基于P-T模型估算雨养大豆田蒸散量

Simulating Evapotranspiration of Rain-fed Soybean Field Based on P-T Model

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。

Based on eddy covariance measurements and the simulating accuracy of evapotranspiration wit ber in Sanjiang Plain is analyzed. Results indicate microclimate observations available from 2005 to h P-T model of rain-fed soybean field from May to that simulated values of evapotranspiration by P-T with conventional parameter （1.26） are significantly the growing season of soybean However, simulated values ar -0.74 mm. d-1. Modelingef model cannot be used in predic higher than observations before emergence and , and the mean bias error e significantly lower than ficiency （ME） of P-T mod ting evapotranspiration of 2007, Octomodel during （MBE） are 1.65 mm ·d-1 and 1.22 mm · d-1. measurements after harvest, with the MBE of el are all negative values, which indicates that the 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 posi- tive 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.