Vegetation Canopy Temperature Research Articles

Observation‐Based Evaluation of Local Climate Effect of Terrestrial Vegetation in Temperate Zones

AbstractTerrestrial vegetation modulates the land‐atmosphere dynamics and is one of the fundamental forces for climatic variability. The vegetation feedback may enhance or offset a portion of ongoing global warming. However, it is challenging to disentangle the role of vegetation from the effect of other drivers, primarily due to lacking suitable reference without vegetation. Fortunately, China has recorded the land surface temperature (LST) on bare land in 2400s weather stations, providing a deal bare‐land reference. Here, we used satellite measurements of the vegetation's canopy temperature, contrasted with the surface temperature of bare land, to quantify the vegetation's effect on LST. Results show that vegetation has a cooling effect during daytime and a warming effect at night, and the magnitude of the former is significantly higher than that of the latter. Consequently, vegetation has a net cooling effect and reduces LST up to 2.18°C in China. Spatially, the cooling effect of vegetation decreases by 0.25°C for a unit increase of latitude. Based on land surface energy balance, the spatial pattern of vegetation's net effect is mainly driven by vegetation‐induced evapotranspiration and albedo. As latitude increases, the decrease of vegetation density significantly reduces the evapotranspiration‐induced cooling effect while having no distinct impact on the albedo‐induced warming effect. As a result, the net cooling effect of vegetation has an evident latitudinal gradient and a clear seasonal contrast. Additionally, the vegetation's effect on LST is sensitive to solar radiation and snow cover changes.

Estimation of surface latent heat fluxes in an oasis utilizing a two-source energy balance model based on land surface temperature decomposition

Latent heat flux is the main indicator of regional water-heat balance and plays an important role in drought monitoring and water resource management. Here, we attempt to estimate latent heat flux using a two-source energy balance model (TSEB). The decomposition algorithm of soil surface temperature and vegetation canopy temperature is discussed, and it is a key factor for calculating the latent heat flux in the TSEB model. Temperature decomposition was conducted using two methods: one is based on a simple linear relationship between the canopy temperature and directional radiation temperature and the other is based on soil latent heat flux expressed by the Priestley–Taylor formula. Then, the soil temperature was estimated using the soil latent heat flux. The estimation of the surface heat flux was based on the soil and vegetation canopy temperatures. The results show that the Priestley–Taylor formula method provided more accurate estimates of the latent heat flux than the linear relation method, and the reliability and precision were improved. The root-mean-squares error of the former method decreased by 38.8% compared with the latter method. The TSEB model was used to estimate the surface heat flux, and it was feasible for monitoring drought in typical drought-prone regions.

The dynamical evolution theory of the isolated oasis system

Assume that an oasis and its surrounding desert consist of an isolated system without mass and energy exchange with the outer environment. The characteristics of oasis evolution have been explored under the condition of system energy conservation. The results show that oasis evolves with two equilibrium states. The first equilibrium suggests a stable expansive and an unstable degraded oasis whereas the second equilibrium indicates a stable shrink and an unstable increase of the oasis area. If one equilibrium state is unstable, the components of the isolated system (oasis and desert) would tend to be no energy exchange and they each reach to energy balance respectively. Oasis would maintain its initial area in this case. Further analyses point out that the two equilibrium states have completely different characteristics. In the first equilibrium state, a higher vegetation albedo, lower soil albedo and larger canopy resistance, and direr soil both contribute to the oasis area expansion, accompanying an excessive large desert soil and vegetation canopy temperature difference (SCTD). In the second equilibrium state, however, a lower vegetation albedo, higher soil albedo and small canopy resistance, and wetter soil benefit the oasis area to stay near its initial value, following a moderate SCTD. The convergent trajectories of the initial values in phase space are influenced by the separatrices of the equilibrium points. Higher temperature is an advantage factor for initial values convergent to the oasis expansion solution.

Deriving the Surface Soil Heat Flux from Observed Soil Temperature and Soil Heat Flux Profiles Using a Variational Data Assimilation Approach

Abstract A novel approach to infer surface soil heat fluxes from measured profiles of soil temperature, soil heat flux, and observations of the vegetation canopy temperature and the incoming shortwave radiation is evaluated for the Cabauw measurement facility in the Netherlands. The approach is a variational data assimilation approach that uses the applied measurements to optimize, on a daily basis, parameter values of a model that describes the heat transport between the vegetation canopy and the surface and within the soil column. Insertion of error characteristics that either are inferred from the field data themselves or are derived from literature leads to valid estimates of the cost function for about 100 days in 2003. The approach gives values of the model parameters that compare well to values derived from the literature, although values for the soil conductivity and the volumetric heat capacity of the soil start to differ from the literature values at the end of 2003, possibly because of specific soil characteristics and the extreme dryness of the summer of 2003. The model gives estimates of the surface soil heat flux that compare well to estimates using the currently operational lambda approach, provided that the latter is adapted to account for the disturbance of the soil heat flux at the locations of the heat flux plates. Only when the surface soil heat flux is very small or very large does the new approach give estimates of the surface soil heat flux that differ from those obtained with the lambda approach.

Relation between vegetation canopy surface temperature and the Sun-surface geometry in a mountainous region of central Italy

Evapotranspiration is the dominant energy exchange process in dense vegetated environments with an adequate water supply. If water is available vegetation canopy temperatures do not respond immediately upon intercepting solar radiation because of the apportionment of absorbed solar radiation into sensible and latent heat. This lag in the thermal conditions of vegetation canopy following the incident solar flux can be even more complex after sunrise because the presence of dew on the foliage requires more available energy investment in evaporating water and less energy spent in warming the foliage. The aim of this Letter, which is based on remotely-sensed thermal data obtained from Landsat Thematic Mapper in the daytime of a clear summer day, is to investigate the relationship between canopy surface temperatures and the incident solar radiation for a forested montainous landscape of central Italy. Results show that, under the conditions of our experiment, a time lag of one hour considerably increases the linear relation between vegetation canopy temperature and local solar illumination angle.

Effects of vegetation canopy structure on remotely sensed canopy temperatures

Remote sensing of vegetation temperatures is a promising technique for inferring plant water stress and yield on a large spatial scale. The effects of vegetation canopy structure on thermal infrared sensor response need to be understood before vegetation surface temperatures of canopies with low percentages of ground cover can be accurately inferred. The response of a sensor is a function of vegetation geometric structure, the vertical surface temperature distribution of the canopy components, and sensor view angle. Large deviations between the nadir sensor effective radiant temperature (ERT) and vegetation ERT for a soybean canopy were observed throughout the growing season. The nadir sensor ERT of a soybean canopy with 35% ground cover deviated from the vegetation ERT by as much as 11°C during the mid-day. These deviations were quantitatively explained as a function of canopy structure and soil temperature. Remote sensing techniques which uniquely determine the vegetation canopy temperature(s) from the sensor response need to be studied.