ENVISAT) for Addressing the Lack of Freshwater Ecosystems Management, Santa Cruz Island, Galapagos

Remote Sensing of Environment 112 (2008) 4131-4147

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Remote Sensing of Environment

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DEM generation using ASAR (ENVISAT) for addressing the lack of freshwater ecosystems management, Santa Cruz Island, Galapagos

Noémi d'Ozouville a,⁎,1, Benoît Deffontaines b, Jérôme Benveniste c, Urs Wegmüller d, Sophie Violette a, Ghislain de Marsily a a Université Pierre et Marie Curie, CNRS UMR 7619, 4 place Jussieu, 75005 Paris, France b Université Marne-la-Vallée, Laboratoire Géomateriaux et Géologie de l'Ingénieur, 5 Bd. Descartes, Champs-sur-Marne, F-77454 Marne-la-Vallée, Cedex 2, France c European Space Agency - ESRIN, via Galileo Galilei, casilla postale 64, 00044 Frascati (Rm), Italy d Gamma Remote Sensing, Worbstrasse 225, CH-3073 Guemligen, Switzerland

ARTICLE INFO ABSTRACT

Article history: Low relief oceanic islands often suffer from scarcity of freshwater resources. Remote sensing has proved to be Received 1 March 2007 an effective tool to generate valuable data for hydrological analysis and has improved the management of Received in revised form 29 January 2008 ecosystems and water. However, remotely sensed data are often tested over areas with existing validation Accepted 23 February 2008 databases and not always where the need is greatest. In this paper we address the need for topographical data to understand the hydrological system of Santa Cruz Island (Galapagos archipelago) so that Keywords: DEM generation management of freshwater ecosystems and resources can take place. No high resolution, high accuracy ENVISAT ASAR topographical data exist for Santa Cruz Island, and its growing population has created an urgent need for Interferometry water resource management and protection of unique and pristine ecosystems. Radargrammetry Inaccessible National Park land covers more than 97% of Galapagos territory, which makes the use of remote SRTM sensing methods indispensable. SRTM data was insufficient in terms of grid size (90 m) to carry out the Hydrology needed data analysis. We used ASAR data (ENVISAT) in VV polarization image mode for Digital Elevation Freshwater Model (DEM) generation, in order to extract drainage network, watersheds, and flow characteristics from a Management morpho-structural analysis. Galapagos archipelago Results show the high potential of these data for both interferometric and radargrammetric generation methods. Although interferometry suffered from low coherence over highly vegetated areas, it showed high precision over the rest of the island. Radargrammetry gave consistent results over the entire island, and details were enhanced by integrating the 90 m SRTM data as an external DEM. Accuracy of the SRTM and the combined radargrammetric/SRTM DEM was similar, with the radargrammetric having a finer pixel-based resolution (20 m). Validation of the extracted drainage networks and watersheds was carried out using ground-based field observations and comparison to mapped river networks visually extracted from aerial photographs and high resolution (1 m) satellite imagery available on GoogleEarth©. For the first time, watershed characteristics and flow paths were made available for an island of the Galapagos archipelago. Furthermore, the drainage network is shown to be strongly influenced by observed and extracted structural discontinuities. Having characterized freshwater flow, water balance calculations were carried out for Pelican Bay watershed, where urban areas, agricultural land and Galapagos National Park land are concomitant. © 2008 Elsevier Inc. All rights reserved.

1. Introduction and climate explain this condition, as the islands are made up mainly of basaltic lava ﬂows which are impermeable but highly fractured. The Galapagos Islands, like many other volcanic oceanic islands Located on the Equator, yet surrounded by cool waters, the low (e.g. Won et al., 2006), suffer from lack of surface freshwater. Geology altitude Galapagos Islands receive much less rainfall than their tropical counterparts (e.g. La Réunion and Hawaii) and evaporation ⁎ Corresponding author. Tel.: +33 1 44 27 51 22; fax: +33 1 44 27 51 25. from the land is very high. Although research in the ﬁeld of E-mail addresses: [email protected] (N. d'Ozouville), evolutionary biology has been on-going since Darwin (1859), [email protected] (B. Deffontaines), [email protected] hydrology and even hydro-ecology have been very poorly studied in (J. Benveniste), [email protected] (U. Wegmüller), [email protected] the islands (Navarro Latorre et al., 1991), perhaps due to the apparent (S. Violette), [email protected] (G. de Marsily). 1 Work carried out as Young Graduate Trainee at European Space Agency, ESRIN, lack of surface freshwater. The Regional Plan for Galapagos (Ingala, Frascati, Italy. 2002) mentions potable water and water for irrigation, but does not

0034-4257/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2008.02.017 4132 N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 address the lack of data and studies or the protection of freshwater a deep well tapping into the brackish basal aquifer and a low-outﬂow ecosystems. Contamination of groundwater resources from lack of a highland spring at the base of a scoria cone (Fig. 1). To date, neither sewage and water treatment system and leaking septic tanks in Puerto freshwater ecosystems nor water resources are being managed. The Ayora (main town of Santa Cruz Island) has been known for over growing pressure on the local ecosystems from socio-economic 20 years (Ingala et al., 1989) and has worsened over the years. The development (tourism industry and population growth rate of 6% aquatic ecosystems known to the island are: i) summital swamps, annually) has forced the local authorities to seek an integrated semi-permanent surface ponds and semi-permanent streams for management system (d'Ozouville & Merlen, 2007). However, this freshwater ecosystems and ii) open coastal fractures known as grietas objective is unattainable without knowledge of the dynamics and and coastal back-beach lagoons for brackish ones. At the beginning of construction of the hydrological system (watershed boundaries, catch- our study, in 2003, the exploited water resources included: the grietas, ment areas, reserves, and residence times). Freshwater ecosystems were

