Model-based assessment of spatial distribution of stomatal conductance in forage herb ecosystems

Stomatal conductance is an important factor which controls carbon and water exchange. By changing stomatal width, a canopy simultaneously controls both the carbon dioxide supply and water loss during transpiration. Stomatal conductance is a parameter of photosynthesis and can help to estimate canopy growth and development in ecosystems. Therefore, it is a necessary component of transpiration models. The aim of this study was to validate a stomatal conductance model using radiometric measurements of energy balance parameters for vegetated surfaces: vegetated surface temperature, sensible and latent heat flux. Considering atmospheric surface layer stability, the crop was assumed to be a «big-leaf», with stomatal conductance influenced by environmental factors. External conditions not only control stomata width, but also directly affect the transpiration processes. We have tested the stomatal conductance model by J.M. Blonquist et al. (2009) based on radiometric canopy temperature and energy balance components such as latent and sensible heat fluxes. The applicability of the model for estimating stomatal conductance using automated ground-based measurements and remote sensing was first shown. Observations were carried out at two locations with forage herbs (60°5¢6²N, 30°25¢27²E and 60°5¢16²N, 30°24¢32²E) at Bugry in the Leningradskaya Province (on May 15 and 31, 2016, respectively). Model inputs, such as air temperature and humidity, atmospheric pressure, wind speed, radiometric temperature and net radiation of vegetated surface were measured with automatic mobile field agrometeorological equipment AMFAE (Agrophysical Research Institute), with measurements taken every 90 seconds. Ground observations were carried out simultaneously with LandSat-8 satellite data surveys. LandSat-8 is an American Earth observation satellite, it contains two instruments: OLI (Operational Land Imager) has 5 visible bands and 4 near infrared bands, TIRS (Thermal InfraRed Sensor) has 2 longwave infrared bands. LandSat-8 data is freely available on the US Geological Survey. Atmospheric correction of satellite imagery was made using the 6S (Second simulation of the satellite signal in the solar spectrum) open source model with publicly available data of aerosol optical depth at 550 nm provided by the MODIS system and the global digital elevation model ASTER GDEM (data is freely available on the US Geological Survey). Components of the energy balance including net radiation, soil heat flux, sensible and latent heat flux were calculated with the SEBAL (Surface Energy Balance Algorithm for Land) model by W.G.M. Bastiaanssen (1998) using the ground observation meteorological data from AMFAE. Obtained maps of net radiation and sensible and latent heat fluxes were used to estimate the spatial distribution of stomatal conductance over the forage herbs. For stomatal conductance calculations the LandSat-8 data for pixel values representing dense vegetation (NDVI > 0.7) were used. As a result of the study, maps of forage herbs stomatal conductance were obtained depending on the canopy temperature and the components of the energy balance with a stratification of the atmosphere boundary layer.