Monitoring the condition or health of the
environment is essential to its proper stewardship and management. Under the Clean Water Act, water resources
are monitored by states and other jurisdictions, but current programs are
unable to monitor all their watersheds, water bodies, and point- and non-point
pollution sources. In reality, the
majority of U.S. water bodies are not monitored regularly or not monitored to
detect all pollution types. This is
both a financial and technical problem, as many monitoring methods are not
cost-effective or technologically efficient enough for monitoring all water
bodies of interest.
The rapid advancement of new technologies, such as remote sensing, may
someday provide methods for monitoring more water quality parameters, in more
water bodies, with improved accuracy, or with reduced per-unit costs. To actively improve the status of monitoring
science, however, monitoring professionals must learn about emerging
technologies as well as those available now, and researchers who develop these
technologies must learn the needs of their clientele. In order to focus and accelerate research and applications of
advanced technologies in water resources monitoring, the U.S. Environmental
Protection Agency (EPA) and the National Aeronautics and Space Administration
(NASA) are convening a workshop to discuss and match monitoring needs with the
appropriate advanced technologies. The
purpose of this workshop is to expose technical and management personnel of
both agencies to (1) NASA's remote sensing science and technology, and (2)
EPA's water resources monitoring requirements and data bases. The goal of the workshop is mutual
education, and the opportunity to explore future collaboration in water
monitoring/remote sensing research and applications.
Nearly three years later, this workshop’s findings remain relevant to water resources monitoring and management. The challenges facing local, state and federal water monitoring programs basically remain the same. The opportunities for NASA and its collaborating researchers to apply remote sensing instruments to water resources monitoring are still significant. Moreover, NASA’s Mission to Planet Earth program, now called Earth Science Enterprise, is considerably closer to applying mission technologies in 1999 than three years ago, and some of the collaborations suggested at the workshop are already taking place. Accordingly the EPA Office of Water has published this workshop report, with minor updates, to share useful findings with water resource managers and researchers.
The EPA / NASA Workshop on Water Monitoring, Remote Sensing and Advanced Technologies was held on the 11th and 12th of December 1996 at the Holiday Inn Capitol at 550 C Street SW, in Washington, DC. Environmental Protection Agency (EPA) and the National Aeronautics and Space Administration (NASA) employees comprised the majority of workshop participants (Table 1.1), but other state and academic institutions were represented. A complete listing of the workshop participants can be found in Appendix C.
Table 1.1: Summary of Participating Agencies and Institutions.
|
AGENCY |
Number of Attendees |
|
EPA/Research |
13 |
|
EPA/Water |
9 |
|
EPA Regions |
9 |
|
EPA Other Offices |
3 |
|
NASA |
14 |
|
NASA Academic/Investigation |
20 |
|
State / Academic |
2 |
|
Other Agencies |
4 |
|
Total Attendance |
74 |
Workshop Structure
This workshop was designed jointly by the agencies to initiate EPA and NASA inter-agency discussions on the topic of monitoring water resources with remote sensing and other advanced technologies. The Workshop Statement of Purpose is provided in the Foreword, located on page iv of this report.
The workshop took place over the course of two days and was structured to maximize the time spent on identifying technologically advanced and appropriate approaches to water resource monitoring. The entire workshop agenda is presented in Appendix A. Workshop participants were sent a background white paper (see Section 3) that summarized EPA’s Clean Water Act mandate and the shortcomings of current monitoring programs, and identified how NASA advanced technologies such as remote sensing might assist EPA in meeting their mandate.
The first part of the workshop provided tutorials in EPA water resources monitoring needs and NASA remote sensing instruments, thereby orienting the participants toward relating the two agencies’ activities. The remainder of the workshop was then dedicated to identifying remote sensing and advanced technologies that might be applied to EPA’s specific water resource monitoring needs. These discussions occurred within Breakout Sessions that were co-moderated by EPA and NASA scientists.
