State of the Canterbury Region Water Resource, October 2003

Report No. U03/80

Prepared by Philippa Aitchison-Earl, Mandy Chater, Michael Dicker, Marc Ettema, Russel Sanders, David Scott, Esther Smith, John Weeber

October 2003

Report No. U03/80

58 Kilmore Street 75 Church Street P O Box 345 P O Box 550 CHRISTCHURCH TIMARU Phone: (03) 365 3828 Phone: (03) 688 9069 Fax: (03) 365 3194 Fax: (03) 688 9067

Website: www.ecan.govt.nz Customer Services Phone 0800 324 636

State of the Canterbury region water resource, October 2002

Executive summary

The state of Canterbury’s water resources in October 2003 before the 2003/04 summer is reviewed.

Rainfall over the winter was below average in all parts of the region. However, a dramatic change to well above average rainfall in September served to replenish soil moisture, river flows and groundwater levels. The greatest rainfall anomalies were in North Canterbury, and about the Hunter Hills in South Canterbury. Consequently the highest river flows are now to be found in these areas. For the very southern part of the region, south of the Hunter Hills, September rainfall was not exceptionally high. This, coupled with the accumulated effects of below average rainfall in this part of the region over the nine months prior to September, has resulted in river flows remaining slightly below average in this area.

Groundwater levels in shallow aquifers are generally average to above average, owing to recharge from recent rainfall and river flow. Groundwater levels in deeper aquifers are currently low to average, and in the West Melton/Yaldhurst area levels have been low for much of 2003, in response to the continuing effects of low winter recharge. A later commencement of the irrigation season has led to deep aquifer groundwater levels being generally higher than levels recorded in Spring 2002. In a long-term context groundwater levels are currently higher than the lows experienced in the early 1970’s, but have been in an extended low period since 1997.

Weather forecasting organisations external to Environment Canterbury state that climatic indicators are currently weak, and that no strong trend in weather conditions is likely for the next three months. However, a period of dominant westerly airflow is expected in October and November, which is usual for the time of year. This will result in warm dry conditions in eastern Canterbury and slightly wetter cooler conditions in alpine areas. A return to more variable conditions, with some periods of cool wet weather in the east, is expected for December. The forecasters have low to moderate confidence in their predictions.

On the basis of these forecasts it is expected that river flows will return to around normal over the months leading up to Christmas. The most significant declines in flow are most likely to be in the very south of the region, and least likely to be areas of the region north of Amberley. On the basis of the average summer conditions that are forecast, water should be used conservatively with anticipation of some river flow restrictions as the summer progresses.

Groundwater levels over most of Canterbury are currently higher than Spring 2002, and with generally average rainfall predicted through the rest of 2003, groundwater conditions can be expected to remain average. The exceptions are in the West Melton/Yaldhurst area where groundwater levels are likely to remain lower than average and in the area south of the Hunter Hills where a decline in river and rainfall may lead to falling levels in the shallow, low storage aquifers. Deeper aquifers have had little winter recharge, and commencement of irrigation may see some areas where there has been a large increase in deep groundwater allocation such as Te Pirita and the Ashburton-Rangitata Plains experiencing low groundwater levels.

Environment Canterbury Technical Report 1 State of the Canterbury region water resource, October 2002

2 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

Table of contents

Executive summary...... 1

1 Preconditions ...... 5 1.1 Rainfall ...... 5 1.2 River flows...... 8 1.3 Groundwater recharge ...... 11

2 Current conditions ...... 12 2.1 September rainfall ...... 12 2.2 Water balance ...... 13 2.3 River flows...... 14 2.4 Canterbury groundwater resources...... 16 2.4.1 Introduction...... 16 2.4.2 Overview of Canterbury groundwater...... 17 2.4.3 Kaikoura Plains...... 19 2.4.4 Waipara/Omihi/Amberley ...... 20 2.4.5 Ashley - Waimakariri Plains...... 21 2.4.6 Christchurch – West Melton ...... 23 2.4.7 Central Plains ...... 25 2.4.8 Rakaia - Ashburton Plains...... 26 2.4.9 Ashburton – Rangitata Plains...... 28 2.4.10 Rangitata – Opihi...... 28 2.4.11 Pareora – Waitaki...... 30

