Development of an open-source ecohydrology model using Landlab with applications in semi-arid landscapes

Thesis (Ph.D.)--University of Washington, 2020Ecosystems are in transition globally with critical societal consequences. Global warming, growing climatic extremes, land degradation, human-introduced herbivores, and climate-related disturbances (wildfires, diseases, insect outbreaks) drive rapid changes in ecosystem productivity and structure, with complex feedbacks in watershed hydrology, geomorphology and biogeochemistry. There is need to develop models that can represent ecosystem changes by incorporating the role of individual plant patches. In my research I developed ecohydrologic components in Landlab, an open source toolkit written in Python (http://landlab.github.io/#/), to study global change drivers in watersheds with emphasis on woody plant encroachment (WPE). I will first discuss the development of Landlab, its design, architecture, and illustrate examples of building models with Landlab. I will then present the development of ecohydrologic components and illustrate examples of coupling these components for simulating local soil moisture and plant dynamics with spatially explicit cellular automaton (CA)-based plant establishment, mortality, fire, and grazing processes. Several key features of arid and semiarid ecosystems will be discussed. Coexistence of tree-grass cover on north facing slopes (NFS) and shrub cover on south facing slopes (SFS) in central New Mexico is attributed to the competitive advantage of trees due to their longer seed dispersal range against shrubs in cooler and moist NFS. Incorporating a rule to represent inhibitory effects of shrubs on grasses enhance modeled shrub cover, while both trees and grasses are favored when runon is included in the local soil moisture model. Feedbacks among livestock grazing, grassland fire frequency and size, resource redistribution on woody plant encroachment are investigated using different ecohydrologic model configurations. These feedbacks have led to a three-phase woody plant expansion processes in the model, with rates of encroachment controlled by the state transition probabilities of vegetation types in relation to plant susceptibility to fires, grazing, and age-related mortality. A critical area of woody plant emerges in the model with which a negative feedback between fire size and woody plant expansion begins, providing a spatially-explict modelling evidence to the alternative stable states hypothesis. Finally, I investigate the transient ecosystem response to climate variability since the Late Pleistocene using paleoclimatic reconstructions of precipitation and temperature in central New Mexico, USA. The interplay between ecosystem state, change in climate, resultant grass connectivity, and hence fire frequency, and topography are explored with an ecohydrologic model discussed earlier. A transition from cool-wet climate to a warm-dry climate leads to shrub expansion due to drought-induced loss of grass connectivity. Shrubs dominate the ecosystem if dry conditions persist longer. Transition back to a tree or grass dominated ecosystem from shrub dominated ecosystem can only happen when climate shifts from dry to wet. The importance of length of dry or wet spells on ecosystem structure is highlighted. Aspect plays a critical role in providing topographical refugia for trees during dry periods and influences the rate of ecosystem transitions during climate change