Integrated Environmental Management: A Case study on shrimp-paddy land use strategies in the southwest of Bangladesh

The aim of this study was to project the choices with regard to land use on the basis of economic, social and environmental considerations. The exercise was carried out as a part of a capacity development process in IEM at EGIS.

The study area consisting of three districts of southwestern Bangladesh, viz., Bagerhat, Khulna and Satkhira with a total area of 10,660 sq. km, comprised coastal districts bordering the Bay of Bengal.

The following key land use strategies were formulated under the study:

The computational framework for the study generates and quantifies the environmental, economic and social trade-offs between shrimp and crop cultivation in the study area through a spreadsheet model. A major part of the computational framework is based on spatial models including a salinity intrusion model and a land use allocation model. The model was calibrated against reported shrimp and paddy production figures of the study area, simulating the values of indicators for different strategies. The value of indicators depends, directly or indirectly, on the different land uses under different salinity regimes according to a particular land use strategy. Relative importance was attached to each type of land use under a salinity regime through the pair-wise comparison technique.

Salinity intrusion model

The hydrodynamic and salinity models were developed for the study area and calibrated by the Surface Water Modeling Centre (SWMC). The hydrodynamic model simulated the flow velocity in the Gorai and associated river systems in the study area. The flow velocities from the hydrodynamic model and measured salinity at the downstream boundaries were used as inputs to an “advection and dispersion model” which simulated the surface water salinity throughout the river network. The simulated surface water salinity at designated points in the river network were collected by EGIS from SWMC. These point data were then used to generate iso-haline surfaces within a GIS environment. The base condition in this case was a situation where the dry season flow of the Gorai was close to zero, which was representative of the hydrological condition of Gorai in the 1997-98 hydrological year. The models were also run for different flow regimes of the Gorai River. The simulated salinity data for Gorai flows of 60, 100 and 150 m³/s were used to produce the corresponding salinity intrusion maps. The different salinity levels available on the continuous iso-haline surface were grouped together into salinity ranges showing areas of <5ppt, 5-10ppt, 10-15ppt, 15-20ppt and >20ppt.

Land-use classification

In the base flow conditions, the major land use classes were generated from the classification of a combination of various satellite imagery. For each flow condition under consideration, the model showed the distribution of the land use classes among the various salinity regimes and allowed for land use allocation based on the strategies. This was done by a GIS overlay of the salinity and land use maps.

A combination of image processing techniques, extensive field information, knowledge of hydrology, ground water condition, existing agricultural practices, GIS data layers of salinity, land type, river networks, etc., were used in the classification. Apart from the satellite images, a map of the Sundarbans, derived from aerial photographs of 1995, containing the spatial distribution of the mangrove species was also used.

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Simulation of strategies

Three different strategies were formulated in this study. One strategy (Max Shrimp) tried to allocate land use in such a way as to maximize shrimp production in the region. The other tried to maximize the production of paddy (Max Paddy) in the region. These two strategies reproduced the two extremes of the trade-off curve of shrimp-paddy production in the region. All other land uses fell on the trade-off curve between these two extremes. One such strategy was formulated which sought to find a balanced use between all production activities. The land use allocations corresponding to different strategies include:

Production under different strategies for base flow condition of the Gorai:

Max Shrimp: In this strategy, all land use under the five different production regimes are reallocated in such a way as to maximize shrimp production from the study area. Accordingly, all land up to salinity level 15 ppt is shifted to the production regime 4 (Golda-Boro) as this regime gives the highest production of shrimp. Beyond 15 ppt, the land use practice suggests that Golda-Boro cultivation is not suitable. Therefore, the rest of the area is shifted to the Bagda-Bagda production regime in order to get the highest shrimp production from this salinity regime. The area under land use other than the production regimes is kept the same.

Max Paddy: According to this strategy, paddy production in the region is maximized by shifting all land use to Aman-Boro. Even in the high salinity regime, Boro cultivation is practiced near the coast, which puts the coastal aquifers at risk to salinity intrusion as irrigation water for Boro cultivation is drawn from aquifers.

Balanced land use: In this strategy, a balanced land use between shrimp production, paddy production and capture fisheries is sought. Accordingly, all Aman-Boro land use was shifted to Golda-Boro in order to increase shrimp production and thereby increase economic returns compared to the base case. This shift however, tends to reduce paddy production due to the loss of Aman land to Golda production. In order to mitigate this effect somewhat, all the Bagda-Bagda land, which is environmentally much damaging, is shifted to the Bagda-Aman regime. The Aman floodplain is kept as it is in order to retain capture fisheries production.

Findings

It was observed that the Max Shrimp is an economically attractive strategy. From a social point of view, however, the Max Paddy strategy is superior. Environmentally, Balanced Land Use is better than the other two strategies. For a set of representative weights assigned to economic, social and environmental objectives, it was noted that the Max Shrimp is the preferred strategy in the study area. Balanced Land Use is the next best strategy followed by the Max Paddy strategy. The rankings do not change with an increasing flow through the Gorai River. However, it was observed that increasing flow through the Gorai would have a better impact on the environmental aspects compared to the economic and social aspects of the study area.

The Max Shrimp strategy in the study area comes out better because of its emphasis on Golda cultivation in the fresh water zone. Golda cultivation is a recent phenomenon in the study area and has less social and environmental impact than Bagda cultivation. A number of supporting strategies were suggested in order to sustain the benefits of Golda cultivation without damaging the environment.

Recommendations

The selection and production of shrimp and paddy varied in this study only according to water salinity. This is not entirely realistic as land characteristics (flood depth, soil properties, etc.) also play a major role in the selection and production of crops.  Therefore, it was recommended that land suitability must be also considered to make the study complete. Furthermore, the management of only land resources was investigated here, water resources management being taken as exogenously determined. Since land management has direct implication on water resources and vice versa, a study combining the management of both land and water resources was also recommended. It was further recommended that the definition of various indicators, e.g., aquatic bio-diversity, be further refined.

Source: EGIS Report on ‘Integrated Environmental Management: A case study on shrimp-paddy land use strategies in the southwest of Bangladesh’