hydrology in the humid tropic environment (proceedings of...

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Hydrology in the Humid Tropic Environment (Proceedings of a symposium held at Kingston, Jamaica, November 1996). IAHS Publ. no. 253, 1998. 81 M o n i t o r i n g a n d m a p p i n g a r e a s a f f e c t e d b y water control projects in coastal Guyana V. SINGHROY Canada Centre for Remote Sensing, 588 Booth Street, Ottawa, Canada K1A 0Y7 Abstract In the cloudy humid tropics, RADARS AT images were used to monitor environmental changes in recently constructed reservoirs. The Abary River Water Control Project in coastal Guyana was constructed in 1981, with the purpose of managing the water needed for coastal agricultural use. This investigation uses June 1993 and April 1996 radar images to monitor surface changes of the reservoir. Fifteen years after the construction of the Abary conservancy, most of the flooded forest has died. Our results have shown considerable changes in SAR (Synthetic Aperture Radar) backscatter over the reservoir from June 1993 to April 1996. In addition, this study has shown that parts of the Guyana coastline, including the area north of the reservoir, has changed from a few metres to half of a kilometre over the past twenty years. This has serious implications for sea defence, commercial agriculture, and drainage and irrigation networks. The radar images provide information on areas of coastal erosion and accretion, mangrove forest depletion, and agricultural and hydrological land uses. These images are particularly useful in updating the existing topographic maps of the coastal areas, and monitoring the changes resulting from the water control project. Monitoreo y mapeo de las areas afectadas por proyectos hidraulicos en la costa de Guyana Resumen En los trôpicos hûmedos y nublados las imâgenes RADARSAT son utiles para vigilar los cambios ambientales en los reservorios de reciente construction. El proyecto de control de las aguas del no Abary en la costa de la Guyana rué creado para administrar el agua necesaria para la agricultura costena. Esta investigaciôn usa imâgenes RADARSAT de Junio de 1993 y Abril de 1996 para detectar cambios en la superficie del reservorio. Cinco anos después del establecimiento de la réserva del Abary, la mayorîa de los bosques inundados han muerto. Nuestros resultados muestran cambios considerables en la retrodifusiôn SAR sobre el reservorio de Junio de 1993 hasta Abril de 1996. Ademâs, este estudio ha mostrado que partes de la demarcaciôn costera de la Guyana, incluyendo el ârea al Norte del reservorio, ha cambiado de unos pocos métros hasta medio kilômetro durante los pasados veinte anos. Esto tiene implicaciones sérias en la defensa contra el mar, agricultura comercial, y redes de desagûe e irrigaciôn. Las imâgenes SAR procuran informaciôn sobre las areas costeras de erosion y acrecentamiento, de destrucciôn de los manglares, y de tierras para usos agrïcolas e hidrolôgicos. Estas imâgenes son especialmente utiles para la puesta al dïa de mapas topogrâficos de las âreas costeras, y para la detecciôn de cambios producidos por proyectos de control de aguas. INTRODUCTION Monitoring water control projects in humid tropical areas requires an integrated approach involving field airborne and spaceborne techniques. This study provides a

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Page 1: Hydrology in the Humid Tropic Environment (Proceedings of ...hydrologie.org/redbooks/a253/iahs_253_0081.pdf · Hydrology in the Humid Tropic Environment (Proceedings of a symposium

Hydrology in the Humid Tropic Environment (Proceedings of a symposium held at Kingston, Jamaica, November 1996). IAHS Publ. no. 253, 1998. 81

