than shallow lakes overall and when grouped by watershed land cover percentages.
Lakes in agriculturally dominated ecoregions in southern and western Minnesota
were more susceptible to decreasing clarity than the rest of the state. Statewide
water clarity remained stable from 1985 to 2005 but decreased in ecoregions where
agricultural is the main land use. Water clarity decreased as agriculture and/or
urban percentages increased and forested land was associated with higher water
clarity.
5 Conclusions
ORS using satellite imagery can be used to measure water quality of inland, marine,
and coastal waters. ORS in marine waters is well established with a large investment in several generations of increasingly sophisticated satellite sensors acquiring
images with large pixel sizes that are ideal for oceanic and most coastal studies but
are too coarse for most inland water bodies. With these systems sophisticated
analytical and semi-analytical algorithms have been developed that retrieve chlorophyll levels from the oceans on a routine, global-scale basis.
Remote sensing scientists focusing on inland waters have had to rely on other
satellites like Landsat, which have adequate spatial resolution but critical deficiencies in spectral and temporal resolution. The spectral bands used to retrieve
chlorophyll levels from oceanic waters do not work in optically complex inland
waters. These deficiencies have limited development of retrieval algorithms for
inland water quality variables by satellite imagery mostly to empirical and semiempirical approaches.
Therefore, use of remote sensing for regional inland water quality has progressed
slowly since the launch of the first Landsat satellite in 1972. Although there have
been many successful regional water quality assessments, these have largely been
limited to water clarity due to the available spectral bands and/or to only very large
lakes (due to the large pixel size of sensors designed to study the oceans). Landsat
8 (launched in 2013) has some significant improvements over its predecessors, but
its spectral and temporal characteristics remain largely unchanged, except for a
shorter wavelength blue band. The next big advancement for remote sensing of
regional water quality of lakes will come from the ESA Sentinel-2 and Sentinel-3
satellites. Improvements in spectral and temporal characteristics of these satellites
will allow for better characterization of chlorophyll, CDOM, and SS min in optically
complex waters.
For effective lake management, it is essential to have long-term water quality
information on a synoptic scale. Combining Landsat and Sentinel satellite imagery
will greatly improve the ability to acquire imagery when needed and should
significantly improve the utility and usefulness of ORS for water resource managers. Landsat 8 and Sentinel-2 imagery can be used for the assessment of all lakes,
and Sentinel-3 can be used for large lakes more often with its higher temporal
resolution. Once reliable water quality products can be produced in a timely fashion
134
L.G. Olmanson et al.
Lakes in agriculturally dominated ecoregions in southern and western Minnesota
were more susceptible to decreasing clarity than the rest of the state. Statewide
water clarity remained stable from 1985 to 2005 but decreased in ecoregions where
agricultural is the main land use. Water clarity decreased as agriculture and/or
urban percentages increased and forested land was associated with higher water
clarity.
5 Conclusions
ORS using satellite imagery can be used to measure water quality of inland, marine,
and coastal waters. ORS in marine waters is well established with a large investment in several generations of increasingly sophisticated satellite sensors acquiring
images with large pixel sizes that are ideal for oceanic and most coastal studies but
are too coarse for most inland water bodies. With these systems sophisticated
analytical and semi-analytical algorithms have been developed that retrieve chlorophyll levels from the oceans on a routine, global-scale basis.
Remote sensing scientists focusing on inland waters have had to rely on other
satellites like Landsat, which have adequate spatial resolution but critical deficiencies in spectral and temporal resolution. The spectral bands used to retrieve
chlorophyll levels from oceanic waters do not work in optically complex inland
waters. These deficiencies have limited development of retrieval algorithms for
inland water quality variables by satellite imagery mostly to empirical and semiempirical approaches.
Therefore, use of remote sensing for regional inland water quality has progressed
slowly since the launch of the first Landsat satellite in 1972. Although there have
been many successful regional water quality assessments, these have largely been
limited to water clarity due to the available spectral bands and/or to only very large
lakes (due to the large pixel size of sensors designed to study the oceans). Landsat
8 (launched in 2013) has some significant improvements over its predecessors, but
its spectral and temporal characteristics remain largely unchanged, except for a
shorter wavelength blue band. The next big advancement for remote sensing of
regional water quality of lakes will come from the ESA Sentinel-2 and Sentinel-3
satellites. Improvements in spectral and temporal characteristics of these satellites
will allow for better characterization of chlorophyll, CDOM, and SS min in optically
complex waters.
For effective lake management, it is essential to have long-term water quality
information on a synoptic scale. Combining Landsat and Sentinel satellite imagery
will greatly improve the ability to acquire imagery when needed and should
significantly improve the utility and usefulness of ORS for water resource managers. Landsat 8 and Sentinel-2 imagery can be used for the assessment of all lakes,
and Sentinel-3 can be used for large lakes more often with its higher temporal
resolution. Once reliable water quality products can be produced in a timely fashion
134
L.G. Olmanson et al.