基于改进SW模型的千烟洲人工林蒸散组分拆分及其特征

蒸散组分拆分是准确评估陆地生态系统生产力以及估算水分利用效率的重要基础。利用改进后的Shuttleworth-Wallace模型,将蒸散拆分为植被蒸腾、土壤蒸发和冠层截留蒸发,并采用Monte Carlo随机参数化方案对模型参数进行优化。将模型与千烟洲亚热带人工针叶林站点的2011年涡度相关及小气候观测资料结合,对千烟洲人工林蒸散及其组分进行模拟。研究结果表明:半小时尺度上蒸散量模拟值与实测值的一致性在晴天和雨天都较高。半小时尺度上全年蒸散模拟值与实测值的决定系数、均方根误差和平均偏差为0.73、1.55 mmol m~(-2)s~(-1)和0.21 mmol m~(-2)s~(-1)。蒸散是该生态系统水分输出的最主要贡献项,占全年降水的80%。在蒸散中,植被蒸腾约占总蒸散量的85%,可推测2011年千烟洲人工林生态系统有较高的水分利用效率。该生态系统的蒸腾量季节变化明显,主要受饱和水汽压差和气温两种环境因素以及植被的叶面积指数影响且与三者均呈正相关;土壤蒸发约占总蒸散量的5%,季节变化平缓;模拟的冠层截留蒸发量约占总蒸散量的10%,季节变化大,与降水量呈正相关,与暴雨频次呈负相关,说明冠层无法有效截留强降水。该模型参数较少、时间分辨率高且可以有效模拟蒸散及其组分特征,是陆地生态系统水分循环过程研究有力的模型工具。

Modeling evapotranspiration and its components in qianyanzhou plantation based on modified SW model

Evapotranspiration (ET) in the terrestrial ecosystem actually includes canopy transpiration (E_c), soil evaporation (E_s), and interception evaporation (E_int). Partitioning of ET is an important basis in order to accurately assess biomass production and estimate water use efficiency in the terrestrial ecosystem. Actually, E_c is the desired component within the water cycling, which being used to enhance plant productivity. Up to now, it is still difficult to accurately partition ET through observation methods in the long period. Instead, partitioning ET with models is effective method in a long period. However, most of them roughly partition ET into E_c and E_s, ignoring E_int. This partition way could lead to deviation in E_c and E_s simulation. In this study, a modified Shuttleworth-Wallace model was used to partition ET into E_c, E_s, and E_int in a forest ecosystem. Monte Carlo, an approach of random parameterization, was performed to optimize the key parameters in the functions estimating soil surface resistance and canopy stomatal resistance in the model. Based on the modified model, we simulated the evapotranspiration and its components in the planted coniferous forest ecosystem at the Qianyanzhou site combining the eddy covariance measurement data (including sensible/latent heat flux and CO₂ flux) with routine meteorological data(including air temperature/humidity, CO₂ concentration, wind speed, net radiation, soil temperature/water content/heat flux and precipitation) in 2011. The results showed that the simulation amount of ET agreed well with the measurement data at 30 minutes temporal scale not matter on sunny days or rainy days. At the same temporal scale, the index of agreement, mean error, and root mean square error between simulation value and measurement data in whole year were 0.73, 0.21 mmol m^-2 s^-1, and 1.55 mmol m^-2 s^-1, respectively. The proportion of ET in annual accumulative precipitation was more than 80%, which meant that the amount of ET contributing to the water outputs is the most in this ecosystem. The proportion of canopy transpiration in ET was about 85%, which suggested that water use efficiency in the forest ecosystem might be quite high in 2011. The seasonal variation of the transpiration was obvious. The environmental factors, vapor pressure deficit, air temperature, and leaf area index controlled transpiration. Transpiration correlated positively with the three factors. Soil evaporation was 5% of the ET and changed little with season. Interception evaporation accounted for 10% of the ET and showed seasonal dynamic. This term correlated positively with precipitation and negatively with rainfall storm events. This result indicated that the vegetation canopy couldn''t effectively intercept the heavy rainfall. The modified Shuttleworth-Wallace model is a powerful tool for studying the hydrological processes in terrestrial ecosystem with the advantages of a few parameters, high temporal resolution, and a good performance in simulating the characteristics of evapotranspiration and its components.