3 Prognosis...... 31 3.1 Climate ...... 31 3.1.1 El Niño Southern Oscillation as a climate indicator...... 31 3.1.2 Sea surface temperatures and El Niño Southern Oscillation as at mid October 2003...... 31 3.1.3 Implications for weather in Canterbury (until end 2003)...... 32 3.1.4 Sources for climate prediction ...... 33 3.2 River flows...... 33 3.3 Groundwater...... 34 3.3.1 Kaikoura Plains...... 34 3.3.2 Waipara/Omihi/Amberley ...... 34 3.3.3 Ashley - Waimakariri Plains...... 34 3.3.4 Christchurch - West Melton ...... 34 3.3.5 Central Plains ...... 34 3.3.6 Rakaia - Ashburton Plains...... 34 3.3.7 Ashburton - Rangitata Plains...... 35 3.3.8 Rangitata - Opihi...... 35 3.3.9 Pareora - Waitaki...... 35

4 Acknowledgements ...... 36

Appendix A: Calculating rainfall trends 1994 to 2003 ...... 37

Appendix B: Groundwater Level Envelope Plots...... 41

Environment Canterbury Technical Report 3 State of the Canterbury region water resource, October 2002

List of Figures

Figure 1.1 Location of rainfall sites ...... 5 Figure 1.2 Winter (June, July, August) rainfall for 2003, as a percent of long-term winter rainfall...... 6 Figure 1.3 Geographic distribution of rainfall anomalies shown in Figure 1.2 ...... 7 Figure 1.4 Location of river catchments considered in analysis ...... 8 Figure 1.5 Winter (June, July August) flows for 2003, as a percent of long-term median winter flows ...... 9 Figure 1.6 Monthly deviations of flows in selected rivers for 2003 ...... 10 Figure 1.7 Recharge monitoring (lysimeter) sites ...... 11 Figure 2.1 September rainfall for 2003, as a percent of long-term average rainfall ...... 12 Figure 2.2 Geographic distribution of rainfall anomalies shown in Figure 2.1 ...... 13 Figure 2.3 Mid September to mid October median flows for 2003, as a percent of long-term median flows for mid September to mid October (note: some sites considered over the winter not considered for this period due to data unavailability) ...... 14 Figure 2.4 Geographic distribution of mid September to mid October 2003 flow anomalies...... 15 Figure 2.5 Example envelope plot, bore L36/0092 ...... 17 Figure 2.6 Rainfall recharge at Darfield and long-term hydrograph of L36/0092, Courtenay Road...... 18 Figure 2.7 Regional groundwater levels, October 2003, compared to normal October range 19 Figure 2.8 Kaikoura Plains location map showing selected monitoring sites and hydrogeological zones ...... 20 Figure 2.9 Waipara/Omihi/Amberley location map showing selected monitoring sites ... 21 Figure 2.10 Ashley - Waimakariri Plains location map showing selected monitoring sites and hydrogeological zones...... 22 Figure 2.11 Flow at Dalley's Weir (note that this spring was dry by the end of September 1998, and that current flow rates are similar to those recorded at the same time in 1997 and 2000) ...... 23 Figure 2.12 Christchurch – West Melton location map showing selected monitoring sites and hydrogeological zones...... 24 Figure 2.13 Central Plains location map showing selected monitoring sites and hydrogeological zones ...... 25 Figure 2.14 Groundwater levels in Te Pirita bores (L36/1157, L36/1226) compared to longer term record for bore L36/0092, Courtenay Road...... 26 Figure 2.15 Rakaia - Ashburton Plains location map showing selected monitoring sites.. 27 Figure 2.16 Ashburton - Rangitata Plains location map showing selected monitoring sites 28 Figure 2.17 Rangitata – Opihi location map showing selected monitoring sites ...... 29 Figure 2.18 Pareora - Waitaki location map showing selected monitoring sites ...... 30 Figure 3.1 El Niño Southern Oscillation Index, 1995 to September 2003 ...... 32

4 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

1 Preconditions

1.1 Rainfall Rainfall trends for Winter 2003

Real time data from Environment Canterbury’s telemetered rain gauges (Figure 1.1) was used to consider rainfall trends in Canterbury for the June/July/August (winter) 2003 recharge period. Although most of these rain gauges have over 15 years of record, variability in length of record from one gauge to the next implies there will be bias, of varying degrees, in some of the results presented. The figures presented below are only intended to illustrate overall rainfall trends for winter 2003. Other rain gauges with long-term records were used for analysing ten-year rainfall trends further on in this section. Generally these other rain gauges are read by local farmers on a daily basis, and the data then submitted on a monthly basis to the National Institute of Water and Atmosphere’s (NIWA) climate database. Thus rainfall data from these long-term gauges was not yet available for considering recent rainfall trends.

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Figure 1.1 Location of rainfall sites

Environment Canterbury Technical Report 5 State of the Canterbury region water resource, October 2002

Figure 1.2 shows, for each telemetered rain gauge, rainfall in winter 2003 as a percent of the long-term mean winter rainfall at the site. It can be seen that rainfall this winter was below average throughout the region. This reflects a dry period that occurred over May and June and to a lesser degree July.

