Mind the gap: modelling event-based and millennial-scale landscape dynamics

This research looks at landscape dynamics – erosion and deposition – from two different perspectives: long-term landscape evolution over millennial timescales on the one hand and short-term event-based erosion and deposition at the other hand. For the first, landscape evolution models (LEMs) are often used, which describe landscape forming processes by geomorphic transport laws, usually on annual temporal resolutions. LEM LAPSUS is used in this research to evaluate the landscape dynamics in a study area in south-east Spain: the Guadalentín Basin. The model is calibrated on dated river terrace levels, which show an erosion – deposition – erosion sequence that the model could reproduce. Annual precipitation in this dryland area shows large inter-annual variability and erosion is supposed to be mainly the results of low-frequency, high magnitude rainfall events. Therefore, in this research, landscape dynamics are also assessed using the event-based erosion model OpenLISEM. Eventually, the role of extreme events in long-term landscape evolution are explored by comparing the two models and by incorporating annual rainfall variability into LEM LAPSUS. Another issue that is being addressed in this study is the relative influence of humans as compared to erosion as a natural process. A conceptual model, derived on the basis of dated sediment archives, is tentatively correlated to periods of human impact on the land. Using LAPSUS, the potential influence of historical tillage erosion is simulated, showing that the relatively slow process of tillage erosion added to floodplain aggradation over thousands of years.</p

A framework approach for unravelling the impact of multiple factors influencing flooding

To have a better understanding of the influence of topographic, climatic, and, especially, anthropogenic factors on hydrological discharge and flooding, this study proposes a new framework approach using a set of methods to answer the questions why, where, when, and how flooding occurs. Including conditional inference tree (CIT), cross-correlation, and double-mass curves analysis, the approach is demonstrated in an application to the Wei River Basin, China. From the CIT analysis, dam construction period was identified as the most important factor (why), and the sub-catchment farthest upstream contributed the most to the flooding of the downstream floodplain (where). We then analysed the effect of the periods of dam construction on the time lag change (when) and the precipitation-discharge relationship (how) using cross-correlation analysis and double-mass curves analysis, respectively. The results suggested that the dam construction delayed the precipitation for 0.4days on average compared to before the dam construction period, and the discharge at the outlet of the basin was reduced by 44%. This framework approach is promising as it can quantitatively evaluate the importance of multiple factors on multiple years of flooding, while many studies evaluate single flooding events.</p

Mind the gap! Het verschil in tijdschaal tussen extreme regenbuien en landschapsevolutie

Erosie en depositie worden in hoge mate bepaald door enkele (heftige) regenbuien, zeker in Mediterrane semi-aride gebieden zoals Spanje. In landschapsevolutie spelen dezelfde processen een belangrijke rol, maar kijken we naar de effecten van die processen over duizenden jaren, bijvoorbeeld de vorming van rivier terrassen. Er zit een groot gat in tijdschaal tussen de regenbuien enerzijds (uren-dagen) en landschapsevolutie anderzijds. Hebben enkele (heftige) regenbuien nog invloed op landschapsevolutie en zo ja, op welke manier? Of is het cumulatieve effect van het grotere aantal kleinere buien uiteindelijk meer bepalend voor hoe het landschap zich ontwikkeld? De afgelopen 5 jaar heb ik me tijdens promotie-onderzoek bezig gehouden met dit thema. Het resultaat, mijn proefschrift, heb ik afgelopen 9 mei succesvol verdedigd aan de Wageningen Universiteit. Een overzicht van mijn onderzoek en de belangrijkste resultaten: 5 jaar in 3 pagina's

Morfología Fluvial.-Controls on Late Quaternary Landscape development in the Upper Guadalentin Basin, Murcia, SE Spain

