Abstracts

A network analysis of patterns and processes in river deltas: Lessons learned from two different systems
Paola Passalacqua (University of Texas)
Sea-level rise, subsidence, hypoxia, and pollutant intrusion in the groundwater are some of the main factors that put many coastal systems around the world, their populations, and ecosystems at great risk. Recent restoration strategies rely on nature-based engineering to allow coastal systems to respond to changes in environmental forcing based on their natural hydrodynamic and morphodynamic functioning. I will cover an overview of current knowledge on delta systems and research challenges and describe a framework based on connectivity to study deltaic systems. This framework, called Delta Connectome, analyzes deltas as networks, which can be composed of physical objects, such as channels and junction/bifurcation nodes, or variables and process couplings. I will quantify different types of connectivity relying on two main study areas and numerical modeling. Most of the field observations have been collected at the Wax Lake Delta, a naturally prograding delta in coastal Louisiana formed by a river diversion for flood control. Another study area is the Ganges-Brahmaputra-Meghna Delta, characterized by a complex surface channel network, significant environmental forcing, and human modifications. Numerical modeling is used for identifying the factors controlling connectivity on the surface, between surface and subsurface, and under scenarios of changing forcing.   
           
Paola Passalacqua is an Associate Professor of Environmental and Water Resources Engineering, in the Civil, Architectural and Environmental Engineering Department at the University of Texas at Austin. She graduated summa cum laude from the University of Genoa, Italy, with a BS (2002) in Environmental Engineering, and received a MS (2005) and a PhD (2009) in Civil Engineering from the University of Minnesota. Her research interests include network analysis and dynamics of hydrologic and environmental transport on river networks and deltaic systems, lidar and satellite imagery analysis, multi-scale analysis of hydrological processes, and quantitative analysis and modeling of landscape forming processes.

Evapotranspiration and cloud variability at regional sub-grid scales
Jordi Vilà-Guerau de Arellano (Wageningen University)
In regional and global models uncertainties arise due to our incomplete understanding of the coupling between biochemical and physical processes. Representing their impact depends on our ability to calculate these processes using physically sound parameterizations, since they are unresolved at scales smaller than the grid size. More specifically over land, the coupling between evapotranspiration, turbulent transport of heat and moisture, and clouds lacks a combined representation to take these sub-grid scales interactions into account. Our approach is based on understanding how radiation, surface exchange, turbulent transport and moist convection are interacting from the leaf- to the cloud scale. We therefore place special emphasis on plant stomatal aperture as the main regulator of CO2-assimilation and water transpiration, a key source of moisture source to the atmosphere.
Plant functionality is critically modulated by interactions with atmospheric conditions occurring at very short spatiotemporal scales such as cloud radiation perturbations or water vapour turbulent fluctuations. By explicitly resolving these processes, the LES (large-eddy simulation) technique is enabling us to characterize and better understand the interactions between canopies and the local atmosphere. This includes the adaption time of vegetation to rapid changes in atmospheric conditions driven by turbulence or the presence of cumulus clouds. Our LES experiments are based on explicitly coupling the diurnal atmospheric dynamics to a plant physiology model. Our general hypothesis is that different partitioning of direct and diffuse radiation leads to different responses of the vegetation. As a result there are changes in the water use efficiencies and shifts in the partitioning of sensible and latent heat fluxes under the presence of clouds.
Our presentation is as follows. First, we discuss the ability of LES to reproduce the surface energy balance including photosynthesis and CO2 soil respiration coupled to the dynamics of a convective boundary layer. LES numerical experiments and constrained and compared with available observations taken at different ecosystems (grass, rainforest). Second, we perform systematic numerical experiments under a wide range of background wind conditions and stomatal aperture response time. Our analysis unravel how thin clouds, characterized by lower values of the cloud optical depth, have a different impact on evapotranspiration compared to thick clouds due to differences in the partitioning between direct and diffuse radiation at canopy level. Related to this detailed simulation, we discuss how new instrumental techniques, e.g. scintillometery, enable us to obtain new observational insight of the coupling between clouds and vegetation. We will close the presentation with open questions regarding the need to include parameterizations for these interactions at short spatiotemporal scales in regional or climate models.

Multi-Scale Modelling and Monitoring of the Kapuas River Delta
Karl Kästner (Wageningen University)
PhD Thesis abstract
Rivers in the humid tropics are those with the largest discharges and sediment loads of the world. Their evergreen and everwet catchments are hotspots of biodiversity and their fertile deltas are acres of plenty for the production of rice, palm oil and rubber. At present, tropical rivers, their catchments, and deltas face growing pressure from rapid economic development and climate change, which may permanently deteriorate their ecosystem services. Yet, despite their importance and advancing degradation, relatively little is known about their physiography. This thesis reduces this gap by contributing to our fundamental understanding of tropical rivers. This thesis in particular addresses fundamental questions regarding the hydro- and morphodynamics of large sand-bedded rivers and their tidally influenced deltas: How can river and tidal discharge be effectively measured? How do the cross-section geometry and bed material change along the fluvial-tidal transition? How do these trends differ between the distributaries? How does the tide propagate up-river river? How can sediment transport be efficiently measured with acoustic instruments? How are the discharge and the sediment divided at river bifurcations? To address these questions, the author undertook a year-long journey to West Kalimantan, during which he surveyed and monitored the Kapuas River. The Kapuas River is a nearly pristine and thus gives a rare insight into the hydro- and morphodynamics of a river that has not yet been restricted by either dams, dykes or groins. Findings from the survey of the Kapuas River are generalized with idealized models.