植物水分来源季节性变化对区域蒸散发模拟的影响

Impacts of the Shift of Plant Water Sources on Regional Evapotranspiration Simulation

植物水分来源会随季节变化。在旱季表层土壤水分不充裕时,植物会逐步使用深层土壤水或地下水,而当雨季表层土壤水分充足时,植物主要使用表层土壤水。然而当前的水文模型、陆面模式和气象模型很少考虑这一现象。本文对分布式陆面-水文模型Par Flow.CLM进行改进,引入以能量差为驱动的根系吸水计算方法,分析植物水分来源季节性变化对区域蒸散发模拟的影响。改进后的模型能够较好地模拟区域蒸散量和土壤水动态,并能较好地再现植物对地下水的使用情况。改进模型结果显示,植物使用地下水的时间主要集中在旱季,植物对地下水的使用是植物应对干旱的重要策略之一。对比美国加利福利亚州Tonzi实验站干旱年2008和平水年2009的结果,2008年植物使用地下水时间比2009年长53 d,用量高36%。改进模型和原模型结果对比显示:当蒸散发主要受到能量限制时,两个模型的模拟结果较为一致;但当蒸散发主要受到水量限制时,两个模型在蒸散发和深层土壤水的模拟结果上有显著差异。改进模型考虑了植物水分来源季节性变化对区域蒸散发模拟的影响,并能模拟地下水位降低对植物用水的影响。结果表明:改进模型中植物腾发量比原模型结果高71 mm/a,占全年总腾发量的43%。若忽略植物水分来源的季节性变化,会对区域蒸散量和深层土壤水的模拟产生巨大影响,且这种影响在旱季或干旱半干旱地区更为明显。

The shift of plant water sources occurs seasonally. During the dry seasons, plants would progressively use deep soil water or groundwater when shallow soil water was depleted. During the wet seasons, when shallow soil water was adequate, plants would mainly use shallow soil water. Few hydrological, land surface and atmospheric models have addressed this phenomenon. This paper aimed to improve the performance of the distributed, land surface-hydrological model Par Flow.CLM. A new simulation method of plant root water uptake based on potential difference was coupled into Par Flow.CLM to analyze the impact of the shift of plant water sources on the regional ET simulation. The improved model was able to simulate the changes of regional ET and soil moisture, and was capable to capture the dynamics of plant groundwater use. The results from the modified model showed that the plants mainly relied on groundwater during the dry seasons. Plant groundwater use was one of the most important plant water-use strategies to mitigate the impacts of droughts. Comparing the results in dry year 2008 with those in normal year 2009,the duration and the amount of plant groundwater use were 53 days longer, 36% higher in 2008, respectively. The comparison between the modified and original model performances showed that, when ET was energy-limited, the results of two models coincided with each other. However,when ET was water-limited, the two models had significant difference in the simulations of plant transpiration and deep soil moisture dynamics.The impacts of the shift of plant water sources on the regional ET and the influences of groundwater level changes on the plant water use were considered in the modified model. As a result, the plant transpiration result from the modified model was 71 mm/a（43% of annual transpiration） higher than the results ofthe original model. If the shift of plant water sources was ignored in the numerical models, it would lead to considerable bias in regional ET and deep soil water simulation. Such bias would become more significant in the simulations during dry seasons or in the arid and semi-arid regions.