## 2 Study Area and Data Sources  
### Study Region and Sites Description  
The study area is located in the Haouz plain in central Morocco. The plain covers 6000 km\({}^{2}\) at 550 m above sea level. The irrigated area is of about 2000 km\({}^{2}\) in which the cereals represent 51% of the irrigated areas while rainfed wheat cover the non-irrigated part of the plain [41]. The climate is semi-arid Mediterranean characterized by high reference evapotranspiration (1600 mm/year) that exceeds the annual average precipitation (250 mm). The air humidity is about 50% and the temperature is characterized by large differences between winter and summer with a daily average ranging between 5 and 35 degrees Celsius [41]. Wheat crop is sown between November and January and harvested in May or June. The irrigation occurs from early winter to spring (December to April).  
Five fields cropped with wheat within three experimental sites (Figure 1) with different irrigation techniques were monitored during several growing seasons:_Drip irrigation_: two fields named D1 and D2, of 1.5 ha, located within a private farm near the city of Chichaoua (Figure 1) were monitored during two growing seasons (2016-2017 and 2017-2018). The sowing occurred in November, 25 th and 27 th and the harvest in May 15 th and June 08 th for 2016-2017 and 2017-2018 seasons, respectively. The soil texture is composed of 32.5% of sand and 37.5% of clay. It should be mentioned that wheat on D2 was impacted by very special growing conditions mainly the spread of weeds, particularly a type of wild thistles characterized by a horizontal structure. In addition, the seeds sowed in this field were a mixture of barley and wheat with higher density in D2 than D1, resulting in long stems that laid down under the effect of wind. Within this context, this field is used when considering in situ measurements to illustrate the effect of particular growing conditions on the accuracy of the irrigation retrieval. By contrast, it is discarded when using S1 SSM estimates because SSM retrievals are of bad quality due to these specific conditions as detailed in [17].  
_Flood irrigation_: three fields named F1 (4.5 ha), F2 (4 ha) and F3 (2 ha) (Figure 1), located in an irrigated perimeter with flood irrigation [42] were monitored during different seasons. F1 was monitored during the season 2002-2003 while F2 and F3 were monitored during 2015-2016 agricultural season. The soil texture is composed of 47% of clay and 33% of loam. Please note that F3 was actually irrigated using the drip technique, but the farmer was applying massive amounts up to ~50 mm by events (8 events during the season), thus mimicking the flood irrigation technique [35]. Please note that F1, which was monitored during 2003 prior to the launch of S1, is included in this study in the evaluation section of the approach with in situ SSM data only (Section 4.2).  
_Rainfed field_: One rainffed field of 1 ha, named R1 (Figure 1) is monitored during four agricultural seasons from 2014 to 2018 where the 2015-2016 season was bare soil (the wheat didn't grow up by lack of rainfall). The soil texture is the same as the flood-irrigated fields. This field is used for an indirect evaluation of the irrigation retrieval method described in this study.  
Figure 1: Location of the study area and the instrumented fields (in red), Haouz plain in the center of Morocco.