Plant and Soil 204: 183–201, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 183 Modelling competition for water in intercrops: theory and comparison with field experiments H. Ozier-Lafontaine 1 , F. Lafolie 2 , L. Bruckler 2,∗ , R. Tournebize 1 and A. Mollier 2 1 INRA, Unit´ e Agrop´ edoclimatologiquede la Zone Caraïbe, Domaine Duclos, BP 515, Petit-Bourg, 97165 Pointe- ` a-Pitre Cedex, Guadeloupe and 2 INRA, Unit´ e de Science du sol, Domaine St Paul, Site Agroparc, 84914 Avignon Cedex 9, France Received 11 March 1998. Accepted in revised form 3 August 1998 Key words: intercrops, maize, model, roots, sorghum, water transport Abstract A knowledge of plant interactions above and below ground with respect to water is essential to understand the performance of intercrop systems. In this study, a physically based framework is proposed to analyse the competition for soil water in the case of intercropped plants. A radiative transfer model, associated with a transpiration-partitioning model based on a modified form of the Penman-Monteith equation, was used to estimate the evaporative demand of maize (Zea mays L.) and sorghum ( Sorghum vulgare R.) intercrops. In order to model soil–root water transport, the root water potential of each species was calculated so as to minimise the difference between the evaporative demand and the amount of water taken up by each species. A characterisation of the micrometeorological conditions (net radiation, photosynthetically active radiation, air temperature and humidity, rain), plant water relations (leaf area index, leaf water potential, stomatal conductance, sap flow measurements), as well as the two-component root systems and water balance (soil–root impacts, soil evaporation) was carried out during a 7-day experiment with densities of about 4.2 plant m −2 for both maize and sorghum. Comparison of the measured and calculated transpiration values shows that the slopes of the measured versus predicted regression lines for hourly transpiration were 0.823 and 0.778 for maize and sorghum, respectively. Overall trends in the variation of volumetric water content profiles are also reasonably well described. This model could be useful for analysing competition where several root systems are present under various environmental conditions. Introduction In intercrops, the sharing of water is the result of dy- namic colonisation by shoot and root systems above and below ground, as well as interactions between environmental conditions and plant growth. The devel- opment of the canopy structure determines the share of radiation intercepted and, to a large extent, the par- titioning of evaporative demand between the two crop components. In return, the ability of each crop compo- nent to satisfy its own evaporative demand for growth will depend on soil water availability, root distribution and functionality, and biophysical regulation of water ∗ FAX No: (0)490316244. E-mail: Laurent.Bruckler@avignon.inra.fr flow. Consequently, a knowledge of above- and below- ground interactions for water is essential to understand how intercrops acclimate to situations where water can be limiting. In his analysis of simulation models for intercrop- ping systems, Caldwell (1995) listed a sample of 19 simulation models for agricultural systems with multi- ple plant species to predict ecophysiological processes at time steps of one day or less, and showed how to integrate those processes over the course of one or more growing seasons. Most of the models used more or less functional approaches to model available soil moisture and plant water uptake. There is a wide range of functional models, which combine simplicity and complexity: one model might be very simple regarding one mechanism, while being more realistic and close