宁夏陆地生态系统水分利用效率特征及其影响因子

生态系统水分利用效率(Water Use Efficiency, WUE)是表征生态系统碳水耦合程度的重要指标,能反映生态系统碳水循环规律及其相互作用关系。基于MODIS数据以及宁夏生态系统类型数据,分析2000—2017年宁夏不同生态系统WUE的变化特征,探讨了NPP和ET两种因子对WUE年际与年内变化的影响。结果表明:(1)全区陆地生态系统的年均WUE为1.03 g·C/kg·H_2O,值域在0.55—2.98 g·C/kg·H_2O之间,总体上呈现南北高、中部低的特征。(2)不同生态系统的WUE差异较大,由高到低为水体及湿地、森林、农田、草地、聚落、荒漠和其他生态系统,在同类生态系统中,植被生物量和盖度越高的亚类生态系统,其WUE也越高。(3)宁夏陆地生态系统WUE存在着每年0.0141 g·C/kg·H_2O的下降趋势,年内WUE呈典型的单峰形态,变化范围在0.02—2.16 g·C/kg·H_2O之间。(4)年际尺度上,宁夏陆地生态系统WUE与年蒸散(Evapotranspiration,ET)有极显著负相关性(P0.01),而与净初级生产力(Net Primary Production,NPP)没有相关性;年内尺度上,WUE变化与ET呈显著正相关(P0.05),与NPP呈极显著正相关(P0.01),这与植被的年内季节性生长过程有关。(5)根据ET强弱和WUE高低,可将宁夏陆地生态系统水分利用效率特征划分为4类,即低ET低WUE区、低ET高WUE区、高ET低WUE区和高ET高WUE区。宁夏的生态恢复工程在增强植被生产力的同时,也增强了区域水分消耗,致使陆地生态系统整体水分利用效率下降,这为宁夏未来水资源调控和生态重建提供了科学依据。

Characteristics of water use efficiency in terrestrial ecosystems and its influence factors in Ningxia Province

Water use efficiency (WUE) is an important indicator to characterize the coupling process between carbon and water cycle in terrestrial ecosystem and can reflect their regulations and interactions. Based on MODIS 8-day composite net primary production data (MOD17A2), evapotranspiration/latent heat flux product data (MOD16A2) and ecosystem type data of Ningxia, the WUE changes in different ecosystems from 2000-2017 were analyzed. We also studied the impacts of two factors, e,g. NPP and ET on the WUE changes at monthly and annually time scales. The results showed that:(1) The average annually WUE of terrestrial ecosystems in the region was 1.03 g·C/kg·H₂O, with the range of 0.55-2.98 g·C/kg·H₂O. The higher WUE levels distributed in the terrestrial ecosystems in north and south of Ningxia province, while the lower WUE levels mainly located in the terrestrial ecosystems in the central Ningxia province. (2) The WUE of different ecosystems varied greatly. The order of WUE value from high to low is water and wetland ecosystem, forest ecosystem, farmland ecosystem, grassland ecosystem, settlement ecosystem, desert ecosystem, and other ecosystem. In the same ecosystem, the higher the vegetation biomass and coverage of the sub ecosystem, the higher the WUE. (3) The WUE has decreased by 0.0141 g·C/kg·H₂O·each year from 2000 to 2017, the monthly change of WUE showed a typical single peak shape during a year with a range of 0.02-2.16 g·C/kg·H₂O in theterrestrial ecosystem. (4) At the annual time scale, there was a significant negative correlation (P<0.01) between WUE and Evapotranspiration (ET) in the terrestrial ecosystem, but had no correlation with net primary production (NPP); at the monthly time scale, WUE significantly positive correlated with ET (P<0.05), and significantly positive correlated with NPP (P<0.01), which is related to the seasonal growth of vegetation in a year. (5) According to the strength of ET and the level of WUE, the characteristics of water use efficiency of terrestrial ecosystem in Ningxia can be divided into four types including low ET with low WUE, low ET with high WUE, high ET with low WUE, and high ET with high WUE. The ecological restoration projects implemented in Ningxia province can not only enhance the vegetation productivity, but also promoted the regional water consumption, which decreased the water use efficiency of the terrestrial ecosystem. The conclusion of this paper could provide scientific basis for water resources utilization and ecological restoration in Ningxia Province in the future.