Fig. 1. Location map of Santa Cruz Island, Galapagos. Inset shows the position of the Galapagos archipelago in the Eastern Pacific and of Santa Cruz island within the archipelago. The whole island consists of Galapagos National Park land except the urban areas and agricultural zone. Permanent water resources are indicated. The lower inset shows the rapid development of the main town of Puerto Ayora (1963, 1985, 2006). The white outline marks the outer limits of the town. Road, tracks, agricultural zone, urban areas and coastline from Charles Darwin Foundation-Galapagos National Park Service (FCD-SPNG) GIS. N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 4133 taken into consideration for the first time in the New Management Plan funds to carry out freshwater management related projects. It is a of the Galapagos National Park in 2005. Here we propose to consider a volcanic island culminating at 855 m a.s.l. (meters above sea level), as freshwater ecosystem as a dynamic entity, where the quantity and type shown in this study. The oldest lavas of the island are dated at 1 to 3 Ma of rainfall play a major role in generating runoff and interlinking the (million years) (Bow, 1979) and the most recent lavas are younger than individual small freshwater ecosystems: feeding swamps and ponds, 250,000 to a couple thousand years (Bow, 1979). The low-lying coastal semi-perennial streams, and recharging the basal aquifer. apron gradually steepens to reach the summit area where the Based on the literature, the horizontal resolution of the two predominant feature is the alignment of volcanic cones along east– existing Digital Elevation Models (DEMs) for Santa Cruz was west normal faults. Large pit craters (greater than 100 m diameter and considered too coarse for hydrological modeling (Endreny et al., 100 m depth) are also aligned along this direction on the upper northern 2000; Garbrecht & Martz, 1999; Valeriano et al., 2006; Walker et al., flank. Distinctive tectonic features such as fault scarps and horst–graben 1999). The aim was therefore to generate a 20 m horizontal resolution structures are visible in the north-eastern and southern lowlands. DEM with 15 m vertical accuracy showing considerable improvement Heavy rainstorms occur during the hot season from January to on the existing data sets. The inter-relationship between the derived June. During the “garúa” season from July to December an inversion data and management issues is characterized at a local level within layer sets in above 300 m a.s.l. The moisture-laden air condenses in the Pelican Bay watershed. This watershed was chosen because it contact with the vegetation giving rise to humid conditions. Annual encompasses Galapagos National Park land, agricultural land and the rainfall varies from 300 mm at the coast to 1500 mm at 630 m a.s.l. most densely populated area. It represents a prime site where (Huttel, 1995). Inter-annual variations can be striking with years with freshwater ecosystem management needs to be achieved. The work no rainfall in the coastal areas and exceptional El Niño years when was carried out in collaboration with local authorities in charge of precipitation can be more than quadruple (Snell & Rea, 1999). The regional planning (Ingala), water distribution (Municipality), ecosys- vegetation distribution on the island is related to the hydrometric tem protection (Galapagos National Park Service), and research potential from the arid coastal zone through a transitional-wet zone to (Charles Darwin Research Station). the very humid summit area. The maximum vegetation height in DEMs have become a vital source of topographical data for dense areas is 10 m. In the agricultural zone, sparsely distributed scientific investigations such as hydrological studies. In most regions, introduced trees grow to 25 m. The highly variable climatic conditions they replace or complement traditional data sources and formats such which prevail in the Galapagos highlight the vital need for freshwater as paper maps. Where topographical data is simply non-existent (e.g. ecosystem monitoring and management. due to remoteness) or unavailable (e.g. due to military security), global coverage elevation data sets, or generating DEMs from remotely 2.2. Existing topographical data sensed data, can be the key to providing the needed information. DEM generation techniques and DEM validation are largely covered by 2.2.1. 1:100,000 thematic maps and digitized DEM literature (Hirano et al., 2003; Stevens et al., 2004), including High resolution accurate topographical data for the Galapagos applications in difficult environments such as insular and volcanic Islands is scarce and rarely covers the entire archipelago when it exists. milieu (Parcharidis et al., 2002). Generation and application of DEMs This is the case for an airborne TOPSAR mission which generated for hydrological modeling has also been widely published (Ludwig & elevation data only for the western islands of Isabela and Fernandina Schneider, 2006; Walker et al., 1999), including analysis of accuracy (Hensley et al., 1994). The only complete topographical data coverage (Kenward et al., 2000), uncertainty (Endreny & Wood, 2001) and grid consists of a series of 1:100,000 thematic maps derived from aerial size (Garbrecht & Martz, 1994, 1997; Martz & Garbrecht, 1992; Meisels photographs taken in the 1960's and 1980's (Ingala et al.,1989). Most of et al., 1995; Wang & Yin, 1998) on the derived parameters (slope, the aerial photographs were of low contrast with abundant cloud cover aspect and drainage network). The generation of DEMs for hydro- in the highlands, giving rise to inaccurate height interpolation and loss logical modeling over areas with limited existing topographical data is of data. Furthermore, the coastal outline was not accurately extracted. rare (Charleux-Demagne, 2001; Hodgson et al., 2003; Valeriano et al., Souris (2001) generated a DEM for the Galapagos Islands by 2006). This paper addresses this issue in regard to an island of the digitization of the contour lines using the GIS software SAVANE Galapagos archipelago and illustrates the application of ASAR data to (©Marc Souris-IRD) with a y–x based rasterization algorithm. The watershed management. accuracy and precision of the DEM is poor (pers. comm. Souris, 2003) We provide: due to the lack of detail and precision of the original maps. Considering that an approximation of the precision of DEM generated from (1) A description of the study site followed by a brief outline of the digitized contour lines can be estimated as one-half to one-third of existing topographical data and data sets which can be used for the distance between contour lines, 50–100 m in this case, the validation. altitudinal precision is estimated to be in the range of 20 to 50 m. A (2) Detailed results of DEM generation methods used and quality shaded relief visualization of this DEM is illustrated in Fig. 2(A). This control of the existing and the generated topographical data. DEM will be referred to as the Souris-IRD DEM in the rest of the paper. (3) An overview of the hydrological network modeling, validation at the island and local scale and the revealed need for 2.2.2. Shuttle Radar Topography Mission data watershed-based management. The Shuttle Radar Topography Mission (SRTM), a single pass interferometry mission flown in February 2000, generated elevation 2. Study site and existing data sets data at 3″ resolution for 80% of the Earth's surface in C-band with a stated vertical accuracy of ±16 m at the 90% confidence interval 2.1. Santa Cruz Island, Galapagos archipelago (Bamler, 1997; Rabus et al., 2003). During the time of this study, the South America data set, 90 m grid resolution, became available (July Santa Cruz is the most populated (11,500 inhabitants) of the four 2003), including the Galapagos Islands (UTM projection zone 15 inhabited islands of the Galapagos archipelago. It has an area of South, WGS84 Reference Geoid). A shaded relief visualization of the 986 km2, of which 70% belongs to the Galapagos National Park, SRTM DEM for Santa Cruz can be seen in Fig. 2(B). Features of the containing many famous endemic species and unique habitats geomorphology of the island such as volcanic cones and faults are (Eliasson, 1984). This island suffers, as do the others, from difficulty of visible on this DEM. Independent validation of the SRTM data around access to the field, extremely limited water resource monitoring and the world to verify the consistency of the data is in progress at a global baseline data (topography, geophysics and geology), and scarcity of scale (Berry et al., 2007; Falorni et al., 2005) and at local scales 4134 N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147

island were deﬁned as: (i) the coastline contour of the island as produced by Fundacion Charles Darwin et al. (2004) (tidal variations in the Galapagos do not exceed 2 m); and (ii) the GPS track of the main asphalted road running through the island from south to north. The validation data used for quality assessment of existing DEMs and those generated in this study were compiled from data collected in the Galapagos and georeferenced using the reference data. The two resulting data sets are described below and illustrated in Fig. 3(A) and (B) respectively.

