Scaling properties of rainfall-runoff generation processes and nutrient flushing mechanisms in the Oregon Cascade Mountain

Abstract:

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 of hydrometric information, water stable isotopes (δ¹⁸O), and concentrations of total nitrogen (TN), ammonia (NH₃), and nitrate (NO₃) within 4 sites in a nested framework at the HJ Andrews Experimental Forest (HJA), OR. Preliminary analysis of 2 storms (54mm and 145mm) indicate highly variable responses across space along with a positive relation between transit time of event water and storm magnitude in all catchments. In addition there appears to be a moisture threshold after which transit time scales with drainage area across the landscape likely related to higher degree of connectivity. We also found a strong correlation between transit times computed based on temporal variability of δ¹⁸O and electrical connectivity (EC). This lead to the analysis of over 50 storm across 10 catchments in the HJA during the last 3 years. In-stream NO₃^- during storm response are highest within the smaller catchments (1-5 km²) and tend to remain elevated throughout the response period. The larger catchments (15-64 km²) demonstrate smaller increases in NO₃^-, the response time lags behind that of the smaller catchments, and the concentration returns rapidly to baseflow conditions rather than remaining elevated. In contrast, in-stream NH₃ show a higher degree of similarity between sites in terms of magnitude and timing of increases in concentration over the duration of the response period. Ultimately we found that fractions of inorganic nitrogen correlate with transit time and drainage area, opening the possibility of a catchment wide model of nutrient export prediction.