VEGetation Generator for Interactive Evolution (VEGGIE)

Over the recent years, it has become more evident that vegetation, as well as its dynamics, is the often-ignored linchpin of the land-surface hydrology. This work emphasizes the coupled nature of vegetation-water-energy dynamics by considering linkages at time scales that vary from hourly to interannual. A dynamic ecohydrological model, [tRIBS+VEGGIE], is constructed, which represents the essential water and energy processes over the complex topography of a river basin and links them to the basic plant life regulatory processes. In this framework, which is particularly focused on ecohydrology of semi-dry environments, soil water is considered to be the key limiting resource affecting vegetation structure and organization. The mechanisms through which water limitation influences plant behavior are related to carbon assimilation via the control of photosynthesis and stomatal behavior, carbon allocation, stress-induced foliage loss, as well as recruitment and phenology patterns.

The system simulates a number of processes that manifest numerous dynamic feedbacks among various components of the coupled vegetation-hydrology system:

  • Biophysical processes: absorption, reflection, and transmittance of solar shortwave radiation; absorption, reflection, and emission of longwave radiation; sensible and latent heat fluxes, partition of latent heat into canopy and soil evaporation, and transpiration; stomatal physiology; ground heat flux.

  • Hydrological processes: interception, throughfall, and stem flow; infiltration in a multi-layer soil; lateral water transfer in the unsaturated zone; runoff and runon with re-infiltration.

  • Biochemical processes and vegetation dynamics: photosynthesis and primary productivity; plant respiration; tissue turnover and stress-induced foliage loss; carbon allocation; vegetation phenology; plant recruitment.

The model simulates the energy and water budgets of both vegetated and non-vegetated surfaces that can be simultaneously present within a given element. In a domain of study, the dynamics of each computational element are simulated separately. Spatial dependencies are introduced by considering the surface and subsurface moisture transfers among the elements, which affect the local dynamics via the coupled energy-water interactions. Consequently, when applied to a catchment system, the model offers a quasi-three-dimensional framework in which lateral moisture transfers may lead to the spatio-temporal variability of states. The model accounts for the hydraulic, thermal, and albedo properties of different soil types.

While the models of biophysical processes operate at an hourly time scale, the routines simulating the processes of infiltration, lateral moisture transfer, and runoff (runon) use a finer time step (7.5-15 min.). Consequently, at the hourly time scale, the stomatal response to environmental conditions is the only vegetation process that affects the water and energy budgets. At the daily and longer time scales, vegetation affects state of the land-surface through the change of its structural attributes (such as leaf area index and height) and vegetation fraction. The latter determines the relative contribution of a given vegetation type to the element-scale fluxes.

Selected Publications

[1] Ivanov, V.Y., Bras, R.L., and Vivoni, E.R. (2008). Vegetation-Hydrology Dynamics in Complex Terrain of Semiarid Areas: I. A mechanistic Approach to Modeling Dynamic Feedbacks, Water Resources Research, in press.

[2] Ivanov, V.Y., Bras, R.L., and Vivoni, E.R. (2008). Vegetation-Hydrology Dynamics in Complex Terrain of Semiarid Areas: II. Energy-Water Controls of Vegetation Spatio-Temporal Dynamics and Topographic Niches of Favorability, Water Resources Research, in press.

[3] Ivanov, V.Y., Bras, R.L., and Curtis, D.C. (2007). A weather generator for hydrological, ecological, and agricultural applications, 43, W10406, doi: 10.1029/ 2006WR005364.

[4] Ivanov, V.Y. (2006d). Effects of Dynamic Vegetation and Topography on Hydrological Processes in Semi-Arid Areas. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, USA.