Young water fractions at diverse time scales are driven by varying runoff generation processes in a Mediterranean small research catchment

<p>The time water resides within a catchment has important implications for the water availability and quality for both ecosystem and human use. Here, we look at the short-term water transport using the concept of young water fraction (<em>F_yw</em>), defined as the proportion of water that is younger than 2-3 months. The study was conducted for the 0.56 Km² sub-humid Can Vila catchment (Vallcebre Research Catchments). During a period of over 58 months, the isotope ratios (²H and ¹⁸O) of rainwater was sampled at 5-mm rainfall intervals and stream water was sampled at variable time intervals (30 minutes to 1 week) depending on flow.</p><p>The early results of this research revealed intense dynamics of<em> F_yw</em> in relationship with discharge: <em>F_yw</em> had values between 0 for low flows and around 1 for the highest flows. Yet, the high variability of discharge and flashy response behaviour in this catchment along with the relatively large discharge sensitivity (<em>S_d</em>) of <em>F_yw</em> implied that even if the maximum sampled discharges were exceeded by only 0.01% of time, about 25% of the <em>F_yw</em> associated to the highest flows were estimated to be missed by the stream water sampling. This behaviour may be associated with a response dominated by saturation runoff generation mechanisms during wet episodes, which are known to drive the main hydrological response of this catchment.</p><p>Nevertheless, these results are obtained when all the samples are lumped for the whole 58 month period, but when different 12-month windows are investigated, the behaviour of <em>F_yw</em> becomes more intricate. Indeed, the wetter year was associated with the largest <em>F_yw</em> and <em>S_d</em> values, but drier years had irregularly varying values poorly correlated to precipitation or runoff statistics. Thus, other runoff generation mechanisms previously identified, including Hortonian-type overland flow in small degraded areas, that lead to runoff of new (and hence young) waters for low to moderate flows, will play a special role.</p><p>Current research is comparing <em>F_yw</em> analyses for groups of events of the same class, supported by hydrograph separation analyses and hydrometric indicators, for better understanding the dynamic and complex response of <em>F_yw</em> in this catchment. Our work further advances the understanding of limitations and opportunities of the <em>F_y_w</em> approach.</p>