Application of geospatial technologies in freshwater resource management
Ying Bo Wang1
1RMIT University, Melbourne VIC 3001, Australia
Unless otherwise indicated, diagrams
included in this report were created by Ying Bo Wang, RMIT University 2013
Abstract
Freshwater is a limited resource and faces global issues
arising from pollution, ever-growing consumption and climate change. To help
each country manage their freshwater supply, it is important to first establish
a knowledge base and subsequently, analyse the collected information.
Geospatial technologies such as hydrography, Global Navigation Satellite
Systems (GNSS) and satellite remote sensing are used to collect data about
various water bodies while Geographic Information System (GIS) is used to store
and analyse this information. This paper gives an overview of how each
technology is used in freshwater management, examine its advantages and
disadvantages and explore its future possibilities. While hydrography and GNSS
play small but important roles in freshwater management, satellite remote
sensing and GIS are critical aspects due to their irreplaceable role. No other
technology currently exists that can work on such a large scale and so quickly.
An underlying theme for these geospatial technologies is their need for expert
users to operate. In the future, there will be an increase in user-contributed
hydrography, better positioning solutions, more complete satellite imagery of
Earth and better datasets to be integrated within various GIS. By giving a brief
summary of each geospatial technology; this paper will contribute to future
research on similar topics.
Key words: geospatial,
freshwater, resource
Introduction
Freshwater is a scarce resource as it is limited. According
to a report by United Nations (2012), there are 35 million cubic kilometres of
freshwater on Earth, which is 2.5% of the world’s total volume of water (see
Figure 1). Within that 2.5%, it is broken down into the categories of ice/snow
(70%), groundwater (30%) and rivers/lakes (0.3%) (see Figure 2). Groundwater
refers to groundwater basins, soil moisture, swamp water and permafrost. The
report notes that the total usable freshwater for humans and the ecosystem is
200 thousand cubic kilometres. This is made up from the rivers/lakes portion
and some of the groundwater.
Figure 1: Total World Water breakdown
Figure 2: Freshwater Breakdown
The importance of freshwater for human livelihood is due to
its role in drinking, sanitation, agriculture, research, industry and
recreation. While humans technically only need freshwater for drinking and
agriculture to survive, for quality of life, the other uses of freshwater is
critical too. Additionally, freshwater sources play a big role in international
politics. A specific example would be a shared river between countries (see
Figure 3). If country X pollutes the water upstream, country Y would have to
deal with the consequences downstream. United Nations Educational, Scientific
and Cultural Organization (2012) states that “148 countries include territory
within one or more transboundary river basins”. A prime example of a
transboundary river basin is the Nile-Kagera river (see Figure 4), which the
countries of Rwanda, Burundi, the Democratic Republic of Congo, Tanzania,
Kenya, Uganda, Eritrea, Ethiopia, Sudan and Egypt share.
Figure 3: Countries sharing a river basin.
Figure 4: Nile-Kagera River. Source: http://www.unesco-ihe.org/stories/networking-nile
The main issues for the freshwater resource are pollution
and the need for a sustainable solution in the near future. Pollution refers to
the act of contaminating the freshwater supply through human actions that
degrade the quality of the freshwater, such that it is no longer drinkable. Pollution
of freshwater has a direct impact on agriculture and subsequently, the food
supply of people. This could also lead to political repercussions, in the case
of shared freshwater sources between nations. The need for a sustainable
solution refers to the prediction that based on current usage rates of
freshwater, there will be not enough freshwater for humans. According to a
report by United Nations Educational, Scientific and Cultural Organization
(2012), in developing countries, up to 90% of wastewater flows untreated into
their freshwater supply. In the same report, it is stated that based on current
human population growth and water consumption rates, that by 2025, 1.8 billion
people will be living in regions with absolute water scarcity and two-thirds of
the world population could be under stress conditions. Another issue would be
the effect of climate change on freshwater supplies. A warmer global average
temperature could change the freshwater distribution across the world, due to
change in rainfall in some areas, ice caps and permafrost melting and more snow
melting from the mountaintops.
To develop a sustainable management solution to deal with
these main issues, it is first necessary to collect data. After the collection
of data, information on how to deal with these issues can be further derived.
To collect data on water resources, geospatial technologies can be applied to
great effect. Geospatial technologies used are hydrography, Global Navigation
Satellite Systems (GNSS), Geographic Information System (GIS), and satellite
remote sensing.
This paper will (i) give an overview of each geospatial
technology; (ii) examine its benefits and limitations; and (iii) discuss its
future applications.
