There is no escaping the weather. The National Institute of Water & Atmospheric Research (NIWA) confirmed last month that the earth's average surface air temperature in 2002 was the second warmest in the recorded observations stretching back to 1860. The ten warmest years recorded have all occurred since 1990, and 2002 will be the 24th consecutive year with a global mean surface temperature above the 1961-1990 normal.
The small amount of rain received by some in the region last week, will do little to alleviate the drought conditions mounting here, except perhaps germinate dormant weed seeds. The Marlborough and Tasman District Councils are both taking steps to assist rural decision makers.
Marlborough District Council, in association with MAF Sustainable Farming Fund and Agriculture New Zealand produce a weekly climatic impact report which provides a current climatic situation, weather forecast predictions and primary production monitoring information which includes rainfall, soil moisture, soil temperature and predicted pasture growth rates. For early autumn they predict the effect of the current El Nino weather pattern in the tropical pacific will continue into April, but it is weakening.
They are forecasting anticyclonic activity over the Tasman Sea with stronger than normal south-westerly winds, and while temperatures are expected near average, the east coast can expect a continuation of below average rainfall.
The autumn/winter outlook reports that while the El Nino oscillation continues to weaken, weather systems approaching the country from the southwesterly direction will bring colder than average temperatures and lower rainfall for the already dry east.
Recommendations for dairy farmers include drying off thin cows still being milked to allow them to perform next season, and securing possible grazing, likely sourced out of district, for dry cows and in-calf heifers. Pasture quality is continuing to deteriorate for beef farmers with a tough autumn predicted for mature cattle especially those still suckling calves, so weaning may be necessary to protect cows.
Maximisation of ovulation rates in sheep will require them to be in good health, so feed supplements or crops should be utilised in preparation for mating. Easing them into new feed gradually while maintaining the old food source will reduce nutritional upsets and a loss of liveweight. Overall, supplementary feed resources will be stretched to the limit and purchasing now may be cheaper than in May or June.
The Tasman District Council is looking further ahead, with regard to planning for future dry spells. The Tasman Regional Water Augmentation Committee (TRWAC) was formed almost three years ago to develop practical solutions to the recurrent problems of water shortages in the Tasman District. Last week they presented a summary report on their findings.
The study was undertaken in three stages 1) land and climate suitability for irrigated crops, 2) likely and extreme demands, and 3) water supply options. The area studied was limited to the area identified as water short, encompassing catchments where the current water sources were limited in their ability to provide reliable water for all potentially irrigable land. The area covered was bounded by Nelson in the northeast, to Motueka and the Hau Plains in the northwest, to Motupiko and Tophouse in the south.
Three reports were prepared using the combined resources of Lincoln Environmental, Agfirst Consultants and Montgomery Watson Herza. Potential crops were grouped into six categories on the basis of sensitivity to critical soil, climate and topography factors. Then the study area was divided into 15 sub-catchment water zones and available water sources were described for each zone and potential future shortages highlighted. Future water demand was assessed using two land use scenarios, likely and extreme, and were developed to envelop potential demand for 20 year and 50 year planning horizons.
While 38,000 ha of irrigable land was identified within the study area, an estimated 9,200 ha is currently irrigated and this was predicted to increase to between 12,000 and 14,000 ha by 2021 and to 16,500 and 20,000 ha by 2051. The predicted increases are predominantly due to increased production of apples, pears, grapes, olives and pasture, mainly for dairying, and most of the irrigation increase is expected to occur in the greater Motueka catchment, particularly Motupiko, Dovedale and lower Moutere sub-catchment zones. Peak demand for irrigation water is expected to increase to between 6,200 and 7,700 litres per second by 2021 and to between 8,600 and 11,200 litres per second by 2051.
Using predicted water demand for 2021 and 2051 and analysis of existing water availability, water supply options were identified for the 15 sub-catchments water zones. The optimum option was selected for each zone based on cost, practicality, reliability of water supply and focusing on in-catchment solutions rather than large multi-catchment options. Needless to say there was a large variation in the cost per hectare irrigated, with some storage costs in the ten's of millions of dollars.
Perhaps predictably, what was established overall, was that the location of water resources throughout the area does not match the area where water is most needed.
A number of catchments are water rich, such as upper, middle and lower Motueka River and to a lesser extent Hau Plains and Waimea Plains, while others are extremely water short such as Moutere, Wai-iti and Dovedale. To fully meet future water demand, the harvesting of high flows and water storage is vital. As such, while this roughly $300,000 report is an extremely useful document, which will be discussed, and refered to for many years to come, one important aspect is the identification and protection of good dam sites from development. Given the recent rapid development of lifestyle properties in the study area, suitable land use controls will need to be implemented to safeguard good dam sites if we are to fully realise the potential for increased irrigation in the Tasman area well into the future.
