Farakka Barrage
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Modeling Sustainable Water Use

 

Background Information

For many societies access to clean water may be the factor most limiting to quality of life.  Water-borne disease remains one of the world's leading causes of death, crops wither for lack of irrigation, and industries cannot grow unless supplies are guaranteed.  Dams may provide additional water, but as discussed in "Farakka" carry a tremendous cost.  Downstream users are perhaps most affected, but even those with access to the water may face challenges unless the increase in supply is adequate to meet the demand.  As discussed in the broadcast, the diversion of water from the Ganges during the dry season is causing hardship to Bangladeshi farmers who have historically used these waters for irrigation.

Maximum Sustainable Yield

The concept of a Maximum Sustainable Yield (MSY) is used to determine the quantities of a resource that can be exploited in a sustainable manner, allowing for the exploitation of that resource at a consistent level indefinitely (although the definition of the word "sustainable" seems to be changing in some circles to include the idea of use for a long period of time; e.g. the sustainable use of oil). MSY is used to determine timber harvests, hunting limits, and catch limits for fisheries.  In the case of waters, the sustainability of the extraction affects both the users above a dam or barrage and those below.

Model Parameters

The first step in building the model is identifying quantitative factors that are important to understanding the dynamics of the system. While not all will be used in most cases, development of the list causes students to reflect on the complexity of the model and gain an understanding of the important relationships associated with the question being asked.

 

2. Building a Model

Computer models lie at the foundation of many environmental questions and decisions.  Beginning with the simplest factors that influence the process under study, the model can be refined through the addition of more variables or by the inclusion of increasingly sophisticated equations.  Some parameters can be varied with precision (amount of water withdrawn from a river), others vary within a range of values (rainfall).  Others require the application of some algebraic equations relating the different variables (the amount of evaporation is largely a function of the surface area of the water stored behind a dam or barrage). 

 

Model Design – developing the question

For Ganga, questions of water resource management frequently center on the use of barrages to reduce the seasonal fluctuations in water supplies to provide more consistent availability for agricultural, domestic and industrial use. As water levels drop over the course of the dry season, more water must be diverted to meet the demands of the users in India.

The Farakka barrage is capable of diverting 40,000 cusecs (cubic feet per second) of water.  Dry season flows of the Ganges at Kanpur were historically approximately 55,000 cusecs, although this figure varied considerably given differences in rainfall patterns.

 

3. ExCel Model

The excel model linked to this page can be used in a variety of ways. First, research is needed into the basic input parameters: what is per capita water usage in an area? How much precipitation reaches a reservoir or aquifer? A basic calculation of sustainability can thus be made, with the effects of the water cycle added to include complexity in the model.

The model can then be made more dynamic by including random variations in the amount of rainfall and water use. Increased population can be factored in over time, and a loss of volume associated with siltation included.

Use the model to explore the following questions:

1.  Assuming that the dry season flow of water through Kanpur is 55,000 cusecs and that 25,000 cusecs of water are currently, how long can a 5% increase in the amount of water diverted be sustained?

2.  Rates of evaporation change as a function of the depth of the water, which can be approximated by the volume of flow.  Using the figures in Table 1, calculate the net effect of withdrawals of water on downstream users based on the volume diverted.

Table 1.  Evaporative water losses as a function of
volume of water diverted.

3.  Siltation is occurring rapidly behind dams and barrages around the world.  If the operators of Farakka commit to withdrawing 25,000 cusecs of water per year, and siltation reduces flows from the current 55,000 cusecs per year by approximately 6% how long can water be diverted before flow rates reach 0 below the barrage?