Modelling Cultural and Natural Hydrology Using Radar Imaging at Angkor, Cambodia Damian EVANS Archaeological Computing Laboratory, Spatial Science Innovation Unit Madsen Building F09, University of Sydney, New South Wales, Australia 2006 email:
[email protected] Matti KUMMU Department of Water Resources, Helsinki University of Technology PO Box 5200, 02015 HUT, Finland email:
[email protected] Abstract: In September 2000 airborne synthetic aperture radar (AIRSAR-TOPSAR) data were acquired over the archaeological site of Angkor, Cambodia by NASA/JPL on behalf of the Greater Angkor Project, a collaborative research program interested in the ecology of Angkor ’s decline. Since 2001 the data have been used in various applications, but have proven particularly useful in the discovery and analysis of Angkorean hydrology. Enhancements and colour composites of the polarimetric data have been used to create a GIS database of the network of canals, reservoirs and embankments used to manage water in Angkorean times. Based on a GIS database and TOPSAR elevation model, a three-dimensional (3D) EIA Flow Model has been set up for simulating water levels and currents, inundation of the flood plains and suspended sediments. The model can be classified as a 3-dimensional baroclinic z- level model and is based on solving Navier Stokes equations (equations X.2-5) in a rectangular model grid. Two hydrological models, a simplified lumped model and a detailed distributed model, have been applied for the watersheds. With the hydrological and hydrodynamic models, it is possible to investigate how the hydrological network was developed, how it worked and why and when it collapsed. This will help us to understand better how the urban complex operated and may offer new information about the decline of Angkor.
Key words: AIRSAR; archaeology; Angkor; 3D flow model; hydrological modelling 1. INTRODUCTION The early historic settlement of Angkor, in Cambodia, flourished for a thousand years between the middle of the first and the middle of the second millennia CE. At its height in the twelfth century, the settlement covered an area of around 1000 km2 and may have supported a population approaching a million according to some estimates. In spite of a traditional scholarly emphasis on the magnificent Hindu-Buddhist structures such as Angkor Wat, the defining archaeological feature of the settlement is its vast hydraulic network, not its temples. The French archaeologist Bernard-Phillippe Groslier (1967; 1974; 1979), among the first to recognise that Angkor was a ‘hydraulic city’, proposed that the settlement’s decline in the 15th and 16th centuries might be attributed to the failure of that water management system. He proposed an integrated programme of archaeological research that took into account both the
‘vertical’ dimension (e.g. traditional excavation techniques) and the ‘horizontal’ dimension, exemplified by his (1979) time-sequence series of maps derived from aerial survey. The Greater Angkor Project (GAP), a collaborative project between the University of Sydney, the Ecole française d'Extrême-Orient (EFEO) in Siem Reap, and APSARA, the Cambodian body responsible for overseeing the monuments at Angkor, has been pursuing many of the same questions since 1999, using more sophisticated remote sensing techniques than were available to Groslier (Fletcher 2001; Pottier 1999). In particular, the AIRSAR-TOPSAR datasets acquired over Angkor by NASA/JPL in September 2000 as part of the PacRim 2 mission have allowed researchers to ‘see’ Angkor in a different way. Building on years of aerial photograph interpretation by Christophe Pottier (1999) of features in the south of the Angkor plain, the radar data have uncovered a network of previously unknown hydrological features – both natural and cultural – in the north of the settlement which have since been mapped in some detail (Figure 1). Combined, the two datasets provide a more comprehensive picture of Angkorean hydrology than has been available in the past, and hypotheses about its operational limits can now be tested with direct reference to GIS databases of hydrological features (both modern and Angkorean) and a precision digital elevation model.
