Young water fractions at diverse time scales are driven by varying runoff generation processes in a Mediterranean small research catchment
Authors: Gallart Francesc , Llorens Pilar , Cayuela Carles , Sprenger Matthias , Latron Jérôme , Saurat Pauline 
Published: March 2020

&lt;p&gt;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 (&lt;em&gt;F&lt;sub&gt;yw&lt;/sub&gt;&lt;/em&gt;), defined as the proportion of water that is younger than 2-3 months. The study was conducted for the 0.56 Km&lt;sup&gt;2&lt;/sup&gt; sub-humid Can Vila catchment (Vallcebre Research Catchments). During a period of over 58 months, the isotope ratios (&lt;sup&gt;2&lt;/sup&gt;H and &lt;sup&gt;18&lt;/sup&gt;O) 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.&lt;/p&gt;&lt;p&gt;The early results of this research revealed intense dynamics of&lt;em&gt; F&lt;sub&gt;yw&lt;/sub&gt;&lt;/em&gt; in relationship with discharge: &lt;em&gt;F&lt;sub&gt;yw&lt;/sub&gt;&lt;/em&gt; 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 (&lt;em&gt;S&lt;sub&gt;d&lt;/sub&gt;&lt;/em&gt;) of &lt;em&gt;F&lt;sub&gt;yw&lt;/sub&gt;&lt;/em&gt; implied that even if the maximum sampled discharges were exceeded by only 0.01% of time, about 25% of the &lt;em&gt;F&lt;sub&gt;yw&lt;/sub&gt;&lt;/em&gt; 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.&lt;/p&gt;&lt;p&gt;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 &lt;em&gt;F&lt;sub&gt;yw&lt;/sub&gt;&lt;/em&gt; becomes more intricate. Indeed, the wetter year was associated with the largest &lt;em&gt;F&lt;sub&gt;yw&lt;/sub&gt;&lt;/em&gt; and &lt;em&gt;S&lt;sub&gt;d&lt;/sub&gt;&lt;/em&gt; 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.&lt;/p&gt;&lt;p&gt;Current research is comparing &lt;em&gt;F&lt;sub&gt;yw&lt;/sub&gt;&lt;/em&gt; 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 &lt;em&gt;F&lt;sub&gt;yw&lt;/sub&gt;&lt;/em&gt; in this catchment. Our work further advances the understanding of limitations and opportunities of the &lt;em&gt;F&lt;sub&gt;y&lt;/sub&gt;&lt;sub&gt;w&lt;/sub&gt;&lt;/em&gt; approach.&lt;/p&gt;