Temporal-spatial Distribution Of The Potential Evapotranspiration And Its Controlling Factors In The Loess Plateau

Potential evapotranspiration（ET₀）is a key factor in calculating crop water demand and the basis for formulating irrigation systems.Understanding changes in regional potential evapotranspiration is important for the rational use of water resources and agricultural production.The Loess Plateau is a typical ecologically fragile area and water resources are tight.As a research on the evaporation of important links in the water cycle,it can control the amount of surface water and affect the water cycle rate,thereby affecting the regional water and heat balance.It is of great significance to regional ecological environment and water resources security.In this study,Penman-Menteith model was used to calculate potential evapotranspiration on the Loess Plateau.The Mann-Kendall trend analysis and wavelet analysis were used to analyze the spatiotemporal changes and periodic characteristics of potential evapotranspiration on the Loess Plateau from 1960 to 2017.,Calculation of contribution rate,cross-wavelet analysis and gray-level correlation analysis method to explore the influence of natural and human factors on potential evapotranspiration in the Loess Plateau.The study has reached following conclusions:（1）The results of temporal analysis shows:During the period from 1960 to 2017,the annual ET₀ showed an increasing trend,and the increasing rate was 7.7 mm/10a.During the study period,the average annual ET₀in the Loess Plateau was 1074mm.Among the three topographical areas（the MR,the VR and the HR）,the annual average ET₀in the hilly region（HR）is the largest,with an ET₀value of 1078mm.In terms of growth trends,the annual ET₀in the three regions shows an increasing trend,with the most significant increase in the valley region（VR）.Among the changes in each season,the ET₀is highest in summer and the lowest value in winter.In terms of growth trends,the growth rate in the HR is the fastest in the spring（7.8mm/10a）,and the fastest growth rate in the summer is in the mountainous region（MR）,and the ET₀in the VR has increased rapidly in autumn and winter.（2）The results of the spatial distribution of ET₀showed that the maximum potential evapotranspiration in the study area appeared in the southwest of Shanxi Province,of which the Xiangning Observation Station had the highest potential evapotranspiration（1467mm）in the past 58 years;the lowest potential evapotranspiration occurred at the Wutaishan Observatory located in the northeast of Shanxi Province.The lowest potential evapotranspiration during the study period was 495mm.The ET₀values are higher in the south and east than in the west.In terms of spatial distribution,the ET₀value in the VR is higher than that in the MR and the HR in spring;In autumn,the high value in the HR is mainly distributed in the west,and the value of the ET₀in the central and eastern areas is small.The high values in the VR are mainly distributed in the Linfen Basin region in the south.In winter,the values of ET₀in the northern Hetao Plain area and the northern HR are lower than those in the southern area.（3）The spatial variation trend of the ET₀and the cycle analysis results shows that the annual ET₀ in space was on the rise,and 93%of the sites showed a significant growth trend in the past 58 years.Compared with the MR and the VR,the trend of the annual ET₀in the HR is more obvious.In terms of the spatial change trend of the four seasons,except for the spring all showing an increasing trend,the other three seasons have a downward trend.In the past 58 years,the annual ET₀has a 30-year main cycle change and a 9-year,49-year sub-cycle change.The seasonal changes of the ET₀in the study area in four seasons are generally consistent with the changes in the annual ET₀,and also show a 30-year main cycle change law,but the small periodic changes in the ET₀in the four seasons are different.（4）The analysis of the factors affecting the ET₀shows:1)Among the basic meteorological factors,changes in relative humidity,temperature,and maximum temperature have significant effects on changes in the area ET₀.The main meteorological factors that affect the changes of the ET₀in the Loess Plateau are relative humidity,temperature,and maximum temperature.In the MR and the VR,the correlation coefficient between the highest temperature and the ET₀is the highest,0.77 in the MR and 0.84 in the VR,both of which passed the 0.1 level test.For the HR,the effect of relative humidity is the largest,with a correlation coefficient of 0.71.Analysis of the contribution rate shows that the contribution rate of temperature across the Loess Plateau reaches 42%;in the MR,the greater contribution rate is the temperature,the highest temperature and the lowest temperature;In the VR,the contribution rate of temperature is relatively high,and the contribution of relative humidity has increased to 15%;in the HR,the highest contribution rate is the highest temperature（39%）,followed by relative humidity（28%）.2)The cross-wavelet results of the ET₀and large-scale circulation factors show that there is a correlation between ET₀and ENSO,PNA,and AMO.Correlation analysis results of ET₀ and large-scale circulation factors show that both the whole Loess Plateau and various topographical areas are greatly affected by AMO,and passed the significance test at the level of 0.1.3)The analysis of human factors and the ET₀shows that the changes in the underlying surface properties of cultivated land,forest land,grassland,water areas,and construction land the effect of the ET₀ is greater.