Canterbury weather, rainfall more of the same

More Of The Same: Cool Dry Weather And Easterlies: Less Water In Rivers, Groundwater Levels Drop

Continued unsettled weather, with average to dry conditions, has meant that groundwater levels are continue to drop in the region, say Environment Canterbury water scientists.

Rainfall throughout much of Canterbury has been generally below average, characteristic of the continued El Niño weather pattern.

“Average to dry conditions coupled with moderate irrigation demand are causing groundwater levels to decline steadily,” said Dr Howard Williams, Environment Canterbury groundwater scientist.

“River and stream flows are generally at or below average, reflecting average to below average rainfall. Soil moisture levels are close to mean values in most areas,” he said.

“Mid January, most catchments within the Canterbury region are experiencing partial restrictions on some of their tributaries,” he said. So far, however, only one part of the West Melton special groundwater zone near Christchurch is under restriction.”

Rainfall

December rainfall was around average in North Canterbury and in alpine areas but was below average in South Canterbury.

January rainfall throughout the region so far is above average for alpine areas, near average for the foothills and below average between Timaru and the Waitaki.

General state of river and streams

December river flows in alpine rivers were above average; flows in other rivers and streams were below average across the region.

January flows are similar to those in December, with alpine rivers above average, foothill and lowland streams below average to well below average.

Restrictions on some takes have been in place in all parts of Canterbury.

General state of groundwater

In general, most groundwater levels are in the average to below average range for the time of year.

Regional detail

North Canterbury

Kaikoura groundwater levels are average to above average range In Waipara, groundwater is illustrating a wide range of levels, from high to very low, with most water levels in the average range. Wells in the Ashley – Waimakariri zone generally display average levels of groundwater.

Mid-Canterbury

Groundwater levels within the Christchurch – West Melton area are slightly lower than average for this time of year. Only one of the Christchurch-West Melton monitoring wells indicates groundwater levels below the first trigger level; currently there is one area under restriction. Both the Selwyn-Waimakariri and the Rakaia-Selwyn groundwater zones are displaying average groundwater levels. Deep wells near the coast and in the Christchurch area are displaying higher than average water levels, those inland, below average. In the Ashburton to Rakaia area groundwater is generally at below average levels and some wells exhibit very low levels.

South Canterbury

In the Mayfield-Hinds groundwater zone, many groundwater readings are below average to very low levels, especially around Lowcliffe. Environment Canterbury monitoring wells show that water levels, historically, increase from late December, however no increase has shown up this year. Lowcliffe is situated down-gradient of the Mayfield Hinds irrigation scheme, and it is recharge water from this scheme that boosts water levels in the later part of the irrigation season. This year there was a very early start to the irrigation season, followed by a break in irrigation early in the summer hence there maybe a greater than usual delay in irrigation–related recharge. The Mayfield Hinds Scheme is now irrigated largely by spray, rather than border strip, resulting in a large reduction in the recharge pulse that has historically boosted water levels at this time of year.

Water levels between the Ashburton River and Rangitata River are below average in most monitoring wells, but not at historical lows, nor do they show unusual rates of decline.

In the Orari-Opihi area the Orari shallow aquifer is exhibiting average groundwater levels, due to rainfall and flow events in mid-January. Deep groundwater in this area is typically low as a result of summer irrigation.

In the Levels Plain area, low groundwater levels are probably due to reduced irrigation recharge derived from the Levels Plains Irrigation Scheme.

In the Pareora-Otaio area, the shallow aquifer exhibits low groundwater levels partly because there have been no marked rainfall and surface flow events in January, unlike further north. In the Waihao area, shallow groundwater is at average levels. Deeper groundwater is at low levels, typically a result of summer irrigation.

Forecast until March 2010

According to the NIWA seasonal outlook website at: http://www.niwa.co.nz/our-science/climate/publications/all/cu/new-zealand-climate-update-127---january-2010/outlook---january-to-march-2010, temperatures are likely to be near or below average, seasonal rainfall is expected to be in the normal range and soil moisture levels and stream flows are most likely to be below normal.

Monitoring data

The most recent monitoring data are available at: http://ecan.govt.nz/services/online-services/monitoring/groundwater-levels/Pages/Default.aspx

Irrigation restrictions

For an updated list of irrigation restrictions go to http://ecan.govt.nz/services/online-services/monitoring/irrigation-restrictions/pages/default.aspx ENDS

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