Within this general trend there are geographical variations, with coastal areas of North and South Canterbury having the driest conditions, while rainfall in alpine regions had the smallest deviations from normal.

Figure 1.3 summarises these spatial variations in rainfall recharge for the 2003 winter. The extremely dry conditions in South Canterbury over winter are part of a longer term dry interval that has prevailed in this part of the region over the last year – as discussed below.

120 above average 100 below average 80 ean 60

40 percent of m 20

0 n s f on on to ook eek eas r y a C ant adlow Ahuriri Kimbell Lincoln rringt Cl H St Mt McR Waipara a runui Sh1 Blueclif C Mt Francis u Mandamus Glynn Wye Luke C Mistake Flat ons Saddle Rocky Gully Lees Valley H 13 Mile Bush Sunny Peaks Waihaorunga Arthurs Pass Ashley Gorge Turt

Figure 1.2 Winter (June, July, August) rainfall for 2003, as a percent of long-term winter rainfall

6 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

x

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Figure 1.3 Geographic distribution of rainfall anomalies shown in Figure 1.2

Rainfall trends 1994 to 2003

Dry conditions over the 2003 winter are part of a longer-term pattern of below average rainfall occurring over much of the region. In Appendix 1 the “Standardised Precipitation Index (SPI)” shows cycles of lower than average precipitation for eastern areas over the last 10 years – as represented by the historic Waipara, Lincoln and Duntroon rain gauges (see Figure 1.1 for gauge locations). The analysis for the Arthur’s Pass historic rain gauge showed cycles of higher than average rainfall over the same 10-year period, differentiating alpine areas from other parts of the region. Although the dry tendency in eastern areas over the last ten years is very evident from the SPI results, the analysis does not provide statistical support for or against long-term climatic change.

In Appendix A rainfall anomalies over different time scales were considered at each of the four sites, as different water resource components are sensitive at different time scales. As a result dry and wet periods could be characterised by their severity (deviation from normal), persistence and the geographical variability of this severity and persistence. Longer-term rainfall patterns, along with more recent rainfall recharge, can help explain the variation in current water resource status. For example in section 2.2 and 2.3 it is reported that below average rainfall this winter, followed by above average rainfall in September, has had variable impact on water resources across the region, due to variations in the persistence of the dry interval in which the below average winter rainfall occurred. In the southern part of the region below average winter rainfall was part of a longer dry interval that persisted in the area for 9 months (see Appendix A). In mid and North Canterbury equally low winter rainfall was not proceeded by such dry conditions as those in the south. Consequently, although recent rainfall has served to raise soil moisture levels and river flows to “above average” for

Environment Canterbury Technical Report 7 State of the Canterbury region water resource, October 2002

most of the region, this is not the case for southern parts, where soil moisture levels and river flows remain “below average”.

1.2 River flows

Flow records for the 2003 winter, from an array of catchments (Figure 1.4), have also been considered.

Figure 1.5 shows 2003 winter median flows, for those catchments in Figure 1.4, as a percent of long-term winter median flow. Note that median values as opposed to mean values are used here as mean values for flow can be positively skewed by a small number of large magnitude flow events – as is characteristic of Canterbury rivers.

The trend for flows reflects that of the rainfall input, with most sites having below average flows over the 2003 winter period. Flows were especially low in the North Canterbury foothill catchments and is eastern areas of South Canterbury south of Timaru.

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Figure 1.4 Location of river catchments considered in analysis

8 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

140

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i a i r a m ) a n e u a i i n h n r y a r s ) i i r e r a o a o o e a ia s t l ton o a t tata e f una hao k m i r s r h e t Cu par k unui i enc oes i Op a t lif hur r a Av r e anton Or a l r a a t Wa c C henu A Ra u ar As y a l K W e S m hbur angi t hbu Pa ( i t H W a S i C s a s n R ew h Do k y am Wa W w h A h A l aer t t Ha ( ar e r M S n o S m y N O lw e S Figure 1.5 Winter (June, July August) flows for 2003, as a percent of long-term median winter flows

Figure 1.6 shows mean monthly flows this year against long term expected mean monthly flows, for three select rivers. Monthly flows in the Ashley and Rangitata rivers can be seen to have fluctuated this year about the long-term expected mean. However, the more southern Pareora River has had below average flows for all months this year, up until September. Recharge in September was significant, but when coupled with the pre-existing very low base flows, flows in September still remained well below those expected for the month.

