Effects Of Rainfall Patterns On Relationships Between Soil Vegetation And Radiation In Prohibited Grazing And Grazing Grassland

Arid and semi-arid grassland is an important part of the global natural ecosystem and an important ecological barrier in northern China.With the change of climate and environment and human development and utilization,the surface state of grassland has also changed,especially the change of near surface energy budget process is more obvious.Local meteorological factors,soil temperature and humidity and vegetation conditions have strong interactions with the land surface,but the understanding of these interactions is seriously limited due to the lack of observation of surface radiation energy flux.In order to deeply understand the response characteristics of the surface radiation energy balance process of semi-arid grassland to the influencing factors under prohibited grazing and grazing conditions,this paper studies the radiation flux observation data of typical degraded grassland in Zhengxiangbai banner,Inner Mongolia from 2020 to 2021.The differences of radiation fluxes（Rsd、Rsu、Rld、Rlu、Rn and Albedo）and vegetation status（NDVI,vegetation coverage）in growth seasons and their responses to meteorological factors and soil temperature and humidity were compared and analyzed.The main research results are as follows:（1）From 2019 to 2021,the total rainfall is 1006.9 mm,and the average annual rainfall is 335.6 mm.There are 142 rainfall times,which are divided into 30 rainfall modes.The mode with the largest rainfall is 10～15 mm two days;the most frequent rainfall pattern is0～5 mm a day.Rainfall events are mostly concentrated in rainfall patterns lasting for one day,two days and three days,while rainfall events lasting for four days or more are relatively few.The soil moisture in the study area has obvious seasonal variation characteristics,and the vertical distribution characteristics are different with the different soil depth.The memory of shallow soil moisture is about 5～10 days.The duration of dry and wet anomaly of soil moisture is short,which may have an impact on short-term weather or climate.The moisture memory of middle soil is about 10～15 days,which is slightly higher in spring,summer and autumn than in winter.The memory of deep soil moisture is maintained at about20～25 days.The abnormality of soil moisture may affect the weather or climate for nearly one month in the later stage.The memory of soil moisture in prohibited grazing areas is slightly higher than that in grazing areas,and the duration of abnormality is relatively longer.In the vertical distribution of soil moisture memory,the soil moisture memory in prohibited grazing areas showed an upward trend with the increase of soil depth;The grazing areas also shows an increasing trend on the whole,with a slight decrease only in summer.（2）During the study period,NDVI in prohibited grazing and grazing areas lags rainfall by 0～5 days or 10～15 days in 2019.In 2020,NDVI and rainfall show a lag effect of 5～10days.There is a good correlation between NDVI and rainfall 0～5 days in advance in 2021.The time of NDVI lagging rainfall in this year is 0～5 days.In a word,the change of NDVI has a significant relationship with rainfall time and with a lag.Under the condition of natural rainfall,the vegetation coverage of prohibited grazing area in the growing season from 2019to 2021 increased by about 1.7%～15.2%compared with the free grazing area in the same period.The vegetation coverage of prohibited grazing rainfall increasing area（JPI）increased by about 8.7%～22.7%compared with the rainfall decrease area（JPD）in the same period.The vegetation coverage of grazing rainfall increasing area（FPI）increased by about 12.0%～26.0%compared with the grazing rainfall decreasing area（FPD）in the same period.It can be seen that the impact of precipitation on vegetation is greater than the management measures of prohibited grazing and grazing.The reflectance spectrum curve of vegetation in the study area has obvious absorption and reflection characteristics in specific bands.The spectral curve characteristics of vegetation in the prohibited grazing area are basically consistent with those in the grazing area.The reflection peak and absorption Valley in different bands tend to move upward.The spectral characteristics of vegetation in the rainfall increasing area are more obvious than those in the rainfall decreasing area,and the reflection peak and absorption valley also move upward.（3）Rsd and Rsu in the study area have significant unimodal diurnal variation characteristics.The Rsu in the prohibited grazing site is generally smaller than that in the grazing site.The response of Rsd to rainfall patterns is timely and significant,and the decline amplitude increases with the duration of rainfall pattern and the increase of rainfall.Under different rainfall modes,the daily variation range of Rld is very small,which is basically maintained at 300～350 W·m^-2,and each rainfall event is bound to increase Rld.The effect of different rainfall patterns on grassland Rlu is very weak.It indirectly affects the change of Rlu by adjusting soil temperature.The influence of rainfall pattern on the diurnal variation trend of Rn is basically same as that on Rsd,and the time of daily peak is also synchronized with the total radiation.The diurnal variation of Albedo of the prohibited grazing and grazing grassland shows a"U"pattern.Each rainfall in the growing season corresponds to the decline of Albedo,and the response of Albedo is also different under different rainfall modes.Albedo was low from May to August,but there was an obvious upward trend in September and October.（4）The radiation flux in the study area has a very significant linear positive correlation with the soil temperature of 0～10 cm in the surface layer and 10～20 cm in the middle and upper layer,and the correlation with the soil temperature of the middle and lower layer and deep layer due to temperature conductivity is gradually weakened.In the prohibited grazing site,VPD and soil surface temperature（TS）have significant negative effects on albedo,and vegetation index（NDVI）has significant positive effects on Rn.However,there was no significant relationship in grazing site.Therefore,VPD,surface temperature and vegetation status are the main factors affecting the process of near surface radiation budget in the prohibited grazing and grazing pastures.