Monitoring wetland ditch water levels in the North Kent ... - CiteSeerX

Hydrological Sciences-Journal-des Sciences Hydrologiques, 46(4) August 2001

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Monitoring wetland ditch water levels in the North Kent Marshes, UK, using Landsat TM imagery and ground-based measurements

D. H. A. AL-KHUDHAIRY, C. LEEMHUIS, V. HOFFMANN, R. CALAON, I. M. SHEPHERD Institute for Systems, Informatics and Safety, Joint Research Centre, Commission of the European Communities, 1-21020 Ispra (VA), Italy e-mail: delilah.al-khudhairvfajjrc.it

J. R. THOMPSON, H. GAVIN & D. L. GASCA-TUCKER Wetland Research Unit, Department of Geography, University College London, 26 Bedford Way, London WC1H OAP, UK e-mail: j.thornpson(g>,geog.ucl.ac.uk Abstract An understanding of hydrology is a prerequisite for ensuring the successful management, conservation and restoration of wetland environments. Frequently, however, little is known about historical hydrological conditions, such as water levels, within wetlands. Moreover, many channel and ditch systems in wetlands are not routinely monitored, except perhaps for research purposes. A methodology is presented herein which makes use of satellite imagery to indirectly provide remotely sensed observations of water levels within channels and ditches. Using multi-temporal Landsat Thematic Mapper (TM) imagery and simultaneous ground-based measurements of water levels, statistical relationships are established between satellite-derived effective wet ditch widths and measured water levels in the drainage system of the Elmley Marshes, southeast England. These relationships can be used subsequently to estimate historical ditch water levels and to monitor contemporary ditch water levels in the wetland. The study shows that satellite imagery has much to offer in monitoring changes in the hydrological regime of wetlands and in providing complimentary approaches to field monitoring. Key words remote sensing; Landsat Thematic Mapper; wetlands; water levels; calibration; hydrological models; satellite-derived hydrological data; water level recorders; North Kent Marshes, UK

Observation des niveaux d'eau dans les rigoles des marais du Nord du Kent, à partir de l'imagerie Landsat TM et de mesures in situ Résumé Une compréhension de l'hydrologie est nécessaire pour garantir la gestion réussie, la conservation et la restauration d'environnements marécageux. Cependant, trop souvent, il existe un manque de connaissance des conditions hydrologiques historiques, notamment en ce qui concerne les niveaux d'eau dans les marais. De plus, de nombreux systèmes de canaux et de rigoles de marais ne sont pas suivis monitorés régulièrement, sauf peut-être pour des objectifs de recherche. On présente ici une méthodologie faisant appel à des images par satellite afin de fournir indirectement des observations du niveau de l'eau dans les canaux enregistrées à distance. Des relations statistiques sont établies entre des largeurs d'eau effectives des canaux mesurées par satellite et des niveaux d'eau mesurés dans les systèmes de drainage des marais Elmley, au sud-est de l'Angleterre, grâce à l'utilisation de les images multitemporelles de Landsat Thematic Mapper (TM). Ces relations peuvent être utilisées ensuite pour reconstituer les niveaux historiques des eaux des canaux et pour observer simultanément les niveaux d'eau dans les marais. Cette étude montre que les images satellitaires peuvent être très utiles pour le suivi monitorage des changements du régime hydrologique des marais et pour fournir des approches complémentaires à l'observation monitorage in situ des champs. Mots clefs télédétection; Landsat Thematic Mapper; marais; niveaux d'eau; calage; modèles

Open for discussion until I February 2002

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hydrologiques; données hydrologiques obtenues par télédétection; limnigraphes; un marais du nord du Kent, Angleterre