Environment Canterbury Technical Report 9 State of the Canterbury region water resource, October 2002

Ashley Gorge mean monthly flow

30 2003 monthly flow

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20 s) 3/

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0 jan feb mar apr may jun jul aug sep oct nov dec

10 mean monthly flow Pareora at Huts 9 2003 monthly flow 8 7 6 5 4 flow (m3/s) 3 2 1 0 jan feb mar apr may jun jul aug sep oct nov dec

Rangitata at Klondyke mean monthly flow 180 2003 monthly flow 160 140 120 100

(m3/s) 80

flow 60 40 20 0 jan feb mar apr may jun jul aug sep oct nov dec

Figure 1.6 Monthly deviations of flows in selected rivers for 2003

10 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

1.3 Groundwater recharge Groundwater recharge from rainfall is recorded at the six lysimeter sites shown in Figure 1.6. With the exception of the Winchmore site, which was established in the 1950s, these sites have operated since 1999 having been installed with the two objectives of supplying data for a FRST-funded research project on natural recharge and establishing a long-term recharge monitoring network.

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RJRE 18/10/2002 Groundwater Recharge 021018.mxd Figure 1.7 Recharge monitoring (lysimeter) sites Measured rainfall recharge at the Winchmore sitefor the year to date has been 30% above the average for recent years. Recharge at Hororata has been slightly below average and significantly below average for the other two sites – the Christchurch Airport lysimeter site recording approximately one third of the average.

2003 recharge Average (1999- Site % of average (mm) 2002) (mm)

Christchurch Airport 60 177 34% Lincoln 66 99 66% Hororata 227 247 92% Winchmore 296 227 130%

Table 1.1 Summary of rainfall recharge monitoring results for 2002 NB (Recharge totals for 2003 and averages are provisional)

Environment Canterbury Technical Report 11 State of the Canterbury region water resource, October 2002

2 Current conditions

2.1 September rainfall In the previous section we considered rainfall over the region during June, July and August, which, due to low evapotranspiration rates at this time, generally tend to be the months of greatest river and groundwater recharge. However, a period of unstable weather this September resulted in very high rainfall for the month across the region. Despite the dry preconditions, it appears the recent rainfall has become the main factor determining current water availability. Therefore, September rainfall is considered here prior to detailing current river flows and groundwater levels.

Figure 2.1 shows, for the telemetered rainfall sites shown in Figure 1.1, September 2003 rainfall as a percent of the long-term mean September rainfall at the site. Above average rainfall for September is evident across the entire region– ranging from 140 to over 350% of average. The largest anomalies were in North Canterbury and in eastern areas south of Timaru. Note, that these are the same areas that had the largest rainfall and river flow deficits over winter, as shown in the previous section. Much of the precipitation was received as snow at higher altitudes, and in one case even on the upper Canterbury plains. Snow reduces the amount of water loss through direct runoff to waterways, and therefore allows for greater infiltration of the precipitation into the soil. Also, lower ground and air temperatures reduce evaporation rates and therefore soil moisture loss. The geographic distribution of the September rainfall anomalies is shown in Figure 2.2

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0 l k l k s 1 s e s e e n e i e s l m y s o i e e w k us n y g o f H g s e e t o r b e l o r a cky eas o l lif t d r y o i o eak S o a anc rt i m R W c unga s C a ngton anton a r Pa i adl t R n e P r Ki 13 M G V G s Mi Cl n u e C r S H Tu ndam F D Mt l r Mc e s t unu y a h u it B r ey haor i h C M Ma u Gl t a hl Luk St unny Lee H Wh s S W Ar A

Figure 2.1 September rainfall for 2003, as a percent of long-term average rainfall

12 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

x

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Figure 2.2 Geographic distribution of rainfall anomalies shown in Figure 2.1

2.2 Water balance The difference between rainfall accumulation and potential evapotranspiration accumulation (basic water balance) provides a general indication of how wet or dry the region is now compared to past years. If rainfall is greater than potential evapotranspiration there is an ‘accumulated surplus’ for the period considered. If rainfall is less than potential evapotranspiration there is an ‘accumulated deficit’. In the situation of accumulated surplus, rainfall that is not lost to evapotranspiration contributes to surface runoff and drainage, and soil and groundwater recharge (this usually only occurs in winter months).

The impacts of the past dry months on the three-month accumulated water balance were considered, as at mid October 2003, for eight sites in Canterbury for which the National Institute of Water and Atmospheric Research (NIWA) supply rainfall and potential evapotranspiration data. The information was sourced from NIWA’s “Climate Now” publication.

Sites at Kaikoura and Culverden show a current water balance that is well above average for this time of year. In late August and September there were several instances where rainfall was well in excess of the soil’s ability to absorb water, which resulted in surface ponding and runoff, at these sites.

