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Impact of the Farakka Dam on Thresholds of the Hydrologic Flow Regime in the Lower Ganges River Basin (bangladesh)

Animesh K. Gain
Venice Centre for Climate Studies (VICCS), Department of Economics, Ca’ Foscari University of Venice, Cannaregio 873, Venice 30121, Italy

Carlo Giupponi
Venice Centre for Climate Studies (VICCS), Department of Economics, Ca’ Foscari University of Venice, Cannaregio 873, Venice 30121, Italy

Abstract/ Executive Summary:

The variation of river flow within a natural range plays an important role in promoting the social-ecological sustainability of a river basin. In order to determine the extent of the natural range of variation, this study assesses hydrologic flow thresholds for the Lower Ganges River Basin. The flow threshold was calculated using twenty-two “Range of Variability (RVA)” parameters. The impact of Farakka Dam on the Lower Ganges River flow was calculated by comparing threshold parameters for the pre-Farakka period (from 1934 to 1974) and the post-Farakka period (1975–2005). The results demonstrate that due to water diversion by the Farakka Dam, various threshold parameters, including the monthly mean of the dry season (December–May) and yearly minimum flows, have been altered significantly. The ecological consequences of such hydrologic alterations include the destruction of the breeding and raising grounds for a number of Gangetic species, the increase of salinity in the southwest coastal region of Bangladesh and a reduction of fish and agricultural diversity. The major findings in this paper have a number of policy-level implications to aid water sharing mechanisms and agreements between the government of Bangladesh and India. The methodological approach presented in this study is applicable to other river basins.

Keywords: Threshold, River flow, Range of variability (RVA), Ganges, Farakka Dam

Location: Ganges Basin in Bangladesh

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Flood, Problem, Livelihood

Objectives:

To determine the extent of the natural range of variation, this study assesses hydrologic flow thresholds for the Lower Ganges River Basin.

Methodology:

Data: To assess the flow thresholds, hydrological discharge data were collected. The major discharge measuring station of the lower Ganges Basin in Bangladesh is at Hardinge Bridge Point, and long-term observation records of this station are available and accessible from the Bangladesh Water Development Board. The data are of high quality and have been used in major hydrological studies in flood forecasting and other planning purposes. Daily discharge data from this station was collected from 1934 to 2005. India began the operation of Farakka Dam on 21 April 1975. Hence, the data series up to 1975 represents pre-Farakka flow, and data from 1975 onwards represent post-Farakka flow. To assess the hydrological data series, the hydrological year, 1 April to 31 March, instead of the calendar year is considered. Depending on meteorological and geographical factors, the beginning of a “hydrologic year” or “water year” differs from the calendar year. In winter snow-dominated watersheds, the year begins 1 October and ends at 30 September, because snow is deposited in the fall and winter and drains out of the watershed in the spring and summer, and the watershed returns to a “dry” state at the end of the year [49]. However, the situation is completely opposite (rainfall in the spring and summer and drains out in the fall and winter) in summer-monsoon dominated regions. Therefore, in the Ganges-Brahmaputra Basin (situated in the Indian Summer Monsoon), the hydrologic year begins 1 April and ends 31 March. In addition to discharge data, available daily rainfall data were also collected from the Rajshahi station of the Bangladesh Meteorological Department (BMD) for the period 1964–2005. The data series of the Rajshahi station represents the rainfall of the Bangladesh portion (only 4% of the basin area) of the Ganges Basin. The missing rainfall data were filled up using appropriate hydrologic techniques, e.g., the averaged value of surrounding stations. For representing the rainfall of the Indian portion of the Ganges Basin, the results of the trend analysis carried out by Kumar and Jain were considered, in which available daily gridded rainfall data at a 1° × 1° resolution for the period 1951–2004 provided by the India Meteorological Department (IMD) were used. Testing the Natural Condition of Discharge: The first step for determining the flow thresholds is to consider the observation data series that represent natural flow, in which no shifts and trends are found. For testing the natural condition, a linear trend analysis was conducted considering the fact that the natural flow series is trend-free and constitutes a stochastic process whose random component follows the appropriate probability distribution. Gain et al. present a detailed description on the method used for testing linear trends, which can be summarized as follows: Assume that y_t, t = 1, …, N is an annual time series and N is the sample size. A simple linear trend can be written as: y_t = D + Mt. where D and M are the parameters of the regression model. The rejection of hypothesis M = 0 can be considered as a detection of a linear trend. In an altered flow regime (by means of climate change or human perturbation), those parameters should be maintained within the limits of their natural variability, which should be based on extensive ecological information, taking into account the ecological consequences of different flow regimes. However, setting flow targets based on ecological information is very difficult to achieve. In the absence of extensive ecological information, Richter et al. suggested several measures of dispersion (e.g., ±1 or 2 standard deviations, twentieth and eightieth percentile, etc.) to use in setting initial threshold flows. The choice of the most appropriate measure of dispersion should be based on whether each parameter follows a normal or skewed distribution, and in the case of a normal distribution, one could use the standard deviation (SD) from the mean value as an initial threshold flow. In order to select an appropriate measure of dispersion, we tested the distribution of each of the 22 RVA parameters, and we found that all of the parameters follow a normal distribution. Therefore, values at ±1 SD from the mean were selected as thresholds for each of the twenty-two RVA parameters. Any considered parameter should thus stay in the limits: (mean − SD) ≤ parameter ≤ (mean + SD). Exceedance of these limits by a particular parameter may lead to considerable ecosystem stress over long time periods. We used this approach for setting initial flow thresholds in this study.

