STUDY ON CALCULATION OF LAND SURFACE EVAPOTRANSPIRATION
USING REMOTE SENSING
Zhiming.Zhan^, Qiming Qin", Zhaodong.Feng^ , Xin Wang?
? Institute of RS & GIS , Peking University, Beijing, 100871, China
? Department of Earth and Environmental Studies, Montclair State University,
Upper Montclair, NJ 07043, USA
* the Department of Atmospheric Science, Peking University, Beijing, 100871, Chin
Commission WG VII/2
KEY WORDS: Evapotranspiration(ET), Remote sensing, NOAA/AVHRR, Western Chinese Loess Plateau, SEBS,
Land surface parameters.
ABSTRACT:
Evapotranspiration (ET) plays a significant role in regional and global climates through its partitioning in hydrological cycles, and
its estimation is thus of a great importance in assessing ground water and surface water resources, predicting crop yield and planning
land use. For experimenting the possibility to assess the hydrol
ogical responses of ecological restoration in the Western Chinese
Loess Plateau, a daily ET is comprehensively estimated by inputting NOAA/AVHRR data-derived parameters into surface energy
balance system (SEBS) model. The observed data in the Western Chinese Loess Plateau are used to verify the estimates of input
variables. ET estimation in this research shows that ET in the Western Chinese Loess Plateau is primarily determined by vegetation
coverage, which is in turn determined by soil water availability. This study also demonstrates the remote sensing-aided surface
energy balance model can be used to estimate ET in semiarid areas.
1. INTRODUCTION
Evapotranspiration (ET) plays a significant role in regional and
global climates through its partitioning in hydrological cycles,
and its estimation is thus of a great importance in assessing
ground water and surface water resources, predicting crop yield
and planning land use. The importance is even greater in the
western Chinese Loess Plateau where human-induced
deterioration in ecological condition has altered the path of
regional hydrological cycle, further exacerbating the ecological
conditions. The altered land-surface ecological and
hydrological processes greatly influence the exchanges of water
(vapor) and energy (heat) between land surface and atmosphere.
ET is the process that performs the water and energy exchanges.
In order to understand the impacts of the human-induced
deterioration in ecological and hydrological conditions, ET
needs to be reliably estimated for scientifically assessing the
hydrological responses of ecological restoration and for
practically re-planning the land uses in the western Chinese
Loess Plateau.
Unfortunately, the estimation of ET with an acceptable level of
accuracy has been inhibited by lack of high-resolution temporal
and spatial data. However, development in satellite remote
sensing technologies provides us an opportunity to obtain such
high-resolution data. The Estimation of ET using remote
sensing techniques involves a series of deductive processes by
indexing the relevant parameters. For example, the visible
channels can be used to index the surface albedo reflecting the
land surface properties. The infrared channels provide an
estimate of the land surface temperature from which the vapor
pressure deficit can be estimated. Near infrared and thermal
infrared channels can be used to calculate the vegetation index.
In this paper, we present our research results of estimating ET
in the western part of the Chinese Loess Plateau (102-108°E,
34-38°N) (Fig.1) using satellite remote sensing data. This
approach is based on a feedback relationship in which the land
surface parameters (e.g., land surface temperature, albedo,
emissivity, and vegetation — index) obtained from
NOAA/AVHRR are applied to the surface energy balance
system model (SEBS) to estimate the actual evapotranspiration.
The observed data from weather stations in the western Chinese
Loess Plateau are used to verify the estimates of input variables.
These input variables to estimate ET include net radiation, soil
heat flux, sensible heat flux, latent heat flux, albedo, NDVI,
emissivity, surface temperature and evaporative fraction.
2. METHODOLOGY
2.1 Basic Model
Two basic facts made satellite remote sensing technologies
attractive in estimating land-surface evapotranspiration (ET).
First, satellite remote sensing is solely based on electromagnetic
radiation. Second, ET is an important component in outgoing
long-wave radiation from the surface to the atmosphere. Thus,
by inferring the non-ET components of the electromagnetic
radiation, the ET can be estimated. The surface energy balance
is estimated with the surface energy balance system model
(SEBS):
R=lE+H+C, + PH (1)
where R, is the net radiation, H the turbulent sensible heat flux,
and A E the turbulent latent heat flux ( A=2.49*10°, Wm/mm:
here E is ET), Go soil heat flux (W/mm) , PH the energy for
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