T&C full description
The Tethys-Chloris (T&C) model is designed to simulate coupled dynamics of energy, water and vegetation at the land surface and at the hourly time-scale in different environments and climates. All the principal components of the hydrological cycle, such as precipitation interception, transpiration, ground evaporation, infiltration, surface and subsurface water fluxes are accounted for. The model can solve the ecohydrological dynamics over complex topography (e.g., a hillslope or a watershed), explicitly considering spatial variability of meteorological fields and the role of topography in controlling incoming radiation and transferring water laterally through the surface and subsurface. Heterogeneity in soil properties and vegetation are accounted for. In each simulated grid cell, vegetation can occupy two vertical layers to mimic the coexistence of trees and bushes/grasses. Horizontal composition of vegetation is also possible since each element can account for multiple species or plant functional types. The basic computational elements are represented using cells of a regular grid. When the model is run in a “plot-scale” mode, each location is assumed flat without lateral effects of mass and energy exchange and without an explicit areal dimension, essentially a one-dimensional representation.
Shortwave and longwave incoming radiation fluxes are explicitly transferred through the vegetation. The energy, water and carbon exchanges between the surface (soil and vegetation) and the planetary boundary layer are computed with a resistance analogy scheme accounting for aerodynamic, undercanopy and leaf boundary layer resistances, as well as for stomatal and soil resistances. Dynamics of water content in the soil profile are solved using a quasi 3-D approach: the one-dimensional (1-D) Richards equation is used for vertical flow and the kinematic wave equation is used for lateral subsurface flow. Saturated and unsaturated parts of the soil column are explicitly identified. Surface overland flow and channel flow are also solved using the kinematic equation. In case of snow occurrence, snowpack dynamics are computed using the energy balance. Snow can be intercepted by the vegetation or fall to the ground, where it accumulates and successively melts. Runoff generation occurs via saturation excess and infiltration excess mechanisms and depends on lateral moisture fluxes in the unsaturated and saturated zones as well as on overland flow. The soil heat flux is computed solving the heat diffusion equation. Water can pond at the surface modifying roughness, albedo and thermal properties and allowing direct evaporation from surface water.
Photosynthesis is simulated using the Farquhar biochemical model adapted with more recent modifications. The model follows the two big leaves scheme, where sunlit and shaded leaves are treated separately for estimating net assimilation and stomatal resistance. Photosynthesis is upscaled from leaf to plant scale assuming an exponential profile of leaf nitrogen content per unit of area and therefore photosynthetic capacity. The stomatal conductance parameterization accounts for net assimilation rate, leaf internal CO2 concentration and vapour pressure deficit following the Leuning model.
The dynamics of seven carbon pools are explicitly simulated in the model and include green aboveground biomass (leaves), living sapwood, fine roots, carbohydrate reserve (non-structural carbohydrates), reproductive tissues (fruit and flowers), standing dead leaves and heartwood/dead-sapwood. The carbon assimilated through photosynthetic activity is used for maintenance and growth respiration otherwise is allocated to one of the first five pools. The different pools are undergoing tissue turnover in function of tissue longevity and environmental stresses, i.e., drought and low temperatures. Carbon allocation is a dynamic process that accounts for resource availability (light and water) and allometric constraints, e.g., a minimum ratio of fine root to foliage carbon; and an upper limit for the storage of carbohydrate reserve. Carbon allocated to reserves can be subsequently translocated to favor leaf expansion at the onset of the growing season or after severe disturbances. Patterns of plant allocation are influenced by phenology. Transition between phenlogical phases are prognostic and respond to environmental cues as soil temperature, soil moisture, photoperiod length and solar radiation. Soil and nutrient biogeochemistry dynamics and forest growth could be simulated when the soil-biogeochemistry module is activated (T&C-BG), otherwise vegetation is assumed in a mature phase and in equilibrium with its nutritional environment (T&C). Vegetation and soil biogeochemistry dynamics are solved at the daily time scale however biochemical processes of photosynthesis and stomatal physiology are computed at the hourly time scale, as a necessary component affecting the hydrological budget.
The model has been applied to a very large spectrum of ecosystems and environmental conditions worldwide and it has been validated in its capability to simulate energy and mass exchanges and vegetation dynamics with observations collected at flux towers, LTER, and other experimental sites worldwide. The model provides a wide range of outputs at the hourly and daily time scales.