What is a catchment?

Waikato Regional Council water quality specialists Alice Trevelyan and Dr Eloise Ryan describe some of the characteristics of water catchments.

Question for discussion:

• Can you locate a stream or river near you and trace the path from its headwaters to where it discharges?

Transcript

ALICE TREVELYAN

A catchment is shaped by topography. So imagine a basin, and it’s everything that falls within that basin, that steep sides that flows towards that water body. But it’s not just necessarily rainfall. A catchment will have groundwater and surface water sources.

DR ELOISE RYAN

Groundwater is the water that exists underground in the spaces of soils and rocks. It’s much like water fills a sponge, it’s the water that exists in the pores. And some groundwater is pumped above ground for irrigation or drinking water. Everything that happens on the ground can impact the groundwater.

ALICE TREVELYAN

The easiest way to define a catchment is an area where any rainfall in that area all flows into a water source, so whether that be a river, a stream, a lake or a wetland. So catchments can be really variable in size. They can be massive, for example, the Waikato River catchment, or they can be really small, for example, a small wetland on someone’s property. There’ll be hundreds or thousands of catchments across the Waikato region.

DR ELOISE RYAN

To map catchments, you can start at the mouth where your stream discharges and work your way back up by looking on a map. And catchments tend to be bounded by things like mountain ridges or forests. So you can actually look on a map and trace your streams from the mouth up to the source. So a headwater is the source of the stream. That is where the stream starts from and then the water flows down the river and ends up in the lowlands – and that’s low-elevation waterways, and those waters tend to be warmer, slow flowing and more full of nutrients or sediment because they’ve flown quite a long distance through the land, whereas headwaters at the source tend to be more clearer, cooler and faster flowing.

ALICE TREVELYAN

Catchments are really important for water quality. Every activity that’s happening in that area – whether it be related to water, soil or air – is going to impact upon the water quality further downstream, whether that’s good or bad.

Acknowledgements
Alice Trevelyan
Dr Eloise Ryan
Waikato Regional Council
Catchment diagram by Phil Jones, New Zealand Landcare Trust
Stream footage: Dairy NZ
Healthy Farms Healthy Rivers and Waikato Regional Council