Interannual characteristics of the surface hydrological variables over the arid and semi-arid areas of northern China

The characteristics of the surface humid index (SHI) were analyzed based on 160 station data in China from 1951 to 1998. The surface humid index is defined as SHI=(P)/(P_e), where P_e is potential evaporation suggested by Thornthwaite's method. The difference between the evolutionary features of the SHI in typical arid regions of north China (Huabei and the northwest) was compared. The results show that the SHI is decreasing (drying trend) in the Huabei region of north China but is increasing in some areas of northwest China (wetting trend). Under regional warming, the drought in the center of north China mainly resulted from the decrease in precipitation and is partly due to the increase in evaporation. A dry period of about 40 years was revealed from the historical data over the area. Increasing evaporation caused by increasing temperature probably intensified the drought in that area, but is not the main reason for the drought. It is the less precipitation that mainly results in the present drought in north China. In addition, the SHI variations in different seasons were also analyzed; the result indicates the notable difference of SHI variation between seasons. Finally, the geographical distribution of annual SHI variation over China was given.