Isotopic evidence for the composition of runoff in a watershed on the Loess Plateau of China

Abstract:

Determining the sources of runoff can provide important information for water resource management, especailly for regions subject to water scaricity. Isotope mass balance method was used to separate and quantify different runoff component in the Heihe watershed on the Loess Plateau of China. Event-based precipitation samples, monthly-based groundwater samples, and runoff samples for non-flood and flood season were collected to analyze the isotopic compositions. After investigating the feasibility of using mass balance method for hydrograph separation, the contributions of precipitation and groundwater to runoff were quantified for flood and non-flood season and for different reaches of the river. Results showed that the isotopic compositions of precipitation had obvious seasonal variation, and the values in the first half year (-5.75‰) were greater than those in the second half year (-11.10‰); however, the isotopic compositions of groundwater were relatively constant (-10.3±0.6‰). The characterisics of the isotopc compositions of precipitation and groundwater made the mass balance equation feasible for hydrograph separaton. The contributions of precipitation and groundwater to runoff had obvious spatiotmeporal variations. During June, the contributions were both about 50% for the upper reach while they were 20% and 80% for the lower reach, respectively. During August, the contributions were 67% and 33% for the upper reach while they were 43% and 57% for the lower reach, respectively. Runoff in the upper reach was mianly from precipitation, while that in the lower reach was mainly from groundwater. Different measures should be taken to manage the water resoureces of different reach.