Study Area Radar Strips 1-4
Kulen Hills
Foothills (C) Great North Channel (Ends)
Northern Angkor (Mapped in Current Study)
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Foothills Banteay Srei
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Siem Reap River
Puok River Route 6 to Sisophon
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Banteay Sra Bayon West Baray Prei Khmeng
East Baray
Angkor Thom
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Roluos River
Chau Srei Vibol
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Baray (Indratataka) Siem Reap Southeast Channel (D) Prasat He Phka
Phnom Krom
High Water Mark (Dashed Line)
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Figure 1. Overview of the study area at Angkor 2. DATA COLLECTION: AIRSAR-TOPSAR The AIRSAR instrument is built and operated by the Jet Propulsion Laboratory (JPL) under contract to the National Aeronautics and Space Administration (NASA). On 21 September 2000, a series of AIRSAR data were acquired over the Angkor plain. The instrument was mounted in a DC-8 aircraft deployed by NASA/JPL as part of the PacRim 2000 Mission, and covered approximately 7000 sq km of terrain on the day. The Angkor overflight was highly successful and has produced datasets of very high quality, clarity and integrity (Fletcher et al 2002, JPL 2002). 2.1. Specifications AIRSAR is an active sensor that records radar amplitude its and polarisation on transmission
and return. Final products included georeferenced quad-polarimetric data in C band (5cm), L band (24cm) and P band (68cm), and precision elevation data (the TOPSAR DEM) generated from the phase information accompanying the C-band VV-polarisation signal using radar interferometry. The data have a 5x5m ground resolution and can resolve sub- metre differences in elevation in areas of low topographic relief. In common with most radar remote sensing instruments, two broad categories of physical attributes determine the amount of signal backscattered to the SAR instrument: geometric characteristics (primarily terrain elevation and surface roughness) and electrical properties (primarily moisture content) of the surface. 2.2. Advantages of AIRSAR as an archaeological tool In terms of the identification of remnant hydrological networks, radar remote sensing offers several key advantages over traditional methods such as black-and-white aerial photography. Although AIRSAR is in its infancy as an archaeological tool, analysis of datasets has so far indicated the following advantages: -
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The Angkor overflights were taken during the rainy season, where the ground was completely waterlogged. Water is more likely to pool in slightly depressed areas of the ground and therefore acts as a specular reflector (in some bands of the polarimetric data) in contrast to the surrounding ground. This highlights the depressed strips of ground and the traces of linear embankments that often indicate remnant channels. The TOPSAR DEM is extremely effective at locating the same traces; Detecting clearings in forest tha t might indicate the existence of moats; Mapping the existence of groves of large trees in rice fields (usually palms growing on the relatively high ground of house mound remains), which are a useful indicator of archaeological sites; The ability of longer wavelengths to penetrate foliage and reveal sub-canopy features; Subtle differences in elevation caused by old embankments and earthworks in turn create areas of differential soil moisture which: 1. Tend to grow different species of plant, or 2. Are used by farmers in different ways, or make the difference between use and disuse, Differences in soil compactness with the same cause and the same effect; preliminary findings indicate that this aspect may be most useful for determining roadways, pathways, and other evidence of human activity and interaction across the landscape; The existence of remnant stonework beneath the surface may interfere with the root structure of some species rather than others; A recognition by modern Khmer that the locations of local shrines are sacred places. Land use is therefore different within the temple precincts, and the subtle variation in vegetation is just enough to discern their spatial structure within the AIRSAR data.
3. DATA COLLECTION: CLASSIFYING ANGKOREAN HYDROLOGY The first step in modelling hydrology at Angkor was to build on Pottier’s (1999) typology of
large-scale archaeological remains and create a GIS database for features the north commensurate with his GIS database of features in the south. These features include: - Channels, both artificial and natural - Temple and moat locations - Household ponds or trapeang - Larger water storage devices such as the great reservoirs or baray - Angkorean field structures (and thus the extent of irrigated land) 3.1. Automated identification of features Several attempts were made to build classification algorithms as part of this study. However, the diversity of the features and attributes that identify archaeological features in the radar created serious problems. There was a particular issue with different surface phenomena marking the same archaeological features. In addition to this, the same surface attributes also sometimes marked different Angkorean features, and occasionally marked both Angkorean and modern features. The moats of shrines, for