西沙赵述岛地表蒸散发实验

蒸散是岛礁水循环过程中的重要组成部分,对保护珊瑚岛礁生态有重要意义。2018年6月27日—2018年8月8日利用自制的微型蒸渗仪及蒸发皿,在中国西沙群岛赵述岛开展了野外蒸散观测实验,获得了岛礁砂壤土裸地雨后实际蒸发速率,以及不同下垫面及钙质砂质地情形下潜在蒸散特征。其中,砂壤土裸地实际蒸发观测表明,雨后第2日至第4日平均蒸发速率为（1.6±0.2） mm/d,第5日至第7日迅速下降,7日后蒸发速率逐渐稳定在（0.5±0.2）mm/d,实际蒸发过程受土壤含水量调节,裸地实际蒸发（E）与蒸发皿潜在蒸发（E_o）的比值E/E_o与土壤表层含水量关系表明两者呈明显的线性相关。蒸散控制实验表明,蒸渗仪潜在蒸散呈现空旷地草地>空旷地砂壤土>林间带草地>林间带砂壤土的规律,林间带遮挡减少草地蒸散比减少裸地蒸发影响更加明显。裸地蒸发速率受岛礁钙质砂质影响,岛礁钙质砂颗粒越大,快速蒸发阶段持续时间越短,蒸发速率越小。微型蒸渗仪日蒸发量和午间蒸发速率呈一定线性相关,其中细粒钙质砂、中粒钙质砂及砂壤土三种土壤类型两者相关性更加明显。

Field evapotranspiration experiment in Zhaoshu Island of Xisha Islands,South China Sea

Evapotranspiration is an important component of water balances on coral islands,which plays a pivotal role in the ecological protection of coral reefs.A field experiment using a self-made microlysimeter was carried out in Zhaoshu Island,Xisha Islands,China from June20 th,2018 to August 16 th,2018,which obtained the change processes of actual evaporation after rainfall,as well as evapotranspiration characteristics of different underlying surface and calcareous sand particles.Soil on coral islands is mainly calcareous sand,which has high porosity and water binding capacity.The actual evaporation observation of sandy loam bare land showed that the average evaporation rate was 1.6±0.2 mm/d from the 2 nd day to the 4 th day after rainfall,which was followed by a rapid decrease on the 5 th-7 th days.Evaporation rate stabilized at 0.5±0.2 mm/d gradually after the 7 th day.Soil water content took control of evaporation.The ratio of bare surface evaporation rate to pan evaporation rate had a significant linear correlation with surface soil water content,indicating that evaporation of bare land on coral islands could be predicted by pan evaporation and soil water content.Control evapotranspiration experiment showed that potential evapotranspiration rate of different types of microlysimeter followed a rule of ETgrassland in the open field>ETsandy loam in the open field>ETgrassland in forests belt>ETsandy loam in forests belt.The shade of forests could decrease evapotranspiration,however,evapotranspiration of grassland decreased larger than that of sandy loam soil under the forests belt.Evaporation of calcareous sand was affected by the sand textures,the larger the calcareous sand particles.The shorter the duration of rapid evaporation,and the smaller the evaporation rate.Daily evaporation rate(mm/d)and midday evaporation rate(mm/h)showed a significant linear correlation especially for sandy loam soil,fine sand,and medium sand.So,we could estimate daily evaporation of fine sand,medium sand,and sandy loam soil in South China Sea with midday evaporation rate for similar underlying surface and climate conditions.