Large scale application of an environmental tracer approach : spatio-temporal patterns of hydrochemistry in a semi-arid grassland

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Barthold_Frauke_2010_07_09.pdf (3.86 MB)

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2010

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DOI:
http://dx.doi.org/10.22029/jlupub-17018

Abstract

One of the grand challenges in large, ungauged catchments is to quickly and at low costs gain insight into the catchment relevant hydrologic processes. These are necessary in land use change oriented hydrological modeling to evaluate if the model of interest captures all relevant hydrological and biogeochemical processes. This is particularly crucial in rangelands which are situated in semi-arid or arid regions of the temperate zone and where water availability is already limited by definition. The research presented in this dissertation makes use of several methodologies to improve catchment understanding in a large, ungauged basin in the grasslands of Inner Mongolia. In a first attempt to assess the main runoff contributing sources of the study area, i.e. the Xilin river catchment, a top-down approach from first field reconnaissance and data collection over perceptual model development, reservoir model conceptualization, evaluation with hydrochemical data to completion of the field campaign was used. Based on hydrochemical data, the headwater source, a tributary and a shallow groundwater well were identified as important sources for the river. A simple conceptual reservoir model, that we were able to develop with this information captured the general trend of the stream hydrograph but showed a general overestimation throughout the growing season. Evaluation of the model through simulation of the geochemistry led to the rejection of the model structure and raised further questions that were able to guide the next steps in improving catchment understanding. More advanced geochemical analyses, i.e. end member mixing analysis (EMMA), were then applied onto a larger data set to identify and quantify the runoff contributing sources. Isotopic signatures of end member waters were used as an additional tool to characterize isolated water storages. Our findings indicate that the runoff can be explained by three interannually changing end members. In wetter years, the runoff is mainly produced by shallow groundwater sources. Dryer years exhibit a dominance of deeper groundwater aquifers as sources of stream flow. Rain is in all years only of minor importance. However, the 3-year measurement programme lacks the documentation of the full spectrum of hydrologic conditions, e.g. very wet years and very dry years and hence prompts the continued development of long-term measurement programme. This dissertation also addressed the problem of which and how many tracers to include into an EMMA and aimed at assessing the uncertainty of the model structure that results from an EMMA. Our findings indicate that the resulting model concept from an EMMA is highly sensitive to the tracers set size and composition in terms of system dimensionality (end members needed to explain flow in a system), end member combination and contribution to stream flow. A certain consent exists concerning the dimensionality of a system with all remaining tracer sets exhibiting a dimensionality of 3 which is also in line with the results in chapter 3. However, findings were ambiguous in terms of identified end members: for 2006, the most often selected end members were a shallow and a deep groundwater aquifer as well as either the headwater source area or a tributary. The large differences in end member contributions that were produced among the different tracer sets demonstrated that we are still missing at least one important end member. They also indicate that the major elements that are preferably used in many other studies are not always the most useful tracers.The top down approach and the environmental tracer approach proofed to be very valuable in identification of control variables on flow which is one step forward in improving catchment understanding in the Xilin river basin. The automatic EMMA procedure proofed that larger tracer sets that include also minor trace elements and that are employed in an iterative EMMA approach with varying tracer set sizes and compositions have a high potential to avoid false conclusions about catchment functioning. The proposed approach is highly complementary to the traditional EMMA approach.

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