Water Resource Types Considered
Four separate water resource categories were used to structure discussions on monitoring needs and potential remote sensing and advanced technology applications. The four water resource types are: watersheds, lakes, and rivers; wetlands; groundwater; and estuaries and oceans. These are briefly described below in terms of their distribution, their volume relative to the global supply of water, and their residence time (the amount of time necessary for water to move through the water body of interest), which are all factors which influence the utility of monitoring with remote sensing or other advanced technologies.
Watersheds, Lakes and Rivers
Water bodies in this category -- lakes, rivers and streams -- are the principal inland surface water bodies; watersheds are their adjoining drainage areas. Rivers (taken to mean all rivers and streams of any size in this workshop) contain approximately 0.2 % of all available fresh water resources; there are over 3 million miles of rivers and streams in the US. Lakes and reservoirs, which are usually considered in the same category, contain approximately 3.0 % of available fresh water resources. Residence times are highly variable according to water body size. About 40 % of these water bodies do not fully meet state water quality standards.
Watersheds, which define the land area draining into the river or lake, often also include groundwater, soil moisture, biospheric water, and atmospheric water that contribute to the rivers and lakes. In some areas, and during some seasons, snow and glaciers contribute additional sources of water. Soil contains approximately 1.7 % of available fresh water, which has a residence time of 2 weeks to 1 year. Atmospheric water is approximately 0.2 % of all fresh water and has a residence time of 1.3 weeks. Watershed characteristics such as soils, surficial geology, and land use/land cover, and processes such as runoff quality and quantity and pollutant transport, groundwater recharge, and erosion/deposition of sediment, often affect the quality and quantity of lake and river water resources.
Wetlands
Wetlands are a diverse group of surface water resources that are typically found at the interfaces of terrestrial and aquatic ecosystems. They are defined and delineated by the presence of certain saturated soils, specific species of water tolerant plants, and distinct periods of near-surface or surface inundation. Wetlands are often attributed with beneficial functions such as stream flow moderation, shoreline stabilization, sediment trapping and nutrient retention, and wildlife habitat. Wetlands comprise approximately 6 % of the earth’s total land area; over half of the wetland area in the US has been lost since colonial times, concurrent with an undetermined amount of degradation or loss of beneficial wetland functions.
Groundwater
Groundwater resources are subsurface water reserves that are held in saturated soils and geologic formations. Groundwater reserves interact with unsaturated soils, wetlands, rivers, and lakes, and depending on the surface topography or the hydrologic season, these interactions can result in a gain or loss of surface waters from place to place and time to time. Groundwater that is readily retrievable for drinking water comprises approximately 97 % of all freshwater supplies (approximately 9.9 x 106 km3) and has a residence time that ranges from 2 weeks to 10,000 years. In the United States ground water provides 50% of all drinking water (90% in rural areas), 25% of industrial water and 34% of agricultural irrigation. . As human population increases competition for fresh water, particularly ground water in arid and semi-arid lands, will become more intense. Degradation of groundwater resources is not thoroughly documented but monitoring ground water quality is increasing significantly. Impairment or groundwater often occurs from infiltration of surface contaminants, or migration of pollutants from uncontrolled burial or landfilling sites. Loss of groundwater is due to overutilization or reduction in the ability to recharge.
Oceans and Estuaries
Near-coastal ocean waters and estuaries are the focus of EPA’s coastal programs. Estuaries are located at the interface of fresh and saline waters, often defined by the tidally influenced zone at the mouth of a river entering the ocean. Oceans and estuaries comprise over 97 % of all water on the earth and evaporation from these reserves produces the fresh water which later precipitates over land to recharge rivers, lakes, wetlands, and groundwater supplies. Residence times vary with size. Burgeoning coastal populations are creating a greater need for monitoring coastal water quality to detect emerging water resource problems as early as possible.
