Long-term evolution of evapotranspiration components in a semi-arid forest using chambers measurement of soil evaporation

Soil evaporation (Es) is a significant hydrological component in dry ecosystems and its quantification is critical to the understanding of ecosystem response to change. It is, however, often estimated as a residual in the hydrological balance because of measurement difficulties. Here, we use continuous, high precision chamber-based direct measurements of soil evaporation (Es) in a semi-arid Pinus halepensis forest to partition eddy covariance-based evapotranspiration (ET) to Es and tree transpiration (Et) and assess its daily and seasonal dynamics, and for comparison with measurements carried out at the same site ten years earlier. The ecosystem is characterized by a high annual Es/ET ratio of 0.26, and an Et/ET of 0.63. Es diminished in the long dry season, but as much as 74 ± 5% of the residual flux was due to the re-evaporation of nighttime moisture adsorption (negative Es), which may provide critical protection from soil drying in summer. Across the long-term observation period (over 10 years), an increase in the transpiration ratio (ΔTR, where TR=Et/ET) of +29% (from 0.49 to 0.63) was associated with the increase in leaf area index (LAI) of +44% observed. However, the ratio of TR/LAI remained constant at ~0.31, with persistently closed hydrological balance (ET/P of 0.94 to 1.07). Rainfall use efficiency (the ratio of annual net primary production/annual precipitation; NPP/P) was on average 0.82 (g C m^-2/Kg H₂O) across the observation period. The observed mean Et/ET values are similar to the estimated global mean values (0.64 ± 0.13), but are attained at a much higher aridity index of 5.5 than the mean one, reflecting adjustments that indicate the potential for expanding forestation into dry regions, and highlight the importance of soil evaporation fluxes in low-density semi-arid forests.