Earth’s water distribution

A number of human activities can impact on the water cycle: damming rivers for hydroelectricity, using water for farming, deforestation and the burning of fossil fuels. When people use water for irrigation, they are taking water from streams or from the ground and, as a result, the water table drops. It can take a long time for groundwater to be recharged. The flow-on effect is a reduction in the amount of water in rivers or lakes, which then impacts on the environment – animals living in the water may find themselves affected.

Water that seeps through the soil carries particles of nutrients, and these particles will eventually reach the groundwater. Too many nutrients can pollute freshwater storages. Also, the removal of too many nutrients means farmers have to add more chemicals to their soils to keep the plants growing.

However, not all water that falls on the ground will seep into the soil. Some of the water will run off overland and transport soil particles with the water into streams and eventually into lakes. This can affect the stability of the land.

Deforestation or removal of trees changes water take-up and can lead, in extreme cases, to an increase in over-land run-off and erosion of land.

Points of interest Students could consider what farmers could do to reduce over-land run-off. Think about what David Hamilton means when he talks about how we take water for human consumption without considering the water needs for our lakes, streams and groundwater.

Transcript

NARRATOR Human activities can have an impact on the water cycle. In some cases, this may have localised and sometimes global effects. For example, some human actions that affect the water cycle are water use for electricity generation, irrigation for horticultural and agricultural land use, and deforestation

ASSOC PROF LOUIS SCHIPPER The way we manage our lands hugely alters the amount of water that is moving through our systems. What we are trying to do often in a productive system is provide enough water for plants to grow. And sometimes there is too much water, so you have poorly drained soils, so people will drain them to improve the production of plants and make it more trafficable so you can get tractors over it, and cows don't pug it up etc. When you have the opposite, such as when you've got soils that are too dry, that is when you would irrigate.

PROF DAVID HAMILTON In the drier areas of New Zealand such as Hawke’s Bay and Canterbury, there is such a demand for water to irrigate crops and irrigate productive land that it has actually created a water shortage. There are so many people taking water out of streams and out of the ground, and so, as a result, groundwater that sits underneath the soil is being slowly depleted, and that has the potential to create a very, very difficult situation in the future. And if we do deplete that groundwater resource, it will take many years for the groundwater resource to actually build up again. And that, of course, has implications for the inflows to the lake and how much water you can extract for other purposes elsewhere. But it also has implications on the environment, the streams that need to function, the fish, the invertebrates, the smaller animals that need water to be able to survive, particularly over summer in those drier areas. One of the things that we haven't considered when we consider how much water we need is that we often say we will take it for human consumption, but we don't say how much water do we actually need to maintain the health of the stream, or the health of a lake, or the groundwater at a steady level, and that is a very, very important consideration.

NARRATOR Watering our fields and crops not only impacts on the amount of water that is available, but also on the distribution of nutrients that move through the soil.

ASSOC PROF LOUIS SCHIPPER So as the water moves down and carries nutrients with it, the plants are trying to extract those nutrients so they can grow properly, and so there is sort of a battle going on here for the nutrients. And when you have nutrients in excess and you have too much water going down through the soil profile, you can lose those nutrients. So one of the things about the water cycle it that its not just the water that is moving. In soil, there is a myriad of different compounds and nutrients.

NARRATOR Water carries these nutrients along, reflecting the environment it passes through.

PROF DAVID HAMILTON So that means, where does the water run to? Well, it runs to the lowest point on the landscape. That lowest point is typically a lake, and so the quality of the water in a lake reflects the way in which we are looking after the landscape. If we look after it very well and we stop erosion and we prevent nutrients from running off in streams and into a lake, then we can obviously have a pristine or very high-quality lake. If, however, we create a lot of nutrients that run off into the lake and we don't manage our catchment very well, then the lake water quality will be poorer.

NARRATOR Deforestation can also impact on the quality of the water.

DR DAVE CAMPBELL Another way in which humans change the water cycle is by changing vegetation. Where we are sitting right now, 150 years ago would have been a forest. Now, it’s a dairy farm, and the way in which the plants using water, extracting it from the soil, and putting it out into the atmosphere is quite different than the ways trees would be using the water.

ASSOC PROF LOUIS SCHIPPER One of the things that is very important with regards to soil is if your rainfall exceeds the capacity of that soil to take that water up in the pores – its infiltration capacity – and if that is going to happen on hillsides, if you get too much, it will start to move down through that hill slope, and it will pick up particles as it moves down through the hill slope, and you will get erosion. And you will get sediments then being carried and deposited where you may not want them, covering grass, which will be a problem. You will lose some of your soil maybe up on your hills, which is a problem. And if you have very big events, you will end up with lots of sediments moving down through waterways and into rivers and that sort of thing.

Acknowledgements: Alexander Turnbull Library, Wellington, New Zealand Lamiot Lykaestria http://creativecommons.org/licenses/by/3.0/ Mathew Allan, University of Waikato Peter Hall Selina Larsen The New Zealand Biotechnology Hub DairyNZ