The Study On The Crop Evapotranspiration Measurement And Computation

Crop evapotranspiration is composed of two important aspects: soil evaporation and plant transpiration. Since crop evapotranspiration played an important role in water cycle and water balance system, to study the crop evapotranspiration is vital for estimating crop water requirement, proper planning of irrigation scheduling, irrigation system design, hydrological studies and drainage researches. Based on the weather data measured in Daxing experimental station of China Institute of Water Resources and Hydropower Research, this article analyzed and validated the Penman-Monteith model and its subentries. In addition, measuring the actual crop evapotranspirations using advanced instruments, such as lysimeter and eddy covariance , this article contrasted the crop evapotranspirations measured in winter wheat season and summer maize season, and analyzed the discrepancies. The main conclusions are as following:(1) For computing reference evapotranspiration, because the Penman-Monteith model can achieve reasonable results on global level, it is recommended as the standard way to calculate reference evapotranspiration. The reference evapotranspiration calculated by Penman-Monteith model is composed of two important aspects: aerodynamic item and radiative item. Studies show that good correlation can be found between reference evapotranspiration and radiative item. As a result, it is essential to calculate the radiative item exactly. In addition, because it is hard to measure radiation data, a series of equations for estimating radiation data are recommended by Penman-Monteith model. Studies show that, the canopy reflection coefficient shows reasonable results. However, the solar radiation coefficients which can not get acceptable results need to calibrate for different area. Meanwhile, the equations for computing the actual vapour pressure recommended by aerodynamic item show reasonable results, and the three different temporal models for computing reference evapotranspiration recommended by Penman-Monteith model can also get acceptable results.(2) For measuring the actual crop evapotranspiration, based on the different principles, the instruments can be divided as two aspects: water balance method and energy balance method. Studies show that, for horizontal spatial scale, because the instruments stand for different horizontal spatial scales, the values of crop evapotranspiration measured by different instruments are different. For winter wheat season and summer maize season, the values of crop evapotranspiration measured by lysimeter are as two times as that measured by eddy covariance. Meanwhile, for vertical spatial scale, the reference surfaces measuring crop evapotranspiration are different for the different instruments. Because of the effect of crop canopy, the resistances of hydrosphere transfer are increased, and the time for hydrosphere moving between different reference surfaces is delayed. Bad correlation can be found between the crop evapotranspirations measured by different instruments.