Changes Of Evaporation In The Loess Plateau And Its Reasons In The Recently 50 Years

Evaporation is a key procedure which links energy balance and water balance in nature.Estimates of the actual evaporation is an important issue and fundamental work of hydrology,and a challenge,too.One side,precise measurement of evaporation calls for expensive instruments and professional staff.It can only be covered in some specific site experiments,which is not very viable in some hostile environments.Thus,evaporation is always estimated by the relationship between actual evaporation and potential evaporation.The hypothesis of the complementary relationship(CR)between actual and potential evaporation was first proposed by Bouchet in 1963.After decades of development,a number of practical models for evaporation estimation have been established.Those models have been verified in a wide range of time scales and spatial scales.Models based on CR always needs energy measurement and routine meteorological observations only,and do not need surface temperature,stomatal conductance or any vegetation index,which are complicated and hard to obtain.Models based on CR are convenient,and t hey have received widely accepted in hydrology research.Available water resources are mainly distributed into two parts,namely,human use and ecosystem use.For Loess Plateau,which is the most eco-fragile region in the world,estimation of evaporation is unprecedented important for water resources management or relevant policy-making.The aim of this article is to base on the CR theory,to trace the changes of evaporation in the last 50 years in the Loess Plateau,and explain the climatology reason s behind it.First of all,a nonlinear CR model of Brutsaert has been tested in a flux station in Yulin,a city located in northern Shaanxi Province.It is found that the model can describe the monthly evaporation process of the site place very well,and the only parameter of the model stay relatively stable even during a peri od when big changes of land uses have taken place.The monthly evaporation estimated match the CR curves approximately in spring and summer,and after a modification of strong advection,which is more common in autumn and winter,the results of these two seasons obtain better matching relationship to the CR curves.Analysis of the basic climate factors shows that,overall,radiation and temperature experience the most significant changes in t he last decades,of which an obvious downward and upward trend may be seen,respectively.In addition,a relatively slower declining in both humidity and wind speed have taken place in many place.Finally,calculated by 78 stations and 27 basins,the sequence of yearly evaporation of Loess Plateau has been obtained.It is showed that,in the rough,yearly evaporation in the eastern area has decreased significantly,and an area of the Midwest part has experienced an increase of evaporation,but not obviously.With a comparison of the original and detrended annual series of evaporation,it shows that change of radiation is the main reason responsible for the changes of evaporation in the Loess Plateau in the recently 50 years,and changes of temperature and wind speed are secondary reasons.