M o n i t o r i n g a n d m a p p i n g a r e a s a f f e c t e d b y w a t e r

c o n t r o l p r o j e c t s i n c o a s t a l G u y a n a

V. SINGHROY Canada Centre for Remote Sensing, 588 Booth Street, Ottawa, Canada K1A 0Y7

Abstract In the cloudy humid tropics, RADARS AT images were used to monitor environmental changes in recently constructed reservoirs. The Abary River Water Control Project in coastal Guyana was constructed in 1981, with the purpose of managing the water needed for coastal agricultural use. This investigation uses June 1993 and April 1996 radar images to monitor surface changes of the reservoir. Fifteen years after the construction of the Abary conservancy, most of the flooded forest has died. Our results have shown considerable changes in SAR (Synthetic Aperture Radar) backscatter over the reservoir from June 1993 to April 1996. In addition, this study has shown that parts of the Guyana coastline, including the area north of the reservoir, has changed from a few metres to half of a kilometre over the past twenty years. This has serious implications for sea defence, commercial agriculture, and drainage and irrigation networks. The radar images provide information on areas of coastal erosion and accretion, mangrove forest depletion, and agricultural and hydrological land uses. These images are particularly useful in updating the existing topographic maps of the coastal areas, and monitoring the changes resulting from the water control project.

Monitoreo y mapeo de las areas afectadas por proyectos hidraulicos en la costa de Guyana Resumen En los trôpicos hûmedos y nublados las imâgenes RADARSAT son utiles para vigilar los cambios ambientales en los reservorios de reciente construction. El proyecto de control de las aguas del no Abary en la costa de la Guyana rué creado para administrar el agua necesaria para la agricultura costena. Esta investigaciôn usa imâgenes RADARSAT de Junio de 1993 y Abril de 1996 para detectar cambios en la superficie del reservorio. Cinco anos después del establecimiento de la réserva del Abary, la mayorîa de los bosques inundados han muerto. Nuestros resultados muestran cambios considerables en la retrodifusiôn SAR sobre el reservorio de Junio de 1993 hasta Abril de 1996. Ademâs, este estudio ha mostrado que partes de la demarcaciôn costera de la Guyana, incluyendo el ârea al Norte del reservorio, ha cambiado de unos pocos métros hasta medio kilômetro durante los pasados veinte anos. Esto tiene implicaciones sérias en la defensa contra el mar, agricultura comercial, y redes de desagûe e irrigaciôn. Las imâgenes SAR procuran informaciôn sobre las areas costeras de erosion y acrecentamiento, de destrucciôn de los manglares, y de tierras para usos agrïcolas e hidrolôgicos. Estas imâgenes son especialmente utiles para la puesta al dïa de mapas topogrâficos de las âreas costeras, y para la detecciôn de cambios producidos por proyectos de control de aguas.

INTRODUCTION

Monitoring water control projects in humid tropical areas requires an integrated approach involving field airborne and spaceborne techniques. This study provides a

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82 V. Singhroy

case study where radar images were used to monitor surface changes in a recently constructed reservoir. In addition, the images were also used to map the agricultural land use and coastal processes in the region.

The Abary River Water Control Project (ARWCP) is the first phase of a development programme to bring the Mahaica-Mahaicony-Abary (MMA) region of coastal Guyana into full agricultural production. ARWCP is aimed at providing water control for approximately 420 000 acres of coastal agricultural lands. The water stored in the Abary reservoir is used for gravity irrigation of rice and sugar cane plantations through a network of canals. The catchment area of the ARWCP is 808 km 2, with 326 km2 as the submerged area. Total storage is estimated as 390 X 106 m 3. At the end of construction in 1981, 80 km of canals and 90 km of drains were constructed to irrigate and drain the mainly rice and sugar cane coastal areas.

The agricultural lands that benefited from the ARWCP are below sea level, like the other coastal areas of Guyana. In fact, the whole coastal plain of Guyana (425 x 12 km) ranges between 0.5 m below and 2.4 m above sea level. Sea defence and the construction and maintenance of drainage and irrigation canals in these flat areas are therefore vital to the economic growth of Guyana. This coastal plain, which

Fig. 1 (a) Field view of maturing sugar cane (15 weeks); (b) aerial view of maturing sugar cane (note parallel drainage patterns); and (c) field view of rice field (12 weeks).

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Monitoring and mapping areas affected by water control projects in coastal Guyana 83

occupies only 7.5% of the total land area, is the main agricultural region of the country. The region receives more than 2000 mm of rainfall each year. Sugar (Fig. 1(a) and (b)) and rice (Fig. 1(c)) are the principal agricultural exports. These crops require extensive drainage and irrigation to maintain their productivity.