Rangiora, Lincoln, Darfield and Winchmore all currently show a seasonal water balance that is about average for this time of year. Soil moisture has exceeded field capacity (saturation) recently although not to the same degree as Kaikoura and Culverden. It is unusual for soil saturation to occur this late in the year.

Environment Canterbury Technical Report 13 State of the Canterbury region water resource, October 2002

Despite the rainfall excesses shown in Figure 2.1 the seasonal water balance and soil moisture levels at Waimate and Tara Hills (Omarama) remain below average for this time of year. This reflects a dry interval that persisted in the southern part of the region for nine months up until the end of August 2003, as referred to in section 1.1.

2.3 River flows

Mid September to mid October 2003 flows for the rivers shown in Figure 1.5 were considered as an indicator of available surface water in early spring and at the commencement of the irrigation season. Median flows for mid September to mid October 2002 were calculated for each river, and compared to long-term median flows for the same period. (Figure 2.3).

350

300

250 above average 200

150

100

50 below average

percentage of long-term median flow 0

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Figure 2.3 Mid September to mid October median flows for 2003, as a percent of long-term median flows for mid September to mid October (note: some sites considered over the winter were not considered for this period owing to data unavailability)

Figure 2.3 shows that nearly all Canterbury rivers have benefited significantly, in terms of flow, from recent rainfall (Figure 2.1). However, variation in flow anomalies occur over the region, as seen in Figure 2.4. The pattern is attributed to spatial differences in rainfall over the winter recharge period (Figure 1.4), as well as the almost reverse rainfall distribution pattern that occurred over the recent wet period (Figure 2.2).

In North Canterbury all river flows are currently well above that expected for this time of year. Despite lower than average rainfall in the area over winter, especially for coastal parts, the very high rainfall of recent weeks has served to saturate the area allowing high flow rates to be sustained.

In mid-Canterbury flows rates are elevated, but to a lesser degree than those in the north. The rainfall anomalies over winter and the last month have been less extreme here than for

14 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

other parts of the region. The Avon River is an obvious exception, with its current flow being only 80% of that expected for the time of year. The Avon River is spring fed, and flows reflect water levels in the groundwater systems from which they are derived. The effect of recent rainfall has yet to be realised in the spring-fed flow of the Avon River.

In South Canterbury current spatial disparities in river flows reflect spatial variations in recent rainfall totals. The effects of very low rainfall over winter have been superseded for most rivers in the area. This is particularly so for the Pareora, Waihao and Hakataramea rivers. These rivers showed very low flows over winter, but now all have flows that are well above the norm for this time of year. High precipitation about the Hunter Hills during spring (Figure 2.2) has recharged river flows in the vicinity – which includes the Pareora, Waihao and Hakataramea Rivers. However, not all rivers in South Canterbury have fared as well. In the most southern part of the region, rainfall over spring has been only slightly above average. This, coupled with below average rainfall for the nine months ending August 2003, has resulted in flows in the Omarama and Maerewhenua rivers remaining below average.

With slightly below average rainfall over winter and slightly above average rainfall over spring, the alpine river flows are now currently about average for the time of year. This is with the exception of North Canterbury’s Waiau and Hurunui rivers, where flows are significantly above average.

x

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Figure 2.4 Geographic distribution of mid September to mid October 2003 flow anomalies

Environment Canterbury Technical Report 15 State of the Canterbury region water resource, October 2002

2.4 Canterbury groundwater resources 2.4.1 Introduction

This section deals with the current state of the groundwater resource in the Canterbury region. For ease of perusal, the Canterbury region has been divided into several smaller areas outlined in full later in this chapter, these being:

• Kaikoura Plains • Waipara/Omihi/Amberley • Ashley - Waimakariri Plains • Christchurch - West Melton • Central Plains • Rakaia - Ashburton Plains • Ashburton - Rangitata Plains • Rangitata - Opihi • Pareora - Waitaki

Groundwater level records are presented in Appendix B through the use of envelope plots. An envelope plot is a summary graph that has been created to display several years of data in one plot, and to indicate how present groundwater levels relate to past measurements. An example of an envelope plot of bore L36/0092 (see Figure 2.13 for location), which has over 50 years of measurements, is illustrated in Figure 2.5 below.

