Based on the meteorological data from weather stations in the northern and southern regions of Qinling Mountains (NSQ) , we used absolute humidity estimating formula and Penman鄄Monteith equation to estimate absolute humidity and potential evaporation, respectively. Furthermore, we analyzed the spatial and temporal distribution and variation of absolute humidity, and tried to give the possible explanations for the reciprocal feedback between absolute humidity and potential evaporation, the result are as following. (1) Absolute humidity presented a clear pattern that high in south and low in north, the order of it was Bawu Valley (BWV) , Han River Basin (HRB) , northern and southern regions of Qinling Mountains(NSQ) , southern slope of Qinling Mountains (SSQ) and northern regions of Qinling Mountains (NRQ) , the order of seasonal absolute humidity was summer, autumn, spring and winter. (2) In the past 52a, absolute humidity in most regions, except for BWV, had increasing trend. 1986 and 1998 are the change point, in the period of 1960-1986, it had insignificant decreasing trend, and then increased significantly until 1998, and decreased since 1998. (3) On the annual scale, spring and autumn, in most regions, except for BWV, absolute humidity negatively correlated with potential evaporation, meanwhile, in summer and winter, except for HRB and BWV, absolute humidity and potential evaporation also had the similar correlation relationship. Beside from this, the correlation coefficients on the annual scale, spring and autumn became smaller with the latitude decreased. In the period of 1960-2011, on the annual and seasonal scale, absolute humidity and potential evaporation had contrary change trend, however, in the period of 1960-1989, they showed same change trend. From 1990 to 2011, potential evaporation in year, spring and winter increased significantly, meanwhile, absolute humidity increased initially and then decreased. (4) The contrary change trend of absolute humidity and potential evapotranspiration can be explained by the evaporation complementary theory. The actual evaporation increasing will lead to the increase of water vapor content in the air, also for the absolute humidity, which will inhibit water evaporation. Absolute humidity and potential evaporation had complementary relationship in NRQ, SSQ, HRB, and part of BWV, and the correlation relationship decreased with water limitation reducing, ranging from negative correlation to insignificant positive correlation.

Potential evapotranspiration (ET 0) , as an estimate of the evaporative demand of the atmosphere, it has been widely applied in irrigation management and predictions in ungauged basins. Analysis of ET 0 spatial and temporal variation is the basic research on the impact of climate change on water resources, and is also important to the optimal allocation of agricultural water resources. In recent years, change trends in ET 0 and its dominant factors across the different regions of the world have been studied by many researchers. Despite global warming,decreasing trends in ET 0 have been detected in several countries including United States,Russia, India, China, Australia and New Zealand. Decreasing sunshine duration, declining wind speed and increasing relative humidity have been considered to be the main causes for the decreasing ET 0 .

Water vapor content is a highly important variable for ecosystem, hydrology and climate systems. It directly affects the generation of precipitation, and it has a close correlation with the whole content of water vapor in the air. Many researches have been done in water vapor conditions, sources, directions of movement and convergence regarding the formation of heavy rain, and in the relations between these factors and the rainy season and rain belts in the east, south and low-latitude regions of China. However, due to the limitation of available data and computing technologies, it was not until the 1990s that the study on water vapor in Northwest China began to be paid much attention to. Since the latter half of the 1980s, precipitation and temperature in the northern and southern regions of Qinling Mountains increased and decreased significantly, respectively, which are usually attributed to global warming and the climate changed from ' warm and humid爷 to ' warm and dry爷 in this region.