Abstract
Studies of evapotranspiration on remote tropical coral islands are important to explore and sustain scarce freshwater resources. However, there is a significant knowledge gap between research to evaluate evapotranspiration based on remote sensing methods and the influences of different land use types on water dynamics on reclaimed coral islands. This study applied the remote-sensing-based Vegetation Interfaces Processes (VIP-RS) model to estimate actual evapotranspiration (ETa) on Zhaoshu Island, Yongxing Island, and Yongshu Island in the South China Sea from 2016 to 2019. The results showed that the average annual ETa of Zhaoshu Island, Yongxing Island, and Yongshu Island was 685 mm, 530 mm, and 210 mm, respectively. Annual transpiration (Ec) and soil evaporation (Es) exhibited similar patterns on the natural islands; however, Es controlled the water consumption on the reclaimed islands. Water dynamics exhibited seasonal fluctuations due to the uneven distribution of precipitation (PRP). However, ETa of the natural islands was higher than PRP in the dry season, indicating vegetation has to absorb water from the groundwater to sustain growth. The results also agreed with the analysis of dominant driving factors based on partial correlation analysis, which demonstrated that the Normalized Difference Vegetation Index (NDVI) is the most important factor that influences ETa, while relative humidity (RH) controlled the bare land or sparsely vegetated areas on the reclaimed islands. The setting of different land use types showed that vegetation and built-up or hardened roads took control of evapotranspiration and rainwater collection, respectively, which play important roles in water dynamics on corals islands. The evaluation of ETa based on a remote-sensing-based model overcame the difficulty in fieldwork observation, which improves the certainty and accuracy at a spatial scale. In addition, it gave us a new reference to protect and manage scarce freshwater resources properly.
Highlights
Coral islands are widely distributed in the Pacific, Indian, and Atlantic Oceans between30◦ N and 30◦ S (Figure 1a)
Reclaimed coral islands have somewhat large areas of hardening and barren land or sparsely vegetated areas compared to natural islands, which indicates that reclaimed islands consume small volumes of water due to the low Ec caused by the lack of evergreen forests or shrubs
This study evaluated ETa and water dynamics on natural and reclaimed coral islands in the South China Sea from 2016 to 2019 based on a simplified remote-sensing-based VIP
Summary
Coral islands are widely distributed in the Pacific, Indian, and Atlantic Oceans between30◦ N and 30◦ S (Figure 1a). Most of them have ecosystems vulnerable to both natural processes and anthropogenic activities [1,2]. Fresh water is one of the most important resources for the natural ecosystem and the people inhabiting the coral islands. It attracts much attention for its sensitivity to climate change [3,4]. Freshwater resources on coral islands differ from those on the continent: (1) Freshwater resources on coral islands mostly come from precipitation. It is easy for precipitation to penetrate the thin, permeable unsaturated zone. There is nearly no ponding water on coral islands [5,6]
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