秃尾河流域植被覆盖变化及其对径流的影响

黄土高原地区水土流失严重,生态环境脆弱.随着退耕还林还草政策的有序实施,区域植被覆盖率从90年代末开始显著提高,但河川径流也随之显著减少,对黄土高原区的水资源安全造成了重要影响.选取陕北秃尾河流域为研究对象,通过分析流域植被变化特征,检验径流序列趋势变化、变异特征以及其与降水、植被指数(NDVI)的相关关系,探讨了降水...

Vegetation cover variation and its impact on runoff in Tuwei River Watershed

In the Loess Plateau of China，soil erosion is serious with a fragile eco-environment． Along with the orderly implementation of the policy of returning farmland to forest and grassland，the vegetation coverage in the region significantly increases since the end of 1990s，but a significant decrease of river runoff is there and makes an important impact on the safety of water resources within the region of the Loess Plateau． By taking Tuweihe River Watershed in the Northern Shaanxi as the study object，the impacts from the variations of precipitation and vegetation cover on runoff are discussed herein through analyzing the variation characteristics of the vegetation and checking the trend variation and the variability characteristics of the runoff sequence as well as their correlations with precipitation and vegetation index ( NDVI) within the watershed，and then their contribution rates in different periods are quantified by means of the Budyko hypothesis-based elastic coefficient method，from which their physical mechanisms affecting runoff decrease are pointed out． The study result shows that ( 1) the vegetation covering pattern in Tuweihe River Watershed is generally higher in the southeast and lower in the northwest and gradually gets better in recent years，especially，the vegetation coverage in the watershed is significantly increased after 2000，while the annual mean NDVI value reaches 0. 45 till 2013，of which the variation is mainly presents that the unused land in the northwest is transformed into grassland and forestland; ( 2) the runoff of Tuweihe River Watershed significantly and continuously decreases in recent years and exhibits the variations in three phases with the inflection points occurred in 1983 and 1997，while the precipitation and potential evapotranspiration exhibit non-significant decrease and the declined trends of both the potential evapotranspiration and runoff are to be continued; ( 3) the precipitation driven-runoff decreases during the period of 1984～ 1997 with stronger positive correlation，while the vegetation cover in the watershed significantly varies after 1997 with strong negative correlation with runoff and becomes the main causation to induce the significant decrease of runoff; ( 4) the underlying surface parameter n obtained from the calculation made by Budyko equation has the same variation trend with that of the annual NDVI value，thus the applicability of the elastic coefficient method is verified． The quantitative analysis result shows that the variations of both climate and vegetation cover exhibit heterotropic impacts on the variation of runoff，and then the climate change makes runoff decrease and the vegetation variation leads to the increase of runoff． Moreover，the contribution rate of the former is obviously larger than that of the latter． Afterward，the impact from the variation of vegetation on the decrease of runoff is significantly enhanced with the contribution rate of over 75%． It is known from the relevant physical analysis that the variation of vegetation is now gradually dominating the runoff variation in the watershed under the current climatic environment，from which the phenomenon of land-surface turning green is possible to make a potential threat to the regional water security．