Alain Perrier

Including Croplands in a Global Biosphere Model: Methodology and Evaluation at Specific Sites

Abstract There is a strong international demand for quantitative estimates of both carbon sources/sinks, and water availability at the land surface at various spatial scales (regional to global). These estimates can be derived (and usually are) from global biosphere models, which simulate physiological, biogeochemical, and biophysical processes, using a variety of plant functional types. Now, the representation of the large area covered with managed land (e.g., croplands, grasslands) is still rather basic in these models, which were first designed to simulate natural ecosystems, while more and more land is heavily disturbed by man (crops cover ∼35% and grasslands ∼30%–40% of western Europes area as a result of massive deforestation mainly in the Middle Ages). In this paper a methodology is presented that combines the use of a dynamic global vegetation model (DGVM) known as Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) and a generic crop model [the Simulateur Multidisciplinaire pour les...

AERODYNAMIC AND SURFACE RESISTANCES OF COMPLETE COVER CROPS: HOW GOOD IS THE “BIG LEAF”?

The Penman-Monteith equation is based on the assumption that the canopy can be reduced to a “big leaf”. Given the most commonly used formulation of aerodynamic resistance (ra), this “big leaf” is considered to be, implicitly, at the d + zoH level (where d is zero plane displacement height and zoH is roughness length for heat transfer). This can lead to negative values of surface resistance (rs) when the leaves of the top of the canopy (between d + zoH and crop height hc) are the ones that most contribute to total water loss to the atmosphere. To avoid this, ra should be computed from the top of the canopy to the reference height in the atmosphere. Also, one concludes that rs for complete cover crops cannot be computed by simply averaging stomatal resistance since the main condition, the driving force being the same in all of the elements of the “circuit”, is violated.

A semiempirical model of bare soil evaporation for crop simulation models

In crop simulation models the water subroutine computes evaporation and transpiration separately. This property allows a greater confidence in the simulation of the early crop stages in terms of water requirements. In the present article the authors suggest a semiempirical model of the drying stage of soil evaporation that can be easily integrated into various crop models. The model is based on a physical approach, yet it depends solely on permanent soil properties and on some key data on the local climate. The basic theory relies upon the mass balance of a dry surface layer of varying thickness. After successive assumptions on the main limiting factors of the drying process, the authors demonstrate that this process can be described by one equation relating actual to potential evaporation. The advantage of this formula comes from its introducing just one synthetic parameter that can be expressed as the product of two components: a climatic one and a pedological one. When compared with experimental data, the results of the model show that the order of magnitude of the evaporative process is respected, which is required for crop models.

Land Surface Processes: Description, Theoretical Approaches, and Physical Laws Underlying Their Measurements

For more than a decade, evidence has accumulated from climate model experiments that the exchanges of momentum, heat, and moisture at the surface are phenomena that strongly influence the dynamics and thermodynamics of the atmosphere.