Simulation of evapotranspiration and carbon dioxide flux in the wheat‐maize rotation croplands of the North China Plain using the Simple Biosphere Model

Abstract

Abstract:The North China Plain, which is critical for food production in China, is encountering serious water shortage due to heavy agricultural water requirement. The accurate modelling of carbon dioxide flux and evapotranspiration (ET) in croplands is thus essential for yield prediction and water resources management. The land surface model is powerful in simulating energy and carbon dioxide fluxes between land and atmosphere. Some key processes in the Simple Biosphere Model (Version 2, SiB2) were parameterized based on the observations. The simulated fluxes were tested against the eddy covariance flux measurements over two typical winter wheat/maize double cropping fields. A simple diagnostic parameterisation of soil respiration, not included in SiB2, was added and calibrated using the observations to model the carbon budget. The Ball-Berry stomatal conductance model was calibrated using observed leaf gas exchange rate, showing that the original SiB2 could significantly underpredict the ET in the wheat field. SiB2 significantly underpredicted soil resistance at the Weishan site, leading to overpredict the ET. Overall, there was a close agreement between the simulated and observed latent heat fluxes and net CO 2 exchange using the re-parameterized SiB2. These findings are important when the model is used for the regional simulation in the North China Plain.

Optimizing the photosynthetic parameter Vcmax by assimilating MODIS-fPAR and MODIS-NDVI with a process-based ecosystem model

“…Therefore, specifying V cmax is critical and substantially contributes to terrestrial GPP (gross primary productivity) sensitivity to the atmospheric CO 2 concentration and the overall uncertainties of model-predicted carbon fluxes. In wheat, a 40% decrease in V cmax led to a 34% decrease in GPP (Lei et al, 2011), and a 10% increase in V cmax led to a 4% increase in yield in NCP (Mo et al, 2005). Although both parameters are tightly correlated with a plant's photosynthetic capacity, carboxylation capacity (V cmax ) has been measured and studied more extensively (Wullschleger, 1993;Leuning, 1997;Kattge and Knorr, 2007).…”

1984~2006年黄河下游大型灌区碳平衡长期变化

“…土壤参数 包括土壤热传导参数和土壤水分特征参数, 其中热 传导参数由模型计算得到, 水分特征参数来自于实 测. 为提高土壤水分模拟的精度, 我们将 SiB2 中原 有的 3 层土壤改为 10 层土壤, 并采用 Richards 方程 模拟土层之间的水分交换 [38] . 基于华北平原典型冬 小麦-夏玉米轮种制农田的实测数据, 我们对模型内 的 土 壤 蒸 发 进 行 了 重 新 参 数 化 , 并 对 模 型 中 的 Ball-Berry 气孔导度模型进行了参数率定 [38] .…”

“…为提高土壤水分模拟的精度, 我们将 SiB2 中原 有的 3 层土壤改为 10 层土壤, 并采用 Richards 方程 模拟土层之间的水分交换 [38] . 基于华北平原典型冬 小麦-夏玉米轮种制农田的实测数据, 我们对模型内 的 土 壤 蒸 发 进 行 了 重 新 参 数 化 , 并 对 模 型 中 的 Ball-Berry 气孔导度模型进行了参数率定 [38] . 另外, 由于 SiB2 未包含土壤呼吸(Rs)(包括根的自养呼吸和 土壤微生物的异养呼吸)的模拟, 所以无法计算 NEE, 因此我们在模型中引入了土壤呼吸的参数化方案, 表示为表层土壤温度和含水率的函数, 公式的参数 则根据位山通量站的实测资料确定 [38] .…”

“…在模型验证方面, 我们已经采用灌区内的位山 通量站以及栾城通量站的通量及土壤水分、 表面温度 数据对该模型进行了综合评价 [38] . 对于这两个站点, SiB2 对小麦和玉米 NEE 模拟的确定性系数(R 2 )均在 0.8 以上, 而对 GPP 模拟的 R 2 在 0.9 以上; 另外, 对 潜热通量、显热通量、土壤热通量、土壤水分以及表 面温度的模拟精度也较高, 综合表明模型在点尺度 上能够精确模拟小麦-玉米轮种制农田的碳循环过程.…”

“…对于位山灌区典型农田, 虽然通量观测资料已经 表明 SiB2 在点尺度上对碳循环有较高的模拟精度 [38] , 但当模型运用于区域尺度时, 有必要对区域模拟的 精度进行检验. 采用 Huang 等 [44] 提供的方法, 我们能 够通过区域粮食单产统计资料推求 NPP.…”