# Mapping Irrigated and Rainfed Wheat Areas Using Multi-Temporal Satellite Data
## Materials and Methods
### Datasets
#### 3.1.2 HJ Satellite Data  
The HJ satellite (Small Satellite Constellation for Environmental Prediction and Disaster Monitoring) carries two sensors, HJ-1A and HJ-1B, and features two main characteristics, _i.e._, a short revisit time of two days and large spatial coverage. The swath width of the charged coupled device sensor (CCD) aboard the HJ satellite is 360 km, and the spatial resolution is 30 \(\times\) 30 meters. The CCD sensor has four bands (blue, green, red and near-infrared) with a spectral range of 0.43 to 0.90 \(\upmu\)m and a radiometric resolution of 8 bits. Landsat TM/ETM+ imagery has the same spatial resolution, but has difficulty meeting the high frequency demand for its 16-day return cycle and cloudiness. The MODIS imagery has high temporal resolution, but its spatial resolution is relatively coarse for small and scattered agricultural fields. The HJ satellite has the advantages of both Landsat and MODIS and therefore is more suitable for dynamic monitoring of multi-temporal and spatial variations in irrigated areas. \begin{table}
\begin{tabular}{l c c c c c c c c c c c} \hline \hline  & & **Yanhu** & **Yaodu** & **Rukicheng** & **Wanrong** & **Jincheng** & **Fenyang** & **Jiexiu** & **Anze** & **Changzhi** & **Average** \\ \hline Triggated/rainfeed & & Irriggated & Irriggated & Irriggated & Rainfeed & Rainfeed & Triggated & Irriggated & Rainfeed & \\ Elevation (m) & 376.0 & 450.0 & 507.0 & 590.0 & 744.0 & 749.0 & 750.0 & 585.0 & 991.0 & \\ Longitude & 111.02 & 111.50 & 110.71 & 110.83 & 112.83 & 111.76 & 111.93 & 112.25 & 113.06 & \\ Latitude & 35.03 & 36.06 & 34.70 & 35.40 & 35.51 & 37.25 & 37.05 & 36.16 & 36.05 & \\ Sowing dates & 10.7 & 9.28 & 10.17 & 9.25 & 9.72 & 10.6 & 10.16 & 9.30 & 9.23 & 9/30 \\ Maturity dates & 6/6 & 6/6 & 6/12 & 6/12 & 6/13 & 6/26 & 6/22 & 6/24 & 6/20 & 6/15 \\ Yield (bg/ha) & 2011 & 3825.0 & 3750.0 & 4800.0 & 1935.0 & 2658.0 & 5325.0 & 6150.0 & 1950.0 & 3670.5 & 3784.5 \\  & 2006 to 2014 & 3700.5 & 5394.0 & 5286.0 & 2142.0 & 3000.0 & 5674.5 & 5617.5 & 2484.0 & 3621.0 & 4102.5 \\ \hline Cumulated & 2011 & 125.8 & 102.0 & 108.0 & 112.2 & 118.3 & 83.3 & 99.9 & 190.8 & 176.5 & 124.1 \\ precipitation (mm) & 2006 to 2014 & 162.4 & 143.9 & 160.0 & 194.9 & 192.0 & 132.1 & 127.9 & 231.2 & 211.6 & 172.9 \\ \hline Cumulated & 2011 & 2258.4 & 2291.7 & 2027.7 & 2520.1 & 2264.9 & 2220.5 & 2055.7 & 1888.6 & 2068.5 & 2177.3 \\ temperature (\({}^{\circ}\)C) & 2006 to 2014 & 2290.5 & 2309.1 & 2039.8 & 2412.3 & 2178.8 & 2224.0 & 2120.7 & 1910.7 & 2032.6 & 2168.7 \\ \hline Cumulated sunshine & 2011 & 1438.7 & 1679.1 & 1481.8 & 1665.9 & 1905.8 & 1513.1 & 1642.9 & 1835.2 & 1845.1 & 1667.5 \\ duration (hours) & 2006 to 2014 & 1270.1 & 1317.9 & 1372.4 & 1432.6 & 1665.3 & 1522.4 & 1450.8 & 1654.6 & 1725.3 & 1490.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Observed environmental conditions and wheat yield at agorneteorological stations in south central Shnad Province. The HJ satellite datasets span the time period September 2010 to July 2011, covering the whole growth period of wheat (Table 2). Most of the HJ-1 A/B images used in this study are cloud-free. The DN value was converted to radiance values using radiometric calibration coefficients from the China Center for Resources Satellite Data and Application (available at [[http://www.cresda.com](http://www.cresda.com)]([http://www.cresda.com](http://www.cresda.com))). The images were then processed with ENVI FLAASH for atmospheric correction. Landsat TM8 images (Level: L1T) covering Shanxi Province were used for geometric correction with an error of less than one pixel. When using the FLAASH module, the center position, access time and other information are obtained from the header file of each image. We selected the mode rural aerosol and the atmospheric mode as mid-latitude winter or mid-latitude summer, depending on the acquisition time. NDVI (NDVI = (\(\rho\)nir - \(\rho\)red)/(\(\rho\)nir + pred)) is calculated from red and near-infrared reflectance.