What controls differences in Stream Water Evaporation Lines in a nested catchment system?

Catchment properties, together with the hydro-meteorological conditions, influence the transformation of the stable isotopic signal from precipitation to stream discharge and the fraction of isotopic fractionated water (by evaporation) in stream flow. Several studies showed that catchments often exhibit a lower slope of the regression line between δOxygen-18 (δ18O) and δDeuterium (δD) of stream water (Stream Water Evaporation Lines: SEL) compared to the Local Meteoric Water Line (LMWL). Here we hypothesize that the combination of land use, geology, and topography is controlling these differences. In this work we test this hypothesis relying on regression analysis in the nested river setup of the Attert catchment (250 km2), with 9 sub-catchment ranging from 0.45 km2 to 161 km2. We employed a three year time series of bi-weekly stable isotope samples of stream water at the catchment outlets. The different sub-catchments show remarkable different catchment characteristics in terms of geology and land use, while the hydro-meteorological forcing is rather uniform between the sub-catchments. Eventually, we found that an elevation effect strongly controlled the differences in mean δ18O and δD of stream flow between the catchments (-1.2‰ δD/100 m). Stream flow also showed remarkable evaporative enrichment, the slopes of the SEL ranges from 3.2 to 5.1. Sandstone showed the highest explanatory power in simple linear regression with R2=0.46. Increasing fractions of sandstone geology and forest cover generally lead to lower slopes of the SEL, while the extent of alluvial floodplain lead to slopes more similar to the LMWL. None of the multiple linear regression models showed higher explanatory power than 0.7 (adjusted R2) based on alluvial coverage and geological permeability. This contribution shows how different catchment properties influence the relationship δ18O and δD that is generally controlled by non-kinetic fractionation. Further work on the processes that lead to different slopes of the SEL is needed.