| Crop evapotranspiration, as one of important researching content of water balance in agriculture field, provides the scientific base for rationally exploiting and utilizing the local water source, making the reasonable irrigation system and reducing the soil salination due to the excessive irrigation, saves the limited water source and improves water use efficiency.The report is conducted to utilize the large lysimeter to observe the cotton evapotranspiration with the technology of drip irrigation under mulch film, researches variable rule of daily evapotranspiration during the cotton growth stage; analyses the cotton evapotranspiration under different period and weather condition, and daily variation of related weather factor; analyses and compares reference crop evapotranspiration during cotton growth within April to October by Hargreaves method, Priestly—Taylor method, Fao Penman—Monteith, Fao—17 Penman method and Correction Penman method; analyses and calculates the evapotranspiration stage and monthly crop coefficient with five methods.Research indicates that the cotton evapotranspiration with drip irrigation under mulch film steps to the peak period after having bud and reaches the maximum during the bloom and boll stage, which module coefficient of evapotranspiration is 53.8%, daily transpiration is 5—7mm/d, crop transpiration is dominate within crop transpiration stage; after boll opening, transpiration reduces gradually, however, the cotton evapotranspiration between the crops within the seedling transpiration is 6.2% of total of evapotranspiration.Under the different weather condition, such as cloudy, the main factor of affecting cotton transpiration is solar radiation, cotton transpiration is direct propitiation to the temperature and wind speed, is inverse propitiation to air relative humidity; in sunshine, wind speed and air relative humidity influence the cotton transpiration greatly, daily variable tendency of evapotranspiration and temperature rise parabola; as to the crop evapotranspiration in daily variation, solar radiation is mainly influencing factor of variation.Analyzing and comparing reference crop evapotranspiration with different calculation, crop evapotranspiration calculated by various given empirical coefficient and local empirical coefficient a, b due to solar radiation exists different, given empirical coefficient a, b by P—M method and Penman method adapts location, while given empirical coefficient a,b by FAO—17P doesn't suit for the location, the main reasons causing the difference attribute to the different parameter and correction chose by various methods, different consideration for the aerodynamics and radiations, different correction resulting from the factors of geography, meteorology and aerodynamics effecting the crop evapotranspiration.Monthly and 10-day reference crop evapotranspiration calculated by Har method, P—T method, Penman method and FAO—17P method exist the district linear relativity, the relative tolerance simulating calculated value of the other four methods by Har method is less than 10% from May to September and is about 20% in April and October.Cotton crop coefficient with drip irrigation under mulch film according to the "the guide of crop evapotranspiration –calculation of crop water requirement" published by FAO in 1998("FAO—56") is almost the same with fit curve of crop coefficient in initial stage, rapid crop development stage and mid-season stage by P—M method, and exist distinct difference in the late season stage, the relative tolerance of practical 10-day evapotranspiration calculated by monthly crop coefficient from June to September by P—T method, P—M method, Penman method, FAO—17P method and Har method is less than 10%. But there were more tolerance April, May and October, which need to be adjusted. The crop coefficient used in calculating of the first 10 days and the last 10 days in May ,the first 10 days in October can be appled with the average value of that of April and May, May and June , September and Oc...
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