scholarly journals Biophysical impacts of Earth greening largely controlled by aerodynamic resistance

2020 ◽  
Vol 6 (47) ◽  
pp. eabb1981
Author(s):  
Chi Chen ◽  
Dan Li ◽  
Yue Li ◽  
Shilong Piao ◽  
Xuhui Wang ◽  
...  
Keyword(s):  
Land Surface ◽  
Aerodynamic Resistance ◽  
Turbulent Heat Transfer ◽  
Dominant Factor ◽  
Turbulent Heat ◽  
Area Index ◽  
The Earth ◽  
Physically Based ◽  
Annual Scale ◽  
Increasing Trends

Satellite observations show widespread increasing trends of leaf area index (LAI), known as the Earth greening. However, the biophysical impacts of this greening on land surface temperature (LST) remain unclear. Here, we quantify the biophysical impacts of Earth greening on LST from 2000 to 2014 and disentangle the contributions of different factors using a physically based attribution model. We find that 93% of the global vegetated area shows negative sensitivity of LST to LAI increase at the annual scale, especially for semiarid woody vegetation. Further considering the LAI trends (P ≤ 0.1), 30% of the global vegetated area is cooled by these trends and 5% is warmed. Aerodynamic resistance is the dominant factor in controlling Earth greening’s biophysical impacts: The increase in LAI produces a decrease in aerodynamic resistance, thereby favoring increased turbulent heat transfer between the land and the atmosphere, especially latent heat flux.

scholarly journals Physically-Based Retrieval of Canopy Equivalent Water Thickness Using Hyperspectral Data

2018 ◽  
Vol 10 (12) ◽  
pp. 1924 ◽  
Author(s):  
Matthias Wocher ◽  
Katja Berger ◽  
Martin Danner ◽  
Wolfram Mauser ◽  
Tobias Hank
Keyword(s):  
Water Content ◽  
Optical Sensors ◽  
Land Surface ◽  
Hyperspectral Data ◽  
Canopy Reflectance ◽  
Area Index ◽  
Equivalent Water Thickness ◽  
Crop Type ◽  
Physically Based ◽  
Leaf Level

Quantitative equivalent water thickness on canopy level (EWTcanopy) is an important land surface variable and retrieving EWTcanopy from remote sensing has been targeted by many studies. However, the effect of radiative penetration into the canopy has not been fully understood. Therefore, in this study the Beer-Lambert law is applied to inversely determine water content information in the 930 to 1060 nm range of canopy reflectance from measured winter wheat and corn spectra collected in 2015, 2017, and 2018. The spectral model was calibrated using a look-up-table (LUT) of 50,000 PROSPECT spectra. Internal model validation was performed using two leaf optical properties datasets (LOPEX93 and ANGERS). Destructive in-situ measurements of water content were collected separately for leaves, stalks, and fruits. Correlation between measured and modelled water content was most promising for leaves and ears in case of wheat, reaching coefficients of determination (R2) up to 0.72 and relative RMSE (rRMSE) of 26% and in case of corn for the leaf fraction only (R2 = 0.86, rRMSE = 23%). These findings indicate that, depending on the crop type and its structure, different parts of the canopy are observed by optical sensors. The results from the Munich-North-Isar test sites indicated that plant compartment specific EWTcanopy allows us to deduce more information about the physical meaning of model results than from equivalent water thickness on leaf level (EWT) which is upscaled to canopy water content (CWC) by multiplication of the leaf area index (LAI). Therefore, it is suggested to collect EWTcanopy data and corresponding reflectance for different crop types over the entire growing cycle. Nevertheless, the calibrated model proved to be transferable in time and space and thus can be applied for fast and effective retrieval of EWTcanopy in the scope of future hyperspectral satellite missions.

scholarly journals Trends in Land Evapotranspiration over Canada for the Period 1960–2000 Based on In Situ Climate Observations and a Land Surface Model

2007 ◽  
Vol 8 (5) ◽  
pp. 1016-1030 ◽  
Author(s):  
Richard Fernandes ◽  
Vladimir Korolevych ◽  
Shusen Wang
Keyword(s):  
Land Surface ◽  
Land Surface Model ◽  
Surface Radiation ◽  
Soil Conditions ◽  
Surface Model ◽  
Eastern Canada ◽  
Area Index ◽  
Increasing Temperature ◽  
Increasing Trends

Abstract An assessment of annual trends in actual evapotranspiration (AET) and associated meteorological inputs is performed at 101 locations across Canada with available long-term hourly surface climate observations to determine if AET in Canada is increasing in relation to observed increases in air temperature. AET was estimated for the dominant land cover class, with representative soil and leaf area index conditions, within a 50 km × 50 km window around each location for the period 1960–2000. The Ecological Assimilation of Land and Climate Observations (EALCO) land surface model, which simulates coupled carbon, energy, and water cycles, was applied to estimate AET on a half-hourly basis at each location using in situ meteorological measurements and ambient atmospheric CO2 concentrations. Increases in annual AET, of up to 0.73% yr−1, were identified at 81 locations, and decreases, of up to 0.25% yr−1, were found at the remaining 20 stations. Statistically significant increasing trends were detected in 35% of the locations with the majority corresponding to Atlantic and Pacific coastal regions. Increasing trends were generally related to increasing temperature and total downwelling surface radiation trends in eastern Canada and increasing temperature, surface radiation, and precipitation trends in western Canada. In sharp contrast to other studies based on simpler AET models, annual AET trends in the prairie climate zone were mixed in terms of increases and decreases with no locations showing statistically significant trends. Future studies focused on scaling AET model estimates to subbasins or basins are required both to account for this spatial variability in soil conditions and to permit water budget closure validation.

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