Water Use Efficiency And Its Response To T/ET Of Maize Field In Longzhong Semi-arid Area

Water use efficiency（WUE）is the key factor influencing the coupling of carbon and water in terrestrial ecosystems.However,there is a dearth of research on WUE in semi-arid farmland ecosystems on different time scales.Based on three years（2015-2017）of eddy covariance and meteorological measurements,we analyzed the WUE and its influencing factors on different timescales（seasonal and diurnal）.The effects of the current climate background and human activities（precipitation differences and film mulching）on the water use efficiency of the ecosystem were investigated,and the response of water use efficiency to T/ET was discussed in Lanzhou University field observation station for semi-arid basin surface processes and climate change.The main results were as follows:（1）Compared with other farmland ecosysterm,this experiment station was a significant carbon sink during the study period,with WUE values of 4.612,4.594,and 5.226 g C kg^-1 H₂O,with evidence of an increasing trend over the three years.It reached the maximum values in summer（grain filling stage and maturation stage）,and the main factor that affected the change of WUE within three years was water availability.（2）There was a relatively weak response of WUE to environmental factors during the entire growth period;however,there were significant correlations between WUE and environmental factors during grain filling and maturation stage.Within each growing stage,we found the interactions of temperature（T_a）and canopy conductance（g_c）（i.e.,T_a×g_c）in the seedling stage,and of vapor pressure deficit（VPD）,photosynthetically active radiation（PAR）and g_c（i.e.,VPD×PAR×g_c）,to be significant factors explaining the seasonal variation in WUE.（3）Diurnal variations in WUE can be divided into three stages:extreme values in the morning and evening,with WUE slowly decreasing and maintaining low values in daytime.Canopy conductance had the most direct influence on the diurnal variation of WUE.The variation of PAR from 7:00-11:00,VPD from 11:00-16:00,and VPD×PAR×T from 16:00-21:00 provide an explanation for this phenomenon.（4）Ecosystem productivity and WUE are not correlated with total precipitation and that seasonal precipitation is much more important than total precipitation.The effects of precipitation on ET were higher than its effects on GEP.Our study area was located in a valley area and experienced a significant long-term enrichment in precipitation compared with the SWC of other stations in semi-arid areas;in addition,the relationship between WUE and the spring drought was not obvious.The capacity of carbon sequestration in the Maize Farmland ecosystem was increased greatly by the film mulching,but the response to the water use efficiency of the system was not obvious.The hydrothermal condition was an important factor to explain this phenomenon.（5）The underlying water use efficiency can greatly reduce the time lag of the diurnal scale of different environmental factors.The T/ET_uWW calculated by the underlying water use efficiency was more close to the in-situ test value T/ET_sf on the seasonal scale compared with the model method T/ET_s-w.Leaf area index（LAI）was the most important factor affecting T/ET in the whole growing season.The T/ET_uWW had a better correlation with LAI than T/ET_s-w,and this was more obvious in the case of film mulching.Overall,our findings provide both a theoretical and practical foundation for agricultural production in the semi-arid farmland region of central Gansu and potentially elsewhere.