Discussion
Hydrography
Overview
Hydrography is the science of mapping the sea floor.
Hydrographic maps are produced from hydrographic surveying. Ingham (1992) states
that the objective of hydrographic surveying is to represent the relief of the
seabed, including all natural and man-made features. In general, there are two
methods to do this; boats using sonar and airplanes using Light Detection and
Ranging (LiDAR) (see Figure 5).
Figure 5: Sonar
bathymetry (top) and Airborne LiDAR bathymetry (bottom). Source: aeromapss.com
(bottom)
Figure 5 above shows the basics of using ship sonar. The
hydrographic transducer, which is hung from the bottom of the boat, emits and
detects the sonar the return to calculate the depth of the sea floor. On top of
the ship, with a good clear view of the sky, sits the GNSS receiver, typically
one that receives Global Positioning System (GPS) signals. The ship’s GPS
receiver, together with a land-based base receiver, can determine the ship’s
position up to 10 metres (95% of the time) (Australian Maritime Safety
Authority 2007). To account for the ship’s pitch and roll, which will affect
the sonar returns, accelerometers are used. Local tide tables are used to
account for the influence of the tide and known reference datums are used while
mapping. A computer on-board the ship will keep track of all this data.
LiDAR works on a similar principle to sonar, emitting light
pulses and measuring the subsequent returns to measure distance instead of
using sound. The laser scanner, which is mounted at the bottom of the aircraft,
emits and detects near-infrared and green laser pulses. Modern LiDAR systems
can measure up to 5 returns per pulse (Weng 2012). Green light possess a
shorter wavelength, which allows it to penetrate through the water body to
reach the solid seabed, while near-infrared light has a longer wavelength,
which cannot penetrate the surface of the water body. The near-infrared surface
return will measure the distance to the water surface while the green bottom
return will measure the distance to the seabed. The difference between these
two distances will be the depth of the water body at that point. Similar to
ships using sonar, there are various instruments and techniques to determine
and keep track of the aircraft’s position, pitch and roll.
Benefits and
limitations
Through hydrography, hydrographic maps with depth soundings
are produced (see Figure 6). With these maps, organizations can roughly
estimate the volume of water in the waterbody. Additionally, these maps are
accurate enough to be used together with other cadastral maps to determine which
parts of the water body lies within which country’s borders. Hydrography can be
considered a cornerstone of freshwater studies. GNSS, GIS and remote sensing
are all relatively new techniques that started development post-World War 2
while hydrography has existed for much longer. A limitation of hydrography is
that it is labour intensive, requiring many hours of field work and data
processing for each map. The main advantage is its integration with other
cartographic maps, while other digital forms of data might not be so easy.
Future applications
There will most likely be an expansion of crowd-sourced
hydrographic data, where the information is volunteered by users rather than
produced by a company. An example is the Teamsurv website. Like OpenStreetMap,
where users volunteer their time to map streets and features, sailors can
upload their sounding data to a single database while they are fishing or
transporting goods. It arises due to several factors, which include cheaper
technology and greater demand for more hydrographic data than professional companies
can supply. An issue with crowd-sourced hydrography is the possible lack of
authority and loss of accuracy. However, this could be a relative issue as not
all users need high accuracy data for their purposes. The idea is to have some
“poor”-quality data rather than no data at all.
Figure 6: Hydrographic map of surroundings at Headlam Point,
Rakhine, Myanmar. The small numbers represent depth at that point while the
curved lines are contour lines. Source: fao.org
GNSS: GPS
Overview
GNSS refers to a satellite system to determine a user’s
position accurately worldwide. GPS is the system designed and operated by the
United States Department of Defense. Typically, GPS Standard Positioning
Service (SPS) will give a position solution with an accuracy of 15 metres.
There are techniques to improve this accuracy, most common of which is
differential GPS (DGPS). DGPS is commonly used to determine the boat or plane’s
position in hydrography. In an ideal situation, DGPS solutions can give
accuracies of up to 2 to 4 metres (Australian Maritime Safety Authority 2007).
Referring to Figure 7 below, DGPS calculates the position of
the rover receiver, relative to the base receiver. Both the base and rover
receivers must track the same satellites for the same time period and must
remain within a certain distance with each other (usually 15 kilometres). The
base receiver is usually setup on a known point. The idea is to negate the most
common errors, including atmospheric error, satellite clock error and satellite
orbit error. To facilitate users of DGPS, organisations and companies have
setup networks of Continually Operating Reference Station (CORS). These CORS
networks will function as the base receiver to the user’s rover receiver.
