In this blog post I want to give an overview how hydrologists define their study regions as catchments, and to give an example of my one research area, the Raab valley. The Raab catchment is located in south east of Styria, Austria. For hydrologist this would be a typical first sentence for a study region description of a river of interest. So when we, hydrologists, talk about a specific research area, we normally talk about catchments. A catchment is that area where all the water flows into one river. When a rain drop falls on to the ground it can for example start its way through the soil into the river and will be transported to the edge, the pour point, out of the catchment into another, bigger river or into the sea. The border of the catchment (water divide) is the part where the water droplet might ‘decide’ if it is a bit more on one side or the other. Like walking on a steep ridge, if you struggle you might fall to one side or the other. This is the water droplet definition for the surface catchment. But nature is not always what you see from above. So the underground catchment might look different… The bedrock under the soil layer might direct the water in a different direction.
To give you some numbers, my research area, the Raab catchment drains an area of 986 km². So as the water droplet example shows (where/how long does the droplet flow) it is also important to mention until what point the catchment is meant. If there is not a specific point mentioned it is always the ‘last possible point’. That is why the Raab has a total catchment area of 1020 km², but I only follow it until the gauging station Neumarkt. After that, it flows through Hungary into the Danube.
Beside the size of the catchment it is also interesting how much water is flowing in the river. Here it is again necessary to focus on one specific point, in practice a gauging station, which often marks the end of the analyzed catchment. In my area of interest the gauging station Neumarkt is the last station, so it is used for many descriptions and analysis. The long annual mean runoff (1991-2013) of Neumarkt/ Raab is around 7 m³/s, so around the amount of water which fits into 48 bathtubs is flowing every second down the river (assuming a volume of 150 L/per bathtub). But of course there are also extreme values. In the period of 1991 until 2013 the minimum measured flow was 0 m³/s (04.05.2007). And the highest flow of 244 m³/s was measured at the 25.06.2009, which are over 1600 bathtubs per second. This show that the early summer could be pretty dry (May 2007) or there might be a flood like in June 2009. That is why it is important to analyze long time periods if you are interested in the hydrological behavior of a catchment. The gauging station Neumarkt provides data since 1991 which should be sufficient for most of the studies. If longer time periods are needed, the gauging station Feldbach/ Raab (catchment area of 689 km²) provides data since 1951, which is really a pretty long time series.
To conclude, hydrologists shape the area in the same way the water flows and they are interested in how much water normally flows in the river, as well as how the extremes look.