Modeling the water balance and soil water fluxes in a fast growing Eucalyptus plantation in Brazil

A five-layered water balance model, with water movement between layers along hydraulic gradients, was developed and parameterized for a eucalypt plantation (Eucalyptus grandis Hill ex, Maiden hybrids) in Brazil. Available soil water controls stomatal conductance and hence transpiration, which is calculated by the Penman-Monteith equation. The model accounts for changes in the depths of the water table. Calculations are supported by measurements: the test period was from October 1995 to September 1996 in a 9-year-old plantation in an experimental catchment in eastern Brazil. Total transpiration for the year was 1116 mm, with 151 mm intercepted and re-evaporated and another 78 mm soil surface evaporation, giving evapotranspiration of 1345 mm compared to rainfall of 1396 mm. The water balance was closed by net flow below the root zone of about 25 mm and an increase in water storage (in the first layer) of 24 mm. The model also estimated a transpiration deficit (difference between the potential and current transpiration) of 125 mm for the period. Upward flux from the water table was around 82 mm and piezometric measurements showed 2.5 m recession of the water table for the same period. The upward flux into the root zone was about 1 mm day^-1 at the end of a long dry season; that kept the water storage in that zone to about 15% of capacity and helped prevent complete stomatal closure. Comparison between estimated water storage and measurements confirmed that this model is a very promising tool for calculating water use by plantations. It can also provide water balance information and information about stomatal conductance for growth prediction models.