2.3.1. Ground Control Points #1 These points were extracted from annotated paper maps representing a leveling survey carried out with a theodolite along the main road across the island which connects the town of Puerto Ayora (south) to the Ithabaca Canal (north). A theodolite is a precision instrument used in triangulation networks (z-accuracy better than 5 cm). The different maps were scanned, georeferenced and mosaicked with an x–y accuracy of 10 m. 132 theodolite points were extracted with their precise coordinates and height. It is assumed that the measurements were correctly carried out (no metadata available) and that the precision of the data falls well within the requirements for validation.

Fig. 2. Shaded relief view of the two existing DEMs. Vertical exaggeration: 5×. 2.3.2. Ground Control Points #2 Illumination direction: NE. (A) Souris-IRD DEM generated from digitized contour lines. 310 points were obtained from Global Positioning System (GPS) Pixel size: 50 m, low accuracy (Souris, 2001). (B) SRTM DEM from Fundacion Charles data collected in the field during this study (within limits of field ″ Darwin et al. (2004). Pixel size: 3 ; given precision 16 m in height. The Souris-IRD DEM accessibility). Use of differential GPS is preferable where no high (A) shows no detail whereas the SRTM DEM (B) shows distinct surface features such as highland cones. accuracy topographical data are available (Gorokhovich & Voustia- niouk, 2006; Hirano et al., 2003). However, this is unavailable in the Galapagos. Use of conventional GPS points for image rectification is (Gorokhovich & Voustianiouk, 2006; Guth, 2006; Rodgriguez et al., discussed by Smith and Atkinson (2001). The given precision and 2006). Use of SRTM data for applications in hydrology and geomor- accuracy of the GPS Garmin 72 used in this study were 7–8m phology is also increasing (Grohmann et al., 2007; Valeriano et al., horizontally and 15 m vertically in WAAS mode (Wide Area 2006; Wright et al., 2006). As yet no validation of the data set has been Augmentation System). Vertical accuracy was often found to be better carried out over the Galapagos Islands as a whole or over specific than 15 m, as confirmed by data acquired later with an altimeter (5 m islands. SRTM data used in this study have been prepared by precision with in-situ repeated calibration). Using this data set Fundacion Charles Darwin et al. (2004) to ensure consistent permitted a wider range of validation relative to the limited number georeferencing with the existing Galapagos databases. and range (south–north along the main road) of theodolite points.

2.3. Existing validation data 3. DEM generation

Given the absence of altitude reference points for Galapagos within Abundant literature (e.g. Walker & Willgoose, 1999)reﬂects the the Ecuadorian Military Geographic Institute, reference data for this importance of ensuring that DEM generation and use lies within the

Fig. 3. Spatial distribution of the theodolite validation data set (A) and the GPS validation set (B). Note that the theodolite points are distributed evenly along the main road from south to north ranging in altitude from 0 to 600 m a.s.l. The GPS points are distributed throughout Santa Cruz Island broadening the area of validation. N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 4135

Table 1 tropospheric artifacts. SAR images are weather independent and have Summary of ASAR pairs used for interferometric DEM generation (VV-mode) and day and night acquisition in both ascending and descending modes. qualitative analysis of coherence maps: poor coherence: 1/3 of the island has coherence The C-band SAR images used in this study had a spatial resolution of values N0.5; average coherence: 2/3 of the island has coherence values N0.5 30 m. Due to the lack of Galapagos coverage for central and eastern Pair # Orbit dates Orbit type Repeat time Perpendicular Coherence non-active volcanic islands (as opposed to the active western (days) baseline (m) qualitative volcanoes e.g. Amelung et al., 2000) in existing ERS1–ERS2 and SZ-1 14-04-03; Ascending 35 248 Bad coherence Radarsat databases, data from the ASAR sensor (ENVISAT) was chosen. 23-06-03 SZ-2 28-07-03; Ascending 70 224 Bad coherence The Galapagos Islands are part of the background mission of ENVISAT 06-10-03 and the multi-angle capacity of the sensor (C-band) enables acquisi- SZ-3 23-06-03; Ascending 35 279 Average coherence tions for radargrammetric processing. Holzner et al. (2002) have 28-07-03 demonstrated the applicability of ASAR data for interferometry, but SZ-4 02-08-03; Descending 70 247 Average coherence literature is still lacking on applications of ASAR data for both 11-10-03 SZ-5 24-01-04; Descending 35 232 Bad coherence interferometric and radargrammetric DEM generation. 28-02-04 3.1. Interferometric DEM processing restrictions of data availability for generation method and validation: Interferometric processing requires optimal short-time delay in i) the quality and grid resolution of the DEM should be consistent with order to get maximum coherence between acquisitions and perpen- the scale of the model and that of the physical processes under dicular baselines in the range of 150 to 300 m for best height consideration — watershed scale and river flow; ii) the resolution and rendering. Ten ASAR VV polarization image mode scenes acquired in accuracy of validation data must be higher than that of the generated 2003 and 2004 were used, allowing three pairs in ascending orbit DEM. geometry and two pairs in descending orbit geometry. All these scenes DEM generation over Santa Cruz Island was made difficult by the were acquired in image mode swath IS2 (incidence angle range: following factors: 19.2°–26.7°). Table 1 indicates the characteristics of selected pairs and coherence results of the processing. Interferometric processing was (1) the Equatorial climate gives rise to cloud cover during most of carried out using Atlantis Earthview InSar software© with the ENVISAT the year, limiting the use of data from optical sensors; extension, with image data in Single Look Complex format. The steps (2) a lack of archived satellite data due to the remote location and carried out with the software were as follows: (1) geometric and small land surface area of study site compared to the coregistration analysis; (2) master and slave image filtering and slave surrounding ocean surface; image resampling; (3) interferogram generation; (4) coherence image (3) the scarcity of available topographical data to be used as estimation; (5) range and azimuth low-pass filtering and resampling; validation for the generated DEM and; (6) phase unwrapping; (7) phase to height conversion; (8) image (4) the difficulty of accessibility to the field for ground validation of filtering/resampling; and (9) terrain distortion correction and geocod- generated topographical data. ing. Although most of the process can be run automatically, most of Two DEM generation methods were chosen: interferometry the steps in this study were followed in a semi-automatic way. Auto- (Crosetto, 2002; Rocca et al., 1997; Rosen et al., 2000)and coregistration was unachievable due to the nature of the study area. radargrammetry (Marinelli et al., 1997; Toutin & Gray, 2000). Both The surface area is small (42 ⁎ 30 km) compared to the size of the have been largely tested (Kervyn, 2001; Toutin, 2002) and some case scene (100 ⁎ 100 km) and is surrounded by the zero coherence surface studies exist on their applicability to volcanic insular environments of the ocean. The island area had to be carefully identified, cropped (Parcharidis et al., 2002). Interferometry gives better height precision and then manual coregistration could be attempted. Coastline and than radargrammetry, but it requires precise ground elevation points geological features such as small volcanic cones were the predominant for phase to height conversion and is affected by atmospheric artifacts. landforms for identifying coregistration points. The two main Radargrammetry is based on the principle of stereoscopy to generate inhabited areas were clearly visible but not sufficient for the height data. It is not affected by temporal decorrelation and or coregistration. Coregistration to less than 0.2 pixel accuracy was

