| The relationship between vegetation and soil moisture content in the arid area is extremely close.Soil moisture content is the main limiting factor to vegetation growth.At the same time vegetation can also affect the spatial and temporal distribution of soil moisture content.However,the complex relationship between the two still lacks in-depth understanding on the watershed scale.Therefore,exploring the interaction between the two on the watershed scale has become a hotspot and a difficult point in the present macro-ecological hydrology study.The related study result has important practical guiding significance to the optimal allocation of water resources and environmental protection in the arid inland river basins.In view of this,this paper chose the Shule River Basin,an inland river basin in the arid region of Northwest China,as the study area and used meteorological and remote sensing data to estimate the net primary productivity(NPP)of the basin from 2001 to 2015 based on the CASA model.And the remote sensing data and measured data were used to obtain the summer soil moisture content data from 2001 to 2017 in the Shule River Basin based on remote sensing inversion and statistical downscaling.Then the temporal and spatial evolution patterns of the two were analyzed by GIS spatial analysis technology,and the spatial-temporal coupling relationship between the summer NPP and the soil moisture content from 2001 to 2015 in the basin was explored.Based on this,the partial least squares regression model between the summer NPP and meteorological and hydrological factors was established to carry out scenario analysis of NPP based on the change of soil moisture content,which further revealed the response relationship of NPP to soil moisture content.(1)In 2001-2015,the NPP in the Shule River Basin fluctuated upward,and the overall ecological environment of the basin was developing well.However,the NPP in some areas of the Dunhuang and Guazhou irrigation districts in the basin decreased insignificantly.The multi-year average NPP of the basin had obvious spatial heterogeneity.The NPP in most areas of the basin was less than 50gC/(m2 a),while the NPP in the source area of the main stream of the Shule River and each irrigation districts in the middle and lower reaches of the basin were higher,both above 250gC/(m2 a).And the NPP reached the maximum at an altitude of 3,000m to 4,500m.In the basin,the NPP of cultivated land was the largest,followed by forest land.The summer was the best season for vegetation growth,and the summer NPP accounted for 54.58%-64.44%of the annual total NPP.Its spatial heterogeneity was much higher than other seasons,and its rising trend was the most obvious.(2)From 2001 to 2017,the soil moisture content in the Shule River Basin in summer showed an overall upward trend.The soil moisture content in the upstream of main stream and irrigation districts in the middle and lower reaches were the highest in the basin.The soil moisture content in the basin showed an upward trend,except the irrigation district of Dunhuang and the upstream frigid areas.The soil moisture content of June was lower than July,and it was the highest in August.And the rising trend of the soil moisture content in June was the most significant.Overall,the soil moisture content increased with altitude.And the soil moisture content of cultivated land was generally higher than that of grassland and woodland.(3)On the watershed scale,there was a very significant positive correlation between the NPP and the soil moisture content in summer and the correlation coefficient was 0.8942(p<0.01).However,the interactions between soil moisture content and vegetation were different among months,and it was in June that the correlation was most positive.There was a hysteresis effect on the influence of the soil moisture content on NPP in summer and different months had different lag time.Conversely,vegetation in the basin also had a significant impact on soil moisture content,and the correlation between the NPP in July and the soil moisture content in August was the largest.(4)From the perspective of geographical division,there were significant positive correlations between the NPP and the soil moisture content in the summer of 2001-2015 in the south,middle and north of the basin.Among them,the correlation between the two was the highest in the northern Mazong mountains area and the lowest in the central plain.(5)From the grid scale,there were significant positive correlations between the NPP and the soil moisture content in each irrigation district of the middle and lower reaches,the upper mountainous areas(except the frigid areas)in the south of the basin and the central area in the northern Mazong mountains area in the summer of 2001-2015.And there was a significant negative correlation between the two on the saline-alkali land which on east of the irrigation district of Dunhuang.The correlation between the two was the highest between 3,000m and 3,500m above sea level.Besides,the correlation between the two of grassland was higher than that of cultivated land and forest land.There were differences in the relationship between vegetation and soil moisture content in different regions.The response of the NPP in most areas of the basin to soil moisture content had a hysteresis effect,and the influence of the NPP on the soil moisture content was greater in desert and Gobi than other areas.(6)Based on the partial least squares regression model of the summer NPP and meteorological and hydrological factors(temperature,precipitation,solar radiation and soil moisture content),the response relationship between the NPP and the soil moisture content was simulated under two scenarios.The results showed that:1)under scenario 1 of which the climatic conditions remain unchanged,the summer NPP would increase by about 2.08%compared with the 0%scenario(the soil moisture content and the climatic conditions remain unchanged)and the spatial heterogeneity of NPP would strengthen continuously,when the soil moisture content in summer varies by±1%according to the original trend.The most obvious areas where the NPP increased were the areas in the upstream mountains in the south of the basin and near the mountain pass,the irrigation districts in the middle and lower reaches and the central area in the northern Mazong mountains area.2)Under scenario 2 of which the temperature would increase by 1? and the precipitation would increase by 2%,with the increase of the change range of the soil moisture content,the increase of the summer NPP was smaller than that of Scenario 1.It shows that the climate change might weaken the effect of soil moisture content on vegetation growth to a certain extent,especially the forest land.And it would also increase the area in desert and Gobi of the middle and lower reaches where the NPP reduced.In the case of climate change in the future,only timely replenishment of soil moisture content can better promote the increase of the NPP and improve the vegetation productivity of the basin.The above research results deepened our understanding on the coupling relationship between ecological environment and water resources on different spatial and temporal scales in the Shule River Basin,and provided a scientific basis for formulating more detailed water resources optimization allocation and ecological environment restoration and protection measures. |
PDF Full Text Request