| Water shortage has become a serious obstacle to the sustainable development of agricultural economy in arid regions.With the improvement of water-saving irrigation and film-mulching technology in arid area,it is of great significance to study the water transfer mechanism of soil-plant-atmosphere continuum(SPAC)under irrigated farmland with film-mulching.This paper mainly studied the water transfer mechanism and evapotranspiration estimation of maize field under different irrigation regimes(Alternative furrow irrigation(AFI),Conventional furrow irrigation(CFI),Border irrigation(BI)and Alternative furrow irrigation with 50%amount of irrigation(AFI(M/2)))using water isotope analyzer,multi-channel vapor cold-trap equipment,eddy covariance system and sap flow system.Based on the isotope mass balance assessment and water balance,the isotope IsoSource model,Keeling plot model,Craig-Grodon model and Rayleigh distillation model were used to evaluate the soil water movement,water uptake pattern,water transfer mechanism and evapotranspiration estimation.The main results are outlined as follows:(1)The local meteoric water line(LMWL)and soil water line(SWL)were fitted:?D = 7.4?18O+8.0(R2=0.95)and ?D =5.6?18O-9.9(R2=0.90).The hydrogen and oxygen isotopic distributions of soil water,plant water and air vapor and their temporal and spatial variations were clarified.The intercept and slope of the LMWL and the SWL are less than the GMWL(?D=8?18O +10),reflecting the strong evaporation effect on soil water.The ?18O in soil water profile decreased with depth.There were significant differences in the isotopic composition of maize stem and different leaves,and no isotopic fractionation of xylem and veins.The crop transpiration and its water vapor concentration are the leading factors influencing the isotopic composition of background water vapor.(2)The mechanism of alternate furrow irrigation(AFI)to improve crop water use efficiency and yield were further revealed.the depth of maize water uptake under different irrigation regimes was:AFI(M/2)>AFI>CFI>BI.The leading factor affecting maize water uptake was the root distribution within the growing season but was the soil water content within a short watering cycle,respectively.Soil water in wet side was firstly absorbed by maize root and then transported to the dry side.AFI increased the proportion by 4-26%of maize water uptake from wet side soil zone,which reduced water consumption in deep percolation when compared to traditional irrigation method.AFI increased maize yield by 17.0-30.2%,WUE was substantially improved by 13.3-33.8%,and no significant yield reduction when only 50%of irrigation water applied.(3)The isotopic model of soil evaporation estimation under plastic film-mulching was established,and the process of soil water movement and its vapor exchange were analyzed and quantified.Soil evaporating front was presented in 5-10 cm soil layer,and the exchange motions of water vapor occurred in 0-5 cm soil layer before it diffused to the outside.During the whole maize growing season,about 4.5%of soil water in the 0-20 cm soil layer was evaporated,and 72.6%of the evaporation vapor was condensed and 70.0%of this condensation water was evaporated again,with the mean evaporation of 0.80 mm/d.The evaporation rate was relatively high with the ratio of E to ET around 21.2%,although the areas of the holes only taking up 0.5-5.0%of the whole area of the plastic cover.(4)A new method for evapotranspiration estimating and partitioning based on stable isotope was proposed.The isotope compositions of transpiration and evaporation(?T and ?E)under non-steady-state were determined directly and continuously.Results of the new isotope method showed that the transpiration fraction(FT,ratio of T to ET)varied from 52%to 91%with the mean of 78%in maize growing season,and it could be well described by a function of LAI,with the nonlinear relationships of FT=0.71LAI0.14.LAI could be a useful tool to understand the contribution of transpiration to evapotranspiration in field. |
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