Estimating Daily Evapotranspiration On The Grassland Of The Northeast Based On A Simplified Two-layer Model

Vegetation is the general term for plants, including forest, shrub, grass, crops and so on.The function of vegetation is soil conservation, intercepting rainfall, slowing runoff and sand prevention. Grass is a large number of vegetation, according to statistics, the grassland coverage of China has reached 41.7% of the total land area, which is in second place of the world. In the range of ecological processes of our land, photosynthesis, evaporation distribution, nutrient cycling, waste decomposition, and pasture are closely related to the biochemical composition, the most important one is evapotranspiration. Evapotranspiration is a water evaporation process that the uneven spatial distribution of underlying surface transports to the atmospheric surface, including land surface evaporation, water surface evaporation and plant transpiration, is an important parameter of water resource management. Since the mid-20th century, more and more attention has been paid to the research of evapotranspiration for the purpose of water conservation. The conventional method of estimating evapotranspiration on the land surface is by the field measurements, which is obtained through a time and space scales of long-term data accumulation. This method has not only a long and complicated experimental process, but also has serious restrictions on time and region. The rise of remote sensing technology has greatly improved the method for estimating evapotranspiration of vegetation, we can greatly improve the accuracy of estimating evapotranspiration by vegetation spectral reflectance characteristics and some of the ground observation data.The grassland of northeast is rich in resources, which is a typical grassland resources area of scientific research. The research area of this study is in Qianguo County of Jilin Province, which has vast area of grassland, large vegetation coverage, flat land surface and low relative temperature. The evapotranspiration process of grassland is one of the most important biosphere physical and chemical processes. From the economic factors, taking a research on the grassland evapotranspiration of northeast is closely related livestock and industry; from the perspective of sustainable development factors, study the grassland evaporation process of northeast have a great significance on climate, water resources, controlling environmental pollution, etc. The purpose of this study is to achieve read, display high-capacity remote sensing data and to extract information on the application of each band, then combined with header information of remote sensing data, ground information and the information of USGS to estimate the land surface parameters and vegetation parameters, after that establish a simplified two-layer model to estimate the daily evapotranspiration of Qianguo County in 18 September 2007 based on the energy balance principle, at last compare the estimated data with the measured data and take a analysis on them.The application is achieved in VS2005 based on GDAL and MFC, in which the MFC is used to implement the framework, the GDAL is used to implement conversion and reading of the raster data. The main function of the application is to achieve fast reading of the large img file which is got from LANDSAT-5 TM, analyze the information of the remote sensing data's each band, combined with the header information of the remote sensing data, ground information and the information of USGS to estimate each band's radiation and reflectivity, then estimate NDVI, biomass, vegetation coverage, surface emissivity, surface albedo, surface temperature and other parameters.The two-layer model for estimating evapotranspiration is relative in terms of single-layer model. Single-layer model is referring to the plant canopy, the main meaning of single-layer model is estimating the surface fluxes with the water and thermal characteristics of the plant canopy, remote sensing data, ground observation data, air data, aerodynamic resistance, radiation energy and so on. The condition to establish a single-layer model is relatively easy, because the single-layer model only considers the aerodynamic resistance of the plant canopy, but does not consider the differences that the water and thermal characteristics between the soil and plant canopy. So the estimating accuracy of a single-layer model is high only when the study region is completely covered by vegetation, if the vegetation is not completely covered, there will be some errors. A two-layer model will consider the soil and vegetation canopy as a continuous turbulent exchange, calculate the contribution of vegetation canopy and soil by gradient diffusion theory, divide the aerodynamic resistance of the single-layer model into vegetation canopy aerodynamic resistance and soil aerodynamic resistance. A two-layer model improves the accuracy, but increase the number of the parameters that make the two-layer model be difficult to use.The simplified two-layer model will not consider the vegetation canopy and soil as a continuous source of turbulent transport, nor as a completely and independent process of turbulent transport, but change the source of heat flux to the reference height, so the interaction between vegetation canopy and soil will be reduced. In addition, the vegetation coverage is introduced to the simplified two-layer model, on the one hand that the heat flux contribution of the vegetation canopy and soil are related to the vegetation coverage when we consider the interaction between the vegetation canopy and soil being reduced, on the other hand we can use the vegetation coverage to estimate the aerodynamic resistance. The reason of introducing the vegetation coverage to the simplified two-layer model is to reduce the error after we change the source of heat flux to the reference height. The simplified two-layer greatly reduces the number of parameters to improve the speed of estimating daily evapotranspiration, through the study we find that the highest estimating error is in the 1 mm·day-2 that the accuracy is very good and meets the actual needs, it has a good value for estimating daily evapotranspiration and taking a sensible configuration to water source.