RESIDENCE TIME DISTRIBUTIONS AND DYNAMICALLY CHANGING HYDROLOGIC SYSTEMS: EXPLORING TRANSIENT HYPORHEIC FLOW SYSTEMS

Traditionally, observed and modeled residence time distributions (RTDs) of hydrologic systems are evaluated as if the flow is in steady state, even when dynamic flow conditions, induced by spatial and temporal variability of weather and climatic forcings, are present. Regional groundwater systems, watersheds, and stream hyporheic zones are examples of hydrologic systems driven by forcings varying at several time scales, such as daily, seasonal, interannual, decadal and longer. In transient systems, flow paths and residence times can change dynamically. In this case, RTDs are a function of the observation/modeling time and the time of release into the system. A finite element scheme is used to explore this issue by modeling transient flow and the transport of an ideal tracer into a multi-scaled flow domain, illustrating the effect of dynamically changing systems on residence-time estimation. Taking advantage of the linearity of the advection dispersion equation (ADE) operator, the residence time distribution for a transient, harmonic system is estimated by analyzing the evolution of the impulse response function for several release times of the tracer into the system, and for different observation strategies. Results show that the steady-state assumption can lead to misinterpretation of natural variability of RTDs, inherited from the dynamic forcing, as data noise. Moreover, dynamic conditions can disguise or break the power-law behavior observed under steady-state flow.