The left vertical axis indicates the level of the groundwater in metres below ground. The bottom horizontal axis is a year’s time scale in months, in this case from November to October. The blue line indicates the levels measured over the last year, with the most recent reading, 24/9/03, indicated. The green line is the data from the same period in 2002 plotted for comparison. 2002 was chosen as a year within recent memory, where groundwater levels were average at the beginning of the season, but periods of high demand led to record lows in much of the region later in the season. Water levels that fall within the white area of the plot are within one standard deviation of the mean groundwater level for each month, so in theory 68% of all readings will be within this range. This white area represents the ‘normal range’ expected in groundwater levels. The light grey shaded zones are between one standard deviation of the mean and the maximum or minimum recorded for each month. Water levels within this zone indicate that levels are either high or low compared to the normal range. The dark grey shaded area indicates levels above the maximum or below the minimum recorded in an individual month at each bore. Hence for late September 2003, the groundwater level at L36/0092 is presently at –46.80m, which is below what it was in October 2002, and is slightly below the normal range expected. The lowest recorded reading in an October is around -52m, and the highest is around -27m below ground level.

16 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

Figure 2.5 Example envelope plot, bore L36/0092

2.4.2 Overview of Canterbury groundwater

2.4.2.1 Long-term groundwater level trends Bore L36/0092 (Courtenay Road), lies in the upper zone of the Central Plains, between the Selwyn and Waimakariri Rivers (see Figure 2.13 for location). L36/0092 penetrates a deep aquifer, and due to its distance from the Selwyn and Waimakariri Rivers is recharged by rainfall. Because of this rainfall recharge source, and the long-term record of groundwater levels available (since 1951) L36/0092 is a good indicator of overall climate-induced groundwater level trends. The long-term hydrograph from L36/0092, compared with estimated rainfall recharge is shown below in Figure 2.6. The rainfall recharge has been obtained using a simple model, based on a soil with 75mm water holding capacity, rainfall at Darfield and potential evapotranspiration at Christchurch Airport.

Environment Canterbury Technical Report 17 State of the Canterbury region water resource, October 2002

650 600 550 m 500

m 450 400 350 300 250 arge in 200 h

c 150

e 100

R 50 0 -2019 50 1960 1970 1980 1990 200 0 m

n i

l e v

Le -40 r e t a dw

-60

Groun 19 50 1960 1970 1980 1990 200 0 Date

Figure 2.6 Rainfall recharge at Darfield and long-term hydrograph of L36/0092, Courtenay Road

Groundwater levels were relatively stable from 1951 to the mid 1960s, when levels began a steady downwards trend, bottoming out in the early 1970s. These recovered to highs in the late 1970s. Other lows have been experienced in the mid 1980s, the late 1980s to early 1990s, and present lows that have been established since 1999. From the long-term record, it can be seen that water levels, while at the low end of the range, are still not as low as experienced in the past. Levels over the 2002/03 summer dropped to the second lowest level experienced, but have been steadily recovering. The groundwater levels generally follow the recharge trends. A year of very low recharge will generally lead to low groundwater levels (i.e 1969 and 1982), and years of high recharge lead to elevated groundwater levels (i.e 1978, 1986 and 1992).

2.4.2.2 Regional groundwater levels The areal distribution of groundwater lows from the Ashley River to Timaru is illustrated in Figure 2.7, where red triangles indicate levels 1-2 standard deviations below the October mean (as in the grey area on the envelope plot), orange average to below average readings, and blue above average. The map shows some low areas such as in West Melton, and north of the Rakaia, but otherwise levels are average across the region. Shallow aquifers across the region have benefited from the recent rainfall, (and subsequent river recharge), with aquifers such as the Eyre, Selwyn, Asburton and Waihao River systems currenlty at average to high levels. Deeper plains aquifers are slower to respond to recharge events. Due to low winter recharge, antecedent levels were low. Currently levels are low to average, with many wells still recovering from last summers irrigation. The 2003/04 irrigation season commencement has been delayed, due to the recent September rainfall, and this also contributes to the average levels measured in many of the wells (as the irrigation based decline has not started, as it had in the same period last year). The Central Plains deeper aquifers (including Te Pirita) are the only areas that are entering the 2003/04 season at lower levels than the 2002/03 season.

18 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

Figure 2.7 Regional groundwater levels, October 2003, compared to normal October range

2.4.3 Kaikoura Plains

Rainfall data from Luke Creek indicates that rainfall for the period June to August was 72% of the normal. In contrast September rainfall was 133% of the normal. Water levels have risen in all monitored wells since March/April and most are currently higher than for the same time in 1998 and 2002.

There are only three wells with long-term continuous groundwater level records that are currently monitored. The network has been recently reviewed and six additional wells have been monitored since May 2001. Based on the review the area was subdivided into four groundwater zones (Figure 2.8).

