## 2 Data and Study Area  
### Site Description  
The study area, on the North China Plain (NCP), one of China's major grain producing regions, extends from 32\({}^{\circ}\)00\({}^{\prime}\)N to 40\({}^{\circ}\)24\({}^{\prime}\)N and 112\({}^{\circ}\)48\({}^{\prime}\)E to 122\({}^{\circ}\)45\({}^{\prime}\)E, an area of approximately 310,000 km\({}^{2}\) (Figure 1). Seven mega-cities/provinces are situated on the NCP (Beijing, Tianjin and parts of Hebei, Shandong, Henan, Anhui and Jiangsu provinces). The mean annual precipitation ranges between 480 and 1050 mm, concentrated between June and September during the summer monsoon [28]. According to Generic Soil Classification of China and Soil Taxonomy, Inceptisols dominate the North China Plain [29]. The plain has very diverse climatic conditions, from a cold, arid steppe regime to a humid, warm temperate regime with hot summers according to the data derived from the FAO New_LocClim database [30] (Table 1). Most of the region belongs to the warm temperate climatic zone. Farming practices and rain-fed production potential vary accordingly. Multiple cropping systems are mainly practiced in southern portions of the NCP; central areas are generally dominated by the rotation of winter wheat and maize and the relay intercropping of winter wheat and cotton; and northern areas mainly practice a single cropping system, because of the cold climate and short frost-free period [31]. Spring maize and cotton are the main crops in areas with a one-crop-a-year system. The study area is characterized by the cultivation of wheat, maize, rice, cotton, groundnut, soybean, vegetables and other small crops in farms subdivided into very small parcels belonging to multiple families [27, 32]. Figure 2, based on data provided by the China Meteorological Administration (CMA), shows crop phenology for five major crops on the NCP.  
Figure 1: Location of the study area. The images are HJ-1 CCD images.  
\begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline
**Items** & **Anhui** & **Beijing** & **Hebei** & **Henan** & **Jiangsu** & **Shandong** & **Tianjin** \\ \hline Longitude (Degree) * & 116.66 & 116.56 & 115.87 & 114.73 & 119.27 & 116.56 & 117.37 \\ Latitude (Degree) * & 33.41 & 39.72 & 38.41 & 34.27 & 33.56 & 36.91 & 39.31 \\ Altitude (m) & 20 & 20 & 0 & 40 & 0 & 0 & 0 \\ Köppen climate class & BSk & Dwa & BSk & Cwa & Cwa & BSk & Dwa \\  & \multirow{2}{*}{Cold arid} & Snow climate & \multirow{2}{*}{Cold arid} & Warm & Warm & \multirow{2}{*}{Cold arid} & Snow climate \\ Köppen & Climate & & with dry & & & temperate with & & with dry \\  & steppe & & & & dry winter and & dry winter and & & \\  & & climate & summer & & hot summer & hot summer & & summer \\ Budyko & radiation & & & & & & \\ dryness & 2.574 & 1.881 & 2.932 & 1.615 & 1.299 & 2.653 & 2.001 \\ Budyko runoff (\%) & 5.9 & 11.5 & 4.1 & 15.3 & 21.6 & 5.5 & 10.2 \\ Gorczynski & & 62.7 & 59.9 & 68.8 & 59.0 & 60.2 & 60.6 & 60.9 \\ Continentality & & & & & & & \\ Miami model rainfed & & & & & & & \\ NPP (g (DM)/m\({}^{2}\)/year) & & & & & & & \\ \hline \multicolumn{7}{l}{Data are derived from the FAO New\_LocClim database [30]. * These coordinates are the centroid of the intersection area of the NCP and each administrative boundary. Climate features were calculated at each coordinate.} \\ \end{tabular}
\end{table}
Table 1: Climatic characteristics of the North China Plain (NCP) area.