| Water transition between different interfaces is the core of the hydrological cycle in forest ecosystem. Combined with the viewpoints and methods of modern hydrology and ecology, study on the mechanism of the changes among aspects of water transfer is the central subject of modern hydrological processes.In this study, Positional observation and recording of hydrographic features, such as canopy interception, throughfall, stemflow, surface runoff and soil water storage was carried out in the6-1,2testing field of the third team, Rubber Research Institute of Chinese Academy of Tropical Agricultural Sciences. Evapotranspiration was observed continuously by using Eddy Covariance (EC) System, automatic meteorological observing system and micro-meteorological observation system. Underground leakage was estimated by well group method. Evapotranspiration and energy flux transformation law in rubber plantations were explored. Meanwhile, variation characteristics of surface runoff and underground leakage of typical watershed of rubber plantations were interpreted in Hainan Island were interpreted. With Eddy Covariance (EC) System, automatic meteorological observing system and micro-meteorological observation system, evapotranspiration and heat flux of the rubber plantation ecosystems in different space and time scale were calculated in observation station. Then comparison with evapotranspiration results obtained from the water balance method was made in order to reveal the variation characteristics of evapotranspiration of the rubber plantation watershed.This study aims to understand the mechanism of water transition, spatial and temporal distribution in different conditions of rubber plantation ecosystem, and to understand the hydrological role of the rubber plantation ecosystem and its response benefits of environment and meteorological factors.The main results are as follows:(1) Moderate to heavy rain, which rainfall intensity is greater than7mm/h and less than80mm/h, is the major part of precipitation in the study area. Large amount within a short time period is the characteristic of precipitation in rubber plantations.The distribution of annual rainfall amount of rubber plantations is uneven, and the seasonal variation is very large. The total rainfall amount was2530.34mm, of which10.82%was dry season precipitation, and89.18%was wet season precipitation. In addition, the differences of precipitation distribution pattern of rubber plantations among and within months are very significant. The coefficients of variation of rainfall in12months range from36.51%to215.5%(2) Results show that the canopy allocation of rubber plantation ecosystem contains three parts:throughfall accounts for71.63%, stemflow accounts for11.54%and canopy inception accounts for16.83%.Distribution characteristics of hydrological cycle of rubber plantation ecosystem are as follows: surface runoff accounts for4.71%, leakage accounts for26.23%, evapotranspiration (transpiration and soil evaporation) accounts for52.65%, and another16.83%part is canopy inception which output in the form of evaporation. The average variation of soil water storage is-0.43%, the changing amplitude is relatively stable.(3) The relationships between canopy interception and rainfall, leaf area index were established. Then linear regression analyses were carried out between canopy inception of rubber plantations and its influence factors. The multiple linear regression equation shows that: The interception amount of rubber canopy increases with rainfall amount and leaf area index, decreases with temperature, rainfall amount, relative humidity and photosynthetically active radiation in flush period. The importance of possible factors that may influence canopy interception is in such order: rainfall amount>rainfall intensity>photosynthetically active radiation>relative humidity>leaf area index. Stepwise regression equation is I=7.0137+0.1375X1+0.5263X2-0.0649X4-0.0044X5-0.1110X6, the multiple correlation coefficient is0.9382and the adjusting correlation coefficient is0.9304.(4) Surface runoff in the study area is mostly concentrated in May to October of wet season, while in dry season surface runoff is visible only in November. Therefore, surface runoff is related with rainfall allocation of rubber plantations during vegetation season and increases with stand density. The relationships between surface runoff and rainfall amount, penetration amount are close to power function, the equations are y=2E-05x2.3029, R2=0.8868; y=3E-05x2.3082R2=0.8975. Moreover, the correlation coefficient of surface flow and rainfall is less than its correlation coefficient with the penetration of rain simulation equation. Therefore, the influence of forest rainfall on surface runoff is higher than rainfall outside, which reflects the role of the forest canopy on rainfall allocation. Furthermore, estimating soil leakage amount by using well group method, the linear relationship between variations of water level in well and cumulative rainfall amount is y=-0.0004x2+0.2127x+44.538, R2=0.7959, the empirical equation is y=0.0039x1.8994, R2=0.8597.(5) The calculation of evapotranspiration in this study was done by using the eddy covariance method and water balance method. Then these two methods were evaluated, the result shows that data obtained from eddy covariance method are reliable. Related equations were established according to the energy closure situations of different soil layers (-5cm、-20cm、-50cm). The relationship between rubber plantation evapotranspiration and rainfall was analyzed, which along with the composition of different components of energy flux in dry season, reflecting the different situations of the energy balance at different water input stage of rubber plantation ecosystem in the study area. During the season with little precipitation, the sensible heat flux in rubber plantation was significantly higher than the latent heat flux, the amount of net radiation income of study area in a day was about9.23MJ·m-2, the amount of sensible heat flux loss and latent heat flux loss was about6.80MJ·m-2and2.17MJ·m-2, respectively. The humidification cooling function of rubber plantations can only be seen during wet season in which water supply is sufficient. Moreover, the peak time of soil heat flux is almost the same as the peak time of evaporation latent heat and sensible heat flux. Evapotranspiration characteristics and laws of rubber plantations may be explored by analyzing variations of evapotranspiration per day, per month and its constituents. Furthermore, understanding the temporal and spatial variations of evapotranspiration in rubber plantation watershed is of great importance for the assessment of the impact mechanism of the rubber plantation on the hydrological processes, development of hydrological models and improvement of rubber plantation managements.(6) The seasonal dynamics of soil water storage in the rubber plantations is related with seasonal distribution of precipitation during the vegetation season. Soil water storage of wet season is higher than dry season. Soil water storage in0-60cm almost always responds precipitation. Normally, soil water storage in different layer is in the order of20cm<40cm<60cm. Spatial and temporal distribution of soil water storage in different layer is the foundation of researches on soil water movement in rubber plantation ecosystem. |
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