RESULTS

Groundwater elevation maps constructed for 0600 on April 20, 1997 and 0700 on June 6, 1997 with data from the shallowest piezometers are shown on the map to the left, below. While the pattern of flow was similar during both synoptic periods, with strongly converging flow toward the stream at the upstream end and weakly converging flow a downstream, the down valley gradient was about 15% lower in June.

Piezometric contours constructed for the deeper piezometers in each cluster are shown on the map to the right, below. The shapes of the contours are similar for both synoptic periods, but have a distinctly different character than the shallower piezometers in the southern part of the area. There are two possible causes for this difference, both related to the heads in the underlying regional aquifer. Pool, (1997) reported the gradients in the underlying regional aquifer were to the west, and that may be influencing the heads in the deeper parts of the floodplain aquifer. The other possible cause is that the WSF piezometer is open to the floodplain aquifer only a few centimeters above the regional aquifer, and its water level may be being influenced more strongly than the other deep piezometers by heads in the regional aquifer.

In both figures, the April contours are solid, and the June contours are dashed. It is possible that a survey error at the ESC cluster’s shallow piezometer may be contributing to the strongly converging flow patterns at the southern end of the study reach. Within the floodplain aquifer the gradients were upward in most of the area, showing that water was moving into the floodplain aquifer from the regional aquifer in the study reach. The figure below shows that the April gradients were greater.

The probable reason for the lower gradients in June is that the roots of the Phreatophytes were extracting water from zones between the open intervals of the piezometers. In April, before the phreatophytes were operating at “full speed”, the vertical gradients had to be higher to elevate the water table in order to deliver the water entering from the underlying regional aquifer laterally to the stream.

On the graph to the right, water levels over three of the synoptic periods are shown. The April trace shows some diurnal fluctuations as the phreatophytes were beginning to “leaf-out” at that time. August and June show stronger diurnal fluctuations, with the August trace also showing the residual effects of some monsoonal precipitation before the start of the synoptic period. The effect is most noticeable at the start.

Until the problems in analyzing the varying dye injection rate can be resolved, it will not be possible to confidently develop an estimate of the streamflow itself. However, by analyzing the relative changes in dye concentration between the sampling points and combining this information with stream flow measurements made by other means, it is possible to estimate the gains in streamflow between cross-sections. First, a Fourier series model was used to fit curves to the dilution gaging data at each cross-section. The time of travel between sections was estimated by comparing similar points on each curve. A mean stream velocity, assumed constant through the study reach, was determined by minimizing the difference between the estimated gain in discharge between the first and the last cross-section and the cumulative gain computed between consecutive sections. The relative gain in discharge (Q) between two cross-sections was found by comparing the dye concentrations at those cross-sections using a simple mass balance formula:

As can be seen in the two figures above, the discharges computed by the dye dilution calculations show a consistent pattern of gain in both April and June. The higher rate of streamflow in April makes the gains less apparent on that graph than it is on the graph for the June data. It is interesting to note that the amounts of baseflow contribution are of the same order of magnitude in both periods.