Posted on June 26, 2013 by James Douglas
Today I rolled over after a night of extremely strange dreams to see two professor’s cars and one of the group minivans in the lot outside the barn. Thinking I had timed my morning perfectly I checked my phone to receive a rude awakening- it was 10 am! Somehow I had slept through an alarm and all of the morning happenings and missed the cars going out to the field. Luckily, I was able to get a ride from Emily who stayed back at the barn to help those who were analyzing data (because people in the field had forgotten things). Eventually, I was able to make it out to Ka’a’awa valley and get to work measuring a stream cross section and acquiring velocity measurements. I walked with Julianna to one of the locations in the stream with a defined channel to maximize the accuracy of the readings I took. Most of the stream channel throughout the valley is either too flat, or too spread out across a wide channel to get meaningful data. Therefore, this was basically the only place in the valley to get a meaningful sample of stream discharge.
Unfortunately, over the past day or so, heavy rains in the valley had finally managed to bring the streamflow up, meaning that our values would not accurately reflect stream baseflow, which was the entire goal of collecting discharge measurements in the first place. Regardless, we got down to business and started measuring depths and velocities at regular intervals to get what we could get on likely our last field day of the REU. Given that we had been able to take a velocity/depth reading at the same location (at the Thalweg point) at baseflow conditions, a reasonable estimation of baseflow could be estimated by normalizing between the differences in depth and velocity between the previous measurement and today’s. After this was completed, I ventured to the Western extent of the watershed to see how the flow was varying in the tributaries. It was interesting to note the changes in the surface hydrology today given that there had been no discernible differences unto this point despite the occurrence of periodic rain events throughout the study period. This made me think about the importance of vegetation distribution (both species wise and density) to the calculations of PET in the water balance model that I was employing, and how vegetation had been completely left out. It seemed naiive to me that one could ignore the factors of interception and plant moisture demand on water flow patterns in a system as small as the Ka’a’awa watershed when calculating water balance. It is certainly something that I will have to mention both in my final presentation and write up.
On a side note, it was interesting to observe that even with the increase in baseflow, the strange pattern of disappearance and reappearance of flow in the main stream channel was still apparent. Although the water was deeper in the upper section of stream, the water still ran underground at the geologic contact between the old and new alluvium, and reappeared in pretty much the same location along the road West of basecamp. I think that Dr. Becker’s hypothesis of a geology driven disappearance of flow and re-emergence correlated with high density sediment/geology is sound, however it was curious to note that the flow was not significantly higher in the lower reach of stream post-re-emergence. This seems to suggest to me that there is a relatively long residence time in this period of groundwater flow along the main channel, otherwise I would have noticed increased flows in the lower stream reach.
After returning from the field, I finally had access to the 5m DEM that I had been waiting for (still waiting on more accurate precipitation data that I know already exists!), and decided to run the toolset again to compare the differences. I also ran a flow accumulation model for the watershed with the updated DEM to model the drainage area that contributed to the location I sampled discharge from, so that I could compare expected and observed surface flow readings. Basically I just had time to run the toolset before I got too tired, so the analysis and comparison will have to wait until tomorrow.