**2. Study Area and Data**

#### *2.1. Study Area*

Mongolia is divided into five different agro-ecological regions, which reflect distinct geographical patterns of agricultural production and climate. The study was conducted in Darkhan and Selenge provinces, which is located in Selenge-Onon agro-ecological region (N48–51 ◦C and E104–108 ◦C) (Figure 2). Selenge and Darkhan provinces are the principle cropping area for and account for more than 50% of national total grain production (Figure 1). In this region, most of the spring wheat cultivated area is rain-fed cropland. Thus, determining the fluctuation of spring wheat yield is highly dependent on the weather condition. The region averages between 90 to 110 frost-free days and has annual mean precipitation between 250 and 400 mm. In addition, crop growing duration is short (90–140 days) in this region, and depends on location and altitude. Approximately 90% of the nationwide precipitation is lost to evapotranspiration, which is associated with a continental climate. The remaining 10% of the total precipitation has been unable to evaporate, and 37% contribute to soil and underground reserves and streams, while 63% is surface runoff. In other words, 3–4% of total precipitation becomes potentially available as a water resource in the form of soil moisture or groundwater [6]. Additionally, the mean annual temperature is between 0.0 ◦C and 2.5 ◦C with cold temperature in January to –20 ◦C and warm temperature in July to 19 ◦C. Average elevations in this region have ranged between 1500 and 2000 m [45].

**Figure 2.** Study area: (**a**) Agro-ecological regions of Mongolia; (**b**) Digital Elevation Model data (DEM), Spatial distribution of agrometeorological stations in Selenge and Darkhan provinces and cropland.
