Welcome to this lecture on streamflow measurements. In this lecture, we'll describe three different ways of measuring streamflow, using a stick, using a current meter, and using a tracer. In addition, we'll talk about how we obtain data series of streamflow in a particular location. First, we need to determine the area of the stream, this is the cross-sectional area. We determine it by multiplying the width of the stream and the depth of the stream. For the stick method, we throw a little stick or some leaves or something else that's small, biodegradable and floats into the river and then measure how long it takes for this stick to travel a certain distance. So if it takes 10 seconds for the stick to travel 5 meters the velocity is 0.5 meters per second. However, there's a very large variation in velocity across the stream. Usually, the stream flows fastest near the surface and very slow near the bottom and the stream banks because of friction. Therefore, in order to determine the streamflow using our stick method, we multiply the area by the velocity that we measured and a correction factor. Often, this correction factor is 0.85. For the current meter method, we take this variation and velocity into account. We divide our stream into different panels that each have a certain depth and a certain width and then measure the velocity in each panel using a current meter. A current meter is a propeller that measures the velocity that can be used in small rivers and in very large rivers. Here, we see a current meter that's used to measure the streamflow in the Limmat. In the tracer method, we inject a tracer, such as a colored dye or a salt solution, into the river and then measure the concentration further downstream where it's fully mixed. When there is more water in the river, the tracer gets more diluted and the concentrations are lower. So based on the concentrations, we can determine the streamflow. Here, we see the release of a color dye tracer in the river that flows from the Damma Glacier. You see that the dye tracer mixes with water. At some point it's fully mixed in the river, this is where we measure the concentrations. And then this dye passes by and the concentrations go back to normal in background conditions. These measurements give us the streamflow at certain time and certain location. However, very often, we're more interested in how streamflow changes, for example, in response to a rain fall event or snow melt event. Therefore, we need to measure streamflow at a particular station or a particular stream over time. Here, we see the streamflow measurement station at the Glatt. What's done here is that the stream level is measured, and we do this continuously, so that we get a time series of stream level, or stage, over time. We can measure the stream level at the staff gauge, which is a large ruler as shown here in the right picture. Or we can measure the water level continuously with the water level logger. So it's just a simple clock and pen recorder or a pressure transducer. In addition, during the year, at different water levels, we go out and measure the streamflow, for example, using our current meter method or our tracer method. And by doing that we can obtain a stage-discharge relation. This is the relation between the water level, the stage, and the streamflow. We can then combine these two, our time series of our water levels and our stage discharge relation, to obtain the time series of the streamflow. Currently, the Federal Office of the Environment, the BAFU, measures streamflow at more than 200 locations across Switzerland. They measure the streamflow for catchments that are as small as 0.5 kilometer square, to catchments that are as large as 35,000 kilometer square. You may want to go out and find the nearest station near your house. If you're interested in making your own streamflow measurements, you can join the CrowdWater community. You can download the CrowdWater app, make your measurements, and share your data with other people. We look forward to seeing your streamflow measurements.
