During initial phases of landform evolution, the surface of sediment bodies is commonly not yet equilibrated with the environmental conditions and is thus liable to rapid changes by accelerated geomorphic activity. Hydro-geomorphic processes acting during these early development phases often lead to the emergence of surface structures that constitute the boundary conditions for further ecosystem development, and of linear erosion features that act as pathways for water, solute and sediment transport, and thus can increase the system's connectivity. Studies on initial landform evolution are often limited to relatively small-scale experiments or rely on modeling exercises. Larger-scale experiments can be carried out in constructed hydrological catchments with well-known and documented initial conditions. The central aim of this work was to describe the hydro-geomorphic surface structure evolution during the initial years of ecosystem development, its dependence on initial and boundary conditions; and on its relations to hydro-geomorphic connectivity. The data are from the 6-ha, constructed hydrological catchment 'Hühnerwasser'. Hydro-geomorphic development was quantified using a combined analysis of digital elevation models and aerial photographs. Empirical results were compared with those obtained with a numerical landscape evolution model. Phases of rill network growth, contraction and stabilization could be characterized. Results show influences of initial morphology, precipitation characteristics, and developing structure-process-interactions on rill network geometry and connectivity increase in the catchment. The results suggest that phases of hydro-geomorphic surface structure evolution can be related to the spatial organization of surface flow patterns during initial phases of landform development.
During initial phases of landform evolution, the surface of sediment bodies is commonly not yet equilibrated with the environmental conditions and is thus liable to rapid changes by accelerated geomorphic activity. Geomorphic processes acting during these early development phases depend on the initial sediment composition and the subsurface and surface structures of the geosystem; at the same time, these processes lead to landscape structures that constitute boundary conditions for further ecosystem development.
For systems such as soils or watersheds, the initial distribution of sediment mass and composition at the starting point of the development are mostly not well-known. The development of such systems, however, has frequently been assumed to depend on boundary and initial conditions including the solid phase. Here, an experimental watershed of about 6 ha surface area was artificially-created in order to imitate an initial stage just at the beginning of the geo-ecosystem development. However, sediment homogeneity could not be achieved for this large-scale field experiment, and the 3D spatial distribution of structures and components remained uncertain. The objective of this study was to quantify and reconstruct the initial sediment distribution and the first development stages using aerial photographs, point information, and a 3D model. The watershed was made of coarse-textured sediments of quaternary origin with a low permeable clay liner as the bottom boundary. A 2D horizontal digital elevation model (DEM) of the surface and a DEM of the subsurface clay layer are used to construct a 3D triangulated numerical grid of the catchment’s initial spatial structure using the 3D-GIS software GOCAD. Physical and chemical soil properties obtained from borehole samples are assigned to this model and interpolated onto the 3D grid. The volumetric changes in space and time are quantified and related to material properties to obtain the mass changes. Correlations between terrain attributes, sediment properties, and mass changes are explored specifically in regions of differing source materials. A temporal sequence of surface DEMs is obtained from photogrammetric, high-precision ground based laser scanning and airborne laser scanning data. 3D models of elevation change are constructed from these DEMs. The processes following the initial state are mainly characterized by runoff-induced erosive mass relocations related to the distribution of surface and subsurface structures and sediment properties. The results suggest that both sediment structures and mass translocations differ between the western and eastern parts of the watershed and that correlations between initial surface structures and subsequent mass changes exist.
The structure of a hydrological catchment is determined by the geometry of the boundaries and the spatial distribution
of soil and sediment properties. Models of the 3D subsurface structure and the soil heterogeneity have
often been built based on geostatistical approaches and conditional simulations for spatial interpolation between
measurements. Here, an alternative model was proposed that generated 3D subsurface structures by imitating basic
structures resulting from mass distribution processes. Instead of directly assuming stochastic variations of the subsurface structure, the present approach assumed stochastic variations in parameters of the process-based algorithms of the generator models. The constructed hydrological catchment “Hühnerwasser” located in the Lower Lusatia region of Brandenburg, Germany, was used as an example for the development of such a 3D structure generator model. Boundary geometries and changes in the surface topography due to erosion and sedimentation processes were quantified on the basis of digital elevation models (DEMs) derived from aerial photographs and terrestrial laser scanning information. Basic sediment properties came i) from a geological model of the parent material at the outcrop site, ii) from actual soil
sample measurements on-site, and iii) based on stochastic texture variations. Sediment distributions were generated
according to construction processes such as sediment dumping, particle segregation, and soil compaction.
The resulting internal structures reflect the formation of spoil cones and surface compaction by machinery. The
simulated 3D model scenarios of soil texture and bulk density distributions were incorporated in a gridded 3D
volume model using the 3D software tool GoCAD (Paradigm Ltd.). This 3D distributed solid phase structure of
the catchment allowed for a more direct comparison with observations using minimal invasive methods. By including
structural changes over time (e.g., derived from DEM’s), spatially-distributed solid mass balances can be
inferred based on the comparison of 3D structure models for subsequent time steps, which is a basic information
for studying soil development. The 3D structure model as spatial database of the solid phase allows an integrative
analysis, a periodical mass balance, and a mechanistic process-based generation of complex realizations of
distributed hydraulic model parameters for the exemplary catchment.
In landscapes with heterogeneous vegetation structure, interception and throughfall patterns produce spatiotemporal
variability of soil moisture. This variability is important for eco-hydrological processes, in particular on
small spatial scales up to the catchment scale. Throughfall depends on vegetation structure, whereas vegetation
development is presumably co-determined by the spatio-temporal distribution of throughfall itself. In addition
to vegetation structure, meteorological factors like wind speed and rainfall intensity also have an impact on
throughfall.
