The Temporai And Spatiai Variations Of Water Resources Over The Tibetan Plateau

Climate warming and intensification of human activities will influence the allocation and variation of water resources over the Tibetan Plateau. The water vapor and cloud, as the source of the land water resources, are active factors in the water cycle and important variables of climate change. Therefore, it is beneficial to investigate their distribution, temporal and spatial variation for deep understanding of water resource change in this region. Based on long-term series of satellite remote sensing data, reanalysis data and surface observation data, this article analyzes regional climate change firstly, and then analyzes the characters and change of water resource distribution, especially the air water resource from different aspects, such as precipitation, evaporation, water vapor, cloud, ice glacier and snow. Finally, combining the above results, this paper discussed the impact of climate change on water resources over the Tibetan Plateau. All the results show that land water resource of the plateau lost in two ways:(1) loss from the air due to enhanced evaporation with rising temperature;(2) loss from the land for increased melt water with rising temperature. And the former is more obvious than the latter. According to results, we also found the nature has the ability of self-adjustment, however, we should not be optimistic on whether it can delay the loss of the water resource on plateau. The details of these results are as following:The time evolution of average temperature can be divided into four periods:two low temperature periods (1901-1940;1960-1985), two high temperature periods (1940-1960;1985-2011). The average temperature trend is first slow increasing followed by slow decreasing and rapid increasing. As for the whole Tibetan Plateau, it shows a slowly increasing trend at a rate of0.8℃/100a. As the spatial distribution, the warming trend in northwest area is higher than that in southeast area. The annual variations of average maximum and minimum temperature have the similar trend as average temperature variation, but the warming rate of minimum temperature is greater than that of the maximum, which leads to an overall downward trend of daily temperature range. These three temperature parameters, in particular the minimum temperature, are dependent on the altitude.The temporal and spatial variation of precipitation and evaporation is also investigated. The area of increasing precipitation is almost twice the decreasing area, but the increasing ratio is lower than the decreasing ratio. Precipitation in the Himalayan region decreased most significantly, at the rate of-3mm/a or above. Overall, average precipitation shows a weakly increasing trend. From1965to2011, pan evaporation shows a downward trend in most area, and it is in accordance with the’evaporation complementary’ theory, which is further proved by the trend of actual evaporation calculated via empirical formula. Furthermore, the actual evaporation shows an obviously increasing trend, and the value of precipitation minus evaporation shows a slowly decreasing trend from1980s.In680-310hPa layer, The maximum PW appears in the southeast corner of the plateau, it is about10mm, accounting for the half of water vapor in the entire troposphere; along the Himalayas, Qilian Mountains, south of the plateau is the second largest value of the PW, it is about5-7mm; the minimum PW is still in the central of the plateau. For the season, summer has the most PW, followed by fall, spring and winter. In the layer, the highest positive trend occurred over the central region of the Tibetan Plateau, where the maximum increase was as much as2.4mm/10a, approximately half of the PW over the plateau. This result is constist with the trend of precipitation. The farther from the center of the plateau an area was, the lower the positive trend was. The PW over the Tibetan Plateau has been increasing, but the surface of the plateau is getting drier, the increased water vapor was not stored. The stronger evaporation caused by the rapid warming is the most possible reason, which means the water resource has been lost through the air.From1984to2009, the maximum total cloud amount is at Pamirs, southeast of the Plateau and by the eastern of the Plateau. The seasonal distribution of cloud amount is relative with topography and transportation of vapor over the Tiebtan Plateau. The total cloud amount had a decreasing trend. During the day, cirrus is the majority, almost covered the whole plateau; deep convective cloud are the second, mainly distributes in the southeast and northwest of the plateau. Cirrus decrease rapidly, and deep convective cloud shows an obvious increasing trend, Deep convective clouds increase may lead to enhancing the probability of precipitation, which may be a micro-adjustment for the loss of water resources in the air. The cloud optical thickness, cloud water path and cloud top temperature are all increasing. The decreased high cloud leads to the change of cloud top temperature. The change of all these cloud parameters is a negative feedback to the regional climate.The snow on the Tibetan shows a slow increasing trend from Oct.,1978to Aug.,1987, and a decreasing trend from Jul.,1987to Jun.,2008. But there is no clear change during Mar.,2000to Mar.,2012. For the snow regional variations, the Tibetan not only has the increasing trend but also has the decreasing trend. The Tibetan’s glaciers have retreated since1980s with gradually increasing rate, for which the only reasonable explanation is the accelerated warming. Precipitation is the main factor affecting drain runoff over the Tibetan. The drain runoff does not increase with the Tibetan warming, which is probably because the increasing rate of the evaporation is faster than the precipitation. But some of the drain runoff showing an increasing trend over the Tibetan in spring, as another way of water resources loss, is probably due to the increasing melt water caused by Tibetan warming.According to the present situation of climate change over the Tibetan, the Tibetan is still getting warmer, and has no obvious signs of a phase transition in the next few years, so the water resource over the Tibetan will continue to lose. The first and most effective way to deal with the losing is making effective and rational plans to use the existing water resources; the second way is to try our best to influence the spatial and temporal distribution of water resources in the air in order to delay the loss. The evaporation, the most important factor affecting the water resources in the air, is significantly related to the air temperature the most followed by wind speed and sunshine hours. In addition, the dust aerosol in spring and summer over the Tibetan shows a downward trend in these years, and has a significant negative correlation with the precipitation, which is another micro-adjustment for the loss of water resources in the air.