example, often had the same surface attributes as Angkorean rice fields (wet low- lying ground, used by modern farmers to grow rice, bunded edges with shrubs…). Texture analysis was used to try and capture the linearity of channels, rice fields and trapeang edges. Again, this technique was problematic, with scale and clarity being particular issues: many remnant hydrological features did not have linear edges that were large enough or distinct enough (given the 5 metre spatial resolution of the data) to be differentiated programmatically from non- linear surface features, even though they could clearly be seen on the image by a viewer. In summary, an enormous amount of background work would have been required to capture the range of the archaeological assemblage and its surface indicators within identification algorithms. There was also concern about the circularity and a priorism of such a method: an automated process would necessarily be built upon an empirical understanding of the known set of archaeolo gical features, and would potentially overlook features that were previously unknown. Automated processing was therefore possible in principle, but after preliminary testing it was decided that this course was not feasible or appropriate given the extent of the study area. 3.2. Manual Identification of Features Angkorean features were therefore identified visually using 3-band colour composite images, the TOPSAR DEM and various image products derived from these data. Once identified, the manually mapped into a GIS database. This process also allowed for careful differentiation of Angkorean features from modern ones by reference to the ZEMP (RAF 1995) and JICA (1999) GIS databases. 3.3. Ground verification Ground verification was undertaken using maps derived from the GIS database, printed radar images overlaid with the modern map grid, and precision GPS units. The 2003 field season of GAP also made use of an Australian- made ultralight for low-level aerial survey in areas that
were otherwise inaccessible due to land mines or dense vegetation. 4. ANALYSIS: MODELS APPLIED FOR THE ANGKOR AREA The models used for the study were 3-dimensional (3D) EIA Flow Model for the detailed current and sedimentation studies, and two watershed models for simulating the hydrology on the watershed and sub-watershed scale. The models have been developed by Technical Research Centre of Finland and EIA Ltd. (Environmental Impact Assessment Centre of Finland) during the last 20 years. So far, they have been applied in over 230 major modelling projects. One of the latest applications has been made for the Tonle Sap Lake, Cambodia, as a part of the MRCS/WUP-FIN Tonle Sap Modelling Project (Mekong River Commission Secretariat / Water Utilization Program). 4.1. 3D Flow Model The 3D Flow Model has been set up for key points in the study region for simulating elements such as water levels and currents, inundation of the rivers, and suspended sediments and erosion rates. The model can be classified as a 3-dimensional baroclinic z- level model (e.g. Virtanen et al., 1998) and is based on the standard Navier-Stokes equations in a rectangular grid. The complicated hydrodynamic and sedimentological characteristics of the Angkor system necessitate use of a versatile enough model. The EIA 3D model can solve basically any dynamic or static flow and sediment situation. It is possible to calculate 2D solutions although usually 3D solutions are more appropriate for erosion and sediment transport studies (Koponen et al., 2003). Reasonable simulation times are reached by using appropriate algorithms (e.g. time splitting and implicit solvers) and model resolutions. The model also has a GIS user interface. 4.1.1. Grid system of the model In the horizontal direction, the model utilizes a rectangular Arakawa E-grid and uses a finite volume method to solve equations. Because of this, the grid width can vary in x-, y- and z-directions (MRCS/WUP-FIN, 2003). The horizontal grid size of the model depends on the application and typically varies from 5m to up to hundreds of meters for large and small scale studies, respectively. Vertical resolution varies normally from 0.2 m to several meters. It is possible to nest higher resolution grids within grids of lower resolution. The size of the model typically reaches up to 1,000,000 grid points. 4.1.2. Vegetation effects Average vegetation height, cover and friction are given for each land use type. These in turn determine wind and flow friction in different depth zones. Vegetation flow friction affects flow directly only in the layers that are lower than the vegetation height. 4.1.3. Input and output of the model The model input data is composed of model grid: topography (TOPSAR DEM) and vegetation (roughness and fr iction); and model forcing data, e.g., wind and boundary flows (Figure 2). Grid generation programs read GIS-generated DEMs or other elevation data and produce the model grid. Based on the tributaries data the average width and depth of the tributary in each
model grid cell is estimated. Model outputs in the hydrodynamic part include water depth, water elevation, velocity components (U, V), sediment concentrations, and bottom heights in morphological studies (net sedimentation). Output files are read directly into the GIS containing coordinate system and format specifications. Thus, those can be used directly for further analysis. (MRCS/WUP-FIN, 2003).
