By Joel Avruch Goldenfum
The International Hydropower Association and the United Nations Educational, Scientific and Cultural Organization are embarking on a four-year greenhouse gas research project. Through this effort, the organizations intend to develop methods for accurately measuring emissions from freshwater reservoirs and to provide guidance to project developers and owners about how to mitigate emissions at vulnerable sites.
Climate change is one of today’s most important policy subjects for sustainable development. The issue of greenhouse gas emissions from freshwater reservoirs plays a role in these discussions, and there are still many questions to be answered. The Greenhouse Gas (GHG) Research Project, being performed by the International Hydropower Association (IHA) in collaboration with the United Nations Educational, Scientific and Cultural Organization (UNESCO), aims to improve understanding of the effect of reservoirs on natural GHG emissions.
Hydropower was, for a long time, considered an emission-free source of energy. Today, it is known that all energy options have a life-cycle GHG footprint, caused by their construction, operation, and, in the case of thermal power, fueling. Although it is a renewable source of energy, hydropower can be a source of GHG emissions, as hydroelectric reservoirs may emit both carbon dioxide (CO2) and methane (CH4) — two primary GHGs. The real extent of these emissions as part of the global carbon cycle is not well addressed.
All freshwater systems emit GHGs due to decomposing organic material. Lakes, rivers, estuaries, wetlands, seasonal flooded zones, and reservoirs emit GHG. Within a given region that shares similar ecological conditions, reservoirs and natural water systems produce similar levels of emissions per unit area. In some cases, natural water bodies and freshwater reservoirs absorb more GHG than they emit.
Freshwater reservoirs are collection points of material coming from the entire drainage basin area. As part of the natural cycle, organic matter is flushed into this collection point from the surrounding terrestrial ecosystems. In addition, domestic sewage, industrial waste, and agricultural pollution enter the system and produce GHG emissions. Estimates of man-made GHG emissions from a reservoir need to consider only the net change in emissions by subtracting the natural absorption/emission from the inundated terrestrial land, wetlands, rivers, and lakes that were located in the area before impoundment, as well as the emissions caused by distant human activities in the catchment.
The measurements of carbon dioxide and methane from the surface and downstream reaches of some reservoirs indicate that the emissions warrant further investigation. The investigation needs to determine to what extent emissions may have been elevated or reduced by the creation and operation of the reservoir. Currently, there is not enough information or tools to support sound decisions about existing and new reservoirs. There is little experience with possible mitigation measures, particularly with any relevance to global inventories of GHG fluxes.
Data from tropical reservoirs indicates that methane emissions vary not only among reservoirs, but also within each reservoir, as a function of type and density of the submerged soil/vegetation, aquatic macrophyte growth, temperature, oxygen saturation, water level, and other factors.
Only a limited number of sites in the tropical and sub-tropical regions have been investigated. Few reservoirs have been studied in detail anywhere in the world. The methodology being used for measurements are not standardized. Most of the investigations to date have been using surface flux measurements only and lack the rigor and scope to estimate the emission increase or reduction induced by the reservoir.
There have been the inevitable attempts at comparisons between GHG emissions from hydropower and thermal plants. Some studies using extreme cases lead to the conclusion that reservoirs emit considerable amounts of GHGs (e.g. 7 percent of all man-made emissions). However, most still argue that, in the vast majority of cases, hydropower substantially offsets GHG emissions that would otherwise have been produced by generation from fossil fuels.
When hydropower’s GHG emissions are assessed, natural pre-impoundment emissions need to be taken into account. This assessment of natural emissions, including ecosystems that are periodically flooded under natural conditions, is important. Several publications have simply neglected to consider natural emissions, potentially overestimating the role of hydropower’s GHG emissions.
Research project details
The purpose of the IHA/UNESCO research project is to evaluate the carbon footprint (changes in GHG emissions in a river basin due to the construction of a freshwater reservoir) as well as potential mitigation measures. Four objectives are to:
- Develop guidance for net GHG measurements in freshwater reservoirs;
- Promote scientifically rigorous measurements and calculate net emissions from a representative set of freshwater reservoirs;
- Develop predictive modeling tools to assess the GHG status of unmonitored reservoirs and potential new reservoir sites; and
- Develop guidance and assessment tools for mitigation of GHG emissions.
The expected output of this project includes: guidance to ensure standardized GHG measurements in freshwater reservoirs; a standardized, credible set of data from representative reservoirs; a methodology and set of tools for predicting reservoir-induced emissions, including an empirical model and a process-based model for prediction of carbon budget and GHG emission of freshwater reservoirs; and guidance on mitigation options for vulnerable sites.
Critical to this initiative is to develop guidance for measurement methodology, and to choose a robust set of criteria for the selection of reservoirs for measurements.
To have reliable measurements of variables and parameters, a guide is required to create a standard code to be followed in new measurements. It is also necessary to develop guidance in how to use existing measurements and assessments.
Several indicators, such as geographical areas and gradients in characteristics, will guide the selection of sites for measurements and model development and testing. Sites will include all climate types, but with emphasis on tropical regions in Latin America, Africa, and Asia. An expert panel will define the site selection criteria in detail. Possible important criteria include: carbon loading, including natural and anthropogenic contributions; reservoir area; reservoir depth; reservoir residence time; reservoir age; engineering issues such as intake level and location of gates; flooded soil and vegetation; climate; and limnological condition.
Because all combinations of gradients cannot be represented, it is recommended to select one or two parameters as the primary criteria and to determine how representative the chosen reservoirs are according to the other criteria. Practical issues — such as accessibility to the reservoir, local political situations, local facilities, manpower available, and the support/ resources of the host — will also influence the selection process.
The modeling component is still to start, as data is gathered. Different results are anticipated from the empirical and process-based models. Because the empirical models are more easily calibrated, it is not uncommon that they provide better predictive results than process-based models. On the other hand, process-based models are much more powerful to produce more reliable estimates of results due to changes in the system characteristics. In short: empirical models can be better for using as a predictive tool. However, process-based models are superior when the understanding of system dynamics is needed; for example, in the assessment of any mitigation measures.
Viewpoint of IHA
Hydropower is one of the most important available sources of energy. It is renewable, free from the emissions associated with fossil-fueled power plants, such as acid gases, heavy metals, and particulates. Hydropower generation also reduces dependence on imported fuel and fuel-price escalations. Hydropower generation can be switched on and off according to needs, bringing opportunities to support the development of more intermittent renewables and also the means to make thermal power plants more efficient (further reducing emissions). Along with this, reservoirs provide water storage, supplying communities, agriculture, and industry, and help to protect against floods and droughts. It is therefore one of the most climate-friendly of the current energy options.
There are still many unanswered questions related to the real role of hydropower as a source of man-made GHG emissions. The IHA/UNESCO GHG Research Project aims to allow a better evaluation of the carbon footprint, improving the knowledge about the effect of hydropower on natural GHG emissions.
Joel Goldenfum, PhD, is the greenhouse gas project manager for the International Hydropower Association (IHA). IHA is a non-governmental association working to advance hydropower’s role in meeting the world’s water and energy needs.