remote sensing Article Evapotranspiration on Natural and Reclaimed Coral Islands in the South China Sea Shengsheng Han 1,2 , Suxia Liu 1,2,*, Shi Hu 1, Xianfang Song 1,2 and Xingguo Mo 1,2 1 Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; [email protected] (S.H.); [email protected] (S.H.); [email protected] (X.S.); [email protected] (X.M.) 2 Sino-Danish Center, College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected] 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 Citation: Han, S.; Liu, S.; Hu, S.; demonstrated that the Normalized Difference Vegetation Index (NDVI) is the most important factor Song, X.; Mo, X. Evapotranspiration that influences ETa, while relative humidity (RH) controlled the bare land or sparsely vegetated on Natural and Reclaimed Coral areas on the reclaimed islands. The setting of different land use types showed that vegetation and Islands in the South China Sea. built-up or hardened roads took control of evapotranspiration and rainwater collection, respectively, Remote Sens. 2021, 13, 1110. which play important roles in water dynamics on corals islands. The evaluation of ETa based on https://doi.org/10.3390/rs13061110 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 Received: 9 February 2021 manage scarce freshwater resources properly. Accepted: 7 March 2021 Published: 15 March 2021 Keywords: evapotranspiration; coral islands; water balance; VIP-RS model Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. Coral islands are widely distributed in the Pacific, Indian, and Atlantic Oceans between 30◦N and 30◦S (Figure1a). 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. In addition, Copyright: © 2021 by the authors. it also attracts much attention for its sensitivity to climate change [3,4]. Freshwater resources Licensee MDPI, Basel, Switzerland. on coral islands differ from those on the continent: (1) Freshwater resources on coral islands This article is an open access article distributed under the terms and mostly come from precipitation. However, it is easy for precipitation to penetrate the thin, conditions of the Creative Commons permeable unsaturated zone. There is nearly no ponding water on coral islands [5,6]. Attribution (CC BY) license (https:// (2) The fresh water is mainly stored in the form of groundwater, whose availability is creativecommons.org/licenses/by/ highly dependent on its resilience in drought periods. When droughts occur, the water of 4.0/). the coral islands gotten from precipitation decreases, and the vegetation may increase the Remote Sens. 2021, 13, 1110. https://doi.org/10.3390/rs13061110 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 1110 2 of 18 consumption of water from groundwater [7,8]. (3) The low land surface elevation of atoll islands also makes fresh water vulnerable to inundation by seawater, such as storm surges, wave setup, extremely high tides, and tsunamis [2,9,10]. (4) The high population densities of some coral islands have led to unsustainable rates of extraction. Changes in land use types and contamination from human settlements can also significantly affect freshwater resources [3,11]. Figure 1. (a) Distribution of coral reefs in the world, (b) location of coral islands in the South China Sea, and land use maps of (c) Zhaoshu Island, (d) Yongxing Island, and (e) Yongshu Island in 2019. Note: The distribution of coral reefs in (a) is made using Natural Earth (free vector and raster map data @ naturalearthdata.com (accessed on 15 December 2020)). Actual evapotranspiration (ETa), which represents the sum of water evaporation and transpiration from a surface area to the atmosphere, is the least quantified freshwater component of atoll islands’ water budget analysis because of the inherent difficulties in measuring it [1,5]. ETa could be estimated from potential evapotranspiration (ETp) by means of a water balance, taking account of evapotranspiration losses from interception and soil water stores [5], where ETp has been investigated based on the Priestley–Taylor formula, the Penman formula, and pan evaporation [12–14]. The ETa of coral islands could also be estimated by field observations such as eddy covariance, heat-dissipative sap flow, and monitor wells [15–19]. However, these studies only concentrated on a few coral islands, and it was hard to go further in such studies due to the difficulty in data collection and equipment maintenance. Satellite remote sensing data, which consider the vegetation and land use type changes at a long temporal scale [20,21], could be convenient to study hydrological processes on remote islands; however, it has rarely been used in ETa evaluation on coral islands. In this study, the remote-sensing-based Vegetation Interfaces Processes (VIP-RS) model [22–24] was used to evaluate the ETa of coral islands in the South China Sea from 2016 to 2019. The VIP-RS model takes account of the detailed physical mechanism of the exchanges of energy and water between the land surface and atmosphere, assimilated with remote sensing information. We aimed to (1) explore the ETa and water dynamics on natural and reclaimed coral islands in the South China Sea and dominant driving factors Remote Sens. 2021, 13, 1110 3 of 18 for ETa using partial correlation analysis and (2) compare the differences in ETa between natural and reclaimed coral islands and analyze the influences of ETa on different land use types according to the scenario analysis of land use types. This method considers the spatial changes in vegetation and land use types and reduces the hard work of field measurements, helping us evaluate the characteristics of freshwater resources accurately and improve water resource management. 2. Materials and Methods 2.1. Study Area The South China Sea (Figure1b), situated between the Karimata Strait (~3 ◦S) and the middle of the Taiwan Strait (~23.5◦N), has numerous coral islands. These coral islands develop a wide variety of scenery and wildlife and provide convenience to people with fish, tourism, and energy. To study the water cycle in natural and reclaimed coral islands in the South China Sea, three typical islands were selected. Zhaoshu Island (latitude 16◦580N, longitude 112◦160E) and Yongxing Island (latitude 16◦500N, longitude 112◦200E) were chosen as typical natural islands with different island sizes. Yongshu Island (latitude 9◦370N, longitude 112◦580E), reclaimed in 2015, was chosen as a case of reclaimed islands. These three islands could represent most coral islands in the South China Sea in island size and type. Zhaoshu Island (Figure1c) has a natural area of 0.22 km 2, with an area of about 0.07 km2 reclaimed in the west. Lush vegetation is distributed in a ring and band in the natural parts, and the reclaimed area is occupied with greens, a built-up area, and roads. The elevation is approximately 4.4 m, with the core of the island being somewhat low. The soil type is calcareous sand, which is marine sediment originating from coral and other creatures’ debris, with high permeability. It has a tropical monsoon climate that is warm all year, with dry and wet seasons, and the average annual temperature is above 26 °C, while the daily temperature variation in a year is only about 6 °C. The rainfall is mainly controlled by typhoons and convective rains, with an average annual rainfall of about 1500 mm, 85% of which is concentrated in June–November [25]. The water table is between 2.3 and 3.0 m and is influenced by the tides. Yongxing Island (Figure1d) is the largest coral island of the Xisha Archipelago, which has an area of 3.58 km2, with the reclaimed area in the northeast. It is 18 km away from Zhaoshu Island, with approximately 3000 people living there. It has similar climate conditions, morphology, and depositional environments as Zhaoshu Island. However, its altitude is as high as 8.2 m. The vegetation is more diverse than that on Zhaoshu Island, but it has suffered some destruction because of human activities. Yongshu Island (Figure1e) is a reclaimed island that is located in Yongshu Atoll. It was reclaimed from August 2014 to September 2015, with the land area increased from less than 0.2 km2 to 2.23 km2.