the Tibet-Qinghai Plateau, which forms the southern and western boundary of this territory.
The climate of this region is semiarid to arid and in some areas quite cold (SRCG, 1988). The average annual precipitation in most of the region is approximately 200 mm, reaching as low as tens of millimeters—and, in a few instances, even less—and only occasionally exceeding 400 mm. In most areas the aridity index is greater than 1.0, and in some places as high as 30. Thus, moisture is the limiting factor for agriculture and animal husbandry. At higher elevations, in the Tibet-Qinghai Plateau and the Tianshan and Altai mountains, temperature is also a problem, and many places are too cold for livestock. The major climatic factors in the northern grasslands include the following.
Solar Radiation Solar radiation in northern China exceeds that required to support grassland vegetation. Radiation is generally higher in the west than in the east, and higher on the plateaus than on the plains. On the Tibet-Qinghai Plateau, for example, the total solar radiation is 7.9 × 105/cm2, and there are 3600 hours of total sunlight annually. The northwest side of the Tibetan Basin receives the most hours of sunlight of any place in China. Under strong solar radiation and favorable climate, the vegetation on the northern grasslands is high in fat and protein and low in fiber. Grass with these qualities is palatable and supports high livestock productivity.
Temperature Thermal conditions in northern China are complex. The eastern grasslands lie in a region of moderate temperatures, which increase gradually from northeast to southwest. In the eastern extremity, the Daxinganling Mountains, the growing season is only 170 days, and the accumulated temperature (the sum of daily temperatures above 10°C, the base temperature for growth of grass, over an average year) is about 2000°C. Further west, the Loess Plateau of Shaanxi and Gansu and the Alashan Desert in Inner Mongolia are warmer, having a growing period of 250 days and accumulated temperatures of 3500–4000°C. Xinjiang, in the far west, is the warmest of all. The growing period in the southern Tianshan Mountains is 250–280 days, and the accumulated temperature is 4000–4800°C. The growing period in the Turpan Basin is 280 days, and the accumulated temperature is 5600–5700°C. Temperatures on the Tibet-Qinghai Plateau are much lower, coldest in the northwest and increasing as one moves south. Northwest Tibet has a growing period of only 60–90 days and an accumulated temperature of less than 500°C; the central sector, a growing period of 160–200 days and accumulated temperature of 500–1500°C; and the southern part, a growing period of 175–320 days and accumulated temperature of 1500–3000°C.