INTRODUCTION Land drainage and water abstraction for agriculture often fundamentally modify the natural hydrological functioning of adjacent or downstream wetlands. The processes operating within wetlands are often poorly understood and unexpected and deleterious impacts frequently occur following interventions within wetlands or their catchments (Denny, 1995; Van der Kamp et al, 1999; Thompson & Finlayson, 2001). Attempts to restore wetlands are regularly hampered by inadequate knowledge of wetland functioning (Wheeler, 1995). Moreover, since wetlands have only infrequently been included within routine hydrological monitoring programmes, the availability of historical data, such as water levels and the extent of surface inundation, is characteristically limited (Hollis & Thompson, 1998; Thompson & Hollis, 1997). This represents a serious restriction to the establishment of baseline conditions against which current and future hydrological conditions can be compared and assessed. It also limits the availability of input and calibration data for hydrological models (AlKhudhairy et ah, 1999), which have evolved in the last few decades from addressing river basin management issues to continental water processes and global water balances (Engman, 1996). Although remote sensing has great potential for addressing some of the deficiencies of limited hydrological data, applied hydrology has not readily embraced remote sensing as a useful source of data. Engman (1996) explains that this may be because existing techniques and data have only been sufficient for limited applications. In fact, most of the advances in remote sensing for hydrology have been in areas where existing methods were unsatisfactory or limited, or areas where data were scarce or non-existent. Engman (1996) also argues that, to meet the challenges of the needs of modern hydrology and to contribute to its future progress, there is a need for (a) more, better, and different spatial and temporal data that cannot be provided nor maintained by traditional hydrological instruments for various reasons including costs and feasibility, and (b) invaluable, long-term data that can be used for model development and validation. The results reported in this paper form part of a wider project, System for HYdrology using Land Observation for model Calibration (SHYLOC), which was partly funded by the European Commission's Space Technology Programme under the Fourth Framework Programme. The aims of the project were to develop improved hydrological models for wetlands and to develop a remote sensing software, SHYLOC, for indirectly estimating water level data for wetlands using satellite imagery. Satellite-derived data have the potential for the establishment of recent (two decade) historical and contemporary hydrological conditions and for providing calibration data for hydrological models (Shepherd et al., 1999; Al-Khudhairy et al, 2001b). The SHYLOC project established comprehensive hydrometeorological databases for four wetlands in the UK and Greece, which are subject to national, European and/or international environmental legislation. These data were employed within the hydrological models developed within the project and as calibration and validation data for both the remote sensing technique and the advanced hydrological

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models. The methodology for reconstructing historical ditch water levels from satellite imagery and the results of applying it successfully to one of the SHYLOC test sites, the Elmley Marshes in southeast England, are described herein.

REMOTE SENSING OF WATER There are numerous existing sensors, and soon-to-be launched sensors, that are suitable for providing invaluable data for validation of hydrological models and for monitoring hydrological conditions. These sensors vary mainly in (a) the frequency of the observations, which can vary from one over a few days to one every 16 days (e.g. Landsat TM) or more, depending on the satellite sensor and the orbit, and (b) spatial resolution, which can range from 1 m (e.g. IKONOS) to 30 m (e.g. Landsat TM) or more, depending on the sensor. Multi-temporal synthetic aperture radar (SAR) data from active microwave sensors have proved useful in estimating streamflow in a braided glacial river in British Columbia, Canada (Smith et al, 1995, 1996; Smith, 1997). A nonlinear relationship between satellite-derived water surface area and ground measurements of river discharge was identified. Space-borne radar altimetry has also been used to directly determine stage variations in large lakes (Birkett, 1995) and large rivers (Birkett, 1998), such as those in the Amazon basin. Koblinsky et al. (1993) used the Geostat altimeter to estimate river stage change within an error of approximately 0.19-1.2 m. More recently, Alsdorf et al. (2000) improved the resolution of these remotely sensed measurements using interferometric processing of SAR data from the SIR-C (spaceborne imaging radar-C) mission to provide centimetre-scale variations in floodplain water level response to changing river discharge within an error of the order of 0.1 m. Unfortunately, altimetry is only applicable to water bodies greater than about 1 km in width. However, radar altimetry shows great promise for providing enhanced spatial monitoring of river water level changes, and for narrowing the minimum river width limit of >1 km, using future satellite SAR missions such as the European Space Agency's ENVISAT RA 2 (radar altimeter) instrument. Techniques using radar imagery offer a major advantage over visible and nearinfrared sensors for applications in hydrology due to their all-weather and day/night capabilities. However, these approaches still suffer from three main drawbacks when compared with those based on visible and near-infrared sensors, such as Landsat TM. Firstly, the interpretation of SAR imagery is much less straightforward than for the visible/infrared range. In addition, the presence of wind-induced waves or emergent vegetation can roughen the surface of open water bodies, making them difficult to discriminate from other non-flooded land surface types when using single frequency and polarization SAR data (Smith, 1997). Finally, whereas the archives of visible and near-infrared sensors are almost twenty years long, most radar archives capable of yielding information for small channels which characterize many wetlands, only date back as far as the early 1990s. Using Landsat multi-spectral scanner (MSS) imagery Usachev (1983) and Xia et al. (1983) found a positive relationship between satellite-derived estimates of inundated area and ground-based measurements of stage and discharge. Similarly, Kruus et al. (1981) found a generally positive relationship between total inundation

