3.1.1. SMAP

SMAP is an orbiting observatory capable of measuring the amount of water in the top 5 cm of soil at global scales. To meet the various needs of soil moisture monitoring, the SMAP mission uses an L-band radar and an L-band radiometer for concurrent, coincident measurements integrated into a

single observational system and ultimately produce a variety of spatial and temporal resolution SM products [38]. Since the successful launch of SMAP in January 2015, it has provided SM products of many levels worldwide. After validating the accuracy of SMAP products, the SMAP products meet the mission requirements and can also be used to assess hydrologic processes [30,31,39]. In this study, considering the spatial and temporal resolution of the SMAP products required for the study area, SMAP enhanced L3 radiometer global daily 9 km EASE-grid soil moisture version 1 was used as the final data source [40]. The study used the SMAP SM datasets from the end of Feb to the end of May (2015–2018) corresponding to the winter wheat irrigation period in study area. Although the SMAP dataset can provide daily SM products, due to satellite orbits, SM products do not cover the spatial extent of the study area every day due to satellite orbits. To select the SMAP data that can completely cover the study area, 8 control points are used to filter the data that meet the requirements. The eight control points are evenly distributed at the vertices of the study area boundary, and the judgement equation is as follows:

$$X\_{\dot{i}} = \begin{cases} 1, \ 0 < V\_{\dot{i}} < 1 \\ 0, \ V\_{\dot{i}} = \text{null} \end{cases} \tag{1}$$

$$J = \begin{cases} \begin{array}{c} R, \ \sum\_{i}^{8} X \ge 6 \\ D, \ \sum\_{i}^{8} X < 6 \end{array} . \end{cases} \tag{2}$$

where X is the judgement result of the SMAP pixel value (*V*) and null is no-data in this pixel, i is the number of the control point. If the value of the SMAP pixel is between 0 and 1, X = 1; and if the SMAP pixel value is null, then X = 0. J is the judgement result of whether the SMAP data are retained, and R and D represent the retention and deletion of SMAP data, respectively. If the sum of the 8-control point X ≥ 6, it indicates that SMAP data can cover a large area (more than 75% of the study area is covered) of the study area and this SMAP data is retained; if it less than 6, the data are deleted. The programming language for batch filtering, processing and extracting of SMAP data is python 2.7, and the arcpy function provided by ArcGIS 10.4 (Environmental Systems Research Institute in California) is also used. Regarding the extraction of pixel values in this paper, the "ExtractValuesToPoints" function in arcpy is used.
