A Simple Biosphere Model (SiB) is developed for use within General Circulation Models (GCMs) to simulate the transfer of energy, mass, and momentum between the atmosphere and the vegetated surface of the Earth. The model represents vegetation as two distinct layers: an upper canopy layer for trees or shrubs and a lower ground cover layer for grasses and herbaceous plants. Each layer may be present or absent, and their coverage can be fractional or complete. The model incorporates physical and physiological properties of vegetation, including radiation absorption, evapotranspiration, soil moisture dynamics, and aerodynamic transfer of water vapor, sensible heat, and momentum. The SiB model has seven prognostic physical-state variables: two temperatures (canopy and ground cover), two interception water stores (canopy and ground cover), and three soil moisture stores. These variables are governed by equations that describe energy and moisture fluxes between the surface and atmosphere. The model accounts for the effects of vegetation on radiation absorption, evapotranspiration, momentum transfer, soil moisture availability, and insulation. The model uses atmospheric boundary conditions, including air temperature, vapor pressure, wind speed, and incident radiation components. It also incorporates morphological and physiological parameters of vegetation, such as leaf reflectance, transmittance, and soil properties. The model is designed to be as physically and biologically realistic as possible, with vegetation characteristics prescribed as functions of season and location. The SiB model is intended to improve understanding of land surface-atmosphere interactions by simulating the effects of vegetation on atmospheric processes. Future studies aim to make the biosphere model phenologically interactive with atmospheric GCMs, enabling the simulation of climatic effects on surface conditions and atmospheric circulation patterns. The model's equations are derived from physical principles and are designed to be computationally efficient for use in GCMs. The model's structure and parameters are detailed in the paper, with a focus on radiation fluxes, aerodynamic resistances, and the transfer of moisture and heat through the vegetation and soil.A Simple Biosphere Model (SiB) is developed for use within General Circulation Models (GCMs) to simulate the transfer of energy, mass, and momentum between the atmosphere and the vegetated surface of the Earth. The model represents vegetation as two distinct layers: an upper canopy layer for trees or shrubs and a lower ground cover layer for grasses and herbaceous plants. Each layer may be present or absent, and their coverage can be fractional or complete. The model incorporates physical and physiological properties of vegetation, including radiation absorption, evapotranspiration, soil moisture dynamics, and aerodynamic transfer of water vapor, sensible heat, and momentum. The SiB model has seven prognostic physical-state variables: two temperatures (canopy and ground cover), two interception water stores (canopy and ground cover), and three soil moisture stores. These variables are governed by equations that describe energy and moisture fluxes between the surface and atmosphere. The model accounts for the effects of vegetation on radiation absorption, evapotranspiration, momentum transfer, soil moisture availability, and insulation. The model uses atmospheric boundary conditions, including air temperature, vapor pressure, wind speed, and incident radiation components. It also incorporates morphological and physiological parameters of vegetation, such as leaf reflectance, transmittance, and soil properties. The model is designed to be as physically and biologically realistic as possible, with vegetation characteristics prescribed as functions of season and location. The SiB model is intended to improve understanding of land surface-atmosphere interactions by simulating the effects of vegetation on atmospheric processes. Future studies aim to make the biosphere model phenologically interactive with atmospheric GCMs, enabling the simulation of climatic effects on surface conditions and atmospheric circulation patterns. The model's equations are derived from physical principles and are designed to be computationally efficient for use in GCMs. The model's structure and parameters are detailed in the paper, with a focus on radiation fluxes, aerodynamic resistances, and the transfer of moisture and heat through the vegetation and soil.