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areas estimated using seven Landsat MSS scenes and simultaneous measurements of river stage. In addition, Gupta & Banerji (1985) used Landsat MSS-derived data (reservoir areas) to monitor water volumes in water bodies in terrains of known topography. Two shortcomings of Landsat MSS imagery compared to Landsat TM are the coarser spatial resolution (80 m pixels compared to a Landsat TM pixel of 30 m) and the smaller number of spectral bands. Landsat MSS data have only four spectral bands, whereas TM imagery has seven covering the visible, near-infrared, mid-infrared and thermal infrared of the electromagnetic spectrum. It therefore provides extra information in the mid-infrared and thermal infrared bands. This multi-spectral nature of sensors such as Landsat TM provides an additional advantage over radar imagery. Of the seven spectral bands provided by Landsat TM, three (bands 4, 5 and 7, which have, respectively, wavelengths equal to 0.76-0.9 |im, 1.55-1.75 |im and 2.082.35 um) are particularly sensitive to the presence of water. However, with regards to remote sensing of water using Landsat TM data, most of the published studies are related to mapping the extent and frequency of river inundation. For example, Pope et al. (1992) used Landsat TM together with airborne synthetic aperture radar data to successfully identify and map the intermittent flood extent of meandering systems of imperfect surface drainage in an area north of Nairobi, Kenya. To the authors' knowledge, Shepherd et al. (2000) are amongst the first to exploit the use of the partial pixel approach (which assumes that ditch-carrying image pixels consist of only two components: water and homogeneous land cover, which can be grassland, bare soil or cultivated land; see Fig. 1) to measure surface water area using Landsat TM imagery and digitized ditch positions. In Fig 1, the shaded areas refer to non-ditch carrying image pixels, whereas the white pixels crossed by ditches are referred to as ditch-carrying pixels. Shepherd et al. (2000) tested their approach on part of the North Kent Marshes in southeast England and found a positive relationship between multi-temporal values of a ditch index (the proportion of an image pixel that is covered by water) and intermittent ground-based measurements of ditch water levels. This study advances the partial pixel approach, which forms the basis of the SHYLOC software (Al-Khudhairy et al, 2001a), by using the ditch index to estimate total water surface area and thereafter dividing satellite-derived estimates of water surface areas by satellite-derived ditch lengths to deduce effective wet ditch widths at various spatial scales ranging from sections of ditches to an entire ditch system. The

Fig. 1 Illustration of ditch (white areas crossed by a continuous black line, representing a ditch) and non-ditch carrying (shaded areas) image pixels. Rm and Rp are the digital numbers or reflectance of the, ditch- and non-ditch carrying, image pixels.

