生态输水后青土湖绿洲植被覆盖度时空演变规律

Spatial-Temporal Evolution of Vegetation Coverage in Qingtu Lake Oasis After Ecological Water Conveyance

由于多年生态输水,石羊河尾闾青土湖的生态环境发生了显著变化,从植被覆盖度角度评估输水对青土湖绿洲的恢复效应,应用2019、2020年的无人机影像,通过面向对象的随机森林算法提取青土湖绿洲植被,利用2010~2020年Landsat卫星图像计算青土湖绿洲夏季I_(NDVI)图像,建立绿洲植被覆盖度和I_(NDVI)之间的关系函数,并利用关系函数对I_(NDVI)图像进行反演,得到2010~2020年夏季植被覆盖度,分析了青土湖绿洲植被覆盖度的时空演变特征。结果表明,2019、2020年绿洲植被覆盖面积分别为8.54、8.52 km;当I_(NDVI)位于区间(0.06,0.79)时,F_(FVC)随I_(NDVI)呈指数上升趋势;绿洲平均植被覆盖度增长至输水前的9.6倍,19.14 km^(2)的植被覆盖度得到改善;植被覆盖度时空演变特征明显,可分为快速增长阶段和波动稳定阶段。生态输水抬高了当地地下水水位,是促进植被恢复的关键因素。在目前的输水形势下,绿洲的生态水文过程达到动态平衡,植被覆盖度变化会保持波动稳定。研究结果可为青土湖绿洲生态环境治理和保护提供科学依据。

As the terminal lake of Shiyang River, the ecological environment of Qingtu Lake has changed significantly due to many years of ecological water conveyance. The restoration effect of water conveyance on Qingtu Lake oasis was evaluated from the perspective of vegetation coverage. The UAV images in 2019 and 2020 were applied to extract vegetation of Qingtu Lake oasis through object-oriented random forest algorithm. The summer I_(NDVI)images of Qingtu Lake oasis were calculated by using Landsat satellite images from 2010 to 2020. The relationship function between vegetation coverage and I_(NDVI)of Qingtu Lake oasis was established to invert the I_(NDVI)image to obtain the summer vegetation coverage from 2010 to 2020. And then the spatial-temporal evolution characteristics of the vegetation coverage of Qingtu Lake oasis were analyzed. The results show that the vegetation coverage area of the oasis in 2019 and 2020 was 8.54 km;and 8.52 km, respectively. The F_(FVC) increases exponentially with I_(NDVI)when that is located in the interval of (0.06, 0.79). After water conveyance, the average F_(FVC) of oasis has increased to 9.6 times than that before, and the improvement area of F_(FVC) is 19.14 km;. The spatial-temporal evolution of F_(FVC) is obvious, which can be divided into rapid growth stage and fluctuation and stability stage. Ecological water conveyance is a key factor to promote vegetation restoration by raising local groundwater level. Under the current situation of water conveyance, the ecological and hydrological process of oasis reaches dynamic balance, and the change of vegetation coverage will remain fluctuating and stable. The research results can provide scientific basis for ecological environment management and protection of Qingtu Lake oasis.