ALBERT

Description: 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 was previously proposed for ecosystem-level assessment relying on vegetation index and meteorological data (RS-Met) in temperate Mediterranean ecosystems. However, these RS-Met models have not been tested yet in extreme high-energy water-limited ecosystems that suffer from continuous stress conditions. Owing to the lack of ET and CO2 flux estimations in the Eastern Mediterranean, we examined the RS-Met approach using a newly developed mobile lab system and the single active Fluxnet station operating in this region, in seven forest and non-forest sites across a climatic transect in Israel (280–770 mm y−1). The RS-Met models were used with and without the addition of a seasonal drought stress factor (fDS), which was based on daily rainfall, temperature and radiation data. Results show that the RS-Met models with the inclusion of the fDS were significantly improved compared to the non-fDS models (r=0.64–0.91 compared to 0.05–0.80; P=0.06 and r=0.72–0.92 compared to r=0.56–0.90; P〈0.01 for ET and GPP, respectively). These, successfully tracked observed seasonal changes in ET and GPP across sites (ETMOD=0.94×ETEC+0.28; r=0.82; MAE=0.54 mm d−1; N=243 d, and GPPMOD = 0.99×GPPEC + 0.51; r=0.86; MAE=1.03 gC m−2 d−1; N=252 d). Modeled ET and GPP also agreed well with eddy covariance estimates at the annual timescale in the Fluxnet station located in the dryland pine forest of Yatir (266±61 vs. 257±58 mm y−1 and 765±112 vs. 748±124 gC m−2 y−1 for ET and GPP, respectively). Using the RS-Met models, we were able to show the effect of afforestation on water vapor and CO2 fluxes in this region. Afforestation was responsible for a significant increase in water use efficiency (WUE) with positive effect decreasing when moving from dry to more humid environments, strengthening the importance of drylands afforestation. This simple but yet robust biophysical approach show a promise for reliable ecosystem-level estimations of ET and CO2 uptake in extreme high-energy water-limited environments when adjusting for drought stress effects.

Description: 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.

Description: We present a simple model to retrieve actual evapotranspiration (ET) solely from satellites (PaVI-E). The model is based on empirical relationships between vegetation indices (NDVI and EVI from MODIS) and total annual ET (ETAnnual) from 16 FLUXNET sites representing a wide range of plant functional types and ETAnnual. The model was applied separately for (a) annual vegetation systems (i.e., croplands and grasslands) and (b) systems with combined annual and perennial vegetation (i.e., woodlands, forests, savannah and shrublands). It explained most of the variance in ETAnnual in those systems (71% for annuals, and 88% for combined annuals and perennials systems) while multiple regression and modified Temperature and Greenness models using also land surface temperature did not improve its performance (p 〉 0.1). PaVI-E was used to retrieve ETAnnual at 250 m spatial resolution for the Eastern Mediterranean from 2000 to 2014. Models' estimates were highly correlated (R = 0.92, p 〈 0.01) with ETAnnual calculated from water catchments balances along rainfall gradient in the Eastern Mediterranean. They were also comparable to the coarser resolution ET products of MSG (LSA-SAF MSG ETa, 3.1 km) and MODIS (MOD16, 1 km) at 148 Eastern Mediterranean basins with correlation coefficients (R) of 0.75 and 0.77 and relative bias of 5.2 and −5.2%, respectively (p 〈 0.001 for both). The proposed model is expected to contribute to hydrological study in the Eastern Mediterranean assisting in water resource management, which is one of the most valuable resources of this region.

Description: We present a model to retrieve actual evapotranspiration (ET) from satellites' vegetation indices (Parameterization of Vegetation Indices for ET estimation model, or PaVI-E) for the eastern Mediterranean (EM) at a spatial resolution of 250 m. The model is based on the empirical relationship between satellites' vegetation indices (normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) from MODIS) and total annual ET (ETAnnual) estimated at 16 FLUXNET sites, representing a wide range of plant functional types and ETAnnual. Empirical relationships were first examined separately for (a) annual vegetation systems (i.e. croplands and grasslands) and (b) systems with combined annual and perennial vegetation (i.e. woodlands, forests, savannah and shrublands). Vegetation indices explained most of the variance in ETAnnual in those systems (71 % for annuals, and 88 % for combined annual and perennial systems), while adding land surface temperature data in a multiple-variable regression and a modified version of the Temperature and Greenness model did not result in better correlations (p 〉 0.1). After establishing empirical relationships, PaVI-E was used to retrieve ETAnnual for the EM from 2000 to 2014. Models' estimates were highly correlated (R = 0.92, p 〈 0.01) with ETAnnual calculated from water catchment balances along rainfall gradient of the EM. They were also comparable to the coarser-resolution ET products of the Land Surface Analysis Satellite Applications Facility (LSA-SAF MSG ETa, 3.1 km) and MODIS (MOD16, 1 km) at 148 EM basins with R of 0.75 and 0.77 and relative biases of 5.2 and −5.2 %, respectively (p 〈 0.001 for both). In the absence of high-resolution (〈 1 km) ET models for the EM the proposed model is expected to contribute to the hydrological study of this region, assisting in water resource management, which is one of the most valuable resources of this region.