Studies On The Methods Of Utilizing Penman-Monteith Equation To Calculate Evapotranspiration Of Forest

The function of Penman-Monteith has been used to study transpiration of the forest for many years. The paper's data is got from 2004 to 2007. In the year of 2007, an new equipment——sap-flow meter has been used, which made the experiment much easier and the result more accurately. Much mathematic knowledge was used to analyse the parameters in the function. The changing rules of every parameter and their effects to the whole function were studied. The paper tries to give some theory direction to the using of the function Penman-Monteith. Main problems discussed in the dissertation are as follows: changing rules of every parameter in the function; sensitivity of the function to every parameter; parameters that affect transpiration rate; rules of soil evaporation in the forest and bare land; rules of evapotranspiration of different types of stands; distribution coefficient between evaporation and transpiration in growing season.Main research results are as follows:Whole canopy stoma resistance rst has strongest effects to the calculate result of the function Penman-Monteith. With the data got by sap-flow meter in jun.1-st 2007, the paper made an regression of the daily change of rst. The difference between simulation value and the value got by sap-flow meter was only -1.2%. Extinction coefficient and leaf area index were once thought have strong effects on the result of the function, but now the paper proved that whether we got an accurate value of these two parameters has no effects on the result.The changing of albedo of each forest canopy in growing season are consistent. The highest value was got at morning and evenfall, and the lowest was got at 11:00~13:00. The height of the sun is the factor which affects the value of albedo. Big height of the sun leads to small value of albedo. Considering from the whole growing season, Albedo got the biggest value in the middle, but it changed very little during the whole growing season.Extinction coefficient was affected by both height of the sun and density of trees. In general, big height of the sun leads to small value of extinction coefficient. But for the forest with small crown density, sunlight can get through the canopy easily, and that made the extinction coefficient changing in another way. For the trees have an crown like picea crassifolia kom or Larix kaempferi(Lamb.)Carr. , smaller height of the sun made the sunlight gets through the canopy more easily. That may be the main reason why extinction coefficient got bigger when the sun got higher.Air temperature and humidity are the main factors that affect transpiration rate. Sun radiation induces the stoma to open, and it leads to higher temperature and lower humidity.For the stomatal movement, transpiration rate also show some endogenous circadian rhythms. That means transpiration rate will keep on changing even when the environment is steady. Soil water content has a strong effect on transpiration rate. When SWC is not too high, transpiration rate keeps on growing as SWC is. At the beginning, it growing in a high speed, then the speed is lower, at last, it stops growing. When SWC is too high, transpiration rate will slowdown. Regression between transpiration rate and SWC can be in the form of y=ax3+bx2+cx+d or y=ax2+bx+c.