Fig. 4. Result of interferometric DEM processing from pair SZ4. (A) Coherence image showing (1) zero coherence over the sea; (2) very low-coherence southern windward slope due to vegetation cover; (3) high coherence in arid zone with dry vegetation and over recent lava ﬂows; (4) high coherence for urbanized areas. (B) Shaded relief of generated DEM with missing data. Vertical exaggeration: 5×. Illumination direction: NE. The low-coherence zones (A) impeded complete phase to height conversion (B). 4136 N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147

Table 2 the matching window size was significantly larger (128 pixels) which Summary of ASAR acquisitions for radargrammetric processing (VV-mode) means there was no real improvement of the spatial resolution. In Orbit date Orbit type Swath mode (width km) Incidence angle (°) areas where SRTM data were good, the combined DEM can be seen as 06-10-2003 Ascending IS2 (105) 19.2–26.7 a quality checked DEM (improvement in the low frequency parts, 20-09-2003 Ascending IS3 (82) 26.0–31.4 whereas high frequency parts of SRTM are better), brought down to 23-09-2003 Ascending IS5 (64) 35.8–39.4 20 m sampling; and in areas where SRTM DEM might present – 11-10-2003 Descending IS2 (105) 19.2 26.7 significant medium- to large-scale height errors, radargrammetry is 08-10-2003 Descending IS4 (88) 31.0–36.3 09-11-2003 Descending IS6 (70) 39.1–42.8 used to correct these without losing the spatial resolution of the SRTM DEM. The resulting “combined” DEM will be referred to as the radargrammetric/SRTM DEM. The shaded relief view of this DEM is achieved before further processing. Independent filtering was carried shown in Fig. 5(A). Fig. 5(B) shows a detailed view comparing the out in range and in azimuth. Finite impulse response filters were used resolution of details on the SRTM and on the radargrammetric/SRTM and filtering was performed in time domain (Kooij van der et al.,1999). DEM. The overall coherence of the interferometric pairs varied from poor In this study, single pass interferometric SRTM data were used to (one-third of the island had coherence values N0.5) to average (two- improve the radargrammetric processing as a complete DEM could thirds of the island had coherence values N0.5). The zones of lowest not be generated through repeat pass interferometry to fulfill this task. coherence consistently covered the central-southern mountainside of the island and in some case the adjacent western part. These areas correspond to the exposed windward mountainside where lush vegetation and the agricultural zone are found. High coherence was obtained in areas of dry bush vegetation and non-vegetated lava flows. Coastal areas at sea level were used as “ground anchor points” from which phase unwrapping could proceed. The presence of volcanic sea- cliffs in many parts of the island, especially the South-East and North- East, was carefully considered. Final height to DEM conversion was not carried out from the pairs SZ-1, SZ-2 and SZ-5. Suggested reasons for poor coherence of these pairs are: i) data acquisition during the hot season (SZ-1 and SZ-5), when vegetation is in leaf and ii) abundant rainfall between acquisition dates (SZ-2), which promotes rapid vegetation growth. DEMs were generated for processing runs SZ-3 and SZ-4 (Table 1); both lack data for the southern mountainside where coherence was close to zero. These pairs were re-processed using the SRTM data as an external DEM for the interferogram generation and phase unwrapping stages. The coverage of the low- coherence central-southern mountainside was slightly improved. The height data was correctly unwrapped from zero along southern coastline up to a maximum elevation of 596 m above which no data are available. The DEM generated from SZ-4 processing gave the best overall coverage and is shown in Fig. 4.

3.2. Radargrammetric DEM generation

Radargrammetric processing requires image acquisitions with varying incidence angles. Quality of radargrammetric processing depends on the base to height ratios (or intersection angle) of the data pair. Shallow look angles (i.e. angle between vertical and the beam direction N20°) and intersection angles (or difference in incidence angle between the two scenes) around 10–23° are usually considered optimal conﬁgurations for medium to high relief areas (Toutin, 1999, 2000). The acquisition of six scenes was planned in the autumn and winter 2003 using the multi-incidence angle capacity of the ASAR sensor. Images in VV polarization were acquired in swath modes IS2, IS3 and IS6 (ascending orbit) and IS2, IS4 and IS5 (descending orbit). Table 2 summarizes the different acquired scenes. The constraints in choosing the swath modes were: i) aiming to achieve intersection angles between both acquisitions of 10 to 23°; and ii) disregarding the swath modes which only gave partial coverage of the island. The ASAR scenes were processed by Gamma Remote Sensing, Switzerland, using a new radargrammetric chain adapted to ENVISAT data and which combines radargrammetry and space triangulation for the DEM generation (Wegmüller, 1999; Wegmüller et al., 2003; Werner et al., Fig. 5. (A) Result of radargrammetric DEM processing from combination with SRTM data. 2005). As radargrammetry alone was missing many of the ﬁne Vertical exaggeration: 5×. Illumination direction: NE. (B) Detailed view of SRTM DEM (top) and generated radargrammetric/SRTM DEM (bottom) looking from the South- structures found on the SRTM DEM, the latter was incorporated into West. Same vertical exaggeration and illumination direction. Reduction in pixel size the processing chain as an external DEM. The sampling used in the from 90 m to 20 m is visible. Detailed features of the SRTM DEM have been preserved and matching was denser than the 90 m of the SRTM DEM; nevertheless visual interpretation of landscape is enhanced e.g. lineaments on lower left. N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 4137

Fig. 6. Height frequency distribution histograms for the four DEMs. Y-axis represents pixel count for each height value, which are thus correlated with pixel size. Souris-IRD DEM presents artifacts related to contour line digitization. Interferometric, SRTM and radargrammetric/SRTM DEMs show similar distribution.

