Abstract:

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.

Key words: hydro-meteorology, patterns recognition, lake-catchment process, water balance, Hulun Lake