Satellite derived water quality information is essential to assess the ecological state of inland and coastal waters and to identify changes or trends in water quality over time. This service will cover three of the most important water quality indicators that can be readily monitored from space: Chlorophyll concentrations, Total Suspended Matter (TSM), and water temperature.
Chlorophyll, specifically Chlorophyll a, is a vital pigment within cells of phytoplankton and can therefore be used a proxy measure of phytoplankton. TSM is a measure of the concentration of particles, both organic and inorganic, suspended in water, and relates closely to water turbidity. The water temperature is known to affect the metabolic rates and biological activity of aquatic lifeforms and can change the solubility of certain compounds. Higher water temperature can increase the solubility of certain harmful compounds while decreasing the solubility of many gases (e.g. oxygen). Typical water quality products are produced using optical sensors with high revisit rates (e.g. MERIS, Sentinel 3, MODIS) at the scale of 300-500m pixel resolution.
Water quality monitoring using EO data supports water resource management in a number of ways, including: assessing the effectiveness of river basin management policies or the impact of ongoing land use practices, monitoring the impact of large-scale infrastructural development projects, assessing the ecological state of coastal and inland waters (e.g. disease outbreaks, harmful algae blooms, aquatic weeds, point source pollution), or predicting the occurrence of threats to water supply systems. Long-term changes in water temperature can point to more gradual processes (e.g. climate change) while abrupt changes can often be an important identifier of point source pollution from industry.
Although the most commonly used optical sensors for water quality and temperature monitoring have high revisit times to deal with the inherently dynamic water environments and to overcome problems related to persistent cloud cover, the relatively coarse spatial resolution (large pixel size) of these sensors means that monitoring has typically been restricted to coastal regions and large lakes. A further constraint is the presence of strong haze in many water dominated regions and sun glint, which reduces the amount of usable data. Additionally, one of the main impediments to operational monitoring of water quality using EO is the lack of bio-optical data for parameterizing and validating products derived from remotely sensed imagery.
Improvements in EO-based water quality and temperature monitoring are constantly evolving and come in three forms: 1) increased sensor capabilities, 2) better availability of in-situ data for model calibration and validation, and 3) improvements to image analytical techniques.
New multi-spectral sensors such as Sentinel-2 MSI (and to a lesser extend also Landsat 8 OLI) will improve the capabilities for monitoring smaller sized water bodies with higher temporal frequency. Sentinel-3 Ocean and Land Colour Instrument (OLCI) will offer continuation of MERIS time series but with additional bands and a design optimised to reduce sun glint. Sentinel 3’s Sea and Land Surface Temperature Radiometer (SLSTR) will further enhance water temperature monitoring capabilities. When both Sentinel 2 and 3 are operating at full capacity (i.e. paired orbiting A and B sensors) the temporal resolution of the time series will be greatly enhanced. Novel parameter modelling techniques and atmospheric correction methods should allow superior water quality retrievals in turbid waters.