Examples of CORS networks include Singapore Satellite Positioning Reference
Network (SiReNT), Wide Area Augmentation System (WAAS) and OmniSTAR.
Additionally, all GPS position solutions have a time stamp, so it is possible
to measure the flow velocity of a river by attaching a buoy and allow it to
drift with the river.
Figure 7: DGPS concept
Benefits and
limitations
GPS is mainly used as a tool to determine position in the
field, for studies involving freshwater resources. It is also used as a
navigation tool. GPS is a minor technology in the study of freshwater resources
as it has no direct application to manage freshwater. The main benefit of GPS
is its global coverage and it operates all the time. The main limitation is
that to achieve a higher accuracy using DGPS, the user will have to stay within
15 kilometres of a base station, setup on land. This means that DGPS cannot be
used in remote areas with no reference station or areas far away from the
coast. For example, scientists taking data at a freshwater lake in the middle
of the mountains, with no reference station nearby, would have no DGPS service
to determine their position. They would have to settle for the SPS solution,
which gives an accuracy of roughly 15 metres.
Future applications
Figure 8: PPP concept
The next frontier for GPS techniques would be the
development of real-time Precise Point Positioning (PPP). PPP uses accurate
orbital data and accurate satellite clock data to build models for calculating
a single receiver’s position (Witchayangkoon 2000, p2). The lone receiver can
be either single- or dual-frequency, with no need for an additional receiver to
be a ‘base’ station like in DGPS (see Figure 8). Currently, single-frequency PPP
can achieve accuracies of several metres (Chen and Gao 2005). There are ongoing
research efforts into achieving centimetre accuracy using only a lone
single-frequency receiver, in real-time. If this is achieved, positional
accuracy in freshwater studies will certainly improve. Currently, there are only
2 functional GNSS which are GPS and the Russian Federation’s Global Navigation
Satellite System (GLONASS). In the future, additional GNSS like BeiDou
Satellite Naviation System (BDS), operated by People’s Republic of China, and
GALILEO, operated by The European Union, will be operational. Receivers that
read these multiple GNSS signals will have an improved solution, as compared to
a receiver that can only read signals from a single GNSS.
Satellite Remote
Sensing
Overview
Figure 9: Passive Remote Sensing. Source: http://www.crisp.nus.edu.sg/~research/tutorial/optical.htm
Remote sensing is the science and technology of acquiring
information about Earth’s surface and atmosphere using sensors aboard airborne
or spaceborne platforms (Weng 2012). Satellite remote sensing is the
measurement of reflected radiation, radiation that is either the Sun’s
(passive) or radiation that is emitted by the satellite (active) (See Figure
9). Examples of satellites that focus on observing water bodies include Aqua,
Ocean Surface Topography Mission (OSTM), Oceansat-2 and Aquaris. As water can
reflect and absorb energy, remote sensing can be applied to water bodies. In
Figure 10 below, it is shown that water’s reflectance is between 0.4 and 0.7
micrometre. Reflectance readings of water can be used to infer the presence of
organic and/or inorganic material in the water. Water with a large amount of
suspended sediment present will have a higher reflectance compared to clearer
waters (University of Calgary 2013). Factors that have a direct influence of
the reflectance of water include suspended sediment in water, turbulence,
chlorophyll content and temperature.
By looking at the reflectance data and correlating it, users
can indirectly estimate levels of organic or inorganic material. Turgeon et al.
(2013) observed that they were able to correctly predict the general level of
fecal contaminants 85% of the time in water. The general workflow for such a
project is to (i) collect ground truth in a small area and satellite images over
a large area; (ii) Use a majority of the ground truth to create a model. This
model will correlate reflectance and the material you are interested in, for
example: pollutants; (iii) Test the model using some of the ground truth values
that were set aside at the start of the project.
Additionally, remote sensing can be used to map large bodies
of water. Using satellite imagery, Peregon et al. (2009) managed to create an
inventory of the wetlands of Western Siberia, as well as establish the spatial
distribution of various wetland classes.
Figure 10: Spectral reflectance for vegetation, soil and
water. Source: http://www.ucalgary.ca/GEOG/Virtual/Remote%20Sensing/reflectance.gif
Benefits and
limitations
Before satellite remote sensing was developed, users had to
solely rely on aerial photography to get a synoptic view of water bodies.
