Modelling species and spacing effects on root zone water dynamics using Hydrus-2D in an Amazonian agroforestry system P. Schlegel 1 , B. Huwe 1, * and W.G. Teixeira 2 1 Soil Physics Group, Universität Bayreuth, 95440 Bayreuth, Germany; 2 Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA) – Amazonia Occidental, Manaus, Caixa Postal 319, 69011-970-Manaus, Brazil; * Author for correspondence (e-mail: bernd.huwe@uni-bayreuth.de) Received 15 november 2003; accepted in revised form 17 september 2003 Key words: Bactris gasipaes, Model scenarios, Plant spacing, Pueraria phaseoloides, Root distribution, Theo- broma grandiflorum Abstract Modelling the root zone water dynamics in a tree crop agroforestry system is a useful approach to understanding small-scale effects in tree crop systems and may be helpful for optimizing tree spacing in agroforestry system planning. The agroforestry system in this study consists of the species Theobroma grandiflorum Willd ex Spreng Schum Cupuaçu, Bactris gasipaes H.B.K. peach palm and the cover crop Pueraria phaseoloides Roxb. Benth Pueraria. The soiltype is an oxisol. Calibration was conducted for each of the three species separately. Calibration results show good conformity between simulated and measured data. Simulated scenarios examine the influence of different spacing between trees on root water uptake, evaporation and drainage. Mean intercep- tion and crop factors of the whole flow region vary with spacing or are held constant to examine below-ground effects only. Also a fictitious scenario of an older agroforestry system with deeper roots is calculated. In order to overcome restrictions of the computer program Hydrus-2D, correction factors in the root zone were introduced and a calculation scheme for root water uptake of a flow subregion was developped. Below-ground effects of spacing between trees are not or almost not present, but the depth of the tree roots has a significant influence on root water uptake, evaporation and drainage. When mean interception and crop factor vary, drainage increases and root water uptake decreases slightly with spacing. The modelling approach has been found promising for optimizing agroforestry systems although it can only be seen as a first beginning. In an agroforestry systems under drier conditions differences in results will probably be larger. Introduction The Amazon region still contains the world’s largest area of tropical rain-forest Klinge et al. 1981, although it is constantly reduced by deforestation Ministery of Science and Technology, 2001; Neps- tad 1999. This is in a large part due to the establish- ment of pastures and shifting cultivation by small farmers Diegues 1992. The destruction of rain for- est has many adverse effects on the environment among which are disturbances of the regional and continental water balance, but also a contribution to the global greenhouse effect Fearnside 1985; Fearn- side 1988 and loss of biodiversity. The Amazon rain forest is rich of biomass and spe- cies, but stands mainly on extremely poor soils, which are not able to store large amounts of nutrients Fitt- kau 1990. A large part of nutrients is stored in the biomass of the tropical forest and are lost to a great extent when plants are removed to give way to agri- cultural land use systems Fearnside 1985. Agrofor- estry systems are closer to the rain forest than conventional agriculture or pasture systems with re- spect to their structure and ability to store nutrients in Agroforestry Systems 60: 277–289, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 277