Water storage for sustainable development and poverty eradication: Part 2 of 2

June 26th, 2012

Prof. Francisco Luiz Sibut Gomide, Universidade Federal do Paraná, Brazil

This is the second part of a series of articles looking at hydropower and water storage in dams. While the first part provided an introduction to the broad issues involved in dam construction worldwide, the second part focuses specifically on the unique challenges faced by Brazil.

Brazil: The water (and hydropower) continent

The globally averaged annual precipitation over land is less than 800 millimeters1. It does not change much from one continent to the other (Europe, 790 mm; Asia, 740 mm; Africa, 740 mm; North America, 756 mm; Australia and Oceania, 791 mm). The exception is South America, where it is twice as large: 1600 mm. In Brazil, it is even larger: 1800 mm. In the Brazilian Amazon, annual rainfall is more than 2200 mm. For the Amazon as a whole, rainfall is over 2400 mm, more than the triple of those average 800 mm!

More than 25% of the total water flux in the planet occurs in South America: the mean annual global run off is 47,000 km3 and South America’s is 12,200 km3. The Brazilian mean annual run off (5,667 km3) triples the American one (1,787 km3), and the countries (Brazil and contiguous USA) are comparable in area. Furthermore, the three largest concentrations of hydropower potential in the planet are in South America, two of them in Brazil.

The Brazilian electricity sector is – and hopefully will continue to be – basically hydraulic. Thanks to hydroelectricity the Brazilian energy mix is one of the cleanest and most carbon-free in the world. Most of the Brazilian freshwater storage in man-made reservoirs (653 km3 out of 724 km3) has been provided by electricity generation companies.

Assuming that the total freshwater storage in American man-made reservoirs is 810 km3, one may state that this is equivalent to 165 days of long term mean run off. Comparatively, the Brazilian total freshwater storage in man-made reservoirs is small: 47 days. Disregarding the electricity sector, it would drop dramatically to less than 5 days.

The reservoirs belonging to the Brazilian electricity sector have a total surface area of 37,000 km2. For comparison purposes, it may be mentioned that 90% of the 90,990 km3 of freshwater storage in natural lakes is concentrated in just eleven locations, with a mean surface area of 44,800 km2. Eight of these lakes are in the northern hemisphere; three are in Africa and, of course, none in South America.

The storage yield relationship

Estimators of long-term storage requirements are proportional to the standard deviation of the net inflows to reservoirs2. In the context of climatic changes, the importance of reservoirs is increased: floods and droughts are expected to be more frequent, and the progressive concentration of occurrences in the tails of the probability distribution inflates the standard deviation of the net inflows, indicating the need for larger reservoirs.

There is an optimal size for reservoirs: not too small, to be useful, and not too large, due to the diminishing marginal returns. This is well illustrated by the so-called storage-yield relationship (SYR), a well-known hydrologic tool. For any given combination of planning horizon (in years) and risk to be assumed, this curve (SYR) furnishes the storage required for each value of “firm” (or “guaranteed”, or “assured”, or “sustained”) yield of river discharge.

Of course, no storage is needed to assure the minimum (for this specific risk and horizon) discharge. The curve (SYR) is a monotonously increasing function. The maximum storage corresponds to the maximum sustainable firm flow, which is the long-term mean discharge (LTMD). The inclination of the tangent to this curve (SYR) is equal to the duration of the drought (“critical” period). To illustrate the diminishing marginal returns, it can be shown that for a SYR applicable to typical Brazilian conditions, one can “firm” more than 85% of the long term mean discharge, with less than 30% of the storage needed to “firm” 100% of the LTMD3.

It is not always the case that the water intake location is adequate for the creation of a sizable reservoir. The probabilistic design does not change as one moves upstream looking for more adequate sites; but the maximum sustainable firm flow decreases, of course. The comparative analysis of the duration curves (another well-known hydrologic tool) for both river cross sections will define, together with the storage-yield-relationship, the limits and details of the feasible decisions regarding firm flow.

When dealing with energy regulation rather than water regulation, the flexibility is immense: the storage has not even to be upstream! Adequate designed transmission lines will transport energy from one point (where it is available) to other (where it is needed).

Furthermore, as the system increases in size, adding new plants located in hydrologic diverse regions, the standard deviation of the total inflow increases at a lower rate in comparison with the mean. In other words, the coefficient of variation decreases, what that implies is higher storage efficiency3. The synergetic Brazilian electricity sector has been, for decades, an interesting demonstration of this mathematical property of the partial sums of random variables.

Closure

Infrastructure investment is central to the world’s objective of poverty eradication. There can be no poverty reduction without access to water and electricity. Investments in sustainable multi-purpose water storage must be encouraged. Reservoirs are not only useful. They are indispensable. Reservoirs do change the ecological balance, initially. However, the evidence of successful adaptation to the new – and often better – ecological environment is overwhelming.

Each and every human intervention on Nature has an environmental impact. Hydroelectricity is favorably compared with most other generation alternatives. Accordingly, hydropower must be acknowledged as an unambiguously renewable source of energy.

Reservoirs take advantage of hydrologic diversity to bring in synergic gains to the operation of complex hydroelectric systems. Reservoirs and other infrastructure works are indispensable for water regulation (low flow augmentation), water supply, waste treatment and disposal systems and flood control.

The concentration of reservoirs in the upper portion of the hydrographic basins, as a consequence of the search for more adequate dam sites, is consistent with the probabilistic design of the storage requirements and with the environmental protection of the watersheds.

Sustainable development implies in the optimum conversion of the resources of nature to benefit the today and future generations. There is no conflict between the fortunately predominant advanced level of global environmental awareness and the timely convenience of the effective development of the Brazilian extraordinary water resources.

References:

1. Shiklomanov, I.A. e Sokolov, A.A. (1983) “Methodological basis of world water balance investigation and computation”, Proc. Hamburg Workshop, IAHS Publication n 148.

2. Gomide, F.L.S. (1975) “Range and Deficit Analysis using Markov Chains”, Hydrology Papers, v 4, n 79, Colorado State University, Fort Collins.

3. Gomide, F.L.S. (2012) “Sobre Reservatórios e Segurança Hídrica”, to appear.

Francisco Luiz Sibut Gomide was born in Curitiba, Paraná in 1945. He earned a PhD in Hydrology and Water Resources at Colorado State University in 1975 and in 1986 he became a Professor of Water Resources Engineering at the Universidade Federal do Paraná. In a long and varied career Francisco has served as Minister of Mines and Energy of the Federative Republic of Brazil; President and CEO of COPEL – Companhia Paranaense de Energia; and President and Director of the Brazilian Association for Hydrology and Water Resources. Currently, Professor Gomide is the owner of the consultant GMD – Organização Industrial e Engenharia Ltda.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.