These four resource types inevitably have some overlap. Certain characteristics of the watersheds, lakes and rivers category in particular are held in common with the other three categories. Examples of overlap include: 1) the fact that virtually all water bodies have watersheds; 2) the surface and subsurface water exchanges among rivers, lakes, wetlands, estuaries, oceans and groundwater; and 3) the wetland types that are physically similar to or co-located with lake littoral areas and riparian areas of streams and rivers. The report identifies areas where monitoring needs of one category of water resource are comparable to needs in the other categories so that applicable remote sensing and advanced technologies are recommended for all appropriate resource types.
Remote Sensing Technologies Considered
Materials on the earth’s surface, such as water, soil, and plant chlorophyll, have a unique atomic structure that reflects a predictable and often unique amount of radiation at each interval along the electromagnetic spectrum. A material’s unique reflectance curve is often referred to as its spectral signature. Electromagnetic radiation, arriving from the sun or sent by a remote sensor, can range from the very short wavelength gamma rays and x-rays to the visible and near infrared and onto the longer microwave and radio. Only certain parts of the electromagnetic spectrum are useful for certain remote sensing applications. For example, at some wavelengths the material of interest may not reflect any unique signature, while at other intervals the material may display a very unique signature but atmospheric gasses might trap the signal and prohibit its detection by the sensor.
A host of environmental remote sensing instruments have been developed and launched into orbit by NASA and other international space agencies since the 1970s. New and improved technologies are continually developed by these agencies, and due to legislation signed by President Bush in 1992, private companies are now also developing and launching environmental sensing technologies into earth’s orbit.
Electromagnetic Sensors
Electromagnetic sensors are not capable of detecting and measuring all spectra, and instead are designed to measure across a discrete range of the entire electromagnetic spectrum. Using the physical relations of electromagnetic theory along with extensive laboratory and field tests scientists have successfully identified the specific spectra or combinations of spectra most strongly emitted from, and unique to, the environmental resources of interest (e.g. water, suspended sediment or chlorophyll). Sensors commonly used in environmental remote sensing include those designed for gamma radiation, visible radiation, infrared radiation, thermal radiation, and microwave radiation. Remote sensing instruments are often categorized by the electromagnetic sensor’s radiometric resolution, spatial resolution, and temporal resolution.
Radiometric Resolution
Radiometric resolution refers to the band-size or spectral range of the electromagnetic sample received by the remote sensor. Many multi-band remote sensing platforms that sample in the visible spectrum are designed to retrieve signals at red, green, and blue wavelengths, which corresponds to spectral ranges from approximately 0.45 to 0.55 for red, 0.55 to 0.65 for green, and 0.65 to 0.75 for blue. If a single sensor is used to sample at this resolution the sensor is considered broad band, while hyper-spectral sensors are capable of resolving multiple wavelengths within each of these bands.
Spatial Resolution
Spatial resolution refers to the horizontal length scale resolved by each individual electromagnetic sensor pixel aboard the platform. Remote sensing platforms typically have a fixed spatial resolution, and current technologies use resolutions that range from 1-m to over 1-km length scales. During data retrieval events remote sensing platforms will sample an entire scene that is composed of an array of pixel areas, resulting in an entire ground swath area of 100s to 10,000s of km2 at the same point in time.
Temporal Resolution
Temporal resolution refers to the frequency at which the remote sensing platform revisits the same point in space. Geostationary remote sensing platforms are in high orbits (10,000s of km) that position them above the same general area and can therefore retrieve duplicate images at sub-hourly temporal resolutions. Sun-synchronous platforms are in lower (600 to 1000 km), semi-polar, orbits that typically result in temporal resolutions measured in the 10’s of days after the platform has circled the entire earth. More advanced sun-synchronous platforms are sometimes designed with tilting sensors that allow the platform to retrieve duplicate coverage by using a different ‘look’ angle. These platforms can achieve temporal resolutions that are hourly to daily.