Identification of geographic runoff sources in a data sparse region: hydrological processes and the limitations of tracer-based approaches

There exists a growing need to improve our understanding of catchment processes on larger scales. This need is especially enhanced in rapidly developing countries such as China where the turnover of traditionally used land to urbanized and industrialized areas influences water resources. In this study, we chose a multi-tracer approach in a large, ungauged basin in Inner Mongolia to elucidate the processes that generate stream flow. We applied end member mixing analysis (EMMA) to identify and quantify the major runoff generating sources in a three end member system. Stream water and a set of eight possible end members were sampled throughout three consecutive vegetation periods. Samples were analysed with an inductively coupled plasma-mass spectrometry (ICP-MS) and an ion chromatograph (IC) for a suite of 33 solutes including 24 cations and 7 anions. We determined that seven tracers exhibited conservative behaviour, they were Li, Rb, Sr, Na, Mg, Cl and electrical conductivity (EC). Stable water isotopes (delta D and delta O-18) highlight differences between the end members. Our results indicate strong interannual variability of end member composition and contribution between the 3 years. We were able to identify shallow groundwater aquifers as important runoff generating sources in some years and deeper groundwater aquifers in other years which vary in rainfall and discharge. A shallow sand dune aquifer which covers a significant part of the area plays an important role in storing and contributing water to the river. Our results also suggest that the major runoff generating source in the dryer year has not been identified yet. Our results prompt us to focus future work on understanding interannual changes in end member contribution especially in semi-arid regions. Copyright (C) 2010 John Wiley & Sons, Ltd.