Water Consumption Of Maize In The Minqin Oasis Based On Hydrogen And Oxygen Stable Isotopes

The shortage of water resources is threatening the sustainable development of oasis irrigation agriculture in the arid area of Northwest China.The study of farmland water consumption is of great significance to improve agricultural water use efficiency and alleviate the current situation of water shortage in arid area.The Minqin oasis is located in the lower reaches of the Shiyang River Basin,and has a long history of irrigated agriculture.It is currently the region with the highest utilization of water resources and the most prominent ecological and environmental issues in Northwest China.Therefore,it is necessary to further understand the water cycle process and the current situation of farmland water resource utilization in Minqin oasis.Due to its high accuracy and non-interference by external factors,isotope measurement technology has been widely used in the fields of hydrology,water resources and the water cycle,and has become an effective tool to solve many major scientific problems.In this study,we take Minqin oasis maize farmland as an example,using theδ²H andδ¹⁸O data of stem water,soil water at different soil layers,farmland irrigation water and precipitation during the maize growth period in 2018 and 2019,based on the MixSIAR Bayesian isotope mixing model,combined with farmland water balance and isotope mass conservation method.The isotopic composition and variation characteristics of different water bodies in the growth period of maize in 2018 and 2019 were analyzed,the water sources and contribution proportions of maize in different growth periods in 2019 were identified,the infiltration and deep infiltration of irrigation water after irrigation events were estimated and the evaporation and transpiration of farmland during different irrigation events were separated.The conclusions are as follows:（1）The seasonal fluctuation of precipitation isotope composition in the study area is large,and the high value mainly occurs in summer and autumn.The Local moisture waterline（LMWL）equations in 2018 and 2019 are respectively:δ²H=7.18δ¹⁸O+5.39 andδ²H=7.44δ¹⁸O+1.73.The slope and intercept of the LMWL in two years are less than the global and the entire northwest arid area and the Shiyang River basin,indicating that the precipitation process is affected by strong secondary evaporation during the precipitation process,which reflects the local environmental characteristics of climate drought and strong evaporation.（2）The slope and intercept of the soil waterline（SWL）equation in 2018 and2019 are both smaller than those of LMWL in the study area,indicating that the soil water isotope value is enriched due to the influence of continuous soil evaporation.In2019,the soil water isotope values in the early and late stages of maize growth are relatively enriched,and the shallow soil water isotope values are relatively enriched,and the soil water isotope values are slightly reduced from the shallow layer to the deep layer.Film mulching weakens soil evaporation and plays a role in water retention,making the surface soil water isotope value less enriched.The isotopic composition of irrigation water is poorer than that of atmospheric precipitation,irrigation activities can change the redistribution of soil water and the change of isotopic value of soil water.Theδ²H andδ¹⁸O values of maize stem water were mostly on the right side of LMWL and fall near SWL,indicating that corn uses soil water after being affected by evaporation.In 2019,the seasonal changes ofδ²H andδ¹⁸O of maize stem water were similar to the changes of shallow soil water isotope values,indicating that maize mainly absorbs shallow soil water and has visible seasonal variation characteristics.（3）It is estimated that the average contribution rate of 0-10,10-50 and 50-100cm soil water in the entire growing season of maize was 62.1%,24.2%and 13.7%respectively.The contribution rates of 0-10 cm soil water in seedling stage,jointing stage,heading stage,filling stage and maturity stage was 85.6%,67.6%,53.7%,48.9%and 55.4%,respectively.At the jointing and heading stage,the contribution rate of20-50 cm soil water increased to 21.8%and 37.7%.At the filling stage,the contribution rate of soil water at 50-100 cm increased to 24.3%.At the maturity stage,maize tends to use shallow soil water.During the entire growing season,maize mainly uses shallow soil water,but as the maize grows,the utilization rate of deep soil water gradually increases.After irrigation,maize can adjust its water use depth,and the response to 0-10cm and 10-50 cm soil water is obvious.The contribution of 0-10 cm soil water increased,10-50 cm decreased first and then increased slowly,and the contribution of50-100 cm did not change much.（4）Combined with the estimation of soil water balance and isotope mass conservation,29.9±4.2%,29.6±2.9%and 30.3±4.3%of irrigation water infiltrated into 0-10 cm,10-50 cm and 50-100 cm soil layers on the first day after irrigation during the maize growth period in 2019.During the 6 irrigation events,the deep infiltration amount was 58.1 mm,39.6 mm,36.9 mm,22.4 mm,9.2 mm and 32 mm respectively,and the total infiltration amount was 198.2 mm.About 31%of the irrigation water and precipitation leaked to the soil layer below 100 cm.In general,the deep infiltration amount is affected by the amount of single irrigation and the use of maize in different growth periods on soil water in different soil layers.（5）According to the water balance and isotope mass conservation,the transpiration rate of farmland during the irrigation event is always greater than the evaporation rate.With the growth of maize,the transpiration rate is smaller in the early growth stage,larger in the middle and late stages,and slowly decreases in the later stage.During the 6 irrigation events,the transpiration rate was 79.5%,85.2%,87.7%,91%,91.6%and 88.7%respectively,the average transpiration rate was 87.3%;the evaporation rate was 20.5%,14.8%,12.3%,9.0%,8.4%and 11.3%,the average evaporation rate was 12.7%.