流域水量平衡变化机制及其效应的认识是理解湖泊水情变化与制定缓解措施的根本依据。针对2000年后呼伦湖持续水位的急剧下降,并考虑到流域监测资料稀缺的实际情况,借助遥感反演降水和蒸散时空数据序列,利用时空模式分解、趋势分析等方法揭示了流域降水时空变化特征及其引起的蒸散效应。结果表明：流域降水变化主要由空间分布及过程强弱互补的2个降水模式决定,这2个模式的变化解释了流域降水变化的67.4%。2000年后2个模式强弱的变化导致了流域整体降水由两侧山区向中部地势平坦区域的转变。流域中部受地形及气象条件影响,相比两侧山区产流效率偏低。因而降水空间分配的改变形成了更有利于增加陆面蒸散的供水条件,导致了流域蒸散占降水比重上升到较高水平。蒸散变化强度及趋势的时空特征分析也进一步确认了以上现象,这种降水模式的变化及其效应直接导致了流域水量平衡中可供产流的净雨量减少,是呼伦湖流域近年产流减少的根本原因。2000年后流域冻土消融补给的改变则对呼伦湖极端水情的发生起到了推波助澜的作用。

A comprehensive understanding of the catchment water balance change is critical for evaluating lake hydrological regimes and adaption strategies. The abrupt lake water level decrease in Hulun Lake, Inner Mongolia, China, since 2000 has significantly influenced lake eco-environments. Given the sparse gauging data in the catchment, the study pred-ominantly analyzed remote sensing retrieved precipitation and evapotranspiration (ET) data to reveal the spatial-temporal patterns and their dynamics. The results indicate that signals of large-scale precipitation variations can be well-described using the empirical orthogonal function (EOF). Spatially, the precipitation in the Hulun Lake watershed is dominated by two leading EOF patterns at the basin scale, explaining 67.4% of the total changes in precipitation. The two leading patterns present complementary features of the high-value centers in space. The high-value center in pattern 1 is on mountainous slopes, and the high-value center in pattern 2 is in the middle flat plain region. First, the strengthening in EOF pattern 2 and weakening in EOF pattern 1 resulted in a 10% precipitation decrease, which is far less than the decreasing river discharge. Second, the shift between the precipitation amplitudes of patterns 1 and 2 also triggers changes in the spatial distribution of precipitation over the watershed, increasing precipitation in the central part of the watershed but not in the mountain regions on the two sides of the watershed as usual. Such variation weakened the capacity of total runoff production and triggered an increased ET because of spatial heterogeneity in hydrology. The runoff generation condition in the middle area is not smaller than in the upper areas affected by landform and hydrothermal conditions, causing more water returns to the atmosphere by ET. The dynamics and trend patterns in ET confirm this effect. The long-term impact of such changes is an increasing ET to precipitation ratio and decreasing precipitation to runoff generation ratio. Thus, the change in the relationship between precipitation and ET is the principal cause of reduced river runoff and hydrological extremes in the terminal lake during the past decades, impacted by regional climate warming and drying. The ET to precipitation (ET/P) ratio has reached a relatively high value since the early 1990s but did not result in a hydrological extreme. Located on the southern edge of the permafrost zone of the Eurasian continent, the recharge from the permafrost degradation caused by climate warming has offset part of the river channel water deficit before 1999. However, when the recharge effect of permafrost degradation vanished after 2000, river discharge started a fast drop period, causing a sharp decrease in the lake water level since 2000. Therefore, the significant changes in the recharge from the permafrost degradation have worsened the hydrological extremes of the lake.