The evolutional states of seasonally losing unclogged stream-groundwater systems.

• Developing the stream-groundwater system evolutional states theoretical framework; • Refining four different system states; • Proposing critical conditions for different evolutional states; • Revealing the evolutional process of stream leakage flux; • Evaluating the impact of parameters on the critical moment of the system. Losing streams are widespread in arid and semi-arid regions. Exploring the evolutional states of losing stream-groundwater systems is of great significance to understand the exchanges of water, mass and energy in stream-aquifer systems. This study is devoted to investigate the evolutional process of infiltration flux, pressure head and flow states of a seasonally losing stream-groundwater system with the semi-permeable streambeds missing (or without clogging). The paper constructs a new theoretical framework for the evolutional states of the system. In particular, it focuses on investigating the transition among different evolutional states including disconnected state, transitional state, negative-pressure connected state and positive-pressure connected state, and the critical conditions of transition between connected and disconnected states. The critical conditions associated with disconnection or connection are that 1) the zero-flux zone starts to disappear, 2) the unsaturated zone starts to disappear, and 3) the negative pressure zone starts to disappear. Analytical solutions of those critical conditions are proposed. The evolutional process of stream leakage is characterized into two stages: the first stage corresponds to the disconnected and transitional states in which the seepage fluxes decrease with time; the second stage corresponds to the fully connected state in which the seepage flux decreases rapidly again as time developing. Finally, the influence of various controlling parameters on the critical conditions is analyzed. The critical time (t CRI 1) and critical depth (z CRI 1) of changing from the disconnected state to the transitional state have nonlinear correlations with saturated hydraulic conductivity (K s), pore-size distribution index (λ) and flux at the base of the unconfined aquifer (q). The critical time (t CRI 2) and the critical depth (z CRI 2) of changing from the transitional state to the negative-pressure connected state and the critical time (t CRI 3) of changing from the negative-pressure connected state to the positive-pressure connected state all have nonlinear correlations with K s and λ. z CRI 1 and z CRI 2 are independent of saturated water content (θ s) and residual water content (θ r). An innovative physical interpretation is given to system evolutional states and quantifiable criteria for those evolutional states are proposed. The findings of this study can help better understand the evolutional states of seasonally losing stream-groundwater systems, and it lays a foundation for further research on water exchange, mass transport and ecological protection in stream-groundwater systems. [ABSTRACT FROM AUTHOR]