Aereal photography at various scales has been used extensively for coastal land development and hydrological mapping programmes (Daniel, 1986). Partial coverage is available dating back to 1940, and the most recent 1:2 000 black and white scale photography of the coastal belt was acquired between 1979 and 1982. Over the past 28 years (1972-1998), there is only one cloud-free LANDSAT image (October 1977) of the ARWCP area. Using the LANDSAT image, Singhroy & Bruce (1983) produced a regional coastal zone map describing the land cover/land use, surficial geology, geomorphology, and coastal water classes. Interpretation of radar images to assess coastal erosion was reported by Singhroy (1996). This paper provides an interpretation of radar images for monitoring the areas affected by the ARWCP. The results have shown that these techniques can be used for long-term environmental monitoring of reservoirs in humid tropical areas.

OBJECTIVES

The need for a sustainable approach to coastal land development projects has been recognized by the Government of Guyana and a number of international donor agencies, namely the Caribbean Development Bank, the Inter American Development Bank, the World Bank, and the European Economic Community (World Bank, 1995). At present, the multilateral financial agencies have allocated US$44 million for sea defence and coastal agricultural infrastructure, including water control projects. Recently the Government of Guyana, in collaboration with the World Resources Institute (The Carter Centre), has undertaken to develop policies for land use in Guyana (Bishop, 1995). Land use information taken from 1979 aereal photographs desperately needs updating. Special attention is given to the coastal areas where there are serious conflicts created by on-going development. A cheaper and more accurate monitoring technique for land use and hydrological mapping is needed. This case study uses radar images: - to monitor surface changes in the Abary reservoir;

to measure the shoreline changes in the areas controlled by the Abary reservoir; and

- to update land use information obtained from the 1979 aerial photographic coverage.

THE STUDY AREA

The study area includes 50 km of coastline and the entire coastal plain between the Abary and Berbice rivers (Fig. 2). The geology, geomorphology, and agricultural land use of the study area are described in several publications (e.g. Bleackley, 1957; Daniel, 1986; Singhroy & Bruce, 1983). Topographically, the coastal areas have been mapped at a scale of 1:50 000 by the British Ordnance Survey. The study area,

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84 V. Singhroy

Fig. 2 Map of the study area.

50 X 50 km, is below sea level, and rises gradually to 4 m a.s.l. in areas covered by beach deposits. Approximately 30 km inland, the land rises to 10 m a.s.l. in the white sand region. Engineering soil maps at a scale of 1:250 000 and the geological map of Guyana (1:1 000 000 scale) describe the surficial material and bedrock geology.

The surficial sediments include mud flats, fluvio-littoral sand ridges and aprons, alluvial silt and clay, and pegasse (organic) accumulations. These deposits are underlain by recent soft marine fossiliferous clay of the Demerara Formation. Further inland within the coastal zone, interbedded Pleistocene silty clay, sand of the Coropina Formation, and Pliocene coarse sand (White Sand Series), were mapped by Bleackley (1957).

The original vegetation was tropical forest, most of which has been cleared for agriculture and settlement. Mangrove forest is found on the tidal flats. Lowland savanna on white sands and arborescent and pegasse swamps occupy the inland depressions (Huber et al, 1995). The major commercial agricultural crops include sugar, rice and coconuts.

METHODOLOGY AND INTERPRETATION

This investigation used a 1993 ERS image and a 1996 RADARSAT image. The ERS is a C-VV image with an incidence of 23°. The RADARSAT (C-HH, resolution 20 m X 27 m ) image is a standard beam mode 7 with incidence 45-49°. The SAR images were geometrically corrected and enhanced (Singhroy, 1996b) and overlaid to

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Monitoring and mapping areas affected by water control projects in coastal Guyana 85

facilitate visual interpretation of the changes in the Abary reservoir. A 5 X 5 Gamma filter (Lopes et al., 1993) was used to reduce image speckle. Registration of the SAR image to 1:50 000 topographic maps enable us to estimate changes to the coastline due to erosion or deposition. The coastline boundary was digitized from the 1972 (1:50 000) topographic map, and superimposed on the 1996 RADARSAT image to estimate the coastline change in the ARWCP area. The radar image was visually interpreted with information from aereal photographs, a helicopter overflight, and field photographs.