Gelautz et al. (2003) have shown the overall accuracy of radargram- Global Spheroid of 1984 (WGS84) as the referential geoid; (iii) errors in metric processing, the high precision but strong outliers of the planimetric shift were adjusted to the reference coastline data of Santa interferometric technique and the improvement generated by mer- Cruz Island produced by Fundacion Charles Darwin et al. (2004). ging the data from the two methods for an arid volcanic region (Republic of Djibouti, East Africa) containing both relatively flat and 3.3.1. Visual inspection homogeneous regions as well as areas of rough relief. The height frequency histograms of all four DEMs (Fig. 6) illustrate marked differences related principally to the nature of the data or the 3.3. General validation generation method. Distinct data peaks at 50 m, 100 m, 200 m, 400 m, 500 m, and 700 m altitude in the Souris-IRD DEM are a result of the Specific work on the evaluation of DEM error has shed light on the digitized contour lines; such artifacts of the method have already been shortcomings of classical validation approaches (Wechsler, 2000; documented (Stevens et al., 2004). The SRTM histogram has less data Wise, 2000). Charleux-Demagne (2001) highlights the fact that the points due to the pixel resolution of 90 m and is marked by noise or notion of quality is often ignored due to difficulty of representing it speckle which is not seen in the other histograms. The interferometric and the establishment of an error propagation model. We addressed DEM histogram is characterized by the lack of data above 500 m the quality analysis of the existing and the generated topographic data elevation. From 0 to 500 m, the frequency distribution is similar to the using various approaches, taking into account the nature of the data radargrammetric/SRTM distribution and does not show the noise and used, the available validation data and the planned use of the DEM. speckle of the SRTM histogram. The radargrammetric/SRTM histo- For consistency, all the DEMs were referenced to the same system: (i) gram distribution is similar to the SRTM histogram, reflecting the Universal Transverse Mercator (UTM) zone 15 South projection; (ii) World integration of SRTM data as an external DEM in the radargrammetric 4138 N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 processing. These histograms reflect well the geomorphologic shape eastern and northern slopes. In distinct places the topography of the of the island with a low-lying apron below 50 m a.s.l., chaotic Souris-IRD DEM coincides precisely with the radargrammetric/SRTM topography between 50 and 150 m a.s.l., a plateau area between 150 data. This is the case in particular on the southern slope from 250 m a. and 200 m a.s.l., a rise in slope between 200 and 300 m a.s.l., and s.l. to the summit, the cones situated between 350 and 450 m a.s.l. on then a decrease in surface with an increase in elevation up to the top the western slope and the upper part of the eastern slope. This is of the island. important as topography of the Souris-IRD DEM comes from digitized contour lines representing “true surface.” The southern slope is the 3.3.2. Elevation profiles most highly vegetated and the coincidence between the two DEMs The Souris-IRD DEM west–east and south–north profiles have indicates that the radargrammetric/SRTM DEM does not suffer greatly marked differences with the radargrammetric/SRTM DEM profiles from errors due to tree height. (Fig. 7). The Souris-IRD DEM underestimates topography on the lower The SRTM DEM is almost identical to the radargrammetric/SRTM western and southern slopes and overestimates topography on the DEM in profile (Fig. 7). Slight differences appear on the upper parts of

Fig. 7. West–east profiles and south–north profiles comparing all four DEMs. The same horizontal and vertical scale is used in both profiles. The profile lines run through the highest point of the island on the radargrammetric/SRTM profile (inset map). The profiles from Souris-IRD and interferometric DEMs differ from the SRTM and ASAR radargrammetric/SRTM profiles which show an almost exact fit at this scale. N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 4139

Fig. 8. Linear regression validation of theodolite and GPS ground control points for all four DEMs: (A) Souris-IRD, (B) SRTM, (C) interferometric, and (D) radargrammetric/SRTM. An overall excellent correlation is obtained except for the Souris-IRD DEM. Note that the interferometric DEM has reduced data points due to lack of height data incertainpartsoftheDEM.

the south–north profile. This is discussed in detail in Section 3.3.4. The tally. These errors are considered to be related to the incomplete radargrammetric/SRTM and SRTM data profiles are consistent with interferogram. known topographic features on the island. The interferometric DEM elevation profile represents only the 3.3.3. Validation using surveying points data where height conversion was achieved from the unwrapping The elevation error for the four DEMs was evaluated with the two of the interferogram (Fig. 7). It shows that the interferometric data validation data sets (theodolite and GPS described in Section 2.3). do follow the general trend of the topography especially on the Results in the form of linear regressions are presented in Fig. 8. The western and eastern lower slopes; on the southern slope there is a descriptive statistics of error analysis are presented in Table 3. vertical shift of approximately 20 m and on the northern slope the The two data sets give consistent results for Root Mean Square data coincide well but are shifted several hundred meters horizon- Error (RMSE) and absolute value of maximum errors. They show that 4140 N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147

Table 3 essential framework through which freshwater management could be Descriptive statistics of elevation errors on the four DEMs addressed. DEM pixel size Theodolite Min/max GPS RMSE Min/max Comment The validation of this dynamic system was constrained by: i) the RMSE error (m) error (m) accessibility of the terrain, ii) the availability of complimentary data Souris-IRD (50 m) 21 1/50 31 0/113 (high quality aerial photographs and prior hydrological studies) and SRTM (90 m) 9 0/35 12 0/48 iii) the absence of an El Niño event during the time of our study. Interferometric (20.4 m) 25 4/55 43 2/63 Missing data Fieldwork and hydrological monitoring carried out during the time of Radargrammetric/ 9 0/35 11 0/49 SRTM (20 m) our study was used for validation (d'Ozouville, 2007).

