基于Richards方程的区域连续日蒸散量遥感估算

Remote sensing estimation on regional continuous daily evapotranspiration based on Richards equation

蒸散发作为自然界水循环的重要组成部分,时空尺度上的蒸散量估算一直是研究热点。遥感手段可以实现区域尺度蒸散量的估算,但是受到卫星过境时间的限制,难以获取连续时间序列的蒸散量。土壤水分作为蒸散发的重要控制因素,结合土壤水分数据改进遥感蒸散发模型,在提高遥感蒸散量估算精度方面也具有重要意义,但是目前大多数遥感方法对土壤水分胁迫性的考虑仍有不足。针对目前蒸散发研究在土壤水分胁迫和连续性方面的不足,以涡度相关法计算的蒸散量作为实际蒸散量,结合联合国粮农组织推荐的单作物系数法,将土壤含水量信息引入Penman-Monteith(P-M)公式计算实际蒸散量,并用Richards方程进行蒸发条件下一维垂向土壤水分运动过程的数值模拟,实现土壤水分胁迫下的连续日蒸散量的估算,并结合遥感数据实现区域尺度的扩展。结果表明:涡度相关法计算的实际日蒸散量与P-M公式计算的潜在日蒸散量具有很强的相关性,相关系数达到0.918;引入土壤含水量信息后的P-M公式,日蒸散量的估算精度显著提升,均方根误差达到0.133 mm/d;基于Richards方程的土壤水分胁迫下连续日蒸散量的估算结果与实测值较为接近,均方根误差为0.288 mm/d;受研究区南北高中间低的地势影响,日蒸散量的高值集中在研究区中部的水域和耕地区域,不同土地利用类型下的平均日蒸散量水域>耕地>林地>草地>未利用土地,且区域扩展的结果与站点的实测结果在时间序列上表现出一致的变化规律。文章可为土壤水分对蒸散发的影响机理研究以及区域蒸散量的估算提供参考。

Evapotranspiration(ET)is an important part of water cycle in nature,and the estimation of evapotranspiration on spatio-temporal scale has always been a hot issue.Remote sensing can estimate evapotranspiration on regional scale,but it is difficult to obtain evapotranspiration in continuous time series due to the limitation of satellite transit time.Soil moisture is an important controlling factor of evapotranspiration.Improving the remote sensing evapotranspiration model by combining soil moisture data is of great significance in improving the accuracy of remote sensing evapotranspiration estimation.However,most remote sensing methods give limited consideration to the characterization of soil moisture stress.This study used the evapotranspiration calculated by the vorticity correlation method as the actual evapotranspiration.combining with the single crop coefficient method recommended by FAO,the soil water content information was introduced into the Penman-Monteith formula to calculate the actual evapotranspiration.Based on Richards equation,the one-dimensional vertical soil water movement process under evaporation conditions was simulated to estimate the continuous daily evapotranspiration under soil water stress.Combining with remote sensing data,the regional scale evapotranspiration was estimated.The results show that the actual daily evapotranspiration calculated by the vorticity correlation method has a strong correlation with the potential daily evapotranspiration calculated by P-M formula,with the correlation coefficient of 0.918.With the introduction of soil water content information,the P-M formula improves the estimation accuracy of daily evapotranspiration significantly,and the RMSE reaches 0.133 mm/d.The estimated daily evapotranspiration under soil water stress based on Richards equation is close to the measured value,with the RMSE of 0.288 mm/d.The high value of daily evapotranspiration affected by the topography of the study area is concentrated in the water area and cultivated land area in the middle of the study area.The average daily evapotranspiration under different soil use types is water area>cultivated land>woodland>grassland>unused land,and the results on the regional scale show similar change with that measured in the the station in time series.This study provides basic information for understanding the influence mechanism of soil moisture on evapotranspiration and estimating regional evapotranspiration.