1 2 Environmental controls on drainage behavior of an ephemeral 3 stream 4 Kyle

7 Abstract Streambed drainage was measured at the 8 cessation of 26 ephemeral streamflow events in Rillito 9 Creek, Tucson, Arizona from August 2000 to June 2002 10 using buried time domain reflectometry (TDR) probes. An 11 unusual drainage response was identified, which was char12 acterized by sharp drainage from saturation to near field 13 capacity at each depth with an increased delay between 14 depths. We simulated the drainage response using a variably 15 saturated numerical flow model representing a two-layer 16 system with a high permeability layer overlying a lower 17 permeability layer. Both the observed data and the numerical 18 simulation show a strong correlation between the drainage 19 velocity and the temperature of the stream water. A linear 20 combination of temperature and the no-flow period preced21 ing flow explained about 90% of the measured variations in 22 drainage velocity. Evaluation of this correlative relationship 23 with the one-dimensional numerical flow model showed that 24 the observed temperature fluctuations could not reproduce 25 the magnitude of variation in the observed drainage velocity. 26 Instead, the model results indicated that flow duration exerts 27 the most control on drainage velocity, with the drainage 28 velocity decreasing nonlinearly with increasing flow dura29 tion. These findings suggest flow duration is a primary 30 control of water availability for plant uptake in near surface 31 sediments of an ephemeral stream, an important finding for 32 estimating the ecological risk of natural or engineered 33 changes to streamflow patterns. Correlative analyses of soil 34 moisture data, although easy and widely used, can result in 35 erroneous conclusions of hydrologic cause—effect rela36 tionships, and demonstrating the need for joint physically37 based numerical modeling and data synthesis for hypothesis 38 testing to support quantitative risk analysis. 39 40