12 research outputs found
A biophysical approach using water deficit factor for daily estimations of evapotranspiration and CO<sub>2</sub> uptake in Mediterranean environments
Estimations of ecosystem-level evapotranspiration (ET) and CO2
uptake in water-limited environments are scarce and scaling up ground-level
measurements is not straightforward. A biophysical approach using remote
sensing (RS) and meteorological data (RS–Met) is adjusted to extreme
high-energy water-limited Mediterranean ecosystems that suffer from
continuous stress conditions to provide daily estimations of ET and
CO2 uptake (measured as gross primary production, GPP) at a spatial
resolution of 250 m. The RS–Met was adjusted using a seasonal water deficit
factor (fWD) based on daily rainfall, temperature and radiation data. We
validated our adjusted RS–Met with eddy covariance flux measurements using a
newly developed mobile lab system and the single active FLUXNET station
operating in this region (Yatir pine forest station) at a total of seven
forest and non-forest sites across a climatic transect in Israel (280–770 mm yr−1). RS–Met was also compared to the satellite-borne MODIS-based ET and GPP products (MOD16
and MOD17, respectively) at these sites.Results show that the inclusion of the fWD significantly improved the
model, with R = 0.64–0.91 for the ET-adjusted model (compared to 0.05–0.80
for the unadjusted model) and R = 0.72–0.92 for the adjusted GPP model
(compared to R = 0.56–0.90 of the non-adjusted model). The RS–Met (with the
fWD) successfully tracked observed changes in ET and GPP between dry and
wet seasons across the sites. ET and GPP estimates from the adjusted RS–Met
also agreed well with eddy covariance estimates on an annual timescale at
the FLUXNET station of Yatir (266 ± 61 vs. 257 ± 58 mm yr−1
and 765 ± 112 vs. 748 ± 124 gC m−2 yr−1 for ET and GPP,
respectively). Comparison with MODIS products showed consistently lower
estimates from the MODIS-based models, particularly at the forest sites.
Using the adjusted RS–Met, we show that afforestation significantly
increased the water use efficiency (the ratio of carbon uptake to ET) in
this region, with the positive effect decreasing when moving from dry to
more humid environments, strengthening the importance of drylands
afforestation. This simple yet robust biophysical approach shows
promise for reliable ecosystem-level estimations of ET and CO2 uptake
in extreme high-energy water-limited environments
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Effect of Surface Heterogeneity on the Boundary-Layer Height: A Case Study at a Semi-Arid Forest
We investigate the effects of an isolated meso-γ-scale surface heterogeneity for roughness and albedo on the atmospheric boundary-layer (ABL) height, with a case study at a semi-arid forest surrounded by sparse shrubland (forest area: 28km2, forest length in the main wind direction: 7 km). Doppler lidar and ceilometer measurements at this semi-arid forest show an increase in the ABL height over the forest compared with the shrubland on four out of eight days. The differences in the ABL height between shrubland and forest are explained for all days with a model that assumes a linear growth of the internal boundary layer of the forest through the convective ABL upwind of the forest followed by a square-root growth into the stable free atmosphere. For the environmental conditions that existed during our measurements, the increase in ABL height due to large sensible heat fluxes from the forest (600Wm-2 in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer
Recommended from our members
Effect of Surface Heterogeneity on the Boundary-Layer Height: A Case Study at a Semi-Arid Forest
We investigate the effects of an isolated meso-γ-scale surface heterogeneity for roughness and albedo on the atmospheric boundary-layer (ABL) height, with a case study at a semi-arid forest surrounded by sparse shrubland (forest area: 28km2, forest length in the main wind direction: 7 km). Doppler lidar and ceilometer measurements at this semi-arid forest show an increase in the ABL height over the forest compared with the shrubland on four out of eight days. The differences in the ABL height between shrubland and forest are explained for all days with a model that assumes a linear growth of the internal boundary layer of the forest through the convective ABL upwind of the forest followed by a square-root growth into the stable free atmosphere. For the environmental conditions that existed during our measurements, the increase in ABL height due to large sensible heat fluxes from the forest (600Wm-2 in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer