Dynamics And Stable Isotope Characteristics Of Soil Water Under Cinnamomum Camphora Woodland In Changsha

The relationship between forest and water has been one of the hot topics in forestry and ecohydrology fields.As an important component of water resources on the land surface,soil water links precipitation,surface water(lake water and river water),groundwater and plant water.Studies of soil moisture migration in the unsaturated zone can provide reference for scientific management of regional water resources and protection as well as restoration of forest ecosystem.Under the influence of the complex monsoon system,the seasonal droughts occur frequently in Changsha,and the contradiction between water supply and water demand has become increasingly fierce.As the city tree of this area,Cinnamomum camphora is an important economic tree species.However,C.camphora is sensitive to the changes in soil moisture,especially to drought because of its weak drought-tolerance.Therefore,this article takes the C.camphora woodland in Changsha as the research object.Based on the continuous data monitored from March 2017 to August 2019,the meteorological factors(precipitation,air temperature,relative humidity,and solar radiation,etc.),soil moisture content and soil temperature at 0-100cm depth,and the stable isotopic compositions of precipitation,soil water at 0-130cm depth and groundwater,the dynamics and stable isotopes characteristics of soil water had been studied.The results show that:(1)The seasonal variation of soil moisture content in the study area was obvious,which on the whole could be divided into three stages including water-sufficiency period from March to June when soil water storage was large and less variable,water-consumption period from July to October and water-replenishment period from November to following February.At the monthly scale,soil water storage reached to the peak in March and the lowest in August.At the annual scale,the mean value of soil water storage in the first and second year were nearly identical during the observation period(2)There were significant differences in the mean values of soil water content at five soil layers(0-10cm,10-20cm,20-40cm,40-60cm and below 60cm)throughout the observation period.From shallow to deep layers,the soil moisture content and its stability increased.There was seasonal difference in the vertical variations of soil water content,which was the most obvious during the water-consumption period According to the temporal stability of soil water content at different depths,the soil water profile in the study area could be divided into rapid-change layer(0-20cm),active layer(20-40cm),and sub-active layer(40-100cm).(3)The evapotranspiration on rainless days calculated by the soil water balance method had a significant intra-day variation.The peak value of hourly evapotranspiration was from 13:00 to 14:00.The intraday evapotranspiration in summer started to increase early and approached 0 latest,and the variation in winter was the opposite of it.The evapotranspiration was strong in summer or water-consumption period,weak in winter or water-replenishment period.Daily evapotranspiration was significantly positively related to the daily mean air temperature,water vapor pressure deficit and solar radiation.(4)The soil moisture responded well to precipitation events,but was affected by the preceding soil moisture conditions.Soil moisture was more sensitive to precipitation in the deficient preceding soil moisture condition than the sufficient.Based on every precipitation event record during the observation period,the soil water content at deep depth was only sensitive to the precipitation events above the heavy rain level.(5)Influenced by different water vapor sources in the monsoon region,the?18O in precipitation was highest in spring(-3.59‰)and lowest in summer(-9.10‰).The maximum and minimum monthly mean values appeared in April and September,respectively.The seasonal variation of ?18O in soil water gradually minimized and became less variable with depth.The seasonal variation of ?18O in soil water(0-60cm depth)was similar to that in precipitation with a specific time lag.There was a small seasonal variation of ?18O in soil water(60-130cm depth)and groundwater,which retained more stable isotopic information in precipitation accumulated repeatedly during the preceding period.(6)From shallow to deep depths,?18O in soil water decreased first and then increased and became less variable.Based on the lag correlation analysis between?18O in precipitation and that in soil water,?18O in soil water at 0-60cm depth lagged behind that in precipitation by 0.5-2 months,while at 60?130cm depth by 3?3.5 months.Mean ?18O in soil water at all depths was more lowed than that in precipitation,since the contribution of precipitation with more depleted 180 to soil water was large,and it was related to the seasonal replenishment of soil water as well.(7)According to the comparison of lc-excess(?2H deviations from the local meteoric water line,LMWL)in different water pools,the evaporative fractionation effect of the stable isotope in soil water gradually decreased with depth,and the stable isotope in groundwater was little affected by the effect.Correlation analysis showed that the lc-excess in soil water was related to stable isotopic composition in soil water,accumulated evaporation and accumulated air temperature in the preceding periods,and soil water content(8)The slope and intercept of the LMWLP>3.7 established after eliminating the small precipitation events of P?3.7mm obviously increased,and were closer to that of the soil water line at all depths than before.The phenomenon that the slope of the soil water line was greater than that of the LMWL in the study area was related to the inverse seasonal variation between the stable isotopes of precipitation and soil evaporation(the more depleted the stable isotopes in precipitation,the stronger the soil evaporation).