Water Work

It has been a busy week ... with a lot of walking. Our ongoing project at the Cienega involves measuring and recording well and pond water levels to provide long-term data about the water level in and under the area. This Thursday we took our usual walk to collect water level data and added a new site to measure, Honnas Pond. Much of the surface water in the Cienega flows through Arivaca Creek. Honnas Pond is a wide spot in the creek which holds water much of the year.

There had been an active USGS water-level gauge at a small dam (called a weir by the USGS) just above the pond. They had been monitoring water flow in the creek electronically for years using satellite telemetry, but the funding dried up (even if the creek didn't) and the electronic equipment was removed. As part of the hydrology project, this would have been useful data to have since water from the Cienega exits via the creek.

Richard Conway, the retired geologist who has been leading the project, acquired a set of digital logging devices called Leveloggers. They are sealed, waterproof, digital pressure recorders that measure water depth and can record 40,000 entries (about 4.5 years at an entry per hour). They are usually placed in wells, but we will use them to record water levels in the creek. The water level can then be used to compute stream flow. The device is particularly useful for measuring during flood events assuming it doesn't wash away.

The USGS has used Leveloggers to record stream flow in one of the canyons east of us and Richard wants to try that here. It is done by embedding a long concrete and steel tube as an anchor in the stream bed and bolting on a pipe containing the logger. When the rains come and water flows, the logger can record the flow over time. Once the water has receded (which is what happens here as most streams are dry for much of the year), the logger can be retrieved and the data downloaded.

We have a small problem with this approach at the Honnas Pond gauge ... there is still water in the creek. We cannot install the anchor while there is water. It looks like dong that project will not happen while we are here since water levels have been rising for the entire period we have been collecting them.

As an alternative, we are beginning to record flow data weekly using a portable meter that is placed in the water. It works much like a boat speed transducer. It is a pole with a tube containing a propeller at the bottom. It has a digital display at the top. You place the prop in the water at whatever depth you want (there is a scale on the pole) and read out the flow velocity in feet per second.

We are using it to record the flow at the weir, where the water pours over the dam. The dam is basically a concrete slab between two rock walls with a box shaped opening. Attached to the opening is a fixed steel plate with a 120° notch at the top. We are recording the speed where the water enters the box and also at the notch where is pours over the dam (if it is deep enough to submerge the meter probe).

By measuring the dimensions of the box and the notch and given the water depth, we can compute the area through which the water is flowing and given the speed from the meter, we can compute the flow volume. When we did that, the results were very close for both measuring points which was good. We did our second set of measurements this week and they are very close to the first set.

When the USGS was monitoring the water here, they developed a Rating Table. This is a table that gives the flow volume based on the gauge height. There is still a physical gauge (a mounted scale in the water showing depth) mounted on the weir and also next to it. Unfortunately, the flow volume given in the table isn't even close to what we computed. This is a quandary to be further pondered.

Meanwhile, we will keep collecting data for as long as we are here.

This brings up another concern for the project ... who will step in and continue collecting the data after we've left. Richard started the project and is still interested, but wants to step back. Ann, the refuge biologist, is also interested in the continuation of data collection and is probably the ultimate fallback person to do the data collection.

We have, however, interested Rob and Reta, the very active local volunteers, in taking over this task when we leave. They pass the Cienega every week on their way to the refuge. We took them along as we did the measurements this week and they seem to be agreeable to doing it after we leave. It doesn't hurt that they are avid birders and birding can be done while data is collected. Did I mention that the Cienega is one of the best birding spots on the refuge?

It still remains a bit of a question as to any updating and interpretation of the data. I have done some work in creating a better spreadsheet database including some analytical tools, but not as much as I would have liked. I need to at least write up directions on the method being used and a description of the data. Time here grows short.

Photos