Altitude isotope effects in Mediterranean high-relief terrains: a correction method to utilize stream water data

Quantification of the isotope altitude effect from precipitation allows the identification of recharge altitudes in high-relief areas. However, steep terrains frequently limit the use of this precipitation-based approach. Perennial rivers often provide easy, year-round access for water sampling. However, in high-relief regions rivers may not adequately represent regional precipitation isotope patterns because they integrate isotope signals along their course. This prevents the calculation of an altitudinal isotope gradient from river water samples alone. We present a study that shows how a river-based sampling approach served to determine the altitude effect. Replacing the original sample site altitude with the mean upstream catchment elevation as calculated from a digital elevation model allowed us to establish a plausible altitude isotope effect from stream water. The correction approach was verified independently using precipitation isotope data. This approach can potentially be transferred to other regions where precipitation sampling at various elevations is difficult to determine.

对降水同位素高程效应（isotope altitude effect）的定量分析，可用于识别地形起伏显著区域的地下水补给高程。然而，陡峭的地形往往限制了这类基于降水的研究方法的应用。常年河流通常便于开展全年不间断的水样采集工作，但在地形起伏剧烈的区域，河流无法充分反映区域降水的同位素分布特征——其沿程会整合不同点位的同位素信号，导致仅通过河水样本无法计算同位素高程梯度。本研究展示了一种基于河流采样的方法如何用于测定同位素高程效应：通过利用数字高程模型（Digital Elevation Model）计算得到的上游集水区平均高程，替换原采样点的海拔高度，我们得以从河水样本中推导得到合理的同位素高程效应关系。该校正方法可通过独立获取的降水同位素数据集完成验证，且有望推广至其他难以开展多海拔梯度降水采样工作的区域。