Environment Canterbury Technical Report 19 State of the Canterbury region water resource, October 2002

R

E ® V AY 1 I W H R IG E H AT T

I S

A

H

W MT FYFFE FANS

O K

O31/0030

7 O31/0200

7

CENTRAL KAIKOURA PLAINS

O31/0026 7

KAIKOURA KOWHAI RIVER EAST 7

O31/0197

KOWHAI RIVER WEST

01230.5 4Kilometres

RJRE 18/10/2002 Kaikoura 021018.mxd Figure 2.8 Kaikoura Plains location map showing selected monitoring sites and hydrogeological zones

2.4.3.1 Mt Fyffe fans The groundwater level is within its normal range – currently higher than water levels for the same time in 2002 and higher than 1998.

2.4.3.2 Central Kaikoura Plains Inland, the groundwater level has recovered from a March/April low to an above average water level in September. The current water level is higher than for the same time in 2002 and 1998. Although the shallow water table at the coast has recovered since February, it is near its lowest level for this time of year and similar to 2002 levels.

2.4.3.3 Kowhai River East Inland the groundwater level has recovered from a March/April low to the highest level recorded for this time of year, significantly higher than for the same time in September 2002 and 1998. In the coastal confined aquifer the water level has also recovered from a March/April low but is below average, slightly lower than for the same time in 2002 but slightly higher than 1998.

2.4.3.4 Kowhai River West Inland, water levels recovered from a March/April low to a high level for September in response to winter recharge. The current water level is higher than for the same time last year – there are no records available for 1998. The water table aquifer at the coast shows a similar trend to inland water levels.

2.4.4 Waipara/Omihi/Amberley

Data from White Gorge (Waipara River) indicates that rainfall for the period June to August only 55% of the normal. In marked contrast the rainfall for September was 179% of the normal. The water levels in all monitored bores have risen since the summer low and are currently within their normal range.

20 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

Only one bore (M34/0095) in the current monitoring network has groundwater level records longer than four years. It is therefore difficult to determine any long-term trends and whether water levels are abnormally low or high with respect to a measured long-term range. Figure 2.9 shows the location of representative monitoring bores.

N34/0110 ® OMIHI 7 1 AY HW HIG STATE

WAIPARA 7 WAI PA R ER A RIV N34/0049

GLASNEVIN

1 M34/0095

Y

A

W

H

7 G 7 I

H

E

T A M34/0670 T

S

W A IP AR A R 012340.5 Kilometres AMBERLEY IVER

RJRE 18/10/2002 Waipara 021018.mxd Figure 2.9 Waipara/Omihi/Amberley location map showing selected monitoring sites

North of the Waipara River, groundwater levels in most monitored bores have risen since a March low and are currently below average and up to a metre lower than for the same time last year. The greatest water level rise occurred in the upper Omihi Valley (N34/0110). A 66m deep monitoring bore near Weka Creek is indicating signs of recovery only since August.

South of the Waipara River groundwater levels for most monitored bores have recovered from a summer low and vary from average to above average with most levels currently up to two metres higher than for the same time last year.

2.4.5 Ashley - Waimakariri Plains

The Ashley - Waimakariri Plains (Figure 2.10) experienced below average winter rainfall and rainfall recharge. However the rivers that influence this sector of the plains (the Ashley, Eyre and Waimakariri), particularly September and early October flows, and the associated spring rainfall, have impacted groundwater levels such that they are generally at higher levels than they were at the same time in 2002. The exception to this is the Coastal Confined Zones where levels are the same as last year. The effect of September and October flows in the Eyre River means that Eyre River Zone levels are above average for this time of year despite the dry winter. In general these water levels are rising as opposed to declining at the same time last year

Environment Canterbury Technical Report 21 State of the Canterbury region water resource, October 2002

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Figure 2.10 Ashley - Waimakariri Plains location map showing selected monitoring sites and hydrogeological zones

The aquifers associated with the Ashley River in the vicinity of Fernside and Rangiora are average or above average for this time of year (M35/0142). However, recent drought years have shown that groundwater levels in the Ashley River Recharge Zone (and flows in the associated springfed streams such as the Cam River) mainly depend on whether or not the Ashley River goes dry in the Rangiora area in the summer, and the duration of this drying.

The shallow aquifers associated with the Eyre River (including bores as far afield as Bennetts, Cust, and also those to the south of South Eyre Road) are usually the ones most impacted by dry winters. Eyre River flow along its full length in September and October 2003, and in its upper reaches through most of the winter, have ensured that groundwater levels in this area are mostly in normal ranges, or slightly high for this time of year and continuing to rise (M35/0008). They are significantly higher than levels in 1998 which was followed by a summer season of very low water levels and some water shortages and higher than last year when few problems arose.

