Water balance of the olive tree-annual crop association: A modeling approach

Water transfers within mixed crops systems are complicated to understand due to the large number of complex interactions between the various components. Standard techniques fail to provide the proper assessment of the components of the water balance. Experiments and modeling developments are used to understand the dynamics of water transfers within the association of olive trees with annual crops under irrigation in Central Tunisia. The whole system is represented by a unit area made up of three components: a plot with the annual crop, a plot with the olive tree and a plot of bare soil. The modeling approach is based on the concept of reservoir. The model works on a daily time step and accounts for the lateral transfers of water occurring between the components of the system: (i) the water uptake by the roots of olive trees; (ii) the physical flow of water between the irrigated plot and the non-irrigated ones. A field experiment was carried out during 2 years (2002, 2003) and three crop cycles (spring potato, spring pea and autumn potato) in order to calibrate the model and test its validity. Olive tree transpiration was estimated from sap flow measurements and soil moisture in the different compartments was measured by neutron probe technique. The experimental data compare fairly well with the model outputs. The first purpose of the model is to understand the functioning of the olive tree-annual crop association from a water standpoint, but it can be easily extended to other intercropping systems mixing perennial vegetation with annual crops or used as a management tool. The estimates of the water extracted by the olive trees in each reservoir appear to be much more significant than those of the water physically transferred between reservoirs.