4.1. Extracted drainage network and watersheds the errors on the Souris-IRD DEM can be large, and the RMSE is in the The extraction of hydrological networks is now a commonly order of that estimated by distance between contour lines: 20 to 50 m. available tool in many GIS and related software. The D-8 algorithm The calculated RMSE for the SRTM DEM shows that the errors on the (Jensen & Domingue, 1998) was used in this study as part of the DEM over this island of Galapagos are better than the given global Rivertools© software. An exhaustive review of the “pros and cons” of precision of 16 m. The interferometric data are better than the Souris- different methods is presented by Charleux-Demagne (2001). The IRD DEM but have higher uncertainty than either the SRTM or the DEM depressions were filled, the flow direction and the flow radargrammetric/SRTM DEM. There are missing data which makes accumulation were calculated and the threshold for the streams was this evaluation biased, but accuracy of existing data seems high. The defined. The DEMs were processed using the same parameters to radargrammetric/SRTM DEM shows the smallest errors both in terms extract a drainage network of streams whose upstream drainage area of RMSE and absolute maximum error as calculated with the is superior to 0.05 km2. The map of the drainage network calculated theodolite data and the GPS data. This analysis confirms that the from the radargrammetric/SRTM DEM and SRTM DEM is shown in SRTM DEM and the radargrammetric/SRTM DEM are within the limits Fig. 10. Strong similarities are observed between the two extracted of acceptable precision. The improvement of the radargrammetric/ river networks, with the radargrammetric/SRTM DEM giving more SRTM data with respect to the 90 m SRTM data is the reduction in grid details in the lower order streams. The inset in Fig. 10 shows the size from 90 m to 20 m which allows more precise extraction of data existing hydrological map of non-perennial streams prior to this study for hydrological modeling and high resolution analysis of geomor- (Ingala et al., 1989). phological features at a local scale. Watershed boundaries extracted from both DEMs are shown in Fig. 11. The watersheds are characterized as draining to the edge and 3.3.4. Difference map with a surface area superior to 5 km2. 13 out of 38 watersheds do not A difference map was calculated between the radargrammetric/ have the same exit point along the coast. Major differences are found SRTM DEM (final state) and the SRTM DEM (initial state). Differences on the western and the eastern slopes related to the rugged nature of vary from − 15 m to 15 m and have a gaussian distribution centered the topography. The presence of abundant volcanic edifices and around − 1 m. The resulting map is shown in Fig. 9(A). Several distinct structures along this main structural axis results in a distinct features appear: (i) overestimation of SRTM data compared to radar- morphology of the watersheds on these two sides of the island. The grammetric/SRTM data between 2 to 5 m occurs on the SE mountain- inset in Fig. 11 shows differences in the upper watershed boundaries side of the island; (ii) underestimation of SRTM data compared to and the drainage networks whereas the outlets are the same. Errors in radargrammetric/SRTM data between 2 to 5 m occurs over the NW the drainage network can be linked to the 90 m grid size and the mountainside; (iii) underestimation of SRTM data compared to nature of the D-8 algorithm that only permits flow directions which radargrammetric/SRTM data occurs on the northern slope of volcanic are a factor of 45°. Errors are easily produced in flat areas, in this case cones along the W–E axis and on the distinctive north-facing fault scarps corresponding to the smooth agricultural zone. These errors can along the NE coast. These features are correlated to slopes superior to 10° accumulate, especially in the longer channels, which might give rise to on the southern mountainside and superior to 20° on the northern the shifts observed in some of the watershed exits. The inset graph in mountainside (Fig. 9(B)). These results corroborate with the findings Fig.11 compares watershed characteristics from the two DEMs. Table 4 from Bourgine and Baghdadi (2005), Ludwig and Schneider (2006), summarizes the characteristics of the 38 watersheds extracted from Gorokhovich and Voustianiouk (2006) (Equatorial forest in French the radargrammetric/SRTM DEM. Guyana; study sites in USA and Thailand; and a study site in Europe, Visual validation of watershed outlets was carried out using high respectively) which reveal SRTM data error to be directly related to slope resolution (1 m) optical satellite imagery on GoogleEarth©. A number (magnitude) and aspect (sign). For slopes N10°, overestimation of SRTM of outlets coincided with known freshwater outflows to the sea (e.g. elevation data occurs on the SE slopes and underestimation on the NW Pelican Bay, Puerto Ayora) and with the occurrence of back-beach slopes, related to the look angle of the shuttle. The integration of SRTM lagoons. Out of the 38 extracted watersheds, 22 outlets visually data with the radargrammetric processing has resulted in a reduction of coincided with distinct morphologies along the coastline. 10 inherent errors within SRTM data. watershed outlets could not be validated because the area was not covered by high resolution images. 5 watershed outlets were off-set 4. Hydrological modeling and Pelican Bay watershed from what seemed to be their outlet on the satellite imagery. These results show the majority of the watersheds and drainage networks to The freshwater system operating on Santa Cruz Island is highly be active during El Niño years. During the 1982 Niño season, the main dynamic and unpredictable. For most of the year water flow in the road was cut in 18 locations by flood streams. highlands is vertical rather than horizontal (validated by our field- Headwater streams could only be validated visually at a local work). With increased rainfall, surface flow begins and small streams scale, but are corroborated by evidence from the radargram- develop but vanish at mid-altitude due to the fractured nature of the metric/SRTM DEM. Although SRTM data seem acceptable at island bedrock. Uniquely, during the exceptional rainy years (El Niño events) scale, the higher precision and 20 m grid size of the radargram- infiltration capacity of the ground is exceeded and surface runoff may metric/SRTM DEM was necessary to carry out the local scale and reach the coast as violent rivers. Given these characteristics, the sub-watershed analysis with precision: exact extent of water- definition of watershed boundaries and their associated drainage sheds, catchment areas, drainage networks, fractures, and out- network at island scale and sub-watershed scale were considered the flows to the ocean. N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 4141

Fig. 9. (A) Map representing the height difference between the radargrammetric/SRTM DEM and the SRTM DEM. (B) Slope map of the SRTM DEM. The average difference between the two DEMs is −1 m. Large height differences (N|5| m) are related to steep topographic features such as volcanic cones, fault scarps and sea-cliffs.

