07(T) − 0.24	0.97	5

Smith R. C , Baker K. S . Optical properties of the clearest natural waters (200–800 nm). Appl. Optic. 1981; 20: 177–184.

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