|
This paper deals with the development of a water budget model for Eucalyptus forest, using a conceptually simple onedimensional mass balance approach within the root zone of the forest. The model uses Leaf Area Index to quantify the forest structure important for mass and energy exchange, and this represents a key simplification for regional scale applications. Remote Sensing vegetation indexes and mixture modeling techniques were used to estimate LAI. 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 remote sensing derived LAI was used to compute the canopy conductance that drives the Penman-Monteith formulation. The model accounts for changes in the depths of the water table. The test period was from October 1995 to September 1996 in a nine-year-old plantation in an experimental catchment in eastern Brazil. Total transpiration for the year was 1116 mm, with 119 mm intercepted and re-evaporated and another 79 mm soil surface evaporation, giving evapotranspiration of 1314 mm compared to rainfall of 1396 mm. The water balance was closed by net flow below the root zone of about 53 mm and an increase in water storage (in the first layer) of 29 mm. The model also estimated a water deficit of 135 mm (difference between the potential and current transpiration) for the period. Upward flux from the water table was around 81 mm and piezometric measurements showed 1.5 m recession 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.
|
