| The apple planting area in the Loess Plateau of China reaches 1.3 million hm2,accounting for 25%of the world.It is the largest concentrated apple cultivation area in the world,of which 80%is dryland orchard.Different from Robinia pseudoacacia and other ecological forests,apple trees showed significantly different water consumption characteristics during vegetative and reproductive growth stages,which significantly affected soil moisture migration and distribution.In addition,insufficient rainfall led to serious soil drying problems.The rapid increase of apple planting area in the Loess Plateau in recent 20 years has not only changed the soil hydrological process of apple orchard,but also profoundly affected the key water cycle in the region.At the same time,the loess area is dry and less rain and the soil is loose and porous,and the vapor water may play an important role in the profile water migration.Therefore,it is of great significance to understand the process and quantity of rainfall transformation for dry-land apple orchard.Taking the mountainous apple orchard in loess area as an example,the hydrological model simulation and isotope tracer methods were used to study the changes of water and heat and transpiration in deep soil of apple orchard,the migration law of soil moisture and water vapor flux,the response of hydrological variables of apple orchard to extreme rainfall,and the redistribution law of seasonal rainfall infiltration in apple orchard,the water utilization strategy of apple tree in spring,so as to clarify the transformation process of water in apple orchard in soil-plant-atmosphere system,and provide scientific basis for revealing the loess profile and the whole key water cycle process.The main findings are as follows:（1）The STEMMUS model has good accuracy in the hydrological process simulation of mountainous apple orchard in loess area,and the variation law of deep soil water and heat,apple tree transpiration and water vapor flux migration mechanism in apple orchard in loess area were proved.Rainfall has significant effects on the matrix potential gradient,temperature gradient and the transport of liquid water flux and vapor water flux.The soil moisture above 200 cm in apple orchard has obvious temporal variation characteristics,which is the active layer of soil moisture,while the variability of deep soil moisture below 200 cm is relatively low and stable,which is the stable layer of soil moisture.The soil temperature was mainly seasonal,and the transpiration of apple trees increased first,then stabilized and then gradually decreased with the development of growth period.The influence depth of three rainfall gradient on liquid water is 80 cm,100 cm and 60 cm respectively.The influence depth on vapor water were 80cm,160 cm and 60 cm,respectively.67.6 mm d-1 extreme rainfall had the greatest influence on liquid water flux and vapor water flux,and its maximum recharge depth on deep soil were100 cm and 160 cm,respectively.It showed that vapor water flux was the main component of deep soil water transport in arid and semi-arid areas.The vapor water flux is two orders of magnitude smaller than the liquid water flux,and the liquid water flux and vapor water flux are mainly driven by matrix potential gradient and temperature gradient.（2）The characteristics of soil moisture,evapotranspiration and deep leakage in apple orchard under future extreme climate were predicted,and the response of hydrological variables to extreme climate was discussed.Precipitation in extreme wet years can effectively supply 0-300 cm soil moisture,and soil moisture in orchards in extreme wet years is significantly higher than that in extreme drought years.There were significant differences in deep leakage in apple orchards among different precipitation years,but the values were all less than 30 mm.Leakage below 100 cm and 200cm in extreme wet years was 5.7 and 6.5 times higher than that in extreme dry years,respectively.In addition,the total evapotranspiration of apple orchards was higher than 400mm.Compared with the huge difference in precipitation（average 310 mm）,the average difference in evapotranspiration between extreme wet years and drought years was only 35mm.Compared with leakage,the influence of annual precipitation on apple tree evapotranspiration was much lower.But it significantly affected the T/ET ratio,the value of extreme drought years（average 0.58）was significantly higher than that of extreme wet years（average 0.51）.（3）The rainfall infiltration process and seasonal variation of soil water isotope in apple orchard were analyzed,and the water use strategies of apple trees in different growing seasons were revealed.Soil moisture in apple orchard and layland changed significantly with soil depth and sampling date.TheδD value of soil water in each sample changed significantly with different sampling dates in the shallow layer（0-60 cm）,and the fluctuation amplitude in the middle layer（60-180 cm）was relatively small,while that in the deep layer（180-300 cm）was approximately vertical.The infiltration depth of marked autumn rain in apple orchard and abandoned land was 80 cm and 140 cm,respectively.The infiltration depth of abandoned land was much larger than that of marked apple orchard.Marker and control apple xylem water stable hydrogen isotope values in different sampling dates were significantly different（p<0.05）.The main water sources of the control apple trees changed from shallow（0-60 cm）soil moisture to middle（60-180 cm）and deep（180-300 cm）soil moisture during the autumn and winter to spring and summer.The seasonal variation of soil water use sources of marked apple trees was obvious.Compared with the control treatment,there were significant differences in water use strategies in each season,indicating that marked apple trees can better reflect the flexibility of root water absorption of fruit trees.Autumn rain mark test also directly proved that apple trees in spring and summer mainly absorbed and utilized soil moisture from the previous year autumn rain,“Autumn rain and spring use”can improve their resistance to drought environment in spring and summer. |