4.2. Local scale validation (Fig. 12A). First-order streams are not presented for clarity. Two field views of temporal runoff gullies are shown in Local scale validation of derived data from the radargrammetric/ Fig. 12(A). SRTM DEM was carried out over three adjacent watersheds of the (2) Visual extraction of river networks was carried out from two southern mountainside where road access, aerial photographs and georeferenced aerial photographs with low cloud cover. In high resolution GoogleEarth© imagery, provided an independent the lower section of the photographs, the shrub vegetation validation database. and rocky nature of the land surface made identification The analysis of the data and results are synthesized below and in and extraction difficult. The detailed visual river network Fig. 12. extracted over the three watersheds coincides well with the DEM-derived drainage network (Fig. 12B), thus validating the (1) GPS points were collected along the road access and the automated extraction process from the DEM. Extracting river Galapagos National Park boundary where ravines indicating networks from surface visual information is hampered by: the occurrence of runoff were identified. The majority of i) time limits, ii) large areas, iii) lack of aerial photographs, the points coincide with the extracted drainage network and iv) dense vegetation impeding field mapping.

Fig. 10. Santa Cruz drainage network extracted from (1) SRTM DEM and (2) radargrammetric/SRTM DEM for streams with upstream areas superior to 0.05 km2. Note that for clarity first-order streams are not represented on the figure. This network is compared with the existing hydrological map of non-perennial streams by Ingala et al. (1989) (inset). This river network indicates flow paths for runoff. There are no perennial rivers on the island of Santa Cruz. 4142 N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147

Fig. 11. Extracted watershed boundaries draining to the edge of Santa Cruz island and superior to 5 km2. 1: SRTM river network, 2: radargrammetric/SRTM river network, 3: SRTM watershed boundaries, 4: radargrammetric/SRTM watershed boundaries, 5: insets showing detailed areas where river network algorithm has difﬁculty choosing the right ﬂow direction with the 90 m pixel grid of the SRTM DEM. Note that outlets for most watersheds coincide and most discrepancies occur on the western and eastern sides. The inset graph shows the distribution of surface area among the 38 watersheds which is similar for both DEMs. Watershed diameters (furthest distance between any two boundary points) are more variable as can be explained by the inset example.

(3) The southern lower zone of the watersheds were visually extension tectonic regime has created east–west trending examined from GoogleEarth© imagery. Fractures were clearly open fractures which act as preferential infiltration zones for visible in the area, whereas the drainage networks were surface runoff. diffuse and not well defined. The resulting cartography of Table 4 tectonic joints was overlaid with the DEM-derived drainage Descriptive statistics characterizing the 38 main watersheds of Santa Cruz Island network and watershed boundaries (Fig. 12C). This revealed extracted from radargrammetric/SRTM DEM and illustrated in Fig. 11 the strong influence of the fractures on the flow paths and correlation between large fractures and watershed bound- Outlet Basin Strahler Longest Total Drainage elevation area order channel channel density aries. Fractures in the upper part of the watershed could not (m) (km2) length (km) length (km) (km− 1) be mapped because: i) high resolution imagery was not Mean 6.8 20.6 5.4 14.4 193 9.3 available and ii) they are disguised by soil and thick alteration S.D. 9.6 14.3 0.6 4.9 136 0.7 cover; but they appear to influence the drainage patterns Median 5 16.6 5 14.9 148 9.3 there also. The highlighted relationship between tectonic Min −1 5.1 4 6.5 40 7.2 Max 54 58.6 7 22.3 586 10.6 joints and drainage networks is important as the north–south N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 4143

Fig. 12. Synthetic illustration of the three-step validation at a local scale using independent validation sets over three watersheds at proximity to the urban areas. 1 — GPS points, 2, 3, 4 — first, second, and third order networks, 5 — manually extracted drainage network, 6 — manually extracted fractures. (A) GPS field data points of observed gullies and active runoff as seen in the photographs coincide with calculated river network. (B) Extracted drainage network from two georeferenced aerial photographs shown on the middle right fits with the calculated drainage network. Oblique aerial view of such features in the field is shown in the photograph. (C) Map of fractures extracted from a GoogleEarth© mosaic shown on the bottom right shows a clear relationship with the drainage network and watershed boundaries. Oblique aerial view of large open fractures is shown.

4.3. Watershed-based management: Pelican Bay watershed in-situ data from existing pluviometers, rain gauges set up during our study (d'Ozouville, 2007) and evapotranspiration calculations (Thorn- The watershed delineation revealed that the greatest human waite, 1946). The effective rainfall for the year 2006 (similar to an development on Santa Cruz island is concentrated on one watershed “average” climatic year) is distributed in the following way: (i) below alone. Pelican Bay watershed (43 km2)(Fig. 13) stands out among the 150 m a.s.l., there is no effective rainfall and therefore no recharge to the rest as it encompasses: 1) two distinct ecological zones of Galapagos basal aquifer; (ii) from 200 m a.s.l. to 600 m a.s.l., effective rainfall mainly National Park land (18.5 km2); 2) an extensive area of agriculture with occurs during the garúa season, on the order of 100 mm/year, but heavy water needs for irrigation and cattle (22.5 km2); 3) the two main rainfall events can contribute to infiltration and/or runoff; and (iii) above towns where water problems are greatest (2 km2); and 4) the two 600 m a.s.l. effective rainfall is ∼1000 mm/year. Subsequently, the points of water extraction. contribution to runoff of the effective rainfall above 600 m a.s.l. was The distribution of effective rainfall (the part of rainfall available quantified using the outflow measurements acquired during our study for runoff or infiltration) was quantified for this watershed based on at a disused dam (650 m a.s.l.) and the corresponding 0.09 km2 4144 .dOovlee l eoeSnigo niomn 1 20)4131-4147 (2008) 112 Environment of Sensing Remote / al. et d'Ozouville N.

