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Weizmann Institute of Science (2008)

Partitioning the evapotranspiration flux of the Yatir semi-arid forest

Raz Yaseef, Naama

Titre : Partitioning the evapotranspiration flux of the Yatir semi-arid forest

Auteur : Raz Yaseef, Naama

Université de soutenance : Weizmann Institute of Science

Grade : Dissertations (Ph.D.) 2008

Résumé
Global climate change is predicted to be associated with both reduced total precipitation and increased storm intensities over many regions, including the entire Mediterranean region. The combined effects are difficult to predict and can greatly benefit from studies in dry forest ecosystems exposed to large inter-annual climatic variations. In such ecosystems, the evapotranspiration flux (ET) is the major component of the hydrological cycle and often balance most precipitation (P). Therefore, partitioning ET to its components, plant transpiration (T) and soil evaporation (E), is critical for understanding water availability for carbon assimilation, soil respiration, and water use efficiency, as well as for ecosystem and water management. We report on a four-year study (2003-2007) in the Yatir semi-arid pine forest in southern Israel (40 years old P. halepensis ; LAI=1.5 ; mean precipitation 280 mm yr-1 ) that showed resistance to seasonal draughts, and carbon uptake capability close to that of the global average for forests (GPP and NEE mean values of 850 and 230 gC yr-1 respectively). E was directly measured (bi-weekly 14 sites) using a modified soil respiration chamber (LI-COR). Soil water content (θ) was recorded in three pits with TDRs to depth of 130 cm. These measurements were combined with background, continuous measurements of ET (eddy flux measurements), T (heat-pulse measurements), and meteorological measurements (soil and air temperatures, VPD, radiation PAR or Rn and wind speed). Large spatial variability in E (daily SD between sites up to ±47%) was correlated best with PAR and θ. The proportion of forest-floor-shaded-fraction (SF) was a function of tree height, canopy width and tree density, and simulating SF provided a basis for estimating E/ET. Seasonal patterns in E and T were differential : E peaked twice - during early and late winter (up to 0.80 mm d-1 ) and T peaked in spring (up to 1.20 mm d-1 ). Low E (0.10 mm day-1 ) was measured during mid winter (daily max. temp. below 15˚C) and low E and T were measured in summer (θ below 10%). E correlated on the seasonal time-scale with environmental parameters : with θ during mostly in the wetting and drying seasons, and with Rn during winter and the dry summer. The partitioning of ET was influenced by precipitation patterns. 82% of rain events were <5 mm, keeping only the upper soil layer θ above 20% during most of the wet season. In contrast, θ of the root zone (15-130 cm) depended on larger, less common rainstorms. High soil water holding capacities resulted in increased θ at depth only from mid-winter, but maintained in some cases throughout the dry summer. Accordingly, the E/ET ratio varied seasonally with maximum during early winter, and minimum during spring. Ecosystem water use efficiency (WUEe) correlated better with the ratio P/RD (RD=number of rainy days) than with P, presumably because P/RD serves as indicator of precipitation patterns. A complete ecosystem hydrological budget was constructed for Yatir indicating that, on average, P was supplemented by vapor adsorption in late summer (5%) and was balanced by interception (11%), soil storage runoff and deep drainage (6%), E (36%) and T (45%). This study provides the first comprehensive hydrological budget for a semi-arid pine forest. The results suggest that increased storm intensity could at least partly compensate for decreased total precipitation in this region, contributing to the maintenance of relatively high productivity of the forest under the global warming scenarios predicted for this region. הגשם, המאופיין באזור המחק

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