Source of Information: Water 2014, 6, 2501-2518; doi:10.3390/w6082501

Article Link (Online Journal):

Funding Source:

Budget:

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Achievement/Findings:

Our analysis showed that the RVA threshold criteria have been exceeded in most years during post-Farakka period, even after adopting a water sharing treaty between Bangladesh and India. The exceedance of threshold conditions is detrimental to both aquatic and terrestrial ecosystems on which the livelihood pattern of the inhabitants is dependent. As a consequence, the social-ecological system of the basin is negatively affected. The approach of hydrologic threshold flow confirms its potential for use in planning and management of water resources, which have impacts on the coupled social-ecological system. The assessment of the hydrologic alteration of different parameters and of the ecological consequences of such an alteration presented in this study is intended to be used in planning and management of water resources, especially for post-Farakka water sharing between Bangladesh and India. Our results have a number of policy-level implications for government agencies of the Ganges Basin. The calculated threshold flow of twenty-two RVA parameters can be used as initial targets for water resources and ecosystem management in the Lower Ganges Basin, particularly in Bangladesh. The government of Bangladesh and India could consider allowing human perturbation and development activities within the calculated threshold ranges. The calculated thresholds can also be used for water allocation to meet household, agriculture and industrial water demands. In trans-boundary river basin management within an integrated water resources management approach, thresholds of flow variability can be used as a basis for negotiation with other riparian countries. This simple, but effective, approach for evaluating the impact of dam-induced hydrologic alterations presented in this study may also prove to be useful to the policymakers and river basin authorities. In setting ecological threshold flows with the RVA approach, the study is mainly based on statistics. However, further research is required to investigate the physical impact of the hydrologic flow regime on ecosystems in detail. To ensure livelihood security in the lower Ganges ecosystem in both India and Bangladesh, there is a need for close introspection and appropriate action in a holistic manner to restore the hydrology of the river system. In this study, we focused only on the impact of the dam on river flow thresholds. However, in reality, climate change and human-induced perturbation (e.g., development of river infrastructure, such as dams) happen concomitantly and interactively. The extent of hydrologic perturbation associated with human activities and climate change has already been assessed separately in several studies. To investigate the combined impact of climate change and human-induced perturbation, future studies are required, aiming at a more in-depth understanding of the system.

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