Temporal-spatial Variations Of ET0 And The Applicability Of Simplified Calculation Methods In Guanzhong Region Of Shaanxi Province

Reference crop evapotranspiration(simplified as ET0)is the foundation of crop water requirement and agricultural water-saving irrigation planning. Conducting accurate estimation of ET0 is also meaningful for regional agricultural planting patterns and water resources allocation. In this paper, based on monthly climatic data of 6 meteorological stations in Shannxi Guanzhong region from 1961 to 2001, monthly ET0 were calculated by FAO-56 Penman Monteith equation(simplified as PM), the temporal and spatial variation characteristics of ET0 were analyzed. Considering the PM method needs a lot of parameters or some meteorological stations can not provide complete meteorological resources, this paper sorts out many existing evapotransporation methods. Based on the foundation of simplification of reference evapotranspiration parameters, this paper chooses 16 different simplified methods including 6 temperature-based methods、4 radiation-based methods、6 mass transfer-based methods. The article uses the meteorological data of 3 sites in Guanzhong region from 1961 to 2001 as the basis, and takes PM equation as the standard to analyze the adaptability of different simplified methods. So it can provide possible precision irrigation for some area lacking of meteorological data. The main results are as follows:(1)In Guanzhong region, reference crop transpiration from 1961 to 2001 showed a trend of decrease over time, From the monthly analysis, the maximum of reference crop evapotranspiration presentes in June and the minimum in January, the variation curve like a parabola, annual variation in different meteorological station had good consistency. In space, reference crop evapotranspiration presentes the tendency of increasing gradually from west to east. From the seasonal variation, ET0 of summer took a large proportion, the ET0 from May to August took 55 percent of the whole year, this result indicated that summer has a significant influence on reference crop evapotranspiration.(2)Analyzing the adaptability of temperature-based methods in Guanzhong region, the conclusion that the Trajkovic performeed best can be reached. The Trajkovic method overestimated the value of ET0, however, the PE value is the minimum of all sites in Guanzhong region, the deviation ranges from 0.5%~7%. Hargreaves series methods had a good correlation between annual curve and monthly curve, those methods overestimated the value of ET0, the PE value is larger than 20%,which means the error is bigger than other methods, Among these methods, HS performed best, HAR correction method took the second place, Hargreaves2 and Hargreaves3 performed the worst. Mc Cloud method is the only one which underestimates the value of ET0. In the annual order, it had a little deviation from PM value, but had a bigger deviation in monthly order and performed the worst.(3) Analyzing the adaptability of radiation-based methods in Guanzhong region, Turc method performed the worst, both the value of yearly and monthly ET0 overestimated the PM value and fluctuated significantly, in the critical months of plant growth(from April to August), the results between the Turc and PM method are quite different. The deviation ratio of Priestly-Taylor method is the biggest, which is more than 25% in each station. The value calculated by I-A method are closer to the PM results, but overestimated the value of ET0, the curve of the yearly changed gentlely, showing that the method can not respond to the change of some meteorological elements, which could not reflect ET0 variation precisely; except in June and July, the other months overestimated the value of ET0.(4) In mass transfer-based methods, except for WMO method, other methods all overestimated the ET0. Brockamp-Wenner method performed the worst, the PE value were more than 51.9% in all stations, which is the highest in all methods. Maringer and Trabert method perform the best, the value of PE ranges from 1.8%~18%、7.5%~12.8%, the performance of Dalton and Meyer methods are ordinary, the PE value of WMO is more than 20%.(5) The adaptability of 16 simplified calculation methods in Guanzhong region shows that Trajkovic、I-A、Makkink methods are all in the top three in all stations, which illustrated that the three methods have a strong adaptability in Guanzhong region; In mass-transferred methods, Trabert and Maringer method always in top 10, so those methods can be used in Guanzhong region when the temperature and radiation are absent and the wind speed and humidity are available. Among the Hargreaves series methods, HS performed best, HAR correction method took the second place, however, the HAR correction method does not improve the calculation accuracy.