Abstract
Hydrological connectivity is an important characteristic of wetlands that maintains the stability and functions of an ecosystem. This study investigates the temporal variations of hydrological connectivity and their driving mechanism in Baiyangdian Lake, a large shallow wetland in North China, using a time series of open water surface area data derived from 36 Landsat 8 multispectral images from 2013–2019 and in situ measured water level data. Water area classification was implemented using the Google Earth Engine. Six commonly used indexes for extracting water surface data from satellite images were compared and the best performing index was selected for the water classification. A composite hydrological connectivity index computed from open water area data derived from Landsat 8 images was developed based on several landscape pattern indices and applied to Baiyangdian Lake. The results show that, reflectance in the near-infrared band is the most accurate index for water classification with >98% overall accuracy because of its sensitivity to different land cover types. The slopes of the best-fit linear relationships between the computed hydrological connectivity and observed water level show high variability between years. In most years, hydrological connectivity generally increases when water levels increase, with an average R2 of 0.88. The spatial distribution of emergent plants also varies year to year owing to interannual variations of the climate and hydrological regime. This presents a possible explanation for the variations in the annual relationship between hydrological connectivity and water level. For a given water level, the hydrological connectivity is generally higher in spring than summer and autumn. This can be explained by the fact that the drag force exerted by emergent plants, which reduces water flow, is smaller than that for summer and autumn owing to seasonal variations in the phenological characteristics of emergent plants. Our study reveals that both interannual and seasonal variations in the hydrological connectivity of Baiyangdian Lake are related to the growth of emergent plants, which occupy a large portion of the lake area. Proper vegetation management may therefore improve hydrological connectivity in this wetland.
Highlights
IntroductionWetlands are critical and vulnerable ecosystems that provide essential ecological services and account for 47% of the global ecosystem [1]
Licensee MDPI, Basel, Switzerland.Wetlands are critical and vulnerable ecosystems that provide essential ecological services and account for 47% of the global ecosystem [1]
The minimum values of NDWI, MNDWI, AWEInsh, AWEIsh, and WI2015 and the maximum NIR band reflectance value for the sampled water area were applied as the thresholds to distinguish open water surface pixels from nonwater pixels to the seven images representing different water levels and seasons
Summary
Wetlands are critical and vulnerable ecosystems that provide essential ecological services and account for 47% of the global ecosystem [1]. The service values of wetlands are strongly influenced by hydrological processes such as interception, infiltration, evapotranspiration, and surface water flow movement [2]. The interaction between a floodplain wetland and its adjacent river owing to flood pulses is the major control over floodplain biota [3].
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