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W UMa-type contact binaries belong to close binary systems whose components exactly overflow their Roche lobes and share a common convective envelope (CCE). In the last twenty years, the long-term variations of their orbital periods have been thought to depend on several mechanisms. Now, we suggest a new mechanism: CCEdominated mechanism. The CCE-dominated mechanism is found based on our numerical result, especially at high mass ratios, that the orbital periods (P ) of contact binaries change very much with their fill-out factors (f ). Because f is taken as a measurement of the thickness of CCE, the physical cause for the variation of P is a mass transfer between CCE and components. Further, an f -dominated simplification model for this mechanism is introduced. According to it, P may change in a long-term oscillation way with a similar time scale of the thermal modulation, meanwhile q is decreasing slowly till the two components merge. It could be also applied to explain the presence of extremely short period, high mass ratio and deep contact binaries. Moreover, the CCE-dominated mechanism should always work due to mass transfer and mass loss both occurring via CCE. Therefor, the effect of CCE on the variations of orbital periods may have been underestimated before.

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The young water fraction (Fyw) can be reliably determined in heterogeneous river basins, and the seasonal origin index (SOI) can be used to determine the relative contributions of winter versus summer precipitation to runoff. Fywand SOI are also important descriptors of how a basin controls runoff generation and the transport of contaminants and nutrients. In this study, high‐frequency sampling of precipitation, river water, creek water, and shallow groundwater was conducted in the Xiangjiang River basin, in south‐central China, from January 2010 to December 2012. The samples were subjected to stable isotope measurements to determine the SOI and Fywof surface water and shallow groundwater. The principal findings are: (1) The δ2H variations of the river water, creek water, and shallow groundwater were driven by the input of precipitation but were attenuated during runoff confluence processes. Also, the low lc‐excess values of the river water indicated a strong evaporative effect. (2) 21.4% and 19.7% of the river water and shallow groundwater were younger than 1.5 and 1.7 months, respectively, indicating a quicker renewal cycle than for the creek water, which had a fraction of 8.0% younger than 2.6 months, which originated from the adjacent mountains. (3) Winter precipitation was more prevalent in the river water and creek water, indicating the substantial inter‐seasonal carryover of precipitation that supplied the runoff. The average SOI value of the shallow groundwater was near zero, indicating the mixing of the precipitation in different seasons that supplies the shallow groundwater. Our findings emphasize the importance of vegetation and soil conservation in maintaining the inter‐seasonal carryover of water storage held by soil, plants, and rock fractures, and the necessity to assess the risks of streamflow contamination because of the rapid renewal cycle of river water in this flat and intensively cultivated landscape.