ABSTRACT Landscapes in South-eastern Spain have developed in response to tectonics, climatic fluctuations and, more recently, to human action. In the valleys, fluvial and colluvial sediments are found in the form of river terraces, pediments and slope deposits. We studied these sediments to evaluate landscape dynamics and the processes of sedimentation and erosion in the semi-arid region of the Guadalentín Basin, Murcia Province, SE Spain. The objective of the study is to deduce controls on Late Quaternary and Holocene landscape development. Fieldwork was carried out on the reach of the Upper Guadalentin, upstream of the city of Lorca, and two of its tributaries (Rio Velez and Rambla de Torrealvilla). River terrace levels were mapped using GPS and presence of gravel layers in outcrops. For the Rambla de Torrealvilla, more detailed sediment descriptions show their build-up. Charcoal was found and dated (14C) on ten locations, while 15 samples for Optically Stimulated Luminescence (OSL) dating were taken to have an age-control on processes of sedimentation and erosion. While the OSL dating measurements and analysis are still in process at this time, some preliminary results are given. Several terrace levels are identified along the Rio Guadalentin and the Rio Velez. These have formed in response to regional and/or global changes in climate. However, local mechanisms have played an important role in the area, overruling regional dynamics. From finely layered sediments, it is deduced that a lake existed during some time in the area, caused by a blockage of the valley. This lake was filled in with sediments and a delta was build at its end, interfering with terrace levels. When the blockage was broken through, lake sediments have been removed and after incision, younger terraces have developed. The Torrealvilla tributary is draining the Lorca Basin, and sediments seem to be younger than those along the Guadalentin and Velez rivers. Infilling of the basin and incomplete removal has shaped this valley. Outcrops in the gully sidewall show stacked layers of large to fine gravels and fine sediments with smaller gravel layers in between. We will present a conceptual model of landscape development since the Late Quaternary based on the age control (14C and OSL) and field observations. This reconstruction is correlated to climatic fluctuations and rates of sedimentation and erosion are approximated on a millennial timescale. Following this approach of longer-term geomorphological investigation of landscape development, we can ultimately put the relative contribution of human actions in the context of natural erosion and sedimentation processes

Can uncertain landscape evolution models discriminate between landscape responses to stable and changing future climate? A millennial-scale test

In the light of increasing societal interest in the effects of climate change, geomorphologists face the task of discriminating between natural landscape changes and landscape changes that result from human-induced climate change. Landscape Evolution Models (LEMs) are available for this purpose, but their application for prediction of future landscapes is problematic. Calibration of LEMs on a sufficiently long palaeo-record of landscape change solves some of these problems, but large uncertainties in input (e.g. climate) records and process descriptions remain. Using one of the few existing ka-scale LEM studies as starting point, this paper explores how uncertainty in the LEM LAPSUS (LandscApe ProcesS modelling at mUlti dimensions and scaleS, [Schoorl, J.M., Veldkamp, A. and Bouma, J., 2002. Modeling water and soil redistribution in a dynamic landscape context. Soil Science Society of America Journal, 66(5): 1610–1619]) affects its ability to discriminate future one-thousand year landscape change under stable climate from that under human-induced changed climate. Okhombe Valley in South Africa is used as a case study area. LEM uncertainty is characterized by different levels of parameter uncertainty. Results indicate that even under high levels of parameter uncertainty, LEM LAPSUS discriminates between responses to stable and changed climates for some zones in the landscape. Although confidence in model predictions remains limited, some explorative and relative conclusions about the effects of changed climate on future landscape evolution of Okhombe Valley are drawn. Finally, some possibilities and limitations of future studies on landscape evolution under changing climate are discusse

The effect of landform variation on vegetation patterning and related sediment dynamics

Semi‐arid ecosystems are often spatially self‐organized in typical patterns of vegetation bands with high plant cover interspersed with bare soil areas, also known as ‘tiger bush’. In modelling studies, most often, straight planar slopes were used to analyse vegetation patterning. The effect of slope steepness has been investigated widely, and some studies investigated the effects of microtopography and hillslope orientation. However, at the larger catchment scale, the overall form of the landscape may affect vegetation patterning and these more complex landscapes are much more prevalent than straight slopes. Hence, our objective was to determine the effect of landform variation on vegetation patterning and sediment dynamics. We linked two well‐established models that simulate (a) plant growth, death and dispersal of vegetation, and (b) erosion and sedimentation dynamics. The model was tested on a straight planar hillslope and then applied to (i) a set of simple synthetic topographies with varying curvature and (ii) three more complex, real‐world landscapes of distinct morphology. Results show banded vegetation patterning on all synthetic topographies, always perpendicular to the slope gradient. Interestingly, we also found that movement of bands – a debated phenomenon – seems to be dependent on curvature. Vegetation banding was simulated on the slopes of the alluvial fan and along the valley slopes of the dissected and rolling landscapes. In all landscapes, local valleys developed a full vegetation cover induced by water concentration, which is consistent with observations worldwide. Finally, banded vegetation patterns were found to reduce erosion significantly as compared to other vegetation configurations