Research On Water Use Efficiency Of Grasslands In Northwestern China Based On LPJ Model

Grassland is an important terrestrial ecosystem in the world.Grassland ecosystems are facing enormous risks and challenges under the background of climate change.The water use efficiency?WUE?is an important indicator for understanding the coupling relationship of the carbon-water cycle in the ecosystem.On this basis,the rain use efficiency?RUE?is proposed,which can better reflect the degradation or recovery of vegetation under the influence of climatic factors and thus reflect the productivity of the region.Grassland resources in the northwestern part of China are very rich,and it is also a climate sensitive and ecologically fragile area.Therefore,the study of grassland WUE and RUE in the northwest region under the background of climate change can provide a basis for the impact of global climate change on the ecosystem carbon water cycle.At the same time,it provides a theoretical basis for vegetation restoration and ecological environment construction in arid and semi-arid areas.In this study,the LPJ?Lund-Potsdam-Jena?global vegetation dynamics model was used to estimate Net Primary Productivity?NPP?and actual evapotranspiration?ET?,and then validated NPP and ET in Northwestern China.The temporal and spatial variation characteristics of NPP,ET and WUE in grassland were analyzed in Northwestern China from 1961 to 2013.Secondly,RUE_NDVI was obtained from the 1982-2013 Normalized Difference Vegetation Inde?NDVI?and cumulative precipitation ratio,and then analyzed and compared the temporal and spatial characteristics of WUE,RUE and RUE_NDVI.Finally,the climate change characteristics of grassland in Northwestern China and the impact of climate change on WUE and RUE were analyzed.The main conclusions are as follows:?1?The annual average value of WUE in grassland in Northwest China from 1961 to 2013was 0.61 gC·mm^-1 m^-2,which showed a decreasing law from east to west.The WUE of grassland in Northwestern China showed a significant increased trend in spatial and interannual variations.However,among the three different types of grassland,the spatial variation trend of low-coverage grassland was the most significant,while that of high-coverage grassland was the lowest.In terms of the inter-annual variation,the moderate-coverage grassland was the most significantly,and that of the high-coverage grassland was the lowest.?2?The annual average RUE of grassland in Northwest China from 1961 to 2013 was 0.53gC·mm^-1 m^-2,which showed a decreasing trend from east to west.There was a significant increase in all grassland or three different types of grassland,with the highest increase in low-coverage grassland.The RUE_NDVIDVI of the grassland in Northwestern region has spatial heterogeneity;the RUE_NDVI increased in the east and decreased in the west.The trend of increasing the coverage of high-coverage grassland was the most obvious.The annual average RUE_NDVI of grassland showed a slight downward trend,with large annual fluctuations;high and moderate grassland showed a slight upward trend,while low-coverage grassland showed a downward trend.?3?Compared with the WUE,RUE and RUE_NDVI of grassland in Northwestern China,the spatial distribution of WUE and RUE was basically the same,and the interannual variation trend was very similar,but the higher value of RUE_NDVI was distributed in the area with less precipitation,which was different from WUE and RUE.In addition,WUE and RUE showed a significant upward trend with time,while RUE_NDVI showed a slight downward trend.?4?There was a significant positive correlation between WUE and temperature in the grassland of Northwestern China,and the positive correlation between the low-coverage grassland and the temperature was the highest.There was a significant positive correlation between WUE and precipitation in 60.45%of grassland in Northwestern China,with negative correlation in a few areas,and the highest correlation between high-coverage grassland and precipitation.?5?The 70.40%RUE of grassland in Northwestern China was positively correlated with temperature,and the annual average RUE of grassland was significantly positively correlated with annual average temperature.The positive correlation between low coverage grassland and temperature was the highest.In areas with high precipitation or areas with little precipitation,grassland RUE was negatively correlated with precipitation,and areas with less precipitation?100mm-400 mm?,grassland RUE was positively correlated with precipitation.There was no significant correlation between annual average RUE and annual average precipitation in Northwestern China,and the annual average RUE of high and moderate covered grassland was negatively correlated with precipitation,and the RUE of low-coverage grassland was not significantly correlated with precipitation.