Development of a new methodology for the creation of water temperature scenarios using frequency analysis tool.

Thermal quality in river ecosystems is a fundamental property for the development of biological processes and many of the human activities linked to the aquatic environment. In the future, this property is going to be threatened due to global change impacts, and basin managers will need useful tools to evaluate these impacts. Currently, future projections in temperature modelling are based on the historical data for air and water temperatures, and the relationship with past temperature scenarios; however, this represents a problem when evaluating future scenarios with new thermal impacts. Here, we analysed the thermal impacts produced by several human activities, and linked them with the decoupling degree of the thermal transfer mechanism from natural systems measured with frequency analysis tools (wavelet coherence). Once this relationship has been established we develop a new methodology for simulating different thermal impacts scenarios in order to project them into future. Finally, we validate this methodology using a site that changed its thermal quality during the studied period due to human impacts. Results showed a high correlation (r(2)=0.84) between the decoupling degree of the thermal transfer mechanisms and the quantified human impacts, obtaining 3 thermal impact scenarios. Furthermore, the graphic representation of these thermal scenarios with its wavelet coherence spectrums showed the impacts of an extreme drought period and the agricultural management. The inter-conversion between the scenarios gave high morphological similarities in the obtained wavelet coherence spectrums, and the validation process clearly showed high efficiency of the developed model against old methodologies when comparing with Nash-Stucliffe criterion. Although there is need for further investigation with different climatic and anthropic management conditions, the developed frequency models could be useful in decision-making processes by managers when faced with future global change impacts.