Abstract:

Applying Static Chamber/IRGA,this paper measured water vapor concentration of tea plantations of different growing years in Yixin City,on the west of Taihu Lake watershed,aimed to evaluate the effects of growing years on the diurnal variation of evapotranspiration.The results showed that diurnal variation of tea plantations evapotranspiration rate appeared in a single peak curve with the peak(273 mmol·m-2·s-1,236 mmol·m-2·s-1,209 mmol·m-2·s-1in the growing years order),increasing from 6〖DK〗∶00 and reaching a maximum value between 12〖DK〗∶00 and 15〖DK〗∶00,then decreasing rapidly.The three variations had the same trends,while their variation ranges during all time intervals were different from each other.The variation ranges decreased as the tea plantation growed,so the youngest one ’s(3 years old) amplitudes were the highest.Though the youngest’s LAI was the lowest(146＜325＜375 in the growing years order),it’s averge of diurnal evapotranspiration rates was the highest one(129 mmol·m-2·s-1＞121 mmol·m-2·s-1＞115 mmol·m-2·s-1 in the growing years order).Total evapotranspiration from 6〖DK〗∶00 to 18〖DK〗∶00 were also the highest(5052 g·m-2＞4778 g·m-2＞4502 g·m-2 in the growing years order),and the evapotranspiration rate maximum value during a day was in the order similar to the average ones,that the youngest’s was 049 mmol·m-2·s-1and 064 mmol·m-2·s-1 more than 9 years old one and 20 years old one respectively,as the 3 years old tea plantation’s lowest LAI brought most sunshine and surface turbulent flows which enhanced soil evaporation,canceled transpiration’s difference,and conclusively ,improved evapotanspiration.The diural evapotranspiration rate had the same trend with tempetature daily variation,in the morning,temperature was lowest(T〖TX-〗=383℃),evapotranspiration rate was lowest(ET〖TX-〗  rate=007 mmol·m-2·s-1) ,then the rate increased as the temperature rised.From 12〖DK〗∶00 to 15〖DK〗∶00,tempetature was highest(T〖TX-〗=3087℃),evapotranspiration rate came to the highest one (ET〖TX-〗 rate 232 mmol·m-2·s-1)during the day,then the rate declined rapidly as the temperature fell.And evapotranspiration rate of tea plantation was closely related to temperature(R2=081*〖KG-*2〗*).Three tea plantations’ temperatures increased 2700℃,2670℃ and 2740℃ in growing years order from 6〖DK〗∶00 to 12〖DK〗∶00,with little difference,while the amplitudes were 248 mmol·m-2·s-1,127 mmol·m-2·s-1,097 mmol·m-2·s-1 (in the growing years order)respectivily,with significant difference.From 15〖DK〗∶00 to 18〖DK〗∶00,amplitude reductions were 219 mmol·m-2·s-1,191 mmol·m-2·s-1 and 144 mmol·m-2·s-1 (in the growing years order),whose difference was much than the temperature drops.Soil moisture of the three tea plantations declined as evapotranspiration rates increased,both of them showed an opposite trend.In the morning,evapotranspiration rate was low,while soil moisture was relatively high,as the rate increased,soil moisture decreased gradually.When the rate reached the peak(from 12〖DK〗∶00 to 15〖DK〗∶00),all the soil moistures were in low level at noon.Then soil moisture recovered as the evapotranspiration rate declined.Though with a trend relationship between soil moisture and evapotranspration rate,there was no obvious correlation.The reason was probably that average daily soil moistures of the three tea plantations were 2441%,2546% and 2710% respectively,all of which were under wet condition and soil moisture had no significant effect on evapotranspiration.Soil moisture declined immediately as the evapotranspiration rate increased,the time when its low ebb appeared didn’t lag from the one when evapotranspiration reached to the peakas soil evaporation consumpted soil moisture by itself