Wa­ter sup­ply un­der threat

Alexan­dra res­i­dent Roger Browne shares his con­cerns about how ir­ri­ga­tion will be af­fected in a warmer world.

Central Otago Mirror - - WANAKA NEWS -

Cen­tral Otago’s hor­ti­cul­ture and viti­cul­ture in­dus­tries are crit­i­cally de­pen­dent on a re­li­able sup­ply of ir­ri­ga­tion wa­ter.

Much of this wa­ter is sup­plied by dams, which are re­plen­ished by the spring thaw.

In win­ter, snow ac­cu­mu­lates on the sur­round­ing moun­tains such as the Hawk­duns and the Old Man Range.

Un­like the South Is­land’s glacially sculp­tured main alpine chain, our lo­cal moun­tains are called block moun­tains be­cause of their rel­a­tively flat tops.

Dur­ing win­ter, ex­ten­sive snow fields cover these tops and the spring thaw feeds streams as well as el­e­vated wet­lands and string bogs.

Wa­ter continues to flow from the wet­lands and bogs right through the sum­mer.

One re­sult of this is that some of the wa­ter that fell in the form of snow in the win­ter will still be flow­ing into the ir­ri­ga­tion dams through­out sum­mer.

From the point of view of the ir­ri­ga­tion com­pa­nies, the win­ter snow pack is na­ture’s stor­age sys­tem for ir­ri­ga­tion wa­ter.

So what would be the im­pact of an al­tered cli­mate? Be­tween now and the end of this century aver­age tem­per­a­tures are ex­pected to rise, and the snow line with it.

In ad­di­tion, west­erly winds are ex­pected to in­crease in win­ter and spring.

This would ac­cel­er­ate the melt­ing of any snow on Cen­tral Otago’s block moun­tains. The com­bined re­sult is likely to be more run-off in the win­ter months and less wa­ter avail­able for ir­ri­ga­tion in sum­mer, when it is most needed.

How will ex­ist­ing grav­ity-fed ir­ri­ga­tion schemes be able to ful­fil the needs of the fruit and grape grow­ers in the lat­ter decades of this century?

More stor­age ca­pac­ity will be needed to make up for the loss of win­ter snow. Some ex­ist­ing ir­ri­ga­tion dams can be ex­tended, such as is pro­posed for Falls Dam on the Manuherikia.

Else­where stor­age dams may need to be built from scratch.

What­ever the so­lu­tion, it will be an additional ex­pense for ir­ri­ga­tors.

Newspapers in English

Newspapers from New Zealand

© PressReader. All rights reserved.