Reservoir changes

Figure 3 shows a comparison of the (1992) ERS radar image and the (1996) RADARSAT image. The adjacent colour image (Fig. 3(c)) shows the difference in SAR backscatter over the reservoir over a four-year period. Comparing the 1992 and 1996 images, it is clear that the surface of the reservoir has changed considerably. The flooded forest cover, which was dying in 1992, is completely dead in 1996. The bright areas in the RADARSAT image correspond to multiple backscatter of the radar resulting from the diffuse scattering from the standing water, combined with double bounce created by the stumps of the dead trees in standing water. All the dark areas in both images correspond to open water. On the colour image, the red areas correspond to combination of both dead forest and macrophytes (floating vegetation). The yellowish-red colour corresponds to completely dead vegetation in standing water. Green and black correspond to areas of both macrophytes and open water. It is clear that the surface roughness of the reservoir corresponds to open water, macrophytes, and various stages of forest decline. These can easily be identified from radar images.

From a global environmental perspective, macrophytes in tropical wetlands are responsible for a significant production of methane (Bartlett et al., 1993). Recently, several studies have investigated the use of radar images to characterize macrophyte stands in wetlands, and the research is still on-going. (Costa et al., 1996). Separating macophytes from other surfaces in the reservoir is not difficult, as shown from this and other studies. Characterizing the physical properties (height, biomass, etc.) of the macrophytes from radar backscatter is more difficult. In addition, further investigation needs to be done on the decay of vegetation and water quality within the Abary reservoir. For instance, increasing acidity with increasing organic content may lead to the gradual acidification of the coastal agricultural lands.

Shoreline changes

Between 1990 and 1994, there were several cases where the dikes and dams were eroded and broken by coastal storms, resulting in severe flooding. Assessing the risk and damage from coastal flooding requires detailed geomorphological and land use information, which does not exist in Guyana at this time. Shoreline recession has been recorded over the past 200 years at rates varying between five and 20 m a year (Cambers et al., 1994). This may be true where specific recordings were taken, but

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Fig. 3 ERS and RADARSAT images showing changes in the Abary reservoir, Guyana: (a) ERS-1 image, 19 June 1992; (b) RADARSAT-1 image, 12 April 1996; and (c) colour overlay showing the difference in SAR backscatter over the reservoir over the four-year period (red: RADARSAT-1; green: ERS-1; blue: red-green).

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Monitoring and mapping areas affected by water control projects in coastal Guyana 87

regional estimates from RADARSAT images show a larger change (Singhroy, 1996). A comparison between the 1996 SAR images and the 1972 topographic maps shows that parts of the coast have been subjected to severe erosion and accretion and some parts remain unchanged. Our estimate shows that the shoreline has retreated to a maximum of a half of a kilometre in areas between the Abary and Berbice rivers, (Fig. 4, Fig. 5(a) and (b)), over the past 24 years. This is about 25 m a year of coastal retreat. These are priority areas where the dykes have been broken by coastal storm events. The two main erosion areas are the coastline from Georgetown to Mahaica, and the stretch from the Abary to Berbice rivers, where there are inland water control projects. The most stable stretch of shoreline is between the Mahaica and Abary rivers, where there are no water control projects. This indicates that the inland water control project reduces the growth of mangroves, east of the Abary and Demerara rivers. In addition, there is no water control project on the Berbice river, and as such the eastern

Fig. 4 Identifying agricultural land use and shoreline changes in the area controlled by Abary reservoir.

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88 V. Singhroy

Fig. 5 (a) Concrete sea wall (sea defence); and (b) eroding mangrove shoreline and mud flat area, west coast Berbice. (Note the location of sluice gates, 400 m and 1 km in the ocean in (a) and (b) respectively, indicating the former shoreline).

bank of the Berbice river is experiencing a considerable deposition and regeneration of mangroves. Mudflat deposition and mangrove growth have resulted in an increase of the shoreline toward the sea, by an average of half of a kilometre (Fig. 4).

