Observation And Simulation Of Surface Energy And Water Processes By CLM In The Agricultural-pastoral Ecotone Of Northwest China

The agricultural-pastoral ecotone of the Northwest China is a mosaic of grassland,cropland and barrenland,and exhibits rapidly changing land use/cover,and complex energy and water exchange processes and interactions over land surface.However,the land surface energy and water processes are poorly understood in the study area.This paper,based on the in situ observations of Yanchi Observation Station in 2017,analyzed land surface energy and water processes of croplands and grasslands,evaluated and revised CLM4.5 in the study area.The main findings are as follows:First,according to the in situ observations of Yanchi station,solar radiation,atmospheric longwave radiation,reflected radiation,upward longwave radiation and net radiation of the grassland had obvious seasonal and diurnal pattern,which were similar to the pattern of the northwest arid area.Surface albedo was affected by vegetation change and precipitation,and reached its minimum in October with an annual mean of 0.21.Soil temperature at Yanchi station decreased with the increasing depth in summer,and it showed opposite pattern in winter.From July to August,the soil temperature in cropland was lower than that in grassland due to irrigation and larger maize canopy coverage.The soil mositure over grassland was the highest at 10-15 cm at Yanchi station,while the soil moisture over cropland was highest at 5-10 cm.The soil moisture over cropland was higher than grassland caused by cropland tillage.Second,CLM4.5 well simulated the variation of reflection radiation,upward longwave radiation and net radiation of the grassland.The simulated high values of leaf area index(LAI)led to the low reflected radiation in May.In other periods,the simulated values of reflected radiation were slightly higher.The simulated values of the upward longwave radiation were higher than the observed values during the daytime due to the oversimulated surface temperature values.The simulated net radiation was low.CLM4.5 simulated well the vertical distribution and temporal variations of soil moisture and temperature.The simulated soil temperature was high in summer and low in winter due to the oversimulated values of heat conductivity.The undersimulated value of soil moisture compared to the observed values was closely related to porosity.Third,the applicability of CLM4.5 under cropland was evaluated using the observed data.Results indicated that the simulated amount of automatic irrigation in Yanchi Observation Station(3,734 mm)was much greater than the actual local irrigation amount(300-600 mm),which led to the undersimulated soil temperature(-5 ?)in September.After modifying the CLM4.5 by using localizing irrigation time and amount,the correlation coefficient of soil temperature with the observed values increased from 0.83 to 0.89,root-mean-square-error(RMSE)decreased from 4.70 ? to 3.17 ?,and the correlation coefficient of soil moisture increased from 0.26 to 0.70 in growing season.Fourth,on sunny and cloudy days,overly simulated values of soil temperature over grassland and undersimulated values of soil temperature over cropland.In a rainfall event,the performance of soil temperature and upward longwave radiation after rainfall by model was improved.Thus,the model performed better in simulating the ground upward longwave radition and net radiation under the more humid conditions.These findings are helpful for understanding land surface energy and water processes in the northwestern agricultural-pastoral ecotone in China and for improving the applicability of the CLM4.5 to the region.