Figure 2. Model Structure of the 3D Flow Model 4.2. Watershed models Two watershed models have been used: a simplified lumped rainfall-runoff HBV model (e.g. Bergström, 1995) for basic hydrological modelling, and a more detailed grid-based distrib uted VMod watershed model (Lauri & Virtanen, 2002), including water resources management, river water quality, erosion control and land use planning. The inp uts for the VMod are meteorological data (evaporation, precipitation, air temperature, etc.), hydrological data (average outflow), land use and digital elevation model. The output of the models includes the average river flow, corrected evaporation and ground water height (in VMod only). 5. APPLICABILITY OF THE MODEL TO ANGKOR AREA For Angkor area, the models create an excellent tool to model how the ancient water system was developed, how it worked and when and why it collapsed. It is possible to calculate sedimentation and erosion rates with the 3D model based on the land use and soil types. It is also possible to reconstruct the water system using the DEM made by NASA/JPL and the GIS database of the hydrological features of the area (Evans, 2002). When tha t data is complemented by the results of watershed models applied to the area, information obtained from the field studies, coring results and aerial photos, a very detailed 3D model for the ancient water system can be created.
5.1. Uses of the watershed models for the study area The results of the 2D watershed models have been used for the boundary flows of 3D Flow Model. The models have been calibrated for the present hydrological and meteorological conditions of the study region and have been applied for the ancient land use, meteorological, and morphological conditions of the watershed as accurately as possible within the bounds of current knowledge. Also, different scenarios about ancient conditions can be run with the models and then applied to the flow model. Thus, it is possible to define the sedimentation and erosion rates for different parts of the study area for different scenarios using the watershed models as an input for the 3D Flow Model. 5.2. Uses of the 3D modelling system for the study area Some of the most valuable uses of the model in the study area are: - flows and currents in complex channel network - reconstruction of the water system - influence of the land use changes scenario analysis - sediment accumulation and filling of the channels both in short term and centuries timescale - erosion rates in the channels and canals, especially in Siem Reap River and Great North Channel - erosion rates in the Barays - the river floodplain vegetation effects to the sediment concentration and flow These results will be used for the archaeological and hydrological analysis of the area. With the analysis, it is possible to get more information about the water system, an estimation of how significant that system was in the city’s operation, and to indicate whether or not the demise of the hydraulic network implicated its urban environment in the middle of the second millennium CE. 5.3. Benefits and limitations of the modelling For hydro-archaeological studies the hydrological watershed models offer an excellent tool for investigate the hydrological changes in the watershed and the ir consequences, especially in the Angkor area. Also, the effects of possible land use and climate changes can be investigated with the se models. Combining the watershed models and the 3D hydrodynamic model for the study area gives many opportunities for the research. A very detailed sedimentation and erosion analysis can be made for the wide channel network including the barays and temple mounds. It is possible to create different scenarios about the historical conditions and simulate the effects for the water system with the models. The net sedimentation in the ancient channels can be determined with the help of coring (Player, 2002) but the exact sedimentation rates can be defined only from the barays using radiocarbon dating (Penny, 2002). Thus, the model is needed to determine the possible sedimentation and erosion rates. Any model is a simplification of reality, a fact worth bearing in mind when interpreting results. The model is a useful tool for investigating the ancient waterways and the functioning of the
system. But because of the limited amount of information available about the ancient meteorological, morphological, ecological and hydrological conditions, a lot of assumptions have to be made in modelling. Another key problem is the dating of the channels for modelling the ancient system for different eras – the network was built over the course of centuries and very little information is available on the developmental steps of the channel netwo rk. 6. RESULTS AND DISCUSSION 6.1. Analysis of AIRSAR data Some thousands of features were mapped in the northern part of Angkor in this analysis, confirming the value of AIRSAR as a tool for the reconstruction and modelling of ancient hydrological systems. To gether with Pottier’s (1999) maps of the south, a comprehensive picture of hydrology on the Angkor plain – natural and cultural, modern and ancient – has emerged (Figure 3).
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Figure 3. Structure of Angkorean hydrology.
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6.2. Application of models The GIS database produced by interpretation of the AIRSAR imagery, in combination with the TOPSAR elevatio n model, has been used for creating the 3D flow model for hydrodynamic simulation. It has proved to be a very useful tool for calculating the sedimentation and erosion rates in the study area (Figure 4). Also, simulation of the functioning of the ancient water system has provided good results.
