Zhu Luyao, Wang Hongquan, Tong Cheng, Liu Wenbin, Du Benxu
Institute of Agricultural Remote Sensing and Information Technology Application, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China.
Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China.
Sensors (Basel). 2019 Jun 17;19(12):2718. doi: 10.3390/s19122718.
The European Space Agency (ESA) Climate Change Initiative (CCI) project combines multi-sensors at different microwave frequencies to derive three harmonized soil moisture products using active, passive and combined approaches. These long-term soil moisture products assist in understanding the global water and carbon cycles. However, extensive validations are a prerequisite before applying the retrieved soil moisture into climatic or hydrological models. To fulfill this objective, we assess the performances of three CCI soil moisture products (active, passive and combined) with respect to soil moisture networks located in China, Spain and Canada. In order to compensate the scale differences between ground stations and the CCI product's coarse resolution, we adopted two upscaling approaches of Inverse Distance Weighting (IDW) interpolation and simple Arithmetic Mean (AM). The temporal agreements between the satellite retrieved and ground-measured soil moisture were quantified using the unbiased root mean square error (ubRMSE), RMSE, correlation coefficients (R) and bias. Furthermore, the temporal variability of the CCI soil moisture is interpreted and verified with respect to the Tropical Rainfall Measuring Mission (TRMM) precipitation observations. The results show that the temporal variations of CCI soil moisture agreed with the ground measurements and the precipitation observations over the China and Spain test sites. In contrast, a significant overestimation was observed over the Canada test sites, which may be due to the strong heterogeneity in soil and vegetation characteristics in accordance with the reported poor performance of soil moisture retrieval there. However, despite a retrieval bias, the relatively temporal variation of the CCI soil moisture also followed the ground measurements. For all the three test sites, the soil moisture retrieved from the combined approach outperformed the active-only and passive-only methods, with ubRMSE of 0.034, 0.050, and 0.050-0.054 m/m over the test sites in China, Spain and Canada, respectively. Thus, the CCI combined soil moisture product is suggested to drive the climatic and hydrological studies.
欧洲航天局（ESA）气候变化倡议（CCI）项目结合了不同微波频率的多传感器，采用主动、被动和组合方法来获取三种协调一致的土壤湿度产品。这些长期的土壤湿度产品有助于理解全球水和碳循环。然而，在将反演得到的土壤湿度应用于气候或水文模型之前，广泛的验证是先决条件。为实现这一目标，我们针对位于中国、西班牙和加拿大的土壤湿度网络，评估了三种CCI土壤湿度产品（主动、被动和组合）的性能。为了弥补地面站点与CCI产品粗分辨率之间的尺度差异，我们采用了反距离加权（IDW）插值和简单算术平均（AM）两种上采样方法。利用无偏均方根误差（ubRMSE）、均方根误差（RMSE）、相关系数（R）和偏差对卫星反演的土壤湿度与地面实测土壤湿度之间的时间一致性进行了量化。此外，还针对热带降雨测量任务（TRMM）降水观测结果，对CCI土壤湿度的时间变异性进行了解释和验证。结果表明，在中国和西班牙的测试站点，CCI土壤湿度的时间变化与地面测量值和降水观测结果一致。相比之下，在加拿大的测试站点观察到明显高估，这可能是由于土壤和植被特征的强烈异质性，与报道的该地区土壤湿度反演性能较差一致。然而，尽管存在反演偏差，CCI土壤湿度的相对时间变化也与地面测量值相符。对于所有三个测试站点，组合方法反演的土壤湿度优于仅主动和仅被动方法，在中国、西班牙和加拿大测试站点的ubRMSE分别为0.034、0.050和0.050 - 0.054 m/m。因此，建议使用CCI组合土壤湿度产品来推动气候和水文研究。
