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Simulating Evapotranspiration of Rain-fed Soybean Field Based on P-T Model
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摘要: 基于2005—2007年涡度相关系统实测值和小气候观测资料，利用Priestley-Taylor (简称P-T) 模型对三江平原雨养大豆田5—10月的蒸散量进行模拟和分析。结果表明：P-T模型参数α采用常规值1.26时，大豆出苗前和生长期模拟值明显大于实测值，大豆收割后模拟值明显小于实测值，模型不能用于模拟大豆田蒸散量。大豆生长期内参数α与叶面积指数呈对数正相关关系；当饱和水汽压差较小时，参数α与其呈幂函数正相关关系，当饱和水汽压差较大时，参数α与其呈幂函数负相关关系。大豆出苗前参数α与太阳辐射呈正相关关系，与饱和水汽压差呈负相关关系；大豆收割后参数α与风速呈显著正相关关系。依据回归方程修正参数α后，多个用于检验模型模拟效果的统计量均表明：P-T模型对不同时期大豆田蒸散量的模拟精度明显提高，能够较好地估算大豆田蒸散量。总而言之，P-T模型必须修正参数α方可用于估算三江平原雨养大豆田蒸散量。Abstract: Based on eddy covariance measurements and microclimate observations available from 2005 to 2007, the simulating accuracy of evapotranspiration with P-T model of rain-fed soybean field from May to October in Sanjiang Plain is analyzed. Results indicate that simulated values of evapotranspiration by P-T model with conventional parameter (1.26) are significantly higher than observations before emergence and during the growing season of soybean, and the mean bias error (MBE) are 1.65 mm·d-1 and 1.22 mm·d-1. However, simulated values are significantly lower than measurements after harvest, with the MBE of-0.74 mm·d-1. Modeling efficiency (ME) of P-T model are all negative values, which indicates that the model cannot be used in predicting evapotranspiration of soybean field during different periods. The cause may have much to do with the parameter, which is assumed as constant value of 1.26. According to measurements of evapotranspiration, the parameter is derived and shows obviously increasing trend during the whole observation periods. Average values of parameter before emergence, during the growing season, and after harvest are 0.76, 0.86 and 2.20, respectively. It is obvious that the parameter varies according to the growing phase, and it is necessary to modify the parameter based on the measured evapotranspiration of rain-fed soybean field in Sanjiang Plain.Statistical analysis shows that leaf area index (LAI) is an important factor affecting evapotranspiration of soybean field. During the growing season, the parameter is creased with increasing LAI, following a logarithmic equation and a positive correlation. Vapor pressure deficit (VPD) is the direct driving force of transporting vapor from the surface to the surrounding atmosphere. The relationship between and VPD can be described empirically by a piecewise function: When the VPD is greater than 5.05 hPa, it's a positive power function, but when the VPD is lower than 5.05, it's a negative power function. The parameter is positively related to solar radiation and negatively related to VPD before soybean emergency and is positively related to wind speed after soybean harvest.With parameter modified by using linear or non-linear regression equation, the estimation accuracy of P-T model under different periods are improved markedly. Before soybean emergency, MBE and root mean square error (RMSE) are 0.06 mm·d-1 and 0.60 mm·d-1, reduced by 96.4% and 71.4%, respectively. ME is improved from a negative to a positive value (0.57), close to the ideal value of 1. During the growing season, MBE and RMSE are 0.15 mm·d-1 and 0.92 mm·d-1, reduced by 87.7% and 38.3%, respectively, and ME from a negative to a positive value (0.28). After soybean harvest, MBE and RMSE are-0.21 mm·d-1 and 0.41 mm·d-1, reduced by 71.6% and 52.3%, respectively, ME turns from a negative into a positive value (0.42). It indicates that the modified P-T model can simulate the evapotranspiration of soybean field. In conclusion, P-T model is suitable to simulate the evapotranspiration only when the parameter is modified.
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Key words:
- P-T model;
- parameter α;
- evapotranspiration;
- rain-fed soybean field
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图 1 基于P-T模型参数α常规值的大豆田不同阶段蒸散量模拟值与实测值比较
(a) 出苗前，(b) 生长期，(c) 收割后
Fig. 1 Comparison of observed evapotranspirations and those simulated by P-T model based on conventional values of α during different period
(a) before emergence, (b) growing season, (c) after harvest
图 2 不同时期大豆田P-T模型参数α变化特征
(a) 出苗前，(b) 生长期，(c) 收割后
Fig. 2 Variation characteristics parameter α of P-T model over soybean field during different period
(a) before emergence, (b) growing season, (c) after harvest
图 3 大豆生长期内叶面积指数 (a) 和饱和水汽压差 (b) 对P-T模型参数α的影响
Fig. 3 Effects of LAI (a) and D(b) on parameter α of P-T model during growing season of soybean
图 4 不同饱和水汽压差 (D) 对P-T模型参数α的影响
(a)D＜5.05 hPa，(b)D＞5.05 hPa
Fig. 4 Effects of D on parameter α of P-T model under different conditions
(a)D < 5.05 hPa, (b)D > 5.05 hPa
图 5 基于P-T模型参数α修正值的大豆田不同阶段蒸散量模拟值与实测值比较
(a) 出苗前，(b) 生长期，(c) 收割后
Fig. 5 Comparison of observed evapotranspirations and those simulated by P-T model based on modified α value during different periods
(a) before emergence, (b) growing season, (c) after harvest
表 1 大豆发育期状况
Table 1 Developmental stages of soybean
发育阶段 开始日期 2005年 2006年 2007年 播种前 05-08 05-01 05-04 播种期 05-25 05-17 05-20 出苗期 06-07 05-26 06-10 开花期 07-03 07-05 07-12 结荚期 07-28 08-10 07-30 鼓粒期 08-17 08-25 08-10 成熟期 09-12 09-15 09-25 收割期 09-28 10-04 10-05 收割后 10-23 10-23 10-23 注：播种前日期为涡度相关系统观测开始日期，收割后日期为涡度相关系统观测结束日期。 
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表 2 大豆生长期P-T模型参数α的非线性统计模型
Table 2 Non-linear statistical models of parameter α of P-T during soybean growing season
条件 参数a(±标准差) 参数b(±标准差) 参数c(±标准差) R2 样本量 D < 5.05 hPa 0.083(±0.087) 0.195 (±0.105) 1.294 (±0.376) 0.786 80 D > 5.05 hPa 0.108 (±0.128) 5.246 (±6.102) -0.953 (±0.596) 0.679 150
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表 3 大豆出苗前和收割后P-T模型参数α的统计模型
Table 3 Statistical models of parameter α of P-T model before soybean emergence and after soybean harvest
发育阶段 常数项及回归因子 回归系数 (±标准差) 标准回归系数 R2 出苗前 C0 0.965*(±0.137) 0.391 Ra 0.028*(±0.008) 0.450 D -0.100*(±0.016) -0.786 收割后 C0 0.278 (±0.494) 0.392 Ws 0.550*(±0.134) 0.626 注：*表示达到0.001显著性水平。
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