The objective of this study is to quantify the influence of vegetation structure and meteorological variables
on spatial (and in the long run the temporal) variability of throughfall. For that purpose, we developed an
approach combining field methods, image analysis and multivariate statistics. The 6-ha constructed catchment
‚Hühnerwasser‘ (aka Chicken Creek, southern Brandenburg, Germany) offers ideal conditions for the investigation
of eco-hydrological feedback processes. After more than 10 years of development, vegetation structure on the
catchment is spatially heterogeneous and evolves through natural succession. Furthermore, complementary
meteorological data are available on-site.
Throughfall was measured using 50 tipping-bucket rain gauges, which are aligned along two transects in 0.5
and 1 m heights, covering the dominating vegetation types on the catchment (e.g., robinia, sallow thorn, reed,
reedgrass, herbs). The spatial distribution of vegetation structures around each measurement site was recorded
with hemispheric photographs, which were subsequently analyzed using image processing techniques. Two
weather stations provide reference values for precipitation and relevant meteorological variables for wind speed
and direction, air humidity, temperature and irradiation.
The amount and distribution of precipitation measured in scarcely vegetated areas of the catchment widely
correspond with values from the reference weather stations. Under dense vegetation, very heterogeneous values
were recorded, which can be explained by i) canopy interception, and ii) fetching effects. The results of this study
can serve as basis for interception models and may also contribute to complex eco-hydrological models.
Landscapes that are heavily disturbed or newly formed by either natural processes or human activity are in a state
of disequilibrium. Their initial development is thus characterized by highly dynamic processes under all climatic
conditions. The primary distribution and structure of the solid phase (i.e. mineral particles forming the pore space)
is one of the decisive factors for the development of hydrological behavior of the eco-hydrological system and
therefore (co-) determining for its – more or less – stable final state. The artificially constructed ‚Hühnerwasser‘ catchment (a 6 ha area located in the open-cast lignite mine
Welzow-Süd, southern Brandenburg, Germany) is a landscape laboratory where the initial eco-hydrological development is observed since 2005. The specific formation (or construction) processes generated characteristic sediment structures and distributions, resulting in a spatially heterogeneous initial state of the catchment. We developed a structure generator that simulates the characteristic distribution of the solid phase for such constructed landscapes. The program is able to generate quasi-realistic structures and sediment compositions
on multiple spatial levels (1 cm up to ∼ 100 m scale). The generated structures can be i) conditioned to actual
measurement values (e.g., soil texture and bulk distribution); ii) stochastically generated, and iii) calculated deterministically according to the geology and technical processes at the excavation site. Results are visualized using the GOCAD software package and the free software Paraview. Based on the 3D-spatial sediment distributions, effective hydraulic van-Genuchten parameters are calculated using pedotransfer functions. The hydraulic behavior of different sediment distribution (i.e. versions or variations of the catchment’s porous body) is calculated using a numerical model developed by one of us (Caviedes-Voullième). Observation data are available from catchment monitoring are available for i) determining the boundary conditions (e.g., precipitation), and ii) the calibration / validation of the model (catchment discharge, ground water). The analysis of multiple sediment distribution scenarios should allow to approximately determine the influx of starting conditions on initial development of hydrological behavior. We present first flow modeling results for a reference (conditioned) catchment model and variations thereof. We will also give an outlook on further methodical
development of our approach.
Rainfall variability within a storm is of major importance for fast hydrological processes, e.g. surface runoff,
erosion and solute dissipation from surface soils. To investigate and simulate the impacts of within-storm variabilities on these processes, long time series of rainfall with high resolution are required. Yet, observed precipitation records of hourly or higher resolution are in most cases available only for a small number of stations and only for a few years. To obtain long time series of alternating rainfall events and interstorm periods while conserving the statistics of observed rainfall events, the Poisson model can be used. Multiplicative microcanonical random cascades have been widely applied to disaggregate rainfall time series from coarse to fine temporal resolution.
We present a new coupling approach of the Poisson rectangular pulse model and the multiplicative microcanonical random cascade model that preserves the characteristics of rainfall events as well as inter-storm periods. In the first step, a Poisson rectangular pulse model is applied to generate discrete rainfall events (duration and mean intensity) and inter-storm periods (duration). The rainfall events are subsequently disaggregated to high-resolution time series (user-specified, e.g. 10 min resolution) by a multiplicative microcanonical random cascade model. One of the challenges of coupling these models is to parameterize the cascade model for the event durations generated by the Poisson model. In fact, the cascade model is best suited to downscale rainfall data with constant time step such as daily precipitation data. Without starting from a fixed time step duration (e.g. daily), the disaggregation of events requires some modifications of the multiplicative microcanonical random cascade model proposed by Olsson (1998): Firstly, the parameterization of the cascade model for events of different durations requires continuous functions for the probabilities of the multiplicative weights, which we implemented through sigmoid functions. Secondly, the branching of the first and last box is constrained to preserve the rainfall event durations generated by the Poisson rectangular pulse model.
The event-based continuous time step rainfall generator has been developed and tested using 10 min and hourly rainfall data of four stations in North-Eastern Germany. The model performs well in comparison to observed rainfall in terms of event durations and mean event intensities as well as wet spell and dry spell durations. It is currently being tested using data from other stations across Germany and in different climate zones. Furthermore, the rainfall event generator is being applied in modelling approaches aimed at understanding the impact of rainfall variability on hydrological processes.