Scaling properties of rainfall-runoff generation processes and nutrient flushing mechanisms in the Oregon Cascade Mountains. Part 1: Hydrologic modelling incorporating electrical conductivity and water stable isotopes

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Even though it is widely recognized that water quality and availability are crucial to society and wildlife sustainability, we are still not able to predict how much water is moved through a given catchment after a storm event nor what nutrients, solutes, and contaminates are mobilized. We will present preliminary results of a study incorporating hydrometric information, measurements of water stable isotopes (δ¹⁸O), and electrical conductivity (EC) as natural tracers over different seasons in 10 nested catchments (0.1-64 km²) in the H.J. Andrews Experimental Forest. From the analysis of 2 storms (54mm and 145mm in magnitude) we have found that storm response is highly variable across space and time. However the transit time of event water increases with storm magnitude in all catchments. In addition there appears to be a moisture threshold after which the transit time scales with drainage area (higher degree of connectivity) across the landscape. This highlighted the fact that drainage area alone cannot anticipate storm response and the need to include other landscape variables to describe storage potential and degree of hydrologic connectivity.   Finally we found a strong correlation between the assessment of transit times and event water fractions using EC and δ¹⁸O. This opened the possibility of going back in time to model rainfall runoff response based on EC observations for storms over the last 3 years.