# Sensitivity Analysis of Multi-Temporal Sentinel-1 SAR Parameters to Crop Height and Canopy Coverage
## 2 Materials and Methods
### Field Surveys  
In situ measurements were conducted for maize, sunflower and wheat fields in the spring-summer agricultural season of the year 2016. Crop variables and parameters which indicate the growth rate including crop height and CC were collected simultaneously with the SAR data acquisition, and recorded during the field works. Field data collection includes measuring the row and plant cover by still tape in unit area, taking photographs by the camera and recording field characteristics such as soil properties and irrigation status. For calculating CC, photographs were taken in downward position and perpendicular to the ground with 100 cm distance from camera and canopy outmost in the tillering stage when the crop height was less than 100 cm. Considering the study area, 36 test fields that consist of 19 maize, 6 sunflower fields, and 11 wheat fields were dedicated for this research. The variability of development stages for maize, sunflower and wheat has been defined by applying the \"Biologische Bundesanstalt, Bundessortenamt, and CHemische\" (BBCH) [20] indicator for each field that generally consist of vegetative and reproductive stages. Seasonal maize, sunflower, and wheat crop calendar of the study area is presented in Figure 4. According to the defined height and CC thresholds, derived from SAR backscattering and BBCH-scale, we call two stages for crops growth season; the early stage and the later stage. Figure 3: Land use map of the study area (Source: Copernicus Land Monitoring Service [19]). Four field surveys were conducted to obtain accurate ground measurements in late May-mid June, early July, late July, and late August 2016. General growth stages categories (leaf development, stem elongation, heading and flowering), are defined according to the BBCH-scale (Table 1). For calculating the crop height of each test site, five height measurements were obtained and their mean value was calculated to represent the crop height of the relevant test field. From the test sites, plant cover and row were measured and also photographs were captured to determine and evaluate the CC percentage. A synopsis of the maize, sunflower and wheat of different growing stages is given in Figure 5.  
In this study site, the BBCH-scale is considered as 53 when the maize height was in range of 120-150 cm. This growth stage of the maize is at the inflorescence emergence and heading stage. When \begin{table}
\begin{tabular}{c c c} \hline
**Field Surveys** & **Crop Growth Stages (BBCH)** & **SAR Acquisitions Dates** \\ \hline
19 May–12 June 2016 & leaf development & 13 June 2016 \\ \hline
01–02 July 2016 & stem elongation & 07 July 2016 \\ \hline
31 July 2016 & heading 1 & 31 July 2016 \\ \hline
24–25 August 2016 & flowering & 24 August 2016 \\ \hline \end{tabular} \({}^{1}\) Due to variation in wheat growth conditions in different fields, in general heading stage starts at late May and lasts until mid-June.  
\end{table} 
Table 1: Biologische Bundesanstalt, Bundessortenamt, and CHemische (BBCH)-identification keys for field measurements and Synthetic Aperture Radar (SAR) data acquisitions of the study area. Figure 4: Seasonal maize, sunflower and wheat calendar of the region. Figure 5: Different growth stages of crops in the study area. maize height was greater than 220 cm, the BBCH-scale was 69 and represented the end of flowering. Once the sunflower height was greater than 92 cm, the BBCH-scale is found as 79, indicating the end of flowering and the inflorescence reaches full size. The BBCH-scale is considered as 59 after that wheat height reached to 53 cm and inflorescence fully emerged. We observed that different wheat height could have the similar wheat BBCH due to variation of wheat growth conditions which cause to distinction even though they are at the same phenology, in agreement with the study of [PERSON] et al. [17]. Figure 6 shows the relationship between crop height and the BBCH-scale corresponding to the each crop principal growth stages. Scatterplots (d-f) in Figure 6 show SAR response to crop heights.