| The Loess Plateau is one of the best areas for the cultivation of apples in China due to its high altitude,abundant sunlight and large diurnal temperature difference.The Chinese Central Government announced a policy to change land use“from cropland to forest”in the late1990s,apple orchards covering large areas are established in this region.Planting commercially valuable trees,especially apple trees,is an efficient way to resolve the complex problems of low soil fertility,water conservation and the slow economic development of rural areas.The Loess Plateau has a dry climate and precipitation is the main source of water for agricultural production.Levels of both transpiration and soil evaporation are high in apple orchards and evapotranspiration consumes most of the rainfall in them,so the conflict between water supply and demand is very serious.Without irrigation soil moisture levels may become very low and shortage of available water is the main factor limiting apple production.As the planted area continues to expand,the hydrological effect of apple orchards cannot be ignored and the use of water for apple production is expected to change or modify the hydrology of the catchments.Thus,studying the characteristics of soil water dynamics,evapotranspiration and its components in apple orchards is necessary for providing undamental understanding of the partitioning of evapotranspiration and soil water dynamics of apple orchards,sound planning of apple planations with limited water resources,formulating scientific and reasonable management strategies and fully exploiting the productive potential of apple trees.The objective of this study were to:（1）quantify and compare canopy interception in apple orchards with different tree ages,apply the acquired data to assess the importance of canopy interception for water balances of apple orchards on the Loess Plateau,（2）measure sap flow velocity of apple trees using thermal dissipation probes then explore relations of sap flow to tree age,environmental parameters and leaf area index,（3）evaluate and compare the dynamics of the components of evapotranspiration（canopy interception,soil evaporation and transpiration）of apple orchards with different ages,then explore the relationship between evapotranspiration and precipitation during the study periods,（4）characterize temporal distribution of soil water content in the 0-600 cm soil profiles and compare soil desiccation（if any）in the apple orchards of different ages.The main conclusions of this study are as follows:1.Daily and monthly canopy interception of the 20-year-old orchards were consistently higher than the 10-year-old orchard in 2015 and 2016（P<0.05）.Daily canopy interception of the 10-and 20-year-old orchards was 0.1-1.6 and 0.2-2.2 mm d-1,respectively.Corresponding values for the monthly canopy interception ranged from 0.9-7.4 and 1-9.4 mm.During the study periods,canopy interception in the 10-and 20-year-old orchards were both significantly positively correlated with incident rainfall and incident rainfall intensity.Canopy interception was consistently higher in the 20-year-old orchard at given levels of either incident rainfall or rainfall intensity.Percentages of precipitation accounted for by throughfall,stemflow and canopy interception differed between the 10-and 20-year-old orchards.In both orchards,throughfall accounted for the highest percentage,followed by canopy interception and stemflow accounted for the lowest percentages of precipitation.Monthly canopy interception was positively correlated with monthly total precipitation in both orchards in 2015 and 2016while there was no correlation with monthly leaf area index.Annual canopy interception was23.5 and 22.2 mm and 26.8 and 29.2 mm for the 10-and 20-year-old orchards in 2015 and2016,respectively.During the growing seasons in 2015 and 2016,total evapotranspiration（calculated as soil evaporation+transpiration+canopy interception）exceeded total precipitation in both orchards.In 2015 and 2016,annual precipitation was consistently lower than evapotranspiration（including or exclusing canopy interception）in the 20-year-old orchard.The relationship between annual precipitation and annual evapotranspiration calculated with and without consideration of canopy interception in the 10-year-old orchard differed in both years.Ignoring canopy interception would result in underestimation of annual evapotranspiration in both apple orchards,and hence incorrect evaluation of the relationship between water supply and water demand,particularly for the 10-year-old orchard.2.Daily sap flow velocity of the 10-year-old orchard was consistently higher than the20-year-old orchard during the study periods in 2015 and 2016（P<0.05）.Daily sap flow velocity of the 10-and 20-year-old orchards ranged from 0.2-2.2 and 0.1-1.4 mm s-1,respectively.Sap flow velocity for the 10-and 20-year-old trees were negatively correlated with relative humidity but positively correlated with reference evapotranspiration,solar radiation,vapor pressure deficit,wind speed and air temperature.Multiple relationships among daily sap flow velocity and main affecting factors in the two orchards differed during the study periods.For the 10-year-old orchard,sap flow velocity was mainly determined by reference evapotranspiration,relative humidity and wind speed while for the 20-year-old orchard,sap flow velocity was mainly determined by reference evapotranspiration,wind speed and vapor pressure deficit.Daily and monthly transpiration of the 20-year-old orchard were consistently higher than the 10-year-old orchard（P<0.05）and there was no obvious trend in daily transpiration of the two orchards during the study periods.3.Annual evapotranspiration of the 7-year-old apple orchards was 330.2,336.1 and 301mm in 2012,2013 and 2014,respectively,while for the 17-year-old orchard the values were367.6,371.4 and 329.7 mm.During the study periods,precipitation exceeded evapotranspiration of both orchards.Tree age clearly influenced evapotranspiration,daily and monthly evapotranspiration of the 17-year-old orchard were consistently higher than the7-year-old orchard（P<0.05）.Differences in monthly precipitation minus monthly evapotranspiration of the 7-and 17-year-old orchard varied in each year and maximum differences of the two orchards both occurred in the same month in 2012,2013 and 2014 and the corresponding months were June,June and July,respectively.Daily evapotranspiration of the 7-and 17-year-old orchards were positively correlated with reference evapotranspiration while there was no correlation with daily leaf area index.Annual canopy interception,soil evaporation and transpiration differed between the two orchards during the past three years.Soil evaporation accounted for the largest proportion,followed by transpiration in the7-year-old orchard,while for the 17-year-old orchard,transpiration was the main component,followed by soil evaporation.Canopy interception accounted for the smallest propotions in the 7-and 17-year-old orchards.4.Annual mean soil water content in the entire 0-600 cm profiles in 2012,2013,2014and 2015 were 23,22.1,22.9 and 22.6%,respectively,in the 7-year-old orchard while for the17-year-old orchard,corresponding values were 19.2,18.6,19.5 and 20%,respectively.For both orchards,the annual mean soil water content along 0-600 cm soil profile differed among the past four years and annual mean soil water content of the entire 0-600 cm profile was consistently higher in the 7-year-old orchard in each year（P<0.05）.Tree age clearly influenced monthly mean soil water content in the 0-600 cm profiles during the study periods.In 2012,2013,2014 and 2015,monthly mean soil water content along the 0-600 cm profiles initially decreased and then increased with the increase in soil depth in the young orchard,but consistently declined with the increase in depth in the old orchard.Soil water recharge and root uptake depths both differed between the two orchards during the four study years.The maximum soil water recharge depth was higher in the 7-year-old orchard while the maximum soil water uptake depth was higher in the 17-year-old orchard in every year.Annual mean soil water storage and soil water avalible storage in the entire 0-600 cm profiles were consistently higher in the 7-year-old orchard in every year（P<0.05）.A dried soil layer had formed in the17-year-old orchard,but no dried soil layer was detected in the 7-year-old orchard during any of the four study years.The monthly mean dried soil layer thickness of the 17-year-old orchard was 292,328,284 and 280 cm in 2012,2013,2014 and 2015,respectively.Monthly mean soil water content within dried soil layer in the 17-year-old apple orchard were 16.8,17.1,17.1 and 17.2%in 2012,2013,2014 and 2015,respectively. |
PDF Full Text Request