Modelling species and spacing effects on root zone water dynamics using Hydrus-2D in an Amazonian agroforestry system

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 (Cupuacu), 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 interception 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 developed. 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.