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SHYLOC software estimates the proportion of water and non-water features creating the mixed reflectance (Rm in Fig. 1) from ditch-carrying pixels, and converts them into appropriate surface areas. It is worthwhile noting that the satellite-derived wet ditch width is an effective value due to the spatial resolution (30 m) of the Landsat TM images used in this study. In other words, estimates are not being made of actual water surface area, ditch length nor wet ditch width from Landsat imagery. Instead, the satellite-derived data are used to develop satellite-derived statistical relationships between such data and ground-measurements of ditch water level data. THE SHYLOC METHODOLOGY In order to construct the required historical or long time series of ditch water levels a network of water level recording instruments is installed in the area of interest to collect calibration data during a period of 1-3 years, depending on availability of satellite imagery coinciding with dates of ground-based water level measurements. Thereafter, the calibration phase consists of developing statistical relationships between satellite-derived effective wet ditch widths, calculated using the SHYLOC software (Al-Khudhairy et al, 2001a) and ditch water levels acquired from the field instrumentation. After the calibration period, it is possible to re-construct past ditch water levels using satellite-derived information alone. This implies that a short time series of observed ditch water levels can be extended by 20 years in the case of Landsat-derived information (Al-Khudhairy et al, 2001c). APPLICATION OF THE SHYLOC SOFTWARE TO THE ELMLEY MARSHES The Elmley Marshes The North Kent Marshes, southeast England, are grazing marshes which lie along the lower Thames Estuary and on both sides of the Swale, a tidal channel that separates the Isle of Sheppey from the mainland (Fig. 2). The marshes are largely the product of human activities, which go back as far as Roman times. Sea defences comprising embankments and walls, the majority of which are several centuries old, have enclosed and isolated former salt marshes from the sea. During the period 1960-1980, extensive drainage and field amalgamation took place within the marshes to convert them into arable land. Since then, little new drainage and conversion has taken place, as concerns have grown over the loss of traditional grazing marshes, whilst the unstable nature of heavy marsh soils after underdrainage has become increasingly apparent. The Elmley Marshes are located on the southern side of the Isle of Sheppey and form a relatively isolated 8.7 km2 catchment (Fig. 2). The marshes have a complex drainage network which comprises fleets, runnels and ditches inherited from preenclosure salt marshes but which have been extensively modified by human activities. The ditches drain by gravity through tidal sluices in the sea walls that allow water to be evacuated into the Swale at low tide. The marshes are managed for low intensity grazing of cattle and sheep (Willock, 1993). Ditches act as wet fences and provide drinking water for stock and wildlife (Harpley, 1999).

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The Elmley Marshes have high conservation value. They are designated as a Site of Special Scientific Interest (SSSI) and are within a Ramsar Site under the Convention on the Conservation of Wetlands of International Importance. The marshes are included in a Special Protection Area (SPA) under the EEC Birds Directive (79/409) and have recently been designated as a National Nature Reserve (NNR). Agri-environmental schemes, which are administrated by the UK Ministry of Agriculture, Fisheries and Food (MAFF) and aim to encourage environmentally friendly farming, are also in operation in the Marshes. The marshes are managed by the Elmley Conservation Trust in a way sympathetic to nature conservation. Management includes the maintenance of high water in winter and early spring in order to create areas of surface water on specified areas of the marshes (Harpley, 1999). Water level observations and ditch cross-sections Observations of ditch water levels have been obtained from a network of instruments installed within the Elmley Marshes. Eight stage boards were installed in December 1997, whilst a further four were installed in June 1999. Since their installation, observations have been undertaken from the stage boards at 2-3 week intervals. Three automatic water level recorders (AWLR), each comprising a Metrolog 420TA data logger and BTEC G depth transmitter, were installed in July 1998 and have provided water level observations at 20-min intervals. A further AWLR comprising a Druck PDCT 830 pressure transducer connected to a Campbell Scientific CR10X datalogger

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