Groundwater recharge through irrigation losses in New Zealand

A guest blog by Nigel Cowburn. Nigel is a landscape architect based in New Zealand. 


 

I recently attended the New Zealand Irrigation Conference. One topic that came up often was aquifer recharge.

NZ is undergoing a rapid shift from government funded irrigation to private sector schemes. The government-led approach resulted in over-allocation of many aquifers and rivers – low-flows and polluted waterways are increasingly common. But now water is being planned by local water catchment management schemes. Being local their aims include sustaining the water resource for the long-term. Aquifer recharge is a common theme.

Catchment management groups comprise farmers, growers, local politicians, Maori (NZ’s native people), grass-roots organizers, consultants and interested citizens and other water users including factories. Maori have a special place in the process as NZ law embodies protection of water with tight controls on water piped beyond the catchment boundary as this is viewed as weakening the life-force of the river.

This view helps support a robust catchment-based management model as a more self-contained and cyclic system; as water is the lifeblood of society and the real catchment is where we often place wastes it is simpler if we can close the water process loops on a river catchment basis.

Until recently all irrigation was border dyke (flood irrigation) based. In these systems water is moved in open canals (with 20% lost to seepage and evaporation) to fields. The land is regraded as parallel dyked strips with a water gate on the ends enabling water to be let in, trapped to allow infiltration, and then released to the canal again. While border dyke (BD) is very inefficient – about 25% of water gets to the root zone. As originally set up a fixed amount of water was allocated to an area for a number of years.

Border dyke irrigation – bottom of page for image

Pivot irrigation is now becoming the norm with piped networks eliminating losses. Water distribution efficiency is about three-and-a-half times the border dyke system. While there is some expansion of farming with more efficient water use, the original water allocations still exist which is where aquifer recharge comes in – recharge is based on constructing large basins with leaky bases sited above aquifers. Clean water is piped to the basin and the aquifer is recharged as an example . The key outcome of this project was groundwater Nitrate falling from 4g/m3 to less than 1. This is a pilot project; at the conference one group said they were hoping to do a recharge in their catchment of 135 million cubic metres per year.

Pivot irrigator

However aquifer recharge is really a stop-gap: that it is seen to be needed shows we are over-allocating water resources – we live on a finite planet and need to farm within its limits but this leads to some very difficult conversations such as – what foods should we eat, how and where should we farm?

 

 

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