塔里木河流域草地净初级生产力时空分异特征研究

采用MODIS数据和改进的光能利用率模型(CASA模型)对2006-2016年塔里木河流域植被生长季草地净初级生产力(NPP)进行估算,通过一元线性回归趋势分析、变异系数、Hurst指数等方法,分别从时间特征、空间特征、空间稳定性和未来变化趋势4个方面对其时空变化过程进行分析,阐述了2006-2016年塔里木河流域草地NPP的时空格局与变化特征。结论如下:1)时间特征上,2006-2016年塔里木河流域草地NPP总体呈波动上升趋势,其中增长区域占64.1%,负增长区域占35.9%,NPP总量平均增长速度为1.31×10¹¹ g C·m^-2·yr^-1;6-8月为流域内草地的主要生长期,NPP总量占生长季总量的47.3%。2)空间特征上,2006-2016年塔里木河流域草地NPP分布呈现明显空间分异特征,总体分布特征为西北向东南呈递减趋势,草地NPP多年平均值为13.62 g C·m^-2·yr^-1。在垂直变化(海拔)上,呈现出随海拔升高NPP呈“降低-升高-降低”的特点;在水平变化上(经、纬度),草地NPP变化没有明显特征。3)空间稳定性上,塔里木河流域草地NPP变异系数介于0.19~3.17。草地NPP存在明显的空间差异性,草地NPP变异系数大部分地区均属于低值区,其中变异系数在0.5以下的占总面积的94.6%。高波动区域主要集中在“四源一干”和车尔臣诸小河流域。4)未来变化趋势上,塔里木河流域草地NPP Hurst指数介于0.10~0.97,均值为0.57。未来变化趋势呈持续性特征面积占71.6%,反持续性特征面积占28.4%,除开孔河流域北部及车尔臣诸小河西南大部呈反持续性特征,其余草地均呈现出持续性特征,预示着流域草地NPP未来处于持续增加趋势。草地NPP时空分布特征显示,塔里木河流域草地生态系统健康状况总体好转,局部恶化。

Spatio-temporal characteristics of grassland net primary productivity (NPP) in the Tarim River basin

We analyzed spatio-temporal changes in grassland net primary productivity (NPP) in the Tarim River basin, based on an improved light use efficiency model (Carnegie-Ames-Stanford approach model, CASA) and moderate resolution imaging spectroradiometer (MODIS) data from 2006 to 2016. To study the temporal pattern, spatial characteristics, spatial stability and the time trend in grassland NPP were used to formulate a simple linear regression model, and derive the coefficient of variation and the Hurst index. The model analyses the temporal variation in grassland NPP in the Tarim River basin from 2006 to 2016. This analysis of the temporal pattern over the 11 years showed that the grassland NPP was variable with 64.1% of the total area, showing increased NPP and for 35.9% of the total area exhibiting falling NPP in the past 11 years. Overall, the grassland NPP of the Tarim basin was 1.31×10¹¹ g C·m^-2·yr^-1. The main growing season for grassland in the basin is from June to August each year, and NPP during this period accounts for 47.3% of the total annual NPP. A geographic gradient of decreasing grassland NPP from northwest to southeast was observed for these 2006-2016 Tarim River basin data. In the whole basin, the average annual grassland NPP estimated by the CASA model for 2006-2016, was 13.62 g C·m^-2·yr^-1. The altitudinal change in grassland NPP for the 11 years followed a “decreased-raised-lowered” pattern with increasing altitude, and also some smaller features related to latitude and longitude. The coefficient of variation of grassland NPP over the 11 years ranged from 0.19 to 3.17, in various localities, which indicated the grassland NPP has obvious spatial differentiation. The land area for which the coefficient of variation of grassland NPP is less than 0.5 accounts for 94.6% of the total surveyed land area. Areas of high NPP fluctuation include the Kaikong river basin, the Aksu river basin, the Yarkant river basin, the Hetian river basin, the Tarim River main stream basin, and the Qarqan river basin. The Hurst index of grassland NPP for the 11 years ranged from 0.10 to 0.97 and the mean value was 0.57. This indicates the grassland NPP is expected to maintain the same change-trend in the future. In summary, the patterns of spatio-temporal change in grassland NPP showed that the health status of the grassland ecosystem is improving in the majority of the Tarim River basin, but deteriorating in some local areas.