 Hello! This final lecture is about groundwater. Groundwater is a very important source of water. This is because it is relatively reliable. A lot of groundwater bodies have a long residence time. It is often available locally and therefore it is distributed. It has a widespread occurrence and it is not affected by evaporation loss if deep enough. This is a picture of a simple groundwater system. We have groundwater recharged due to deep percolation from the hydrological cycle that feeds the body that can contain the groundwater, the aquifer. And the aquifer has, as it were, a little hole from which the base flow is constantly released. The groundwater is connected to the surface water through the base flow somewhere downstream. If the groundwater is not connected to the river water, we talk about fossil water, which is not part of this lecture series. Humans use groundwater and it is said that if they consume and pump groundwater at a rate that is smaller than the recharge rate, then we think that we are in the sustainable zone. We can abstract groundwater at a certain rate, which is less than the recharge rate. Question. What is the impact if groundwater is indeed abstracted as in this case? Let's say at half the amount that the groundwater is recharged. Would it affect the base flow? Yes, of course it would. Because we have the law of the conservation of Mars. If we take water out of the groundwater by pumping, it cannot become base flow water in the rivers. So groundwater abstraction will always have an impact on the base flow water in the river. What if groundwater is abstracted at a larger rate than the groundwater recharge rate? What will be the impact? The impact would be, of course, that slowly the groundwater body is mined and slowly the groundwater will be depleted as well as the entire base flow in the river. This is a situation that happens in several parts of the world where consistently the amount of groundwater pumped from the groundwater is higher than the natural recharge rate. Eventually this will have devastating impacts. Some challenges with respect to groundwater. The use of groundwater is generally an energy intensive and therefore expensive because of the large pumping cost involved. Groundwater may naturally have chemical elements that may be harmful to humans, for example arsenic and fluoride. Groundwater is also sensitive to pollution and it may take a long time once the groundwater body is polluted that it is clean again. This again is related to the residence time, the large residence time of many groundwater bodies. They may have residence time of 10 years, 100 years or even more. Therefore groundwater exploitation has often long lasting impacts. If we over abstract now, it may take time before the groundwater body is recovered from the exploitation. Also, and we have seen that in many places of the world, including for instance in Mexico City or Jakarta, that groundwater exploitation may cause land subsidence and or salinization. Furthermore, it is challenging to manage groundwater because of its invisibility. Do we actually know how large the groundwater body is and how much the recharge is and how much the various different users who have all kinds of pumping stations spread all over, how much is being used. So this also causes a collective action challenge because we have to manage the resource as a collective. But then if there is no joint and collaborative efforts and understanding, we may over exploit the resource. And just a warning, be careful with the notion of safe yield. The safe yield of a groundwater body is the amount that you can abstract from the groundwater, but it will always have an impact on the area downstream of that groundwater body. Now let's go back to our simplified groundwater system. We have an aquifer and there is somewhere a little hole in the aquifer. And slowly the groundwater seeps back as base flow water into our rivers. It becomes surface water again. In the beginning, if the aquifer is full, the base flow will be a bit high. But if the base flow is a bit high, the aquifer will quickly empty because of the pressure. But if the aquifer is getting emptier, the pressure in the vessel is less and the base flow decreases and further decreases and further decreases. As a result, we see often that the depletion, the outflow of a groundwater body has an exponential function, a depletion function, which can be expressed as such. The E is the number 2.71, which is an important number in nature. And the T is the time, which is expressed here in the horizontal axis. And the K is the characteristic factor for groundwater body. This can be expressed like this. But I don't want to scare you with these exponential functions. But you can see it in the hydrograph. This is a random river. This is the time on the horizontal axis. The vertical axis is the runoff. And these are hydrological years, year 1, year 2, year 3, year 4, etc. So you have a rainy season when the flow in the river is high and you have a dry season when the flow in the river is low. And that repeats every year. And some years are more wet and some years are more dry. Years are devastating throughout the year. What is interesting is the shape of the depletion curve of this river. It has more or less the shape of this. And they look alike. And this shape is related to actually this function. So the base flow depletion rate in the river during the dry season seems to follow a very nice exponential function. And we can simulate this also in an exercise. Let's do an exercise. You simulate groundwater seepage flow in a very simplistic way. In this exercise we simulate the flow of water out of an aquifer back to the river known as seepage flow or base flow simply by using a leaking bucket with water in it. See this video and replay as many times as needed. Hope you liked the video. Make a table of the volume of water stored over time. This on the horizontal axis the time and the vertical axis the volume of water stored. Note the time at each centimeter or half centimeter decrease of the water level. Make a graph for instance in Excel. What do you see? Find a trend line that best fits your data points. Does this conform to the theory? Success.