桉树人工林主要生态水文过程及其对产流的影响机理

The intensive and frequent vegetation change and soil disturbance in Eucalyptus plantation broke the water cycle in forests and became a potential threat to the regional water supply. Because of the lack of understanding of the important eco-hydrology processes such as soil evaporation and water storage, previous study was incapable of explaining and predicting the impact of Eucalyptus plantation on regional water resource. This study will study the water evaporation and storage of forest soil as the key hydrology process controlling the surface runoff from Eucalyptus plantation. The water storage capacity of Eucalyptus plantation at different age will be studied under storm water events. This study involves continuous monitoring of soil evaporation, plant transpiration, surface runoff as well as the measurement of soil moisture and other physical characteristics, weather condition, and forest structure. The objects of this study are 1) to elucidate the dominant factors of the eco-hydrology of Eucalyptus plantation; 2) to identify the relationship between evapotranspiration and soil water storage capacity and establish the time-dependent soil moisture function; 3) to establish the rainfall-runoff function and criteria of water demand to generate surface runoff from drainage basins with different forest stands. Result from this study is anticipated to reveal the mechanism the influence of soil water storage on the surface runoff, soil evaporation and plant transpiration. This study will provide scientific foundation for the effective management of regional water resource and soil water in planted forests.

桉树人工林因其剧烈而频繁的植被变化与土壤扰动，打破了原有相对稳定的森林水循环，对区域水资源供给构成潜在威胁。以往研究因缺乏对土壤蒸发、土壤水分库容量变化等重要生态水文过程的了解而未能阐明桉树造林对区域水资源的真实影响。项目将土壤蒸发、土壤水分库容量和土壤产流需水阈值变化作为桉树人工林影响产流量的关键水文过程进行重点研究。通过极端暴雨和模拟降雨下研究不同林龄桉树林对土壤水分库最大容量的影响规律；通过连续的土壤蒸发、植被蒸腾、径流量等观测，结合土壤水分、土壤物理性质、气象和林分结构等监测结果，阐明桉树林生态水文过程的关键影响因子及其影响特征，明确蒸散发与土壤水分库现存量之间的消长关系，建立土壤水分库存量的时间曲线；探讨不同林分林地和集水区产流的土壤需水阈值，揭示土壤水库变化对产流量、土壤蒸发和植被蒸腾的影响机理，建立不同植被集水区的雨量-流量响应模型，为区域水资源及人工林土壤水分管理提供依据。

近十多年来，桉树人工林在中国的快速发展引发了公众的担忧：会否引起区域水资源量的减少？项目建立了配对集水区，对集水区林地土壤水文物理性质、土壤水分饱和差、土壤蒸发、乔木蒸腾、产流量等进行了全面观测与研究，基本量化了桉树人工林主要生态水文通量。研究结果发现，由长周期人工林改造为短周期桉树人工林后，在一个轮伐期（6年）内，集水区产流量减少14%，植被蒸腾量减少4.7%，土壤蒸发量增加68%，土壤蒸发量增加是产流量减少最主要的原因。幼龄林期产流量会增加，产流量减少主要出现在中龄林和成熟林，其主要水文过程与机制为：植被盖度降低→土壤蒸发增强→土壤水分饱和差增大→林地土壤产流需水阈值增加→产流量减少。