Plant roots are highly adaptable, but their adaptability is not included in crop and land surface models. They rely on a simplified representation of root growth, which is independent of soil moisture availability. Data of subsurface processes and interactions, needed for model setup and validation, are scarce. Here we investigated soil-moisture-driven root growth. To this end, we installed subsurface drip lines and small soil moisture sensors (0.2 L measurement volume) inside rhizoboxes (length × width × height of 45 × 7.5 × 45 cm). The development of the vertical soil moisture and root growth profiles is tracked with a high spatial and temporal resolution. The results confirm that root growth is predominantly driven by vertical soil moisture distribution, while influencing soil moisture at the same time. Besides support for the functional relationship between the soil moisture and the root density growth rate, the experiments also suggest that the extension of the maximum rooting depth will stop if the soil moisture at the root tip drops below a threshold value. We show that even a parsimonious one-dimensional water balance model, driven by the water input flux (irrigation), can be convincingly improved by implementing root growth driven by soil moisture availability.@en

We use the results of a one-dimensional morphodynamic model and the basis of the “Lagrangian equilibrium state” (Maan et al., 2015, https://doi.org/10.1002/2014JF003311) to derive a quantitative relationship between the progradation speed of tidal flats and the suspended sediment concentration in their adjacent waters and show that the speed increases more than linearly with the concentration. We also show that horizontally prograding flats rise vertically with sea level rise at the expense of their horizontal speed via a linear relationship. If accretion rates are insufficient to keep up with sea level rise, however, the intertidal flat submerges and retreats landward at the same time. We apply the obtained relationships to the Yangtze Estuary to estimate the critical sediment concentration level below which a shift from progradation to retreat can be expected.@en

The prediction of the erosion of mudflats is hampered by inaccurate estimates of the erodibility distribution of the sediment bed. To investigate how erodibility varies in space and what the vertical distribution over the sediment depth is, comprehensive observations of the sediment properties, hydrodynamics and bed-level changes were conducted on an intertidal flat in the Western Scheldt Estuary, the Netherlands. The erosion potential on a mudflat is determined by the critical shear stress for erosion (τ_e), erosion rate coefficient (M) and local hydrodynamic conditions. A clear difference in hydrodynamic forcing was observed, leading to significant bed level variations at the low water line, where erosion often occurs during very shallow water condition, and a nearly constant bed level at the upper part. The erosion parameters τ_e and M could be determined over a sediment bed of 12 cm at the low water line. The erosion coefficient M can be considered constant with depth, although there is a large spreading. A clear vertical variation of τ_e was found: τ_e increased significantly downward from 0.10 Pa at the sediment surface to 1.13 Pa at 12 cm below the surface. Additionally, there was a strong indication that the presence of diatoms enhanced τ_e in the upper 2 mm of sediment by five times of the abiotic τ_e (from 0.09 Pa to 0.46 Pa). These findings lead to the following improvement for predicting morphological changes of tidal mudflats: (1) very shallow conditions should be better simulated, (2) the vertical distribution of τ_e should be considered. Otherwise, erosion rates can be overestimated, especially during extreme events, because exposure of the deeper well-consolidated layer likely occurs; and (3) an appropriate description of the effect of diatoms should be considered as part of the bottom boundary condition.

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We study the coupled action of water uptake and root development of maize in potting soil under greenhouse conditions. To this end, we apply subsurface irrigation strategies that are constant over weeks. We perform synchronous realtime measurements of the co-evolving soil moisture fields and root distributions. Will constant irrigation regimes eventually lead to constant root distributions and soil moisture profiles? In this contribution we report on the preliminary results of a study on the soil-root system behavior and underlying feedback loops. Understanding of the feedback loops between the soil moisture distribution and root development opens new pathways for boosting natural adaptation and climate resilience of plants. We compare two soil-root-systems that differ in irrigation depth; one with a constant irrigation depth and one with a step wise increasing irrigation depth. We also compare a bare soil system without roots.

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Decreasing sediment availability, in combination with sea level rise and human fixation of the coastline, results in losses of the intertidal environment (lying in-between the mean low water and mean high water spring tide). This means a loss of biodiversity and an increased coastal vulnerability to extreme events and sea level rise. Thereforeit is of utmost importance to understand the dynamics of the intertidal wetlands; their response to sea level rise and to different types of human interferences. The better we understand the processes that underlie the evolution of the intertidal system, the more effectively we can manipulate the system, to stimulate its rise and maintain its elevation relative to mean sea level.The long-term morphodynamics is difficult to understand due to the interdependencies of the underlying processes; the morphology is shaped by the hydrodynamic forces,while it influences these forces at the same time. Due to the feedback loops, the components are strongly entangled and the whole system cannot be reduced to the sum of itsparts and solved by the traditional reductionist method.In this thesis, system theory and system analysis are applied to get towards an understanding of ‘the intertidal morphodynamical system’. This is the philosophy that states arise that are understandable and possible to determine exactly, despite the many interactionsbetween the variables and the apparent complexity of systems. To describe these states, I follow a top-down approach, where I learn from the observed system behavior. Hence, the observation of conserved properties leads to the important question:‘why are they conserved?’ The answer to this question can reveal much of the system’s dynamics.@en

We apply a 2-D horizontal process-based model (Delft3D) to study the feedback mechanisms that control the long-term evolution of a fringing intertidal flat in the Western Scheldt Estuary. The hydrodynamic model is validated using a comparison with measurements on the intertidal flat and the sediment transport module is calibrated against long-term morphology data. First, the processes that lead to net sediment exchange between channel and flat are studied. Then, long-term simulations are performed and the dependency of sediment fluxes on the tidal flat bathymetry, and the corresponding morphodynamic feedback mechanisms are explained. In the long run, relatively stable states can be approached, which are shown to be typical for wave-dominated fringing mudflats. The system behavior can be explained by the typical feedback mechanisms between the intertidal bathymetry and the hydrodynamic forces on the flat. In the subtidal domain, the impact of small (5–10 cm) wind waves increases with a rising elevation due to decreasing water depths. In the intertidal domain, the wave impact
increases with increasing cross-sectional slope due to wave shoaling. These relationships result in negative (stabilizing) morphodynamic feedback loops. The tidal current velocities and tide-induced bed shear stresses, on the other hand, are largely determined by the typical horizontal geometry. A stabilizing
feedback loop fails, so that there is no trend toward an equilibrium state in the absence of wind waves.@en