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.

References

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