Metrics of Surface Water Quality: Introduction

Although satellite imagery cannot measure all aspects of lake physical-chemical and biological characteristics, it can efficiently measure the following optically related properties, which are key indicators of lake water quality and ecological status: chlorophyll, phycocyanin (characteristic pigment of cyanobacteria), suspended solids, turbidity, and colored dissolved organic matter (CDOM). These variables absorb and/or scatter visible light (and infrared radiation) at different wavelengths and thus affect the amount and spectral character of light reflected back to satellite sensors. The reflected light measured by sensors can be used to measure these water quality variables.

Reflectance spectra for waters dominated by various optical properties.

Above: Reflectance spectra for waters dominated by various optical properties. The spectrum for water is from a highly oligotrophic, very clear mine pit lake in northern Minnesota, and its spectrum is close to that for pure water. The chlorophyll and CDOM-chlorophyll spectra have minima in reflectance at ~670 nm, where chlorophyll has an absorbance peak and have secondary minima around 620 nm due to phycocyanin (thus indicating the presence of cyanobacteria in the waters). The three spectra labeled with CDOM all have low reflectance in the blue region (< ~500 nm) because of high CDOM absorbance in this region. The very low (red) spectrum is for a water that is optically dominated by very high CDOM, which absorbs nearly all incoming light, leaving very little to be reflected, even at longer wavelengths.sensors can be used to measure these water quality variables.

As the figure on the right shows, the properties of light reflected from inland waters depend on the relative abundance of three important “optical” constituents: plant pig­ments (chlorophyll and phycocyanin), sus­pended matter, and colored dissolved organic matter (CDOM).

Retrieval of information on the abundance of these constituents in inland waters from satellite sensors typically uses sensor bands that measure character­istic absorption or reflectance features of these constituents and that have minimal inter­ference from other optical prop­erties. For example, retrieval equations for chlorophyll typically use the ratio of the scattering peak for phytoplank­ton cells at ~ 705 nm and the reflectance trough at ~ 670 nm, which is where peak chlorophyll absorbance occurs. These wavelengths generally are not affected by CDOM absorbance, as shown by the difference between the yellow and green spectral lines in the figure. Marine remote sensing scientists measure chlorophyll using bands related to chlorophyll absorbance in the 450-550 nm region, but CDOM levels are low in the oceans and do not interfere.

The abundant aquatic resources of the Upper Great Lakes region are critically important for recreational activities, transportation, agriculture, industry, and drinking water supplies. They also serve as recipients of treated waste water. The heavy reliance of region’s economy on its water resources translates into high vulnerability to potential changes in water quality, and human activities can impact surface water quality in many ways. Lake eutrophication (nutrient over-enrichment resulting in excessive production of algae and cyanobacteria) is a pervasive problem in the region. Its primary causes are nutrient inputs from municipal wastewater, urban and agricultural runoff, and near-shore land use. Suspended sediment loadings related to land management practices are a cause of poor water clarity in southern Minnesota; many organic and heavy metal contaminants also are associated with suspended sediment loadings. Mercury pollution is a widespread problem, especially in northern Minnesota, and mercury levels in fish are associated with CDOM levels. Understanding the links between surface water quality and environmental drivers is a major focus of water resources research.

Monitoring surface water quality is a major activity for government water management agencies and for many citizen groups. Such efforts provide a vital service for understanding and manag­ing surface waters, but because of cost and human resource limitations, only a small fraction of lakes and rivers can be sampled annually; fewer still can be assessed at frequencies high enough to detect rapid change. Satellite remote sensing can be a cost-effective way to gather spatially comprehensive water quality information.

Long-term water quality information at broad regional scales is a fundamental need for effective environmental planning and management. The extraction of historic and current water quality data from satellite images, coupled with existing data collection, facilitates the development of comprehensive regional databases (e.g., LAGOS) that can be used to evaluate regional differences and trends over time, as well as to evaluate the effects of land use practices. Results of such analyses will help local and state agencies make informed policy decisions and improve the management of our surface water resources.