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1. The land that a river drains is called the catchment, and a river of this size could collect the rain that falls on an area of several hundred square kilometres. 2. The fate of each drop of rain depends on where and when it lands. Some of the rain evaporates back into the atmosphere from the surface of plants. Water hitting the ground can also evaporate, as well as soak into the soil. Once in the soil it can be used by plants before being passed back into the atmosphere. 3. The amount of rainfall that becomes run-off depends on how much rain there is and when during the year it falls. It also depends on the type of land on which it falls. Ploughing, for example, will reduce run-off by holding-up the water, allowing it to soak into the ground. 4. Run-off is one way for water to reach the river but if this was the only way then rivers would dry-up when the rain stopped. This doesn’t happen, so there must be some other source of water. To understand we need to follow the path of raindrops which reach the ground surface and pass into the soil. 5. What happens when water hits rock depends on the nature of the rock. If it’s made up of grains, like this sandstone, with spaces in between which are connected, then water will pass through and the rock is said to be permeable. 6. It’s not just sandstones that are permeable. Chalk, made up of the remains of countless tiny shells is also permeable mainly because there are cracks in the rock. 7. The percentage of the rock that is made up of spaces is known as the rock’s porosity. The porosity of sandstone can be 20% or more, which means that a cubic metre of rock say the size of a large fridge, can absorb 200 or more litres of water. 8. Water, that has passed through the soil will continue to move down through the underlying rock until it reaches a level where all the pore spaces are full of water. This level is called the water table. Water moving in the rock, below the water-table is called groundwater. Layers of rocks that are permeable and allow a significant amount of water to flow through them are called aquifers. 9. These are the principal aquifers of the UK. Chalk and particular sandstones, make up the 2 most important aquifers. About a third of Britain’s public water supplies come from aquifers, but in some regions of the country it’s as much as three quarters. 10. The shape of the water-table follows that of the ground surface. Groundwater flows from areas where the water-table is high, to areas where it’s low. When the water table meets the ground surface, water leaves the rock, either as a spring or by seeping directly into rivers. The time the groundwater takes to travel to the river will depend on how permeable the rock is and on whether it follows a shallow or a deep path. Either way it can be in the ground for a long time, from months, up to centuries in some cases. 11. The reason the stream dries-up is because the water-table that feeds it with groundwater falls below the bed of the river during the summer. As it drops, the point at which it cuts the ground surface moves down the valley. Where slopes are gentle, that might be kilometres away from the source of the stream. 12. It’s not surprising that if millions of litres of water are being extracted daily from this borehole, that the pumping will have an effect on the water-table. The greatest drawdown in the water-table occurs around the borehole but the effects may be seen kilometres away. It is possible that the fall in the water-table can reduce the amount of groundwater that feeds into nearby rivers and streams so reducing their flow. 13. However, it’s often difficult to say how much the reduced flows that occur at times in some rivers are due to pumping and how much they are due to natural changes in the water-table. Where it is recognised that groundwater pumping is having a bad effect on particularly important stretches of river, water companies are working together with the environment regulators to find ways of improving the flow. 14. These are only a few examples but they highlight the challenges that we face to meet the ever-increasing demands of society while keeping the impacts on the environment to an acceptable level. This is particularly important given the impact that climate change is likely to have on the country’s water resources.