Influence Of Vegetation Coverage On Evapotranspiration Process Of Alpine Meadow In The Head Of The Yangtza River And Determination Of Crop Coefficients

To investigate the actual evapotranspiration(ET) under different vegetation coverage, three plots of alpine meadow were established on the places where the topography was of the same slope, direction and shape and where the vegetation coverage was 92%,65% and 30%, respectively, in the head of the Yangtza River. The lysimeter method is used to calculate ET of alpine meadow under different vegetation coverage. Analyses the difference of ET and the dominating factors on four period.The results show that:(1)The total evapotranspiration of alpine meadow in 92%,65% and 30% plots are 314.7mm,289.5mm and 246.2mm in 2007 and 347.5mm,309.3mm and 267.3mm in 2008.The change trend of the actual evapotranspiration is conformably under different coverages. maximum evapotranspiration at the growth period, second largest at the post-growth period, moreover is the per-growth period, freezing period is minimum. ET decreases with the reduction of vegetation coverage, and the change of vegetation coverage influenced ET of the growth period very obviously.(2)The evapotranspiration process of the alpine meadow performances monthly variation. The daily evapotranspiration increases with the reduction of vegetation coverage in 1～4 month and decrease with the reduction of vegetation coverage in 5～12 month.The daily evapotranspiration under different coverage reaches maxi-mum in July.(3)As the reduction of vegetation coverage, the moisture surplus cumulant is 62.3,37.2 mm and 105.6 mm respectively in 2007. The most moisture loss in August and the most moisture surplus in July. The moisture surplus cumulant is 155.2mm,182.5mm and 223.4mm in 2008. And the most moisture loss in July, the alpine meadow wanes the most moisture loss at 92% of high coverage in September. This is mainly caused by the difference between monthly rainfall.(4) When the observation period, the alpine meadows under different coverages also showed obvious space change and diurnal variation.The dominating factors varies with different period. The atmospheric temperature, the geotemperature and the surface soil moisture are the main factors which Influences the evapotranspiration at the per-growth period and post-growth period. At the growth period are the atmospheric temperature, the geotemperature,the relative humidity, the precipitation and the soil moisture. The meteorological condition played a leading role in the per-growth period and the post-growth period, and the underlaying surface conditions played a leading role in the growth period.(5)The actual evapotranspiration of the alpine meadows is measured by lysimeter, combining reference crop evapo-transpiration which is calculated by the FAO Penman-Monteith formula, both make business to get fixed crop coefficients.But the method that the use of fixed crop coefficients to calculate the actual evapotranspiration is not accurate. The actual evapotranspiration measured by lysimeter and calculated by dynamic crop coefficients to fit. And gained regression model of Kc=p1Rn+p2Ta+p3u+p4W+p5 in the per-growth period and the post-growth period; the regression model of Kc=p1Rn+p2Ta+p3P+p4u+p5W+p6 in the growth period; the regression model of Kc=p1Rn+p2Ta+p3u+p4 in freezing period, where p1,p2,p3,p4,p5,p6 are fitting parameters, the feasibility of which were determined through validity testing between calculated and observed data.