PW06 Life-Cycle Assessment
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(PW064) Up- and downstream water quantity conflicts in land use impact assessments.

Heuvelmans, G1, Garcia, J1, Muys, B1, Feyen, J1, 1 KULeuven, Department of Land Management

ABSTRACT- Exergy, i.e. useful energy or energy able to do work, has been proposed as a framework for quantifying the impact of land use practices on ecosystem functioning. It is in this context assumed that land use systems with a higher exergy content cause a relatively lower impact. The exergy level of a land use system depends on the biomass, the complexity of food webs, the genetic diversity and the buffering capacity. The evapotranspiration rate of a land use system is often used as a measure of its exergy because evapotranspiration is related to hydrological buffer capacity and biomass production. So the impact of a land use system is considered inversely proportional to the amount of water use. However, high evapotranspiration rates can lower stream flow volumes endangering the functioning of downstream aquatic ecosystems. So trade-offs have to be made between upstream impacts, handled in the land use impact category, and downstream impacts. It is proposed that an extension of the exergy concept from terrestrial ecosystems to the joint terrestrial-aquatic environment could assist in the balancing of up- and downstream impacts. To this end, the monthly stream flow volumes with an exceedance probability of 95% (low flows) and 50% (average flows) can be used as indicators of the exergy level downstream. The natural climax vegetation and the associated stream flow regime are considered as the state with the highest possible total exergy content and thus the lowest possible environmental impact. Some land use types e.g. plantation forestry might possess a higher upstream exergy level (higher evapotranspiration and biomass production), but only at the expense of the exergy level of the downstream ecosystem (lower stream flow volume).

Key words: exergy, LCA, water balance, land use

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