In this article I try to answer the questions why there is water flowing in rivers, even in dry summers and what all this has to do with my own research.
As far as everybody knows, there is more or less always water in our rivers (at least in our areas/ or the area I do my research in, the Raab valley in South-East Styria, Austria). So let’s follow the way of a rain drop, or better, many rain drops. The rain falls to the ground and since there is space and air in the soil the rain drops infiltrate into the soil matrix. When more and more rain is falling, more and more water is infiltrating into the underground. Probably there will be some earthworm tunnels, which help the water to infiltrate even faster (macropore flow). Some of the rain water will flow through the soil into the river (interflow). But when it is raining cats and dogs and not all the water is able to infiltrate (fast enough) into the ground, there will be overland flow which takes all the rain drops directly into the river. So from the overland flow the river gets this typical brown and muddy color (Picture 2). That is because a lot of material, like dust and soil is also transported into the river by the fast flowing water.
But we know that there is also water flowing in the river when there is no rain at all… To see where this water is coming from, we need to follow the rain drops through the soil (unsaturated zone) and even further to the groundwater (saturated zone). So when the rain infiltrates into the soil and further to the underground it fills all the empty pores and gaps. A bit like if you drop water on a sponge. The sponge can hold a lot of water, but when there is too much of it, it starts to leak. But where does the leakage water go? Back to the rain drops which are on their way through the underground. After a while it might not be possible to go further down by gravitation, because there is e.g. a clay layer (impermeable layer). Most of the water will be blocked by this layer and more and more water will be stored above this layer, in other words, the groundwater level rises. So this water flows slowly and after a while finds its way out, often at the side of the hills, and forms a spring, which feeds the river continuously. The groundwater part of the river is called base flow. So the base flow comes from all these little springs, which are feeding the river especially in summer time. When it gets dryer and dryer, fewer springs are bringing water to the river, so the water level goes down. But the storage capacity in the underground is so huge and the water is flowing slowly through the underground, there is usually some water to feed the rivers on their way to the ocean.
But will it always be like this? And why I am explaining all this? You might think I should do my research in natural science and not only think about what the single rain drop is doing in its life. But all these processes of the flowing water are exactly the things I’m working on at the moment. I use the hydrological model WaSiM (www.wasim.ch), which includes all this different flow types and processes. There is a unsaturated zone/soil module which includes marcopore flow through the earthworm tunnels, and a saturated zone module with the base flow. There is a calculation of overland flow, inter flow, base flow and groundwater recharge. All these processes are included in the hydrological model which I setup for the Raab catchment. To read more about what I mean with a catchment, see my blog article. But why is it important to include all these complex processes in a model? The simulations with the model will help me to understand how the catchment reacts and which processes are most important for specific parts of the catchment. When I have an idea about the dominant processes, I can also change some boundaries/parameter/inputs and analyze the changes. For example, if the storage capacity is huge enough to feed the river all summer long even though there might be less rain and warmer temperatures. Hopefully, I can answer questions like this when I have a running and calibrated WaSiM model setup for the Raab catchment in Styria.