University of Twente Student Theses


A hillslope hydrology analysis of the landscape evolution observatory within Biosphere2

Heuvel, D.B. van den (2016) A hillslope hydrology analysis of the landscape evolution observatory within Biosphere2.

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Abstract:The Landscape Evolution Observatory (LEO) is a modern and multidisciplinary research project aiming to shed light on the role that coupled processes play in landscape evolution. These processes are generally attributed to the disciplines of (a) hydrology, (b) ecology, (c) geochemistry and (d) geomorphology. The LEO project comprises three identical artificial hillslopes upon which artificial precipitation may be cast in an endeavor to improve understanding of these coupled processes’ nature and relations. The hillslopes sit in a fully controlled environment, overcoming the barrier of unknown boundary conditions and internal structures that field experiments tend to encounter. Previous experiments at LEO have revealed that water transit times are significantly longer in the western slope than in the central slope. In turn, preliminary analysis of the results has led the scientists to believe that the central and east slopes’ responses are more resemblant. Considering that the hillslopes were designed to be fully identical in shape, technical equipment and soil characteristics, this difference should not exist. This research aims to identify what differences between the central and west slopes may have brought about the glaring discrepancy in the two slopes’ responses to rainfall-runoff experiments. A thorough analysis of data that was previously gathered shows that water transit times are indeed significantly longer in the west slope, because of its tendency to discharge water more slowly. Since the slopes’ geometry and technical equipment had previously been found to be similar, the research’s focal point was shifted to the soil characteristics. Examination of newly plotted soil water retention curves fitted to the Van Genuchten model reveals considerable variation in the fitting parameters α–1 and n. It was therefore decided to concentrate on differences among these two parameters and the soil’s saturated hydraulic conductivity, Ks, which partially depends on n. Through simulations of the central and west hillslope using already known east slope input parameters in the hydrological model CATchment HYdrology (CATHY), this thesis illustrates that the central and east slopes bear less resemblance than was previously assumed. In fact, this simulation shows that the west slope is more similar to the east slope than the central slope. Subsequently, the three parameters are calibrated for both slopes using CATHY as part of this research. To do so, about 1,200 simulations have been generated with CATHY under randomized values for α–1, n and Ks within certain ranges. These ranges were inspired by analysis of the various soil water retention curves which had been plotted. The twenty parameter combinations which yielded the best goodness-of-fit as compared with observed data are then compared in both cases. From these results the conclusion is drawn that mainly the parameter α–1 exhibits a sharp distinction between both hillslopes, indicating a difference in the depth of the slopes’ capillary fringe. The value of α–1 was found to be in the order of –0.2 m in the central slope and approximately –0.45 m in the west slope. The Van Genuchten model suggests that greater absolute values of α–1 enhance water retention characteristics, thus explaining why the west slope discharges water more slowly than its centrally located counterpart. While values of n and Ks did show slight variation between the two slopes, these differences are deemed too small to be accountable for the substantial discrepancy in responses as was observed. It was impossible to make conclusive claims as to why exactly the depth of the capillary fringe appears to differ in both hillslopes. However, the hypothesis that the west slope contains more fine soil particles than the central slope has been formulated, explaining why the west slope discharges more slowly despite its similar soil porosity. It is suggested that this hypothesis be tested in further research to deliver conclusive proof with regard to the found physical difference among both slopes.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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