Fig. 13. Detailed view of Pelican Bay watershed and water management issues that can be addressed at different levels in the watershed through analysis and exploitation of data available through the DEM. 1 — Adventive cones; 2 — small dam at Cerro Gallito; 3 — exploited grietas (open fractures); 4 — deep well; 5 — fractures; 6 — river network; 7 — agricultural zone. Road and urban areas are as in Fig. 1. Rainfall and potential evapotranspiration graphs on the left hand side of the figure illustrate the lack of available rainfall during the first half of the year and in the lowlands. Data from Charles Darwin Research Station and hydro-climatic network set up during this study (d'Ozouville, 2007). N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 4145 catchment area deduced from the radargrammetric/SRTM DEM. For the identification of watershed boundaries and flow patterns, necessary to year 2006, less than 10% of the effective rainfall contributes to runoff in ensure that surface freshwater and water storage in aquifers continue the summital part of the island, in agreement with the high infiltration in terms of quality and quantity. Without this knowledge, future water capacity of soils in the area (Adelinet et al., 2007). shortages and loss of unique ecosystems may result. Water balance calculations are essential to characterize the water availability for the 5. Discussion and conclusion ecosystems and their vulnerability to climatic change, land-use change, contamination, and invasive species. A close inter-relationship 5.1. DEM generation over a Galapagos island for hydrological analysis between freshwater flow on the surface and the densely fractured nature of the environment was identified. Freshwater ecosystems ASAR Image Mode data from ENVISAT allowed both interferometric such as ponds, river channels or brackish ones such as grietas or and radargrammetric processing to generate a DEM. Neither technique backwater lagoons are inter-linked through the activation of the alone was able to provide a DEM meeting the requirements for drainage networks which are themselves conditioned by fractures and hydrological modeling, due to the environmental conditions (inter- watershed boundaries. Management of freshwater resources and ferometry) or the intrinsic nature of the method (radargrammetry) ecosystems on Santa Cruz Island therefore needs to focus on a wa- (Toutin, 2002). Interferometric processing was conditioned by low tershed-based management concept within an inter-institutional coherence over densely vegetated areas, typical of equatorial tropical framework, rather than a user-based management, as human environments. In agreement with Zebker et al. (1994),lackof development borders Galapagos National Park land and the different coherence was not systematically correlated to the time delay between zones are locked into one another. User needs and protection of scene acquisition but dependent on environmental factors (season and ecosystems cannot be separated, as the freshwater system runs from rainfall). Although it is unlikely that overall good coherence can be the summit of the island to the coast through the different zones. obtained using repeat pass interferometry in C-band, areas with high To conclude, the aim of digital elevation model generation was to coherence such as recent lava flows give good results and inter- establish the basis of freshwater ecosystem management on Santa ferometry can provide up-to-date elevation data where changing Cruz Island (Galapagos archipelago). In achieving this objective, this morphology will make SRTM data obsolete (e.g. the eruption of the work integrates subjects which are usually treated individually: DEM Sierra Negra volcano in October 2005). Furthermore, localized lack of generation, validation, hydrological modeling, and freshwater ecosys- coherence in arid zones can serve to evaluate the unsuspected tem and resource management. The inter-disciplinary approach of presence of water and green vegetation growth. The 90 m SRTM data incorporating remote sensing to answer an environmental issue is for Santa Cruz Island are within the stated accuracy range in the indispensable because of the lack of existing data, difficulty of access Galapagos but the grid size was insufficient for the hydrological to the field and non-existent management schemes. But remote application. Adding the single pass interferometrically derived SRTM sensing on its own is not sufficient, and combination with other DEM into the radargrammetric processing chain provided the required disciplines such as hydrology, tectonics or structural morphology is accuracy (reduction of errors related to slope), spatial resolution (20 m) necessary. Given this inter-disciplinary context, the novel results and coverage for enhanced monitoring and management of freshwater include: (i) DEM generation over an area with no existing high ecosystems and resources at the watershed scale. Advantages of data definition topographical grid to compare and validate results; (ii) use fusion, although more demanding in terms of image quantity and of ASAR data (ENVISAT) for both interferometric and radargrammetric processing time, have previously been published (e.g. Crosetto & Perez processing; (iii) validation of SRTM data over a Galapagos island and Aragues, 1999). The potential of 90 m SRTM data for this purpose demonstration of a method to reduce pixel size for better hydrological because of the global coverage, consistency and lack of atmospheric and geomorphological analysis without loss of accuracy; (iv) a map of artifacts has also been mentioned (Gelautz et al., 2003; Roth et al., watershed boundaries and river network at island scale, local scale 2002). In remote areas where high resolution data sets are required, and watershed scale for a Galapagos island; and (v) the possibility of a integrating SRTM data with radargrammetry and/or interferometry is a watershed-based and sub-watershed-based management for Galapa- vital tool. Improvements in extracted river network and watershed gos based on understanding hydrological system from derived data in boundaries by reduction of pixel size were shown by visual validation order to ensure protection of unique and fragile water-dependent of watershed outlets along the coast. The limitations of automated ecosystems and sustainable use of freshwater resources. The meth- river network extraction algorithm were highlighted and discussed. odological steps used in this paper could be applied to other islands of Field-based validation and manual extraction of river networks and the Galapagos archipelago and other study sites worldwide where tectonic features validated the results at a local scale. accurate watershed delineation and quantitative water balance assessment are impeded by the lack of topographical data. The 5.2. Using DEMs to understand hydrological processes and address absence of such data impedes watershed-based management and this freshwater ecosystem management is a handicap for many remote places while it is becoming a key recommendation at the international level. The research described in this paper generated the first map of flow paths and watershed boundaries for Santa Cruz Island. The map Acknowledgments and its validation with existing watershed outlets show that this drainage network does represent a true network which can be active This work was carried out in collaboration with the Galapagos at specific moments in time (catastrophic rainfall of El Niño events). National Park Service, the Charles Darwin Foundation, the Municipality The strong influence of fractures on the drainage network system was of Santa Cruz, and the Galapagos National Institute in Galapagos as part revealed through the morpho-structural analysis of the DEM. The high of the on-going project to study the hydrology–hydrogeology of the accuracy and detailed pixel size of the DEM has enabled the Galapagos: “Galapagos Islands: Integrated Water Studies.” We thank hydrological processes at island scale, watershed scale and sub- ESA-ESRIN for the funding of Noémi d'Ozouville under the Young watershed scale to be looked at and quantified. Having access to Graduate Trainee program and New Opportunities for Women (2003– information at the island scale is important as more than 70% of the 2004); “Fondation de France,”“Naturalia & Biologia,” Schlumberger land surface area consists of protected area within the Galapagos Foundation, Schlumberger SEED program, “Fondation d'Entreprise National Park. This land surface area is not readily accessible, yet Veolia Environnement” for funding of the project; and “La Chancellerie equally needs to be understood in terms of the hydrological processes des Universités de Paris” for the funding of Noémi d'Ozouville (2006). that take place. The value of defining the catchment areas lies in the The authors are most grateful to Frederic Kaveh, “Université de Marne- 4146 N. d'Ozouville et al. / Remote Sensing of Environment 112 (2008) 4131-4147 la-Vallée” and Godfrey Merlen, Galapagos National Park Service for Holzner, J., Eineder, M., & Schattler, B. (2002). 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