Figure 4. Surface (left) and bottom (right) sediment concentrations calculated with 3D Flow Model, West Baray. 7. CONCLUSION Using the GIS database, TOPSAR elevation model, and powerful hydraulics and hydrological models it is possible to investigate how the hydrological network was developed, how it worked and why and when it collapsed. Bangkok The results tend to confirm Groslier’s (1979) assertion that Angkor was a ‘hydraulic city’. A distributed network of channels spans the entire Angkor plain, with channels in the northern (up slope) half tending to converge on the great storage reservoirs in the centre while channels in the southern (down slope) half tend to radiate outwards. The powerful hydrodynamic model combined to hydrological models, GIS database, TOPSAR digital elevation model, comprehensive field work, meteorological and hydrological data, and coring results creates an excellent tool for investigating the ancient water system in Angkor area. Sedimentation and erosion rates can be defined for the area using different historical scenarios for landuse and climate, allowing archaeologists to test hypotheses about the decline of Angkor against rich and complex datasets. It also lends historical perspective to current systems of land- use and hydrology on the Angkor plain, and will allow decision- makers in modern communities there the opportunity to avoid repeating the mistakes of the past.
8. REFERENCES Bergström, S. 1995. The HBV model. In Computer Models of Watershed Hydrology, edited by Singh, V. P. Water Resources Publications, Colorado, U.S.A. pp. 443-476. ISBN 0-918334-91-8. Evans, D. 2002. Pixels, Ponds and People: Urban Form at Angkor from Radar Imaging. Honours Thesis for the Department of Archaeology at the University of Sydney, Australia. 107 pages. Fletcher, R, D.H. Evans, and I. J. Tapley. 2002. "AIRSAR's contribution to understanding the Angkor World Heritage Site, Cambodia - Objectives and preliminary findings from an examination of PACRIM2 datasets." Proceedings of the 11th Australasian Remote Sensing and Photogrammetry Conference, Brisbane, Australia September 2-6 2002. Fletcher, R. 2001. "A.R. Davis Memorial Lecture. Seeing Angkor: New views of an old city." Journal of the Oriental Society of Australia 32-33:1-25. Groslier, Bernard Philippe. 1967. Indochina. Geneva: Nagel. Groslier, Bernard Philippe 1974. "Agriculture and Religion in the Angkorean Empire." Etudes-rurales 53-56:95-117. Groslier, Bernard Philippe 1979. "La Cite Hydraulique Angkorienne." Bulletin de l'Ecole Francaise d'Extreme Orient 66:161-202. JPL. 2002. AIRSAR: Airborne Synthetic Aperture Radar. URL: http://airsar.jpl.nasa.gov/. Koponen, J. Virtanen, M., Lauri, H., van Zallinge, N., and Sarkkula, J. 2003. Modelling Tonle Sap Basin for Sustainable Resources Management. (will be published in, The 1st Yellow River Conference, China) Lauri, H. and Virtanen, M. 2002. Distributed modelling of lake Tonle Sap catchment. Environmental Software 2002, Eds. C.A.Brebbia et al., WIT Press, Swansea, 7 pages. MRSC/WUP-FIN, 2003. Modelling Tonle Sap Watershed and Lake Processes for Environmental Change Assessment. Model Report. MRCS/WUP-FIN, Phnom Penh, Cambodia. Draft, January 2003. Penny, D. 2002. Sedimentation rates in the Tonle Sap, Cambodia. Report to the Mekong River Commission. Pottier, C. 1999. Carte Archéologique de la Région d'Angkor. Zone Sud. Unpublished PhD Thesis, 3 vols. Paris: UFR Orient et Monde Arabe, Universite Paris III - Sorbonne Nouvelle. Player, S. 2002. Summary Report of Exploratory Coring on the South East Canal of the Angkor Plain, Cambodia. A field study undertaken in January 2002 by the Greater Angkor Project. The University of Sydney. Virtanen, M., Koponen, J., and Nenonen, O. 1998. Modelling the systems of three reservoirs, rivers, lakes, coastal area and the sea in northern Finland. International Review of Hydrobiology 83, pp. 705 - 712.
