Streamflow timing and estimation of infiltration rates in an ephemeral stream channel using variably saturated heat and fluid transport methods

摘要

Ephemeral streamflow infiltration through alluvial channels has been identified as an important source of aquifer replenishment in arid and semi-arid environments. In this dissertation, two field methods were developed for monitoring streamflow timing in ephemeral stream channels. The first streamflow timing method exploits differences in the advective and conductive thermal transport mechanisms during the presence and absence of streamflow. The second method of streamflow timing utilized the relationship between soil water content and electrical conductance. Electrical resistance sensors were designed to detect saturated soil conditions and thus to infer streamflow timing during periods of saturation. Both methods were field-tested in Rillito Creek, Tucson, Arizona. The electrical resistance method proved more suitable than the temperature method because it was not depth dependent and was able to more accurately infer streamflow timing with less data post processing. Transient and steady state infiltration fluxes were simulated in a coarse-grained alluvial channel to determine the relative contribution the onset of streamflow provides to potential recharge. Water content, temperature, and pore pressure measurements were incorporated into a variably saturated heat and fluid transport model to simulate infiltration. Infiltration fluxes at the onset of streamflow were about 2-3 orders of magnitude higher than steady state fluxes and were inversely proportional to antecedent water content. The time duration from the onset of streamflow to steady-state infiltration ranged from 1.8 to 20 hours. Two transient and steady state periods were observed indicating a lower permeable layer at depth. During steady state periods, infiltration fluxes averaged 0.33 meters per day and ranged from 0.14 to 0.45 meters per day. A long-term decline was observed in all three events. Higher frequency diurnal and episodic changes were prompted by fluctuations in atmospheric temperature and discharge. The simulated steady state values were consistent with the effective vertical conductivity values (0.22 meters per day) of an underlying less permeable layer. The average contribution from the cumulative transient infiltration for the events was approximately 18 percent. Therefore, it is apparent that potential recharge calculations for alluvial channels that do not consider infiltration during the onset transient period may underestimate the true potential for recharge.