Accounting for seasonal isotopic patterns of forest canopy intercepted precipitation in streamflow modeling

Forest canopy interception alters the isotopic tracer signal of precipitation leading to significant isotopic differences between open precipitation (delta OP) and throughfall (delta TF). This has important consequences for the tracer-based modeling of streamwater transit times. Some studies have suggested using a simple static correction to delta OP by uniformly increasing it because delta TF is rarely available for hydrological modeling. Here, we used data from a 38.5 ha spruce forested headwater catchment where three years of delta OP and delta TF were available to develop a data driven method that accounts for canopy effects on delta OP. Changes in isotopic composition, defined as the difference delta TF-delta OP, varied seasonally with higher values during winter and lower values during summer. We used this pattern to derive a corrected delta OP time series and analyzed the impact of using (1) delta OP, (2) reference throughfall data (delta TFref) and (3) the corrected delta OP time series (delta OPsine) in estimating the fraction of young water (F-yw), i.e., the percentage of streamflow younger than two to three months. We found that F-yw derived from delta OPsine came closer to delta TFref in comparison to delta OP. Thus, a seasonally-varying correction for delta OP can be successfully used to infer delta TF where it is not available and is superior to the method of using a fixed correction factor. Seasonal isotopic enrichment patterns should be accounted for when estimating F-yw and more generally in catchment hydrology studies using other tracer methods to reduce uncertainty. (C) 2017 The Authors. Published by Elsevier B.V.