Technical note: An improved discharge sensitivity metric for young water fractions

• Main Authors: F. Gallart, J. von Freyberg, M. Valiente, J. W. Kirchner, P. Llorens, J. Latron
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-03-01
• Series: Hydrology and Earth System Sciences
• Online Access: https://www.hydrol-earth-syst-sci.net/24/1101/2020/hess-24-1101-2020.pdf

<p>Recent virtual and experimental investigations have shown that the young water fraction <span class="inline-formula"><i>F</i>_yw</span> (i.e. the proportion of catchment outflow younger than circa 2–3 months) increases with discharge in most catchments. The discharge sensitivity of <span class="inline-formula"><i>F</i>_yw</span> has been defined as the rate of increase in <span class="inline-formula"><i>F</i>_yw</span> with increasing discharge (<span class="inline-formula"><i>Q</i></span>) and has been estimated by the linear regression slope between <span class="inline-formula"><i>F</i>_yw</span> and <span class="inline-formula"><i>Q</i></span>, hereafter called DS(<span class="inline-formula"><i>Q</i></span>). The combined use of both metrics, <span class="inline-formula"><i>F</i>_yw</span> and DS(<span class="inline-formula"><i>Q</i></span>), provides a promising method for catchment inter-comparison studies that seek to understand streamflow generation processes. Here we explore the discharge sensitivity of <span class="inline-formula"><i>F</i>_yw</span> in the intensively sampled small Mediterranean research catchment Can Vila. Intensive sampling of high flows at Can Vila allows young water fractions to be estimated for the far upper tail of the flow frequency distribution. These young water fractions converge toward 1 at the highest flows, illustrating a conceptual limitation in the linear regression method for estimating DS(<span class="inline-formula"><i>Q</i></span>) as a metric of discharge sensitivity: <span class="inline-formula"><i>F</i>_yw</span> cannot grow with discharge indefinitely, since the fraction of young water in discharge can never be larger than 1. Here we propose to quantify discharge sensitivity by the parameter of an exponential-type equation that expresses how <span class="inline-formula"><i>F</i>_yw</span> varies with discharge. The exponential parameter (<span class="inline-formula"><i>S</i>_d</span>) approximates DS(<span class="inline-formula"><i>Q</i></span>) at moderate discharges where <span class="inline-formula"><i>F</i>_yw</span> is well below 1; however, the exponential equation and its discharge sensitivity metric better capture the non-linear relationship between <span class="inline-formula"><i>F</i>_yw</span> and <span class="inline-formula"><i>Q</i></span> and are robust with respect to changes in the range of sampled discharges, allowing comparisons between catchments with strongly contrasting flow regimes.</p>