Gross CO2 and CH4 emissions (degassing and diffusion from the reservoir) and the carbon balance were assessed in 2009-2010 in two Southeast Asian sub-tropical reservoirs: the Nam Ngum and Nam Leuk Reservoirs (Lao PDR). These two reservoirs are within the same climatic area but differ mainly in age, size, residence time and initial biomass stock. The Nam Leuk Reservoir was impounded in 1999 after partial vegetation clearance and burning. However, GHG emissions are still significant 10 years after impoundment. CH4 diffusive flux ranged from 0.8 (January 2010) to 11.9 mmol m(-2) d(-1) (April 2009) and CO2 diffusive flux ranged from -10.6 (October 2009) to 38.2 mmol m(-2) d(-1) (April 2009). These values are comparable to other tropical reservoirs. Moreover, degassing fluxes at the outlet of the powerhouse downstream of the turbines were very low. The tentative annual carbon balance calculation indicates that this reservoir was a carbon source with an annual carbon export (atmosphere+downstream river) of about 2.2±1.0 GgC yr(-1). The Nam Ngum Reservoir was impounded in 1971 without any significant biomass removal. Diffusive and degassing CO2 and CH4 fluxes were lower than for other tropical reservoirs. Particularly, CO2 diffusive fluxes were always negative with values ranging from -21.2 (April 2009) to -2.7 mmol m(-2) d(-1) (January 2010). CH4 diffusive flux ranged from 0.1 (October 2009) to 0.6 mmol m(-2) d(-1) (April 2009) and no degassing downstream of the turbines was measured. As a consequence of these low values, the reservoir was a carbon sink with an estimated annual uptake of - 53±35 GgC yr(-1).

Climate change and the needed reductions in the use of fossil fuels call for the development of renewable energy sources. However, renewable energy production, such as hydropower (both small- and large-scale) and wind power have adverse impacts on the local environment by causing reductions in biodiversity and loss of habitats and species. This paper compares the environmental impacts of many small-scale hydropower plants with a few large-scale hydropower projects and one wind power farm, based on the same set of environmental parameters; land occupation, reduction in wilderness areas (INON), visibility and impacts on red-listed species. Our basis for comparison was similar energy volumes produced, without considering the quality of the energy services provided. The results show that small-scale hydropower performs less favourably in all parameters except land occupation. The land occupation of large hydropower and wind power is in the range of 45-50 m(2)/MWh, which is more than two times larger than the small-scale hydropower, where the large land occupation for large hydropower is explained by the extent of the reservoirs. On all the three other parameters small-scale hydropower performs more than two times worse than both large hydropower and wind power. Wind power compares similarly to large-scale hydropower regarding land occupation, much better on the reduction in INON areas, and in the same range regarding red-listed species. Our results demonstrate that the selected four parameters provide a basis for further development of a fair and consistent comparison of impacts between the analysed renewable technologies.

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The river Surna has been regulated for hydropower production since the late 1960s, with a minimum flow requirement of ca. 30% of the mean annual flow downstream of the power plant outlet, leaving a large by-pass section with residual flow. This river has recently been selected as a national salmon watercourse by the Norwegian Parliament. Mitigations to protect the Atlantic salmon stock will be given priority in these rivers, and hence an environmental flow assessment (EFA) has been started to optimize hydropower production in relation to salmon habitat. A suite of methodologies for EFA like habitat-hydraulic models, indicators of hydrological alteration (IHA), mesohabitat analysis, temperature simulations and optimalization of habitat improvements, has been applied across scales and seasons in this catchment. The EFA has been related to various hydropower schemes, to suggest more salmon-friendly hydropower operation. A temperature-adjusted running of the hydropower plant was found to have a major influence on the production of salmonids. Downstream of the hydropower station, rapid ramping has often occurred. This is possibly harmful for juvenile fish inhabiting the shallow parts of the river due to stranding. Based on our habitat modelling toolkit, we have suggested ramping guidelines to drastically reduce stranding of juvenile salmonids. Our EFA has indicated a great potential with mitigations to increase salmon production in the river Surna. For maximum gain, combinations of more gentle flow ramping, habitat improvements and/or environmental flow requirement (EFR) in the by-pass section and temperature alteration by new intake arrangements and/or temperature-adjusted run of the power plant are needed.

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