黄土高原植被降水利用效率对植被恢复/退化的响应

基于2000-2014年MODIS NDVI数据及气象数据，运用累计降水利用效率变化差异（CRD，cumulative rain use efficiency differences）估算模型和基于地形要素降水量插值法，探讨2000-2014年黄土高原RUE（降水利用效率rain use efficiency）对植被变化的响应，以期为黄土高原生态可持续发展提供数据支撑。结果表明：黄土高原大部分地区植被覆盖得以改善，其面积约占总面积的81%，区域边缘植被覆盖退化严重。黄土高原降水利用效率RUE与累计NDVI的相关性总体表现为“东南呈正相关，西北为负相关”的空间格局，全区相关系数以正相关为主。黄土高原CRD与植被变化趋势的相关性显著，其中，植被退化背景下，植被退化程度越严重，RUE越低；植被恢复背景下，RUE受“退耕还林还草”作用显著，2000-2005年，RUE呈上升趋势，2007年后，随着退耕还林还草政策的工作重心转移，RUE呈波动变化。

Spatial pattern of vegetation rainfall use efficiency and its response to vegetation changes on the Loess Plateau

Relying on RUE (rainfall use efficiency), CRD (cumulative rain use efficiency differences)indicates the development trend of vegetation in arid and semiarid region productivity. Based on the NDVI and meteorological data, using the method of CRD and combining it with the spatial of vegetation cover organically, this paper analyzed the influence of vegetation cover to RUE, and focused on the characteristic of "Process-Service" of vegetation cover and RUE. The precipitation interpolation, which is based on the topographic features, reduced the chance of precipitation interpolation errors and enhanced the scientific accuracy of the data. We probed the spatial pattern of RUE and its response to ecological changes on the Loess Plateau during 2000-2014. The results show as follows: (1)The vegetation was improved in most areas on the Loess Plateau which accounted for 81% of the total area from 2000 to 2014. The spatial of vegetation degradation in the research area presented the trend of marginalization. Overall spatial distribution of CRD on the Loess Plateau presented "high in east and low in west, high in south and low in north" from 2000 to 2014. (2)The positive and negative correlation between RUE and NPP co-existed, but the main correlation was positive from 2000 to 2014. The area that had significant positive correlation and extremely significant positive correlation between RUE and NPP accounted for 24.7% and 6.5% of the total area, respectively; that had significant negative correlation accounted for no more than 8.5%. (3)In order to analyze the internal-external connection between RUE and vegetation, by using the precipitation on the Loess Plateau, we explored the change of RUE and the correlation between RUE and NDVI. In the area that lacks precipitation, both precipitation and NDVI had significant negative correlation with RUE; in the area with abundant precipitation, both precipitation and NDVI had significant positive correlation with RUE. (4)The correlation coefficient of CRD and the trend of NPP were significant. Under the background of vegetation degradation, vegetation rainfall use efficiency was positively correlated with the degradation degree from 2000 to 2014; under the background of vegetation restoration, vegetation rainfall use efficiency was closely correlated with "Grain for Green" project. It shows that the improvement of regional vegetation led to the increases of RUE, drought trend was further enhanced and the burden of water resources had increased. But, the calculation of RUE was over-reliance on precipitation, so the rapidly increasing or decreasing of precipitation might cause pseudo degradation or pseudo recovery of vegetation. Currently, there is no better method to estimate RUE; in subsequent studies, we will probe the formation mechanism of RUE, and then provide a scientific basis for the change of vegetation.