Simulated Surface Runoff by the biogeography model MAPSS using S. Hostetler's (USGS) climate data (detailed information available at http://regclim.coas.oregonstate.edu/domains.html), created using RegCM3 with GFDL boundary conditions.
MAPSS (Mapped Atmosphere-Plant-Soil System) is a static biogeography model that projects potential vegetation distribution and hydrological flows on a grid (http://www.databasin.org/climate-center/features/mapss-model). MAPSS has been used widely for various climate change assessments including the 2000 National Assessment Synthesis Team's report.
MAPSS uses long term, average monthly climate data (mean monthly temperature, vapor pressure, wind speed, and precipitation) as well as soils information (texture, depth). Based on a set of climatic thresholds, the model defines the following plant functional types: evergreen needleleaf or broadleaf trees/shrubs, deciduous broadleaf or needleleaf trees/shrubs, C3 and C4 grasses. The model uses thresholds of LAI and climatic zone thresholds to identify potential vegetation types.
The model simulates surface runoff, infiltration, saturated, and unsaturated percolation through the soil profile. Transpiration is constrained by available soil water, leaf area, and stomatal conductance. The model calculates the leaf area index (LAIs) for both woody (either trees or shrubs) and grass lifeforms competing for light and water, while maintaining a site water balance consistent with observed runoff (Neilson 1995). Water in the surface soil layer is apportioned to the trees and grasses in relation to their relative LAIs and stomatal conductance, i.e., canopy conductance, but trees have access to deeper soil water while grasses do not. Stomatal conductance varies as a function of potential evapotranspiration (PET, a surrogate for vapor pressure deficit) and soil water content.
An elevated CO2 has been documented to affect vegetation responses to climate change through changes in carbon fixation and water-use-efficiency (WUE, carbon atoms fixed per water molecule transpired). The MAPSS model has been run under the A2 emission scenario imposing an increase in water use efficiency such that at 700pm CO2, transpiration is reduced by 25%.
