tailieunhanh - Evapotranspiration Remote Sensing and Modeling Part 9

Tham khảo tài liệu 'evapotranspiration remote sensing and modeling part 9', khoa học tự nhiên, công nghệ sinh học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Guidelines for Remote Sensing of Evapotranspiration 229 W m-2 . Radiation fluxes are positive when directed towards the land surface the other fluxes are positive when pointed away from the surface. The partition of energy between the terms is largely controlled by the availability of water or moisture in the system. When moisture is not restricted E reaches a maximum and H is small. In order to estimate ET Eq. 1 is solved for ẤE. When applied to remote sensor retrievals Rn is solved entirely from a combination of radiation counting at sensor level and few ground information. Ground or soil heat flux is a minor component in densely vegetated areas but a large term in non-vegetated or sparsely vegetated areas Heusinkveld et al. 2004 . The importance of a better evaluation of the soil heat flux is gaining attention mainly to ensure the EBE closure in such areas. The evaluation of H is the major difficulty. There are several models and approaches to solve for H SEB models and a number of parameters and assumptions are still under debate. The remote sensing models for ET mainly differ in the way H is treated. In the following sections the individual terms of the EBE Eq. 1 will be discussed in further detail. A theoretical description is presented for each term in the EBE in combination with a discussion on the feasibility of data acquisition from remote and ground sources. Net radiation Net radiation Rn is the dominant term in the EBE since it represents the source of energy that must be balanced by the thermodynamic equilibrium of the other terms. The net radiation can also be expressed as an electromagnetic balance of all incoming and outgoing radiation reaching and leaving a flat horizontal and homogeneous surface as Rn S ị -S T L ị -L T 2 Where S is the shortwave radiation nominally between to 3 pm and L is the long wave radiation nominally between 3 to 100 pm. The arrows show the direction of the flux entering ị or leaving T the system. Equation 2 is very