The Study Of Evapotranspiration Of The Subalpine Meadow In Tianlaochi Catchment In The Upper Reaches Of Heihe River

Grassland accounts for about 25% of the world’s land area, is an important part of the ecosystem, and the ratio of grassland area of total area of Q ilian National Nature Reserve reached 50.54%, the subalpine meadow distributed widely and is the main part of the grassland ecosystem, which plays an important role in ecosystem service function in Q ilian Mountain area. The study area as water conservation forest has the most abundant vegetation and complete biological communities, which is located in the depths of the Q ilian Mountain. Subalpine meadow accounts for 7.45% of the total area of the study area, the ecological environment is very overgrazing, rodent activity is frequent, resulting fact the evapotranspiration of the grassland. Therefore, the study of subalpine meadow evapotranspiration on the local grazing has important guiding significance, also provide some reference for local authorities.The study using self- made lysimeter observe the evapotranspiration of subalpine meadow in the study area, ma inly includes: 1. observation of evapotranspiration at 8 am and 8 pm every day(including water evaporation evaporation); 2. observation of evapotranspiration on hour scale once a week(if weather conditions is bad, such as rainfall, push back a day, including water evaporation observation); 3. the relationship between evaporation and meteorological factor; 4. the characteristics of evapotranspiration obserced by lysimeter and calculated with vorticity data and its relationship with model simulation values; 5. with improved crop coefficient calculation method to calculate the coefficient of subalpine meadow, and tests this with the observed value. Draw the following conclusion:1. On the month scale, the order of the the average daily evapotranspiration is 2013>2011>2014>2012;In the year 2011 and 2012, the order of the the average daily evapotranspiratio of every month is June>August>July, and in the year 2013 and 2014, the order respectively are August >June>July and July>August>June. The average daily evapotranspiration has a certain relationship with the rainfall, the time of rain in one day is different, have different effects on evapotranspiration. The relationship between daily evapotranspiration and meteorological elements is wind speed>saturated vapor pressure>relative humidity>net radiation, the R2 is 0.4174, 0.3598, 0.3461 and 0.3322, all four meteorological elements have affect on daily evapotranspiration. The evapotranspiration of non- growing season(from the middle of October to mid-april the following year) calculated with vorticity data from September 16, 2013 to September 6, 2014, overall showed a trend of fluctuations, is small, basic below 1 mm/d, average daily evapotranspiration is 0.64 mm/d, the growing season(from the middle of October to mid-april) probably started in midapril, and the evapotranspiration began to increase, basically reaches the maximum on June, July and August, and then begin a stable state of fluctuation, the average daily evapotranspiration of growth season is 2.98 mm/day. The evapotranspiration and precipitation is very small on the non- growing season and is very large on growing season, so there may be a certain relationship between evapotranspiration and precipitation.2. On the evapotranspiration observed by the lys imeter, FAO-Penman-Monteith model is the best on day scale, and other three models(i.e., Penman, FAO-Penman, and Priestley-Taylor) are also good, FAO-Radiation model is bad; Priestley-Taylor model is the best one on hour scale, other four models are not s uitable for estimating hourly evapotranspiration in the grassland of the study area, the order of the input parameters significantly influencing evapotranspiration is radiation > air pressure > air humidity > air temperature>wind speed. When the radiation changes ±10%, the simulation values change about 20% and when other parameters change ±10%, the simulation values change less than 8%. Therefore, the radiation is the most sensitive factor.3. On the evapotranspiration calculated with vorticity data of growing season, the simulation value of FAO-Radiation model is bad and the simulation value other four models is light larger than the evapotranspiration calculated with vorticity data, so appropriate adjustments is made for the four models to simulate the evapotranspiration. And on the non- growing season, it is same to the growing season, the simulation value of FAO-Radiation model is bad and the simulation value other four models is light larger than the evapotranspiration calculated with vorticity data, so appropriate adjustments is also made for the four models to simulate the evapotranspiration. The simulation of evapotranspiration of non- growing season has a great significance for the study area.4. The improved crop coefficient formula recommended by FAO is suitable to be used in the study area, the daily ET0 estimated by,FAO Penman-Monteith model ranges from 0.50 mm/d to 7.26 mm/d, the ET0 has interannual difference and the sequence of ET0 in the study period is 2011 > 2014 > 2012 > 2013. From measurement data, we find soil evaporation having a little interannual change, that means the transpiration of grass is a key component to control interannual different of ET0. From the study, we can draw a conclusion that the actual evaportranspiration of grass in the subalpine meadow can be estimated by crop coefficients and reference evapotranspiration, both depending on weather data which are easily obtained by automatic weather station.5. Analyse the relationship between hourly actual evapotranspiration and pan evapotranspiration based on a typical sunny day, pan coefficient can be divided into 8:00-14:00 and 14:00-20:00 two periods and the whole time 8:00-20:00 to calculate, the pan coefficient of 8:00-14:00 and 14:00-20:00 is greater than 1 and less than 1, and the pan coefficient of 8:00-20:00 is less than 1.