基于Triple Collocation方法的微波土壤水分产品不确定性分析与时空变化规律研究

Error Characteristics of Microwave Soil Moisture Products based on Triple Collocation and Its Spatial-temporal Pattern

土壤水分是连接地—气系统的重要状态变量,微波遥感为准确获取大面积土壤水分信息提供新的技术手段。准确解读微波土壤水分产品质量、深入了解其误差的时空分布特征是通过数据同化等方法将其融入陆面模型,从而成功应用于地球科学领域的重要先决条件。基于Triple Collocation(TC)方法检验了风云三号C星(FY-3C)、土壤水分主被动卫星(SMAP)及高级微波散射计(ASCAT)这3种常用微波土壤水分产品在中国陆域的质量,并通过Hovm?ller图评估了3套产品捕捉土壤水分时空变化的能力。结果显示:①TC方法得到的分析结论与地面实测资料的验证结果一致,整体上SMAP优于ASCAT和FY-3C,不同土地利用类型下SMAP信噪比均最高,三者的TC信噪比分别为1.668 dB、-0.316 dB和-2.182 dB,同时三者与实测值的相关系数分别为0.514、0.501和0.209;②FY-3C和ASCAT产品的精度在中国西北地区整体优于南部地区,3种产品均能较好地刻画土壤水分随纬度和经度变化的情况,3种产品展现的季节波动整体高于实测,其中FY-3C的季节波动在3种产品中最为剧烈;③FY-3C的质量比ASCAT和SMAP更易受到植被影响,但在裸土区FY-3C优于ASCAT。本研究基于TC分析提供了全国范围内3种主流微波土壤水分产品的误差和信噪比的空间分布,并通过Hovm?ller图评估了其描述土壤水分时空变化的能力。研究结论可为微波土壤水分产品的同化研究提供一定参考。

Soil moisture is an important state variable connecting the land surface-atmosphere system,and its information can be efficiently acquired by the new technique of microwave remote sensing.Accurate interpretation of the microwave soil moisture products qualities and in-depth understanding of their temporal and spatial distributions are important prerequisites for their successful application in earth science through data assimilation.In this study,three microwave soil moisture products,FengYun-3C(FY-3C),Soil Moisture Active Passive(SMAP)and Advanced Scatterometer(ASCAT),were evaluated over China based on the triple collocation(TC)method.The abilities of three products to obtain temporal and spatial variations of soil moisture were illustrated by Hovm?ller diagram.The results show that:(1)SMAP generally outperforms ASCAT and FY-3C,with highest TC-based signal-to-noise ratio(SNR)under different land use types.The TC-based SNRs are 1.668dB,-0.316dB and-2.182dB for SMAP,ASCAT and FY-3C respectively;and their correlation coefficients with ground observations are 0.514,0.501 and 0.209,respectively.(2)The accuracies of FY-3C and ASCAT in Northwest China are overall higher than those in the southern China.All three products can capture the latitudinal and longitudinal gradients of soil moisture,whereas their seasonal fluctuations are higher than those of in-situ measurements.Among three products,FY-3C shows highest spatial gradient and strongest seasonal fluctuations.(3)FY-3C product performance is more susceptible to vegetation coverage than AS‐CAT and SMAP,but it outperforms ASCAT in barren areas.The results of our study could provide useful insights for assimilating microwave soil moisture products into land surface models to improve hydrological prediction.