Water abstraction and agriculture
How do agricultural activities potentially influence water flows?
Abstraction or diversion of water using ditches, pipes, and streams for irrigation is a popular method of turning unproductive and/or dry areas of land into productive agricultural enterprises. Large areas of land, such as in parts of the Canterbury Plains, which were used primarily for sheep grazing, have now been converted to large scale intensive agricultural operations, the dominant one of these being dairy.
When natural seasonal flow variations are interrupted by taking water from streams, rivers, and lakes, extreme care must be taken to maintain the amount of water needed to support healthy ecosystems. The amount of water needed is called environmental flow, which considers maximum and minimum flow levels to support a healthy ecosystem. Failure to provide an environmental flow can have serious consequences for water quality and mahinga kai.
Taking water for agriculture or industry reduces the natural flow of a river or stream, or reduces water levels in a lake. This impacts on mahinga kai by changing and reducing available habitat and food supply. The taking of water for irrigation increases farm productivity but also reduces water flow and has the added potential to increase both point and non-point sources of pollution into surface waters.
Applying water (irrigation) to land to improve production can result in the following impacts:
- Increased stocking rates can result in more nutrients (faeces and urine) deposited on the land by high stock numbers. Stock trampling and pugging of soils, especially around unfenced streams and rivers, will increase erosion and affect water quality and clarity (see riparian vegetation).
- Changes in the type of stock that can be carried by the land, i.e., allow dairy farming to occur in formerly dry stock land with potential effects on erosion or water quality.
- Increases in effluent discharge - some irrigation practices can produce significant surface water runoff and increase the contaminant loads that reach streams and rivers.
- Increases in nutrients reaching waterways from higher fertiliser application rates.
Impacts of water take (abstraction) on water quality and mahinga kai can include:
- Changes in flow - changes in water levels and flow variability alters available mahinga kai habitat and the invertebrates they feed on.
- Reduction of habitat - a decrease in water levels reduces habitat for fish and can impact feeding and spawning success.
- Reduction of specialist habitats - a decrease in water levels reduces flow to riparian wetlands, backwaters, and intermittent streams.
- Decreases in species abundance and diversity - aquatic species have developed life history strategies in direct response to natural flows, for example, diadromous fish species migrate up and down the river at various times of the year and rely on preferred velocities and depths.
- Changes in sediment accumulation - flow reduction affects movement and deposition of sediments in streams and rivers.
- Changes in water quality parameters - for example, turbidity and temperature levels can increase with reduced flows in rivers.
- Increases in algae accumulation - algae respond to changes in temperature and nutrients, which are likely to increase with reduction of flow, especially during summer months.