Agricultural and Forest Meteorology 100 (2000) 137–153 Modelling of daily fluxes of water and carbon from shortgrass steppes Y. Nouvellon a,∗ , S. Rambal b , D. Lo Seen a , M.S. Moran c , J.P. Lhomme d , A. Bégué a , A.G. Chehbouni e , Y. Kerr f a CIRAD, 34093, Montpellier cedex 5, France b DREAM-CEFE, CNRS (UPR 9056) 34293, Montpellier cedex 5, France c USDA-ARS, Phoenix, AZ, USA d ORSTOM/CICTUS, Hermosillo, Mexico e ORSTOM/IMADES, Hermosillo, Mexico f CESBIO-CNES, Toulouse, France Received 27 October 1998; received in revised form 20 September 1999; accepted 5 October 1999 Abstract A process-based model for semi-arid grassland ecosystems was developed. It is driven by standard daily meteorological data and simulates with a daily time step the seasonal course of root, aboveground green, and dead biomass. Water infiltration and redistribution in the soil, transpiration and evaporation are simulated in a coupled water budget submodel. The main plant processes are photosynthesis, allocation of assimilates between aboveground and belowground compartments, shoots and roots respiration and senescence, and litter fall. Structural parameters of the canopy such as fractional cover and LAI are also simulated. This model was validated in southwest Arizona on a semi-arid grassland site. In spite of simplifications inherent to the process-based modelling approach, this model is useful for elucidating interac- tions between the shortgrass ecosystem and environmental variables, for interpreting H 2 O exchange measurements, and for predicting the temporal variation of above- and belowground biomass and the ecosystem carbon budget. Published by Elsevier Science B.V. Keywords: Simulation model; Water and carbon fluxes; Shortgrass ecosystem; Arid environment; SALSA program 1. Introduction Arid and semi-arid rangelands constitute nearly one third of the earth’s land surface (Branson et al., 1972). The broad extent of arid and semi-arid regions and their sensitivity to climatic variations and land-use changes make it imperative to improve our under- ∗ Corresponding author. Present address: USDA-ARS-USWCL, SW Watershed Research Center, 2000 E. Allen Road, Tucson, AZ 85719, USA; Tel.:+1-520-670-6380; fax: +1-520-670-5550. E-mail address: yann@tucson.ars.ag.gov (Y. Nouvellon). standing of the hydrologic, atmospheric and ecolog- ical interactions and sustainability of these systems. The Semi-Arid Land Surface Atmosphere (SALSA) program was conceived as a long-term, multidisci- plinary, monitoring and modelling effort to understand the complex interactions between hydrometeorolog- ical, biological and ecological processes occurring in semi-arid areas (Goodrich, 1994). The Upper San Pedro River Basin (USPB) was selected as the focal area for SALSA experiments. It spans the Mexico–US border from Sonora to Arizona and includes such major vegetation types as desert shrubsteppe, riparian 0168-1923/00/$ – see front matter Published by Elsevier Science B.V. PII:S0168-1923(99)00140-9