Evapotranspiration Partitioning and Response to Abnormally Low Water Levels in a Floodplain Wetland in China

Evapotranspiration Partitioning and Response to Abnormally Low Water Levels in a Floodplain Wetland in China Xiaosong Zhao and Yuanbo Liu Key Laboratory of Watershed Geographic Sciences, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China Received 23 October 2015; Revised 18 February 2016; Accepted 27 March 2016 Academic Editor: Nir Y. Krakauer Copyright © 2016 Xiaosong Zhao and Yuanbo Liu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Evapotranspiration (ET) is an important component of the wetland water budget. Water level declines in Poyang Lake, the largest freshwater lake in China, have caused concerns, especially during low water levels. However, how wetland ET and its partitioning respond to abnormally low water levels is unclear. In this study, wetland ET was estimated with MODIS data and meteorological data. The wetland ET partitioning and its relationship with abnormally low water levels were analyzed for 2000–2013. The results showed that the water evaporation rate () was larger than the land ET rate (); the / ranged from 0.77 to 0.99. When the water level was below 12.8 m, the ET partition ratio was larger than 1, which indicates that wetland ET comes from land surface ET more than water evaporation. The negative standardized water level index (SWI) was used to represent an abnormally low water level in the wetland. Although the monthly wetland ET decreased as the negative SWI decreased, was higher than the average under negative SWI conditions from September to December, when the water level decreased. The abnormally low water level induced more water loss from the land surface, especially when the water level decreased, which reduced the available water resources along the wetland shore. 1. Introduction Evapotranspiration (ET) is the major factor that controls water balance in nearly all wetlands. Floodplain wetlands are hydrologically complex components of the landscape and are located at the transition between aquatic and terrestrial ecosystems [1, 2]. Water vapor fluxes, especially evapotranspiration, can be highly variable because of intermittent flooding and a diversity of plant communities, which may differ in their water use characteristics and responses to water level changes. Changes in the proportion of water lost via ET from land surfaces versus evaporation () from water bodies relative to the total wetland ET affect ecological, biogeochemical, and hydrological cycles at multiple temporal and spatial scales in floodplain wetlands [3]. Many studies have highlighted the linkage between ET partitioning versus water level and vegetation cover in wetlands [4–8]. Changes in water levels for shallow lakes can transform an aquatic environment into an exposed environment, or vice versa. Land surface covers are typically reflected in the shifting dynamics, productivity, and spatial distribution of wetland communities [7, 9, 10]. Vegetation cover and soil conditions are also important factors that affect the rate of water loss [11]. Booth and Loheide [5] showed that changes in ET partitioning occurred because of changes in Leaf Area Index (LAI), plant water stress function, and available storage in the upper soil layer. Decreasing the LAI increased the partitioning of ET toward evaporation. Sánchez-Carrillo et al. [7] indicated that increased vegetation cover did not significantly reduce evapotranspiration but did increase the transpiration to evaporation ratio and the open water area, which increased the wetland evapotranspiration. Huxman et al. [12] identified the partitioning of ET as one of the most important ecohydrological challenges in understanding vegetation dynamics and succession in floodplain wetlands. Poyang Lake is the largest freshwater lake in China [13, 14]. It is a typical floodplain wetland with dramatic seasonal water level variations that cause inundation areas to vary remarkably from several thousand square kilometers during the summer to less than one thousand square kilometers during the winter [15–17]. Poyang Lake wetland is well known and was listed on the Ramsar Convention List of Wetlands of International Importance (the Ramsar Convention 2012) to protect migration bird species and habitats for rare and endangered species [18–20]. Unfortunately, the water level and inundation area in Poyang Lake wetland have significantly decreased; moreover, the duration of extreme low water events has increased in the previous decade [13, 15, 21]. These changes have affected the water supply, water quality, wetland, and aquatic habitats [22]. The above problems have caused great concern for researchers. Some studies have focused on investigating recent low water levels and explaining their causes [13–15, 23]. Liu et al. [13] revealed a regime shift in the lake size that began in 2006; the trigger for the recent lake level declines was principally attributed to low water levels in the Yangtze River and enhanced outflows from Poyang Lake. Zhang et al. [24] related the low water level to the effect of local catchment and lake-river interactions. Lai et al. [23] indicated that increasing the discharge ability of Poyang Lake into the Yangtze River at low water levels enhanced the drought risk in Poyang Lake. Other studies have examined the effect of recent water level declines on vegetation dynamics and hydrological processes in Poyang Lake wetland [25–28]. Zhang et al. [26] analyzed the relationship between hydrological conditions and the distribution of vegetation communities, whereas Yu et al. [28] indicated that recent low water levels have accelerated positive succession in wetland vegetation. Zhao and Liu [27] examined the impact of water level fluctuations on wetland ET, which indicated that the annual ET decreased because of falling water levels. The effect of water level changes on vegetation and hydrological processes has been attributed to water allocation and hydrological gradients in the wetland. Water vapor loss along the shore of Poyang Lake wetland was the primary component of ET when the water level decreased, which affected the water budget and vegetation growth [27]. Some studies have indicated that the long-term exposure of the wetland shore promoted vegetation growth from September to December, during which the water level decreased [25]; this high biomass may increase the land surface ET. However, the long-term exposure of the land surface also causes soil water deficits [7], which typically decrease the land surface ET. How the wetland ET and its partitioning in Poyang Lake wetland respond to abnormally low water levels in different seasons remains unclear. A better understanding of the response of wetland ET partitioning is essential for the rational utilization of water resources and wetland (...truncated)