1981-2013年气候因子变化对西藏拉萨河径流的影响

采用1981-2013年西藏拉萨河流域2个气象站降水量、气温、蒸发量的实测数据,以及拉萨水文站径流序列资料,分析拉萨河流域降水、气温变化及其对径流量的影响.结果显示:近33 a来,拉萨河流域降水量呈增多趋势,冷季增多趋势显著,倾向率达到3.51 mm·(10a)^-1;年、季平均气温、平均最高、最低气温呈显著增高趋势.平均气温倾向率年尺度为0.58℃·(10a)^-1、暖季0.42℃·(10a)^-1、冷季0.74℃·(10a)^-1;年、季蒸发量呈显著减少趋势,倾向率达到年127.7 mm·(10a)^-1、暖季82.2 mm·(10a)^-1、冷季45.5 mm·(10a)^-1.20世纪80年代降水量偏少、气温偏低、蒸发量大,是一个比较寒冷干燥的时期;90年代降水增多、气温增高、蒸发量减少,到21世纪初,降水、气温均达到各年代最高值,蒸发量为各年代最小,拉萨河流域进入一个相对温暖湿润的时期;拉萨河径流量年际变化较小,其变化趋势与降水、气温基本一致,20世纪80年代径流量最小,之后逐年代增大,21世纪初,年、季径流量达到各年代最大.1983年全流域出现的干旱少雨天气,导致20世纪80年代拉萨河年和暖季径流略偏枯,其他时段年、季径流无明显的丰枯变化,处于一个比较平稳的状态;拉萨河流域降水量的大小直接影响着径流量的大小,且暖季降水在拉萨河年径流的形成上起主导作用;气温的显著升高和人类活动对下垫面条件的改变,削减了降水量增多、蒸发量减少对径流形成的有利影响.

Effect of climate factors on the runoff over Lhasa River basin during 1981-2013

In this paper, the variations of air temperature, precipitation and evaporation, together with their effect on runoff, over the Lhasa River basin in Tibet were investigated by using the monthly precipitation, air temperature and evaporation data observed at two meteorological stations and the runoff data measured at a hydrological station in Lhasa during 1981-2013. It is found that the precipitation has increased in the past 33 years over the Lhasa River basin, especially the precipitation during the cold season, reaching an increasing rate of 3.51 mm·(10a)^-1. The annual and seasonal mean temperatures exhibit significant increasing tendency while the evaporation shows significant decreasing tendency. The annual temperature changing rate is 0.58℃·(10a)^-1, the warm season temperature changing rate is 0.42℃·(10a)^-1 and the cold season temperature changing rate is 0.74℃·(10a)^-1; the annual evaporation changing rate is 127.2 mm·(10a)^-1, the warm season evaporation changing rate is 82.2 mm·(10a)^-1 and the cold season evaporation changing rate is 45.5 mm·(10a)^-1. The 1980s is pretty cold and dry period with less precipitation, low temperature and large evaporation, followed by more precipitation, higher temperature and smaller evaporation in the 1990s, then precipitation and temperature reached the highest values and smallest evaporation at the beginning of the twenty-first century. Thus, Lhasa River basin has entered a relatively warm and humid period, when annual variation of runoff was quite small, and the annual and seasonal runoff varied consistent with the tendency of precipitation and temperature, namely runoff in the 1980s was the smallest and then increased with time, and appeared the maximum at the beginning of the twenty-first century. The annual runoff and the runoff during the warm season in the Lhasa River basin exhibited low water level since less precipitation in 1983; however, runoff was extremely stable for the whole basin during other periods without remarkable variation. Precipitation in Lhasa River basin directly controls the magnitude of runoff and warm season precipitation plays an important role in the formation of runoff. The pronounced temperature rise and the variation of underlying surface due to human activity might cut down the contribution coming from the increasing precipitation and decreasing evaporation on the runoff formation areas.