20 Remote Sensing of African Lakes: A Review
411
Wooster et al. (2001) used Channels 3, 4 and 5 from nighttime AVHRR for Lake
Malawi to estimate lake surface temperatures from May 1993 to September 2004.
Using nighttime images and including information from Channel 3, they were able
to show that, based on field vertical temperature profiles, the near-surface values
were indeed representative of the temperature of the mixed epilimnion.
Chavula et al. (2009a) used MODIS images to infer lake circulation patterns from
surface temperature. MODIS Terra daytime images for the period May to November
2006 were downloaded as both Land Surface Temperature (i.e. a data product coded
as MOD11a1) and SST. Compared with field observations, SST was a better estimator
(r
2
= 0.75) of actual lake surface temperature than MOD11a1 (r
2
= 0.68). Based on
this agreement, they were able to infer the location of an upwelling zone with potential
for fisheries.
20.5 Lake Properties: Biological
20.5.1 Visible and NIR Sensing of Macrophytes
Proliferation of invasive aquatic macrophytes has significantly impacted several
African lakes. The rapid expansion of Eichhornia spp. (water hyacinth) and other
plants in Lake Victoria has received much attention. Given the vast spatial dimensions of floating plant biomass and their elevated reflectance in visible and infrared
bands, there have been numerous uses of remote sensing to monitor macrophyte
extent.
Cavalli et al. (2009a) used MERIS and ASTER to map aquatic weed infestations
in Winam Gulf, Lake Victoria, after building a spectral library (350–2500 nm) of nine
common weed species. Two ETM+ scenes (1999 and 2001) were used to perform
spatial and temporal mapping of the major invasive aquatic plants (water hyacinth,
Nile lily, Nile cabbage). The authors then used 3-band (visible and NIR) ASTER
images to follow the evolution of weed cover from 2002 to 2006 in selected parts of
Winam Gulf. Because of the limited spectral resolution of ASTER compared with
ETM+, individual species could not be identified but the clear patterns and changes
in weed cover were identified.
Cavalli et al. (2009b) used an 18-band (410–1040 nm) image generated by the
Compact High Resolution Imaging Spectrometer (CHRIS), onboard the Project for
On-Board Autonomy (PROBA) satellite, (spatial resolution = 17 m) along with a
contemporary ETM+ image to identify specific plant species based on the spectral
library described in Cavalli et al. (2009a). The much wider swath width of ETM+
allowed mapping of macrophyte distribution over the whole gulf.
Fusilli et al. (2011) used MODIS images to map floating vegetation in Winam Gulf
at a minimum half-weekly frequency over a 10-year period of 2000–2009. NDVI was
calculated and used to distinguish floating vegetation from water and land. A Landsat
TM image with higher spatial resolution was used to help determine the appropriate
NDVI cut off value for floating vegetation (> 0.4). Additional information on rainfall,
411
Wooster et al. (2001) used Channels 3, 4 and 5 from nighttime AVHRR for Lake
Malawi to estimate lake surface temperatures from May 1993 to September 2004.
Using nighttime images and including information from Channel 3, they were able
to show that, based on field vertical temperature profiles, the near-surface values
were indeed representative of the temperature of the mixed epilimnion.
Chavula et al. (2009a) used MODIS images to infer lake circulation patterns from
surface temperature. MODIS Terra daytime images for the period May to November
2006 were downloaded as both Land Surface Temperature (i.e. a data product coded
as MOD11a1) and SST. Compared with field observations, SST was a better estimator
(r
2
= 0.75) of actual lake surface temperature than MOD11a1 (r
2
= 0.68). Based on
this agreement, they were able to infer the location of an upwelling zone with potential
for fisheries.
20.5 Lake Properties: Biological
20.5.1 Visible and NIR Sensing of Macrophytes
Proliferation of invasive aquatic macrophytes has significantly impacted several
African lakes. The rapid expansion of Eichhornia spp. (water hyacinth) and other
plants in Lake Victoria has received much attention. Given the vast spatial dimensions of floating plant biomass and their elevated reflectance in visible and infrared
bands, there have been numerous uses of remote sensing to monitor macrophyte
extent.
Cavalli et al. (2009a) used MERIS and ASTER to map aquatic weed infestations
in Winam Gulf, Lake Victoria, after building a spectral library (350–2500 nm) of nine
common weed species. Two ETM+ scenes (1999 and 2001) were used to perform
spatial and temporal mapping of the major invasive aquatic plants (water hyacinth,
Nile lily, Nile cabbage). The authors then used 3-band (visible and NIR) ASTER
images to follow the evolution of weed cover from 2002 to 2006 in selected parts of
Winam Gulf. Because of the limited spectral resolution of ASTER compared with
ETM+, individual species could not be identified but the clear patterns and changes
in weed cover were identified.
Cavalli et al. (2009b) used an 18-band (410–1040 nm) image generated by the
Compact High Resolution Imaging Spectrometer (CHRIS), onboard the Project for
On-Board Autonomy (PROBA) satellite, (spatial resolution = 17 m) along with a
contemporary ETM+ image to identify specific plant species based on the spectral
library described in Cavalli et al. (2009a). The much wider swath width of ETM+
allowed mapping of macrophyte distribution over the whole gulf.
Fusilli et al. (2011) used MODIS images to map floating vegetation in Winam Gulf
at a minimum half-weekly frequency over a 10-year period of 2000–2009. NDVI was
calculated and used to distinguish floating vegetation from water and land. A Landsat
TM image with higher spatial resolution was used to help determine the appropriate
NDVI cut off value for floating vegetation (> 0.4). Additional information on rainfall,