Abstract
Typhoon storm and plum rain are two typical rainfall types in the lower regions of the Yangtze River Basin, which frequently cause flood disasters in China. New information in stable water isotopes offers the opportunity to advance understanding of runoff mechanisms and water source dynamics in response to these two typical rainfall types. We intensively monitored two representative rainfall events in a small bamboo forestry watershed in 2016. Results showed that precipitation isotopic variations during the event were generally larger than those of other monitored compartments (including throughfall, surface overland water, groundwater and river water) and also larger for the plum rain than for the typhoon event (δ18O varied in 5.2‰ and 3.7‰, respectively). Importantly, the differences of isotopic temporal variation between rainfall and throughfall showed significant impacts on the two-component hydrograph separation for both rainfall types (e.g., if not considered, the pre-event water fractions were 26.6% and 15.3% higher for the typhoon and plum rain events, respectively). Furthermore, we evaluated the role of soil water on the three-component isotopic hydrograph separation model; results revealed that soil water accounted for 10.9% and 28.3% of the total discharge in typhoon and plum rain events, respectively. This underpins the important role of soil water dynamics during the rainy season in this humid region.
Highlights
The lower region of the Yangtze River Basin, as one of the most active economic regions of China, has experienced dramatic development over recent decades; high risks of social-economic damages have been raised by flood-induced disasters, which might occur regularly due to the regional climate pattern [1]
The objectives of this study are: (1) to examine the temporal variations and characteristics of isotopic composition in different water sources during different rainfall types at event scale and to illuminate the reasons for the differences; (2) to calculate the hydrograph separation based on isotopes in precipitation and throughfall, and compare the differences of using constant isotopic values and temporal variations during the event; (3) to evaluate the soil water contribution of the two different rainfall types using a three-component hydrograph separation model; and (4) to explore the runoff generation mechanism between rainfall types based on the isotopic signatures and hydrograph sepa
We found that the difference was generally lower for flood peaks than for the whole average values for both rainfall types
Summary
The lower region of the Yangtze River Basin, as one of the most active economic regions of China, has experienced dramatic development over recent decades; high risks of social-economic damages have been raised by flood-induced disasters, which might occur regularly due to the regional climate pattern (i.e., heavy rainfall events occurring during the plum rain and typhoon seasons) [1]. For precise flood predictions and optimal water resource management, it is of highly importance to advance understanding of the regional hydrological cycle and catchment mechanistic responses to the typical plum rain and typhoon events. Stable water isotopes and their temporal variations have been widely integrated into discharge hydrograph separation, revealing event-scale water source portioning and its dynamics. It is much less intensively studied in the lower region of the Yangtze River Basin with regular plum rain and typhoon events. Existing IHS studies highlighted the importance of the old water (pre-event water) in the runoff generation [3,4], which contrasts to the traditional perception that “rainfall was considered to dominate storm hydrographs”. Many hydrologists have focused on five assumptions in the traditional two-component hydrograph separation [5,6,7] to improve the understanding of different dominant runoff components in different watersheds
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