Ecohydrology

Landlab: Component-Based Software Architecture for Computational Landscape Modeling [Funding: NSF, G. Tucker (UC, Boulder), N. Gasparini (U Tulane), E. Istanbulluoglu (UW)]

Landscape models compute flows of mass, such as water, sediment, glacial ice, volcanic material, or landslide debris, across a gridded terrain surface. Science and engineering applications of these models range from short-term flood forecasting to long-term landform evolution. At present, software development behind these models is highly compartmentalized and idiosyncratic, despite the strong similarity in core algorithms and data structures between otherwise diverse models. We introduce a component-based approach to software development for landscape models that will provide the platform to integrate range of earth science and engineering fields. We call this new modeling framework “Landlab”.

Currently in Landlab we adapt and enhance existing landscape model code from the CHILD landscape evolution model [Tucker et al., 2001] and CATGraSS ecohydrology model [Zhou et al., 2013], to provide a set of independent, interoperable components. Landlab includes: (1) a gridding engine to handle both regular and unstructured meshes, (2) an interface for space-time climate forcing input, (3) a surface hydrology component, (4) an erosion-deposition component, (5) a vegetation component, (6) crater impact component to study planetary processes, and (6) a simulation driver. The components are tested with a trial application that addresses runoff and erosion on post-wildfire landscapes, post-wildfire vegetation regeneration, and ecohydrology of woody plant encroachment in the southwestern USA.

Figure Coutesy of Greg Tucker

BGM (Bucket Grassland Model)

BGM is a zero-dimensional ecohydrology model based on a vertically averaged representation of soil moisture in the root-zone, coupled with grass dynamics represented by live and dead above and below ground biomass pools. Five variants of the model, combined the different time steps (daily vs interstorm), representations of potential evapotranspiration (PET), and constant versus variable water use efficiency (WUE) were developed and tested across the Nebraska Sand Hills (NSH) (Istanbulluoglu et al., 2011). Water table is also added in the model as a lower boundary condition (Soylu et al., 2011).

Modeled and observed live and total biomass of an interdunnal valley in the NSH: DS1: daily time step simulation with calculated WUE and ETR; DS2: daily time-step simulation with a constant WUE and calculated ETR; IS1: inter-storm time step simulation with a constant WUE and calculated ETR; IS2: inter-storm time-step simulation with a constant WUE and ETR-COS. Despite its simplicity IS2 performs almost as good as DS1. Data is from the Barta Brothers Research Ranch of the University of Nebraska (Courtesy of Prof. David Wedin)

CATGraSS (Cellular Automata Tree-Grass-Shrub Simulator)

CATGraSS is developed to simulate tree-grass-shrub coexistence and their dynamics on the complex terrain. The salient aspect of this model is the explicit treatment of topography on the distribution of solar radiation, leading to spatial variations in evapoatranspiration, soil moisture, and plant water stress on the landscape. Plant establishment and mortality are driven by water stress and competition for space. Developed by: Xiaochi Zhou and Erkan Istanbulluoglu.

CATGraSS model flow chart with three model state variables and their linkages to various ecohydrological processes. Heterogeneities due to topography influence local water balance through modulating climate forcing received by plants. PFT is plant functional type.

The figure below illustrates a preliminary result of the cellular-automaton vegetation model in central New Mexico, Sevilleta National Wildlife Refuge LTER site. The model reproduces the aspect-induced differences in vegetation patterns. The top picture shows the existing topography and vegetation patterns where the model is run.

CATGraSS is used to simulate future vegetation patterns under climate change. In the example below the model is forced with downscaled GCM outputs and 50 ensemble members of climate change are used.

Modeling of vegetation dynamics in a semiarid site (Walnut Gulch, southeastern Arizona)

We integrate ecohydrology models with the Channel and Hillslope Integrated Landscape Development (CHILD) model. Below are examples from the validation of ecohydrology model implemented in two study sites, Walnut Gulch basin Arizona (by Homero Flores), and the Sevilleta National Wildlife Refuge, New Mexico (by Omer Yetemen).

Sevilleta National Wildlife Refuge, New Mexico (Collaborator Enrique Vivoni, ASU):

 

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