Dynamics of evapotranspiration partitioning in a semi-arid forest as affected by temporal rainfall patterns

We extend our recent study of the effects of tree density on evapotranspiration (ET) partitioning in a semi-arid pine forest by examining the influence of the temporal patterns in rainfall (P) on the dynamic contributions of tree transpiration (T _t), soil evaporation (E _s) and rainfall interception (I _P) to total ET. Soil evaporation accounted for 39% of average annual ET over the four-year period, and was associated with soil moisture content in the upper 5cm and solar radiation, therefore peaking during the wetting and drying seasons (up to 0.75mmday ^-1). In the dry summer, E _s diminished and as much as 50% of the residual flux was due to re-evaporation of moisture condensed at night (adsorption). Tree transpiration accounted for 49% of average annual ET, and was associated with soil moisture at a depth of 10-20cm. Transpiration peaked only in late spring (1.5mmday ^-1), after the accumulation of large storms allowing infiltration below the topsoil. Moisture at these depths was maintained for longer periods and was even carried over between rain seasons following a high precipitation year. Interception was 12% of annual ET but was larger than 20% during the rainy period. The results indicated that both T _t/ET and E _s/ET could vary between 30% and 60% due to their differential response to seasonal environmental drivers. Annual T _t/ET, a major parameter indicating forest productivity and survival, was more influenced by the occurrence of large storms (>30mm; P ₃₀/P ratio) than by P itself. In an assessment of the potential warming and drying trends predicted for the Mediterranean region in the next century, changes in both total precipitation and in its temporal patterns must be considered.