Soil Moisture Dynamics Under Typical Plantation And Its Response To Rainfall In The Loess Plateau

Since the implementation of the "Grain-for-Green" Project in 1999,a large area of artificial plantations has been constructed in the Loess Plateau,which has achieved remarkable results in preventing and controlling soil erosion.However,with the increase of plantations area,the water demand of plantations is also increasing.There were some problems such as "imbalance of soil moisture,and"soil drying in the artificial plantations,even including the plantations growth decline and river runoff decrease.Therefore,the relationship among plantation vegatation,rainfall and soil water has become a hot spot for ecological restoration.This study takes artificial Platycladus orientalis plantations,artificial Pinus tabulaeformis plantations and aritificial Robinia pseudoacacia plantations in the loess area of western Shanxi as the research objects.The relationship between soil water and rainfall in soil and water conservation plantations land are studied with statistical analysis method through long-term soil moisture and rainfall monitoring.The main conclusions are as follows:(1)The precipitation in the study area varies greatly between the years and the month,and the precipitation in the rainy season from May to October accounts for 87.02%of the annual precipitation.Light rain is the main type of precipitation event in the study area,but moderate rain has the greatest contribution to the total precipitation in the study area.Moreover,the frequency of occurrence of special precipitation years in the study area has increased over the past 25 years,which indicates that the fluctuation of atmospheric precipitation has been large in recent years.(2)The water storage of 0-200 cm soil layer in the Platycladus orientalis plantations,Pinus tabulaeformis plantations,and Robinia pseudoacacia plantations is significantly different(p<0.01).The water storage in the Platycladus orientalis plantations,the Pinus tabulaeformis plantations,and the Robinia pseudoacacia plantations are 425.63 mm,373.89 mm,and 312.43 mm,respectively.(3)The terrace can significantly increase the water storage capacity in Platycladus orientalis plantations and Pinus tabulaeformis plantations.After 22 years of afforestation,the terrace can still store rainfall and prevent soil erosion,while the terrace has no effect on the soil water storage capacity in Robinia pseudoacacia forest land.The water storage capacity of 0-200 cm soil layer in the terrace of the Platycladus orientalis plantations,Pinus tabulaeformis plantations and Robinia pseudoacacia plantations is 482.81 mm,430.33 mm and 316.22 mm,respectively,with significant difference(p<0.01).The water storage capacity of 0-200 cm soil layer on the slope is 368.44 mm,317,45 mm and 308.64 mm,respectively.The soil water storage in the slope area of the Platycladus orientalis plantations is significantly larger than that in the Pinus tabulaeformis plantations and the Robinia pseudoacacia plantations.The difference of water storage between the Pinus tabulaeformis plantations and the Robinia pseudoacacia plantations is not significant.(4)Optimal partition for ordinal specimen method is used to analyze the annual and interannual changes of soil moisture.Soil water storage in the study area is divided into stable period(January-March),volatility period(April-June),rapid growth period(July-September).and rapid consumption(October-December).During the four periods,the duration of the four periods in different hydrological years are different.(5)The water content of 0-200 cm soil layer show different characteristics under different hydrological years.In the wet year,the soil water storage is mainly restored by high-intensity heavy rain.The water storage capacity of 0-200 cm soil layer in each forest land can be restored and supplemented.The recharge amount of each forest land is 169.86 mm in Platycladus orientalis plantations>130.30mm in Pinus tabulaeformis plantations>31.04 mm in Robinia pseudoacacia plantations.The water storage capacity of 0-200 cm soil layer in each forest land show different degrees of decline.The loss of each forest land is 91.19 mm in Robinia pseudoacacia plantations,70.51 mm in Pinus tabulaeformis plantations and 48.83 mm in Platycladus orientalis plantations.In the normal rainfall year,the recharge effect of rainwater in different plantations is different.The Platycladus orientalis plantations can be replenished to 140 cm soil depth,the water storage increased 71.04 mm,and the Pinus tabulaeformis forest land can be replenished to 120 cm soil depth,and the water storage increased 35.82 mm,the water storage capacity of the Robinia pseudoacacia plantations at the end of the rainy season reduced 5.52 mm.(6)The total infiltration of each forest land during the rainy season is 374.05 mm in the Platycladus orientalis plantations,375.88 mm in the Pinus tabulaeformis plantations,and 313.04 mm in the Robinia pseudoacacia plantations,respectively.The average infiltration rate of rainfall is 66.18%in the Platycladus orientalis plantations,66.50%in the Pinus tabulaeformis plantations.55.39%in the Robinia pseudoacacia plantations,respectively.The infiltration and infiltration depth of single-field rainfall generally increased with the rainfall and rainfall intensity,and the terrace can significantly increase the infiltration and infiltration depth during rainfall.(7)The linear regression of soil water replenishment and rainfall in different forest land has a good fitting effect.The minimum precipitation threshold required for infiltration replenishment of each forest land can be estimated by fitting equations,which is 2.61 mm in the slope of Platycladus orientalis plantations.7.53 mm in the terrace of Platycladus orientalis plantations,5.59 mm in the slope of Pinus tabulaeformis plantations,2.31 mm in the terrace of Pinus tabulaeformis plantations,2.66 mm in the slope of Robinia pseudoacacia plantations,and 7.14 mm in the terrace of Robinia pseudoacacia plantations,respectively.(8)The soil water storage capacity of 0-200 cm soil layer can be replenished in each forest land when the monthly rainfall reaches 340.8 mm,and the maximum change of soil water content decrease with the soil layer depth.The maximum change of soil water content with soil layer depth show a good linear relationship.The maximum recharge depth of each forest land under heavy rainfall is 234.83-249.85 cm in the Platycladus orientalis plantations,247.83-289.44 cm in the Pinus tabulaeformis plantations,and 251.82-286.60 cm in the Robinia pseudoacacia plantations,respectively.The results show that the recharge depth of each forest land can not exceed 300 cm even in extreme rainfall event.