Actual Evapotranspiration Modelling And The Sustainability Of Veegetation Water Use In The Loess Plateau

Loess plateau is a typical ecological sensitive area in China,and there are two major problems: drought and soil erosion.Jointly influenced by the natural factors and the artificial protection measures in resent years,the vegetation coverage of Loess Plateau has been significantly increased.The Loess Plateau is located in arid and semi-arid region,and water is the most important factor to restrict vegetation restoration.To maintain the balance between precipitation and water consumption is an important foundation for sustainable development of the ecology remediation.Based on this,this study used Budyko Framework to simulate the annual actual water consumption from 1990 to 2014 and analyze the temporal and spatial evolution of the regional precipitation and the actual evapotranspiration(AET).Then the study estimated monthly AET in three typical hydrological years based on the remote-sensing-based ET-estimation model.This study also introduced rainwater utilization potential indicator(RUP)to quantify the sustainability of water balance in Loess Plateau.The main conclusions in this study are summarized as follows:(1)The Budyko theory framework was introduced in this study to simulate the regional AET from 1990 to 2014 in the Loess Plateau.The spatial-temporal patterns of precipitation,potential evapotranspiration and actual evapotranspiration from 1990-2014 of Loess Plateau were all analyzed in this study.Based on the results,the average values of precipitation and actual evapotranspiration in the entire Loess Plateau were showing a significant increasing trend.In the spatial distribution,the AET is decreasing from the southeast to the northwest in the Loess Plateau.Meanwhile,the high-AET area(AET>400 mm)was also extremely enlarged from 39%(1990)to 73%(2014)in Loess Plateau after the Grain-for-Green Project was implemented.However,the average values of potential evapotranspiration had no obviously change in the study period.(2)Due to the serious uneven distribution of precipitation in the loess plateau,it is not possible to analyze the sustainability of vegetation water consumption only on the annual scale.Based on this,the remote-sensing-based SEBAL model was introduced in this study to estimate the regional AET in three typical hydrological years.Based on analysis of the annualdistribution of the monthly precipitation and AET in different hydrological years,the vegetation water consumption in the spring(March to April)is generally much higher than the precipitation,which may indicate that the spring drought is a potential threat for vegetation growth in Loess Plateau.(3)Based on the estimated AET result,this study also analyzed the relationship between NDVI and ET efficiency(EEC)and the relationship between NDVI and rainwater utilization potential indicator(RUP).The RUP values also have a significant positive relationship with NDVI,indicating that more water will be available for vegetation growth and ecological restoration with the vegetation recovery in the Loess Plateau.The average values of RUP were 0.132,0.137,0.149 and 0.163 respectively in the 4 periods of 1990-1999,2000-2004,2005-2009 and 2010-2014.The rise of RUP is due to that the increasing rate of precipitation is statistically higher than the AET.Consequently,Loess Plateau is showing a wetting trend in the period of 1990-2014,especially after the Grain-for-Green project was implemented.(4)The results of this paper were proved by using three different depths of Era-Interim soil water content data in the same period and the measured runoff data of 24 different hydrological sites.It was found that the soil water content in the Loess Plateau did not change significantly during the study period,and the annual runoff of most hydrological sites showed an increasing trend from 2002 to 2012.Although previous studies showed that compared to the average annual runoff from 2000 to 2010,1960-1969 and 1990-1969,the runoff of two phase showed a decreasing trend,but the period of vegetation recovery in the Loess Plateau is after 2000,so large-scale afforestation and ecological restoration are unlikely to be the main reason of resulting in the decreasing of runoff in loess the plateau.On the contrary,the vegetation should be able to conserve water,and the reduction of measured runoff should be attributed to the dam,reservoir,or other human activitivity.