## 3 Results and Discussion  
### Natural Characteristics and Water Deficit Distribution of Winter Wheat in Different Subregions  
The topographic conditions, soil types, annual average rainfall, annual average temperature, and annual evaporation of Shaanxi Province are presented in Figure 1. Shaanxi Province lies between 105\({}^{\circ}\)29\({}^{\prime}\)-111\({}^{\circ}\)15' E and 31\({}^{\circ}\)42\({}^{\prime}\)-39\({}^{\circ}\)35' N and has a total area of 205,600 square kilometers. Its terrain is high in the north and south and low in the middle, and it is composed of a mixture of the plateau, mountain, plain, basin, and other landforms. The south of Shaanxi includes the south foot of the Qinling Mountains, the north foot of the Ta-pa Mountains, and the River Han. Its altitude is generally 700-3000 m. The central part of Shaanxi is north of the Qinling Mountains and mainly made up of plains on both sides of River Wei, with an altitude of 320-800 m. The northern part of Shaanxi largely consists of Loess Plateau, hills, and gullies, with an altitude of 900-1500 m.  
In addition to the south of Shaanxi Province, the soil texture changes from sandy soil to cohesive soil from north to south. The main soil types are sandy loam, loam, silt loam, and silt clay loam. When examining the province as a whole, rainfall is substantially more prevalent in the south than in the north. Annual rainfall in the Ta-pa Mountains at the southernmost end, for example, averages 1240 mm, while annual rainfall at the northernmost end averages only 340 mm. The precipitation in all parts of the Shaanxi Province is concentrated from July to September, with the lowest precipitation in winter, accounting for only 2-5% of the total annual precipitation. Evaporation also gradually increases from south to north. The lack of precipitation and high evaporation in northern Shaanxi makes it  
Figure 1: Characteristics of topography, soil, precipitation, evaporation, and temperature in Shaanxi Province. difficult or even impossible to meet the water demand for crops. In addition, the terrain in the northern area is fragmented. The local surface groundwater resources are seriously lacking, the maximum available water in the whole year is only 40-60 mm, and 70% of the surface runoff of the river is mainly from July to September. Therefore, there is no large ground water source nor suitable conditions within which to build a large irrigation canal and pipeline irrigation system, and it is also difficult to transfer water from outside the region. Thus, there is an urgent need to use extremely limited water resources to develop deficit irrigation in the critical period of crop water demand. Due to the different topography, soil, precipitation, evaporation, and temperature across Shaanxi Province, rice is mainly grown in the south, while winter wheat and summer maize are grown in the central and north regions. Notably, both sides of the Wei River in central Shaanxi Province have a flat terrain and favorable water source conditions. There are nine large-scale irrigation systems here, which mainly use full irrigation for winter wheat. Due to the low temperatures in the northernmost part of Shaanxi Province, winter wheat growth is difficult. However, other regions across the middle and northern parts of Shaanxi Province must develop deficit irrigation with a small amount of water for winter wheat. This study focused on determining the optimal irrigation period of winter wheat in the deficit irrigation regions with a small amount of water of Shaanxi Province mentioned above. As the natural conditions of the region vary considerably, the study region was divided into four winter wheat planting subregions (I, II, III, IV) according to terrain characteristics, soil characteristics, annual precipitation, and crop water demand during the growth period of winter wheat (Figure 2).