Lambs L, Loubiat M, Richardson W
Federation of Research in Ecology, Laboratory of Biodiversity and Dynamics--CNRS, 29 Rue Jeanne Marvig, Toulouse, 31055, France.
Isotopes Environ Health Stud. 2003 Dec;39(4):301-10. doi: 10.1080/10256010310001621100.
Before the confluence of the Tarn, the Garonne valley was the driest area in the entire south-west of France, due to the relatively low rainfall and low summer discharge of the Garonne River and its tributaries. The natural abundance of the stable isotope of oxygen (18O) and ionic charge of surface and ground water were used to estimate the water source for the Garonne River and phreatic subsurface water. We also measured these constituents in the sap of trees at several flood plain sites to better understand the source of water used by these trees. 18O signatures and conductivity in the Garonne River indicated that the predominance of water was from high altitude surface runoff from the Pyrenees Mountains. Tributary inputs had little effect on isotopic identity, but had a small effect on the conductivity. The isotopic signature and ionic conductivity of river water (delta18O: -9.1 per thousand to -9.0 per thousand, conductivity: 217-410 microS/cm) was distinctly different from groundwater (delta18O: -7.1 per thousand to -6.6 per thousand, conductivity: 600-900 microS/cm). Isotopic signatures from the sap of trees on the flood plain showed that the water source was shallow subsurface water (<30 cm), whereas trees further from the river relied on deeper ground water (>1 m). Trees at both locations maintained sap with ionic charges much greater (2.3-3.7x) than that of source water. The combined use of 18O signatures and ionic conductivity appears to be a potent tool to determine water sources on geographic scales, and source and use patterns by trees at the local forest scale. These analyses also show promise for better understanding of the effects of anthropogenic land-use and water-use changes on flood plain forest dynamics.
在塔恩河汇流之前，加龙河谷是法国整个西南部最干旱的地区，这是由于加龙河及其支流的降雨量相对较低以及夏季流量较小。利用氧的稳定同位素（18O）的自然丰度以及地表水和地下水的离子电荷来估算加龙河和潜水层地下水的水源。我们还在几个洪泛平原地点测量了树木汁液中的这些成分，以便更好地了解这些树木所使用的水源。加龙河中的18O特征和电导率表明，主要水源是来自比利牛斯山脉的高海拔地表径流。支流的汇入对同位素特征影响不大，但对电导率有轻微影响。河水的同位素特征和离子电导率（δ18O：-9.1‰至-9.0‰，电导率：217 - 410 μS/cm）与地下水（δ18O：-7.1‰至-6.6‰，电导率：600 - 900 μS/cm）明显不同。洪泛平原上树木汁液的同位素特征表明，水源是浅层地下水（<30厘米），而离河流较远的树木则依赖更深层的地下水（>1米）。两个地点的树木所保持的汁液离子电荷都比水源的离子电荷大得多（2.3 - 3.7倍）。结合使用18O特征和离子电导率似乎是一种有效的工具，可用于确定地理尺度上的水源以及当地森林尺度上树木的水源和使用模式。这些分析也有望更好地理解人为土地利用和用水变化对洪泛平原森林动态的影响。
