Study On Evapotranspiration And Its Influencing Factors In Loess Plateau Of Northern Shaanxi Province

Evapotranspiration is an important part of the water cycle,and it is also an important variable of energy cycle and material exchange.The strength of mutual influence of the regional gas in a certain area can be well represented by the size of evapotranspiration.And evapotranspiration is directly or indirectly affected by factors such as human activities,vegetation,and climate.Therefore,analyzing the changes in various factors in the study area can better understand the evapotranspiration-related issues.With the rapid population growth and rapid economic development,land use patterns in China have changed significantly.The research on hydrological effects and other related fields caused by land use/cover changes has become the focus of current research.However,the Loess Plateau in northern Shaanxi has suffered serious soil erosion and wind erosion for a long time.It is a typical ecologically fragile zone in China and a key area for water and soil conservation in the upper and middle reaches of the Yellow River.In 1999,China started to implement the project of returning farmland to forests(grass)in the western region,and the Loess Plateau in northern Shaanxi was the first area to implement the project of returning farmland to forests(grass).After several years of hard work,the local vegetation has been greatly improved.restore.This paper selects the actual evapotranspiration data from the Loess Plateau from 2000 to 2013,and analyzes the temporal and spatial characteristics of the changes and trends by using software such as ArcGIS,ENVI,Matlab,and SPSS;and analyzes the impact factors of land use/cover change,NDVI and spatial-temporal characteristics and trends of meteorological factors such as temperature,precipitation,sunshine hours,relative humidity,etc.;and analysis of changes in evapotranspiration,NDVI and evapotranspiration,meteorological factors caused by changes in land use/cover types.The correlation with evapotranspiration,the research findings are as follows:(1)The annual evapotranspiration of the Loess Plateau in northern Shaanxi from 2000 to 2013 showed a weak upward trend,and the evapotranspiration in the northern Shaanxi Plateau changed most from 2000 to 2005,and it was the highest in Fuxian,Luochuan,Huanglong and Huangling counties and the south of Yichuan County.The district is a district with high evapotranspiration and is also the region with the greatest change.The standard deviations of Shenmu,Suiyang,and Hengshan counties in the Yulin area are areas where the evapotranspiration value is small,and it is also a stable area.From 2010 to 2013,the evapotranspiration changes from 2005 to 2010 were the smallest.There was no significant change in evapotranspiration in most of the regions in spring,and evapotranspiration in summer was an increase in large areas.Evapotranspiration in major sectors was significantly reduced in autumn,and evapotranspiration in winter was significantly reduced.Seasonal performance of ET is: summer> autumn> winter> spring.(2)Grassland and cultivated land are the most widely distributed in the Loess Plateau in northern Shaanxi,followed by shrub forests and sandy land.The area of sparse woodland and forested land is relatively small.The main types of land distributed in Dingbian,Jingbian and Hengshan counties of Yulin are arable land.Grassland is distributed in all counties and districts of Yan’an and is the most widely distributed.There are large sandy areas in Shenmu and Xiangyang districts of Yulin.Although there is a small area of woodland,in the spring,summer,and autumn,there is the largest evapotranspiration in the woodland,followed by the shrubbery,followed by the evapotranspiration in the open woodland,and the lowest in the sandy land;the evapotranspiration in various types of land in winter.The difference is not so much,because the low temperature in winter makes the transpiration of plants lessen.From the change analysis,shrubbery and sandy land are the two types of land use with the largest number of changes,and the conversion of cultivated land to grassland> grassland conversion> sandy land conversion to grassland.Changes in evapotranspiration caused by the conversion of different types of land use/coverage: conversion of open forest land to grassland> conversion of cultivated land to sandy land> conversion of shrubs to grassland.(3)In the past 14 years,the NDVI in the study area was the highest in summer,followed by spring,followed by autumn,and the season with the lowest average NDVI for many years was in winter.The entire area rose significantly during the study period.The NDVI increase area is distributed in the east of Shenmu County,Jia County,Mizhi County,Zichang County,Wubao County,Fugu County,Hengshan County,Zichang County,etc.;NDVI reduction area is mainly in Jingbian County,Luochuan County,The south side of Baota District,the east side of Fu County,Liyang District,the west of Fu County,Huangling County and Huanglong County.The spatial correlation between ET and NDVI is mainly positively correlated and negatively correlated,and is mainly positively correlated in spring and summer,and negatively correlated in autumn and winter.This is the result of the joint action of human activities and climate change.The high NDVI area is also a high value area for evapotranspiration.The spatial correlation between evapotranspiration and NDVI was mainly positively correlated and more significant negatively correlated,and positive correlation was dominant in spring and summer,and negative correlation was dominant in autumn and winter.(4)The M-K trend test was conducted on the five climate factors including mean temperature,precipitation,average wind speed,sunshine duration,and average relative humidity.The results showed that the average temperature and sunshine duration decreased,and the average wind speed,average relative humidity,and precipitation were presented.The increase was significant and the precipitation increased most significantly.The most important climatic factors affecting evapotranspiration are average relative humidity,average wind speed,and sunshine hours.The mean temperature,wind speed,and sunshine hours are negatively related to evapotranspiration.The average relative humidity and precipitation are positively correlated with evapotranspiration.In the study area,the temperature of the thermal factors was negatively correlated with evapotranspiration,indicating that the higher the temperature,the smaller the evapotranspiration,but the impact on the spatial extent is not significant.The effects of sunshine hours are both positive and negative.Precipitation and mean relative humidity in water conditions are positively correlated with evapotranspiration,indicating that the greater the precipitation,the greater the evapotranspiration or the higher the average relative humidity,the greater the evapotranspiration,and the effect on the spatial extent: the humidity factor.>Precipitation.The influence of wind speed on evapotranspiration is mostly negative,ie,the higher the wind speed,the smaller the evapotranspiration.(5)The influence of climate and vegetation on the actual evapotranspiration of the Loess Plateau in northern Shaanxi has some differences.During the study period,the contribution of vegetation to evapotranspiration gradually increased,but the climatic factors are always the main reason affecting the evapotranspiration in the study area.