To reduce the cost of sea defence maintenance, serious consideration is given to the management of the mangrove forest. Mangroves are suitable for coastal protection and induce sedimentation in some areas (Tomlinson, 1986; Hussain, 1990). The entire coastline of Guyana was once fringed with mangrove forest. In the Abary-Berbice river corridor (Figs 4 and 5(b)) the mangroves are rapidly being reduced by coastal erosion and the ARWCP. East of the Berbice River where accretion has favoured the growth of mangroves, the shoreline has increased from half to one kilometre since 1972, with mangrove colonizing all the new mud flat areas. The interpretation of the RADARSAT images also shows the extent of mangrove forest (Mg) in areas where the mangrove forests are at risk from coastal erosion. Pastakia (1991) recommended that the planting of mangroves to extend the existing mangal seaward, would further stabilize some of the shoreline. Sites for replanting and regeneration can be delineated from the SAR data.

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Monitoring and mapping areas affected by water control projects in coastal Guyana 89

9 n H H Ë M H H B

H H H H • M B M i M M O M i M

Fig. 6 (a) Coconut plantation (note this rough open surface produces a high backscatter); (b) aerial view of close Wallaba forest canopy in the White Sand area adjacent to the coastal plain; and (c) former swamp forest, now a flooded reservoir with dead stumps (ARWCP) producing a high backscatter (double bounce) on SAR image.

Land use mapping

The only available land use information in coastal Guyana was documented on the 1:50 000 topographic maps, produced from 1979 photographs. There is an obvious need for accurate and reliable land use information, to update the current topographic maps, given the importance of agriculture to the Guyanese economy. The RADARSAT image (Fig. 4) has shown considerable promise in providing information to produce coastal agricultural land use maps in the ARWCP region. Areas of rice, sugar and coconut plantations, pasture, mangrove and inland forest, forest cut over, wetlands, and flooded reservoir were identified. The field

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90 V. Singhroy

photographs of all the major land uses are also included (Figs 1 and 6). Similar land cover information was obtained from ERS-1 data in adjacent French Guyana (Rudant etal, 1994; Rosaz et a/., 1994).

Sugar (Fig. l(a),(b)), rice (Fig. 1(c)) and coconut (Fig. 6(a)) plantations are identified on the RADARSAT images (Fig. 4) based on field size and SAR backscatter of crop conditions: sugar cane plantations (S) are characterized by large rectangular fields with beds separated by drainage ditches (D) (Fig. 1(b)) (Philipson & Liang, 1982). Three types of cropping pattern within the sugar plantations can be recognized on Fig. 4: (a) abandoned flooded fields (A) with parallel drainage ditches, smooth texture and, therefore, a dark appearance; (b) maturing cane (M) (Fig. 1(a)) which varies from 25 to 40 weeks old, approximately 2.5 m in height, with a medium texture and dark to light tone on the SAR image; (c) harvested cane (H) with a bright appearance, because of the rough texture produced by the dry stumps of the cut cane.

The obvious separation of rice from sugar plantations, from a remote sensing standpoint, is the difference in size of the fields. In Guyana, rice is cultivated in small elongated plots, compared to large rectangular plots of sugar cane. The cropping patterns of rice involve ploughing, seeding early growth in flooded fields, maturing fields and harvested fields. However, three stages of rice cropping pattern were identified from the April 1996 RADARSAT images (Fig. 4): these include rice fields that have not been ploughed (P) and contain a rough surface (light tone) of stubble from the previous harvesting; rice fields that have been recently ploughed, and flooded fields where seeding has taken place (dark tone); and maturing rice fields (MR) with uniform texture.

Coconut plantations (Fig. 6(a)) have a rough surface and therefore appear as a light tone on the RADARSAT images (Fig. 4). They flourish on the sandy soils of the beach ridges and cheniers.

CONCLUSION

This paper has demonstrated that radar images can be used to monitor and map areas in the humid tropics that are affected by water control projects. The images were used to: - monitor the changes in the surface of the reservoir;

estimate changes in the shoreline, which are linked to inland water control; and - identify different agricultural crops for the revision of land use maps.

Acknowledgements The author would like to thank Robert St. Jean and Ron Pietch for producing the image maps used in this paper. Invaluable field support and technical advice were provided by Navin Chanderpaul, Lake Chatterpaul and Andrew Bishop of the Environmental Protection Agency, Office of the President, Guyana.

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