Additionally, aerial photographs were analog data in the form of pictures and
only had information in the visible wavelength. Remote sensing provided digital
data and information outside the visible wavelength. As freshwater bodies can
span dozens kilometres in length, satellite remote sensing is very useful in
mapping the horizontal area of the water bodies. This gives organizations a
good idea of the volume and extent of water that they are dealing with. Equally
important is the use of satellite imagery to observe levels of organic and/or
inorganic material in the water. Satellite remote sensing is very important to
the field of freshwater studies. This is because of (i) its timely coverage,
where the satellite will pass over all areas frequently; (ii) synoptic overview
of the area; and (iii) efficient data acquisition, as compared to regular field
work. Its main limitations are the huge expense needed to build and operate
satellites and the need to have experts to interpret the raw data. Such experts
need to know the theory of how remote sensing works, local knowledge of the
area and knowledge of uncertainties in measurements.
Future applications
There are currently plans to launch more satellites to
observe the Earth’s surface and atmosphere. They will replace the older
satellites, giving equal or better spatial and spectral resolution. Examples of
upcoming satellites include the ASANARO and GCOM-C1. There is also an ongoing
initiative called the Global Earth Observation System of Systems (GEOSS) that
combines and disseminates multiple countries’ satellite data (Normile 2004).
The concept is similar to how the Internet is a network of networks, where all
the information on various networks is accessed from a single entity. By
pooling all this data together, the goal is to build a more complete picture of
Earth. Subsequently, this gives a better observation of Earth’s freshwater
supply.
Geographic
Information System (GIS)
Overview
‘A geographic information system (GIS) is a computer system
for capturing, storing, querying, analyzing, and displaying geospatial data.’
(Chang 2012). GIS functions as a digital framework to store and work with all
the spatial and attribute data. This includes data about freshwater bodies,
like location, area, depth, reflectance, temperature, flow velocity, etcetera.
It can also be used to perform flood forecasting. Liu et al. (2004) built a model that combines
soil, elevation and land use data within a GIS and predicts the flood
hydrograph over a river basin.
The most prominent example of GIS is the ArcGIS product from
Environmental Systems Research Institute (ESRI). Besides ArcGIS, there are
other GIS products in the market, some of them being freeware. Among these,
MapWindow (See Figure 11) is particularly interesting as it is used by the US
Environmental Protection Agency, due to its watershed analysis and modelling
capabilities.
Figure 11: MapWindow GIS displaying Idaho Falls, Indiana. Source: http://mapwindow4.codeplex.com/wikipage?title=MW4GIS
Various organizations have built their own GIS to suit their
needs. An example is the Australian Bureau of Meteorology’s Geofabric (See
Figure 12). It is an Australia-wide GIS that combines Landsat imagery, cartographic
maps, a digital elevation model (DEM) and multiple networks. One of the more
interesting products from Geofabric is the Topographic Drainage Divisions and
River Regions map. It shows the water drains and flows across the whole of
Australia.
Figure 12: Geofabric GIS, displaying the Australian country.
Source:
http://mapconnect.ga.gov.au/MapConnect/Geofabric/
Benefits and
limitations
Before GIS, users had to use transparency sheets and overlay
them over paper maps and aerial photographs. With GIS, users could match
various attributes to their spatial data, model simulations, had improved
database organisation, ability to integrate other data sources easily and much
faster to respond to queries and toggling of data than just using
transparencies (Lyon 2003). GIS is well suited for the management of water
data, as are all resources. The main limitation of GIS is its need for expert
users. Users need to have basic knowledge of cartography and the ability to
integrate various data sets, which may not have the same format or coordinate
system. Local expert knowledge of the area is not necessary but would be very
helpful.
Future applications
The next phase for GIS would be moving from the desktop
environment to a cloud-based environment. This means that the GIS would not be
installed on the desktop directly; rather the users would access the GIS
through the Internet. An example would be ArcGIS Online. Users can create maps
and do basic analysis on ESRI’s servers without the need to install ArcGIS. In the
future, a complete GIS with advanced analysis capabilities could be available.
In addition, more and better quality datasets could be integrated into existing
GIS. This could improve the accuracy of the analysis operations within GIS. An
example would be Geofabric’s plans to replace their 9-second DEM with a
1-second DEM. This improved DEM would lead to the borders of the Topographic
Drainage Divisions and River Regions map to be more accurately defined.
Conclusion
With pollution, ever-growing consumption of freshwater and
climate change, there is a strong need for sustainable management of every
country’s freshwater supply. Geospatial technologies can be used to help
achieve this purpose. They can collect, store, display and help analyse the
freshwater data. As such technology becomes cheaper, there will be a lower
barrier, allowing more users to use geospatial technologies to manage their
freshwater resource. Another geospatial technology that was not covered in
great detail is aerial photography.
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