Canterbury groundwater, rainfall and river levels
September 5, 2008
Canterbury groundwater, rainfall and river levels update
“The state of the groundwater resource has improved markedly as a result of the intense rainfall in July and August,” says Dr Howard Williams, ECan groundwater scientist.
“Monitoring of groundwater levels across the region from mid-August to early September indicated that most of the region’s monitoring wells have average to above average groundwater levels,” he says. “Some deep wells far from rivers have yet to respond to the recent intense rainfall events and recovery in their water levels is expected to be recorded in the next few months.”
Intense rainfall in North Canterbury boosted river flows and has led to raised groundwater levels in associated shallow aquifers (less than 50 metres deep). Groundwater levels in Waipara have recovered, with many monitoring wells displaying average to above average groundwater levels. Similarly, wells in the Ashley – Waimakariri zone display generally average to above average levels of groundwater.
Within Christchurch City, groundwater levels vary from average to above average for the time of year. In the West Melton area, groundwater levels in all but one monitoring well are now higher than trigger levels.
In the Central Plains there has been a marked increase in groundwater levels from last month with groundwater now generally recovered to above average levels in shallow, moderate and deep wells where close to the Rakaia, Selwyn and Waimakariri rivers. Elsewhere, deep wells (100 metres or more) are recovering, but less dramatically.
Groundwater levels in the Ashburton-Rakaia zone showed a strong recovery over July levels with many above average levels for the time of year.
In the Mayfield-Hinds zone, August groundwater levels in shallow wells were at or above average levels. In deeper aquifers, groundwater levels were at average levels. Further south in the Rangitata to Opihi area, groundwater levels in the majority of shallow wells between Coopers Creek and the Rangitata River, and between Coopers Creek and the Waihi River are at above average levels.
The most recent monitoring data are available
Rainfall amounts for July were at or near record amounts across Canterbury region, due to a south-east storm at the end of the month. The Lowry Hill recorder, inland from Cheviot, between the Hurunui and Waiau Rivers, received 307 mm for the month, the most rain in 37 years of record.
Average recurrence intervals for this event were in the range of mean annual to a one in 20 year levels. The storm predominantly affected areas from the Selwyn River, Central Canterbury, north to Kaikoura, but its effect was felt across the region.
Rainfall amounts in August were similarly affected by a south-east storm that occurred between August 25 to 28, flooding large areas of Canterbury. But the main focus of the storm was in the Kaikoura area where Environment Canterbury’s four rain gauges recorded maximum recorded rainfall levels for 24, 48 and 72 hour periods for records of 14 to 40 years. The Luke Creek rain gauge recorded 541 mm for the month of August which is the highest August rainfall on record since 1968 and is more than four times the August average.
The Snowflake raingauge inland Kaikoura recorded its maximum amount ever, almost half a metre of rain or 459.5 mm over 48 hours, 314 mm over 24 hours.
River flows across Canterbury increased as a result of these two major rainfall events in late July and August. In some cases the rivers recorded over twice the average August flows. The Selwyn River stands out at over six times the long-term August flow. In contrast, the Rangitata River was flowing at just 78 percent of its long term mean for August although it has risen in recent days.*
River flows associated with this rainfall are tabulated below.
Name Flow (m3/s) period (yrs)
Avon 25 10 13/3/07
Selwyn at Coes Ford ~400 10-20 27/7/94
Cust Main Drain ~100 20 6/8/95
Ashley Gorge 430 5 6/2/97
Waipara at Gorge 250 10 13/1/02
Hurunui at SH1 1770 20 13/1/02
River flows associated with the August rainfall were not as high as in July because the most intense rain fell further north.
River flow information on floods is provisional because measuring the flow during a flood event is difficult and at times imprecise. During a flood event, rivers frequently scour out their bed and the relationship between river height and flow is not stable. More accurate values will be calculated once the data has been processed.
Note that the estimation of average recurrence intervals is based on statistical distributions that river flows normally fit. The estimates should be treated with caution as a measure of how large the flood event was relative to normal river flows and not interpreted as an absolute return period. It is possible to get two floods with an average recurrent interval of 20 years occurring within the same year; just not very likely.
Regional soil moisture levels are now generally at average or above levels for the time of year in all areas.
*For Selwyn River flows at Coes Ford, cut and paste this link then use the drop down box to get a three month view:
For Rangitata flows, cut and
paste this link, then use the drop down box to get a three