Variations of the Surface Wettability Index over the Tibetan Plateau During 1971-2005

Based on 1971-2005 monthly mean maximum/minimum temperature, wind speed, relative humidity, sunshine duration, and precipitation data at 25 stations over the Tibetan Plateau, a study of the largest potential evapotranspiration (LPE) is performed by using the Penman-Monteith model. The surface wettability index (SWI) is calculated and examined, together with its space distribution, interannual and seasonal variations, as well as associated causes. The results suggest that the annual area rainfall exhibits a pronounced increasing trend at 15.0 mm per decade; the annual LPE shows a different-degree decrease at -4.6--71.6 mm/10 yr. In the southwestern Ngari prefecture and Nyalam county, the annual SWI displays insignificant decline trends compared to increasing trends in other areas of Tibet (0.02-0.09 per decade). For Tibet, on average, the SWI experiences a noticeable rise at 0.04/10 yr, particularly in 1981-2005. On a seasonal basis, the SWI shows increasing trends, especially in summer. In the 1970s-1980s, the interannual variation is characterized mainly by lower temperature and lower humidity. From the 1990s, air temperature keeps on rising, leading to an appreciable increase in SWI, displayed as a type of warm and humid climate. The salient increases (decrease) of precipitation and relative humidity (mean temperature daily range) are the principal causes of the greatly enhanced SWI in the region. The pronounced decrease in mean wind and sunshine duration also plays an active role.