Of limited concern in the Ashley-Waimakariri Plains is the flow of the Ashley River in the forthcoming summer and the sustainability of flows in some of the associated springfed streams (Cam River, Taranaki Creek, Waikuku Stream). However, at this stage there is no indication to suggest that this will be an issue.

Springfed streams away from the Ashley River are likely to be maintained above minimum flows. Data from a spring in the Ohoka Stream (Figure 2.11) shows that flows are currently at similar levels to those recorded in September 2000 after which minimum flows were not recorded. These flows are significantly higher than in September 1998 when this spring was almost dry and restrictions on groundwater abstraction in the Ohoka catchment were considered.

22 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

10.0

9.0

8.0

7.0

6.0

5.0 Flow (l/s)

4.0

3.0

2.0

1.0

0.0 Jul-97 Dec-97 Jul-98 Jan-99 Jul-99 Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03

Figure 2.11 Flow at Dalley's Weir (note that this spring was dry by the end of September 1998, and that current flow rates are similar to those recorded at the same time in 1997 and 2000)

2.4.6 Christchurch – West Melton

2.4.6.1 Unconfined aquifer zone Groundwater levels in the unconfined aquifer area to the west of Christchurch are generally lower than normal for this time of year. This is consistent with the lower than average long- term rainfall experienced in this part of the region. For many of the bores in the area, groundwater levels are beginning to rise in response to recent rainfall recharge. Groundwater use within the West Melton/Yaldhurst Groundwater Management Zone is managed in terms of specified trigger levels in five monitoring bores (Figure 2.12). Groundwater levels in four of the monitoring bores (M35/1000, M35/5696, M35/1691 and M36/0217) have been rising over recent months and are currently above the uppermost trigger level. The exception is M35/1100 where groundwater levels have remained below the first trigger level for much of the year and have only begun to respond to recent recharge within the last month.

Environment Canterbury Technical Report 23 State of the Canterbury region water resource, October 2002

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Figure 2.12 Christchurch – West Melton location map showing selected monitoring sites and hydrogeological zones

2.4.6.2 Spring zone Groundwater levels within the zone most significant for springflow are generally lower than normal for this time of year and this is reflected in the Avon River flow which is currently approximately 20% below the average seasonal rate. Nevertheless, groundwater levels have risen in several bores in response to recent rainfall recharge and spring-fed stream flows can be expected to benefit as a result.

2.4.6.3 Confined aquifer zone Over the last few months groundwater levels in the confined aquifer zone have generally been below average. However, recent water level increases have resulted in above average levels in most confined aquifer monitoring bores with levels generally slightly higher that at this time last year.

24 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

2.4.7 Central Plains

2.4.7.1 Selwyn Zone The Selwyn River has been flowing several times over its entire length due to snowfall and rain in the foot hills. This has recharged the (shallow) aquifers in this zone to above average for the time of year. As a consequence of this the Irwell River - which fell dry last year - has started flowing again. The effects of the recharge extend up to the Leeston and Greenpark area.

®

W A IM A KA RIR I R IVER Upper Selwyn Zone DARFIELD

L36/0064

CHRISTCHURCH L36/0092 L36/1226 7 7 7 ROLLESTON AY 1 HW IG E H S AT E ST LW TE PIRITA YN RI 7 V ER L36/1157 R A M36/0354 K A IA

RI VER IRW 7 ELL R IV E R 7 Lower Selwyn Zone M36/0355 LAKE ELLESMERE TE WAIHORA AY 1 HW IG E H AT ST

03691.5 12Kilometres

RJRE 18/10/2002 Central 021018.mxd Figure 2.13 Central Plains location map showing selected monitoring sites and hydrogeological zones

2.4.7.2 Central Plains Most deep wells (Figure 2.13) in this area are still below average and even below normal lows, as a consequence of the low rainfall over last winter. The recent rains have not yet recharged these aquifers.

2.4.7.3 Te Pirita Last year the groundwater level in the Te Pirita area showed the effects of increasing irrigation abstraction (Figure 2.14. The groundwater level in L36/1157 reached its lowest level ever since measurements started in 1996. It also took a long time to recover from this drawdown and still has not reached the level of the beginning of last irrigation season. The yearly amplitude has become larger over the last 3 years especially when compared to the less affected L36/0092. Well L36/1226, which is located north of the Te Pirita area also shows some increasing effects of irrigation. If the trend of increasing irrigation demand and average or less then average rainfall/recharge years continues this area might be experiencing some more water supply problems over the coming year(s).

Environment Canterbury Technical Report 25 State of the Canterbury region water resource, October 2002

-70 -35

L36/1157 Te Pirita L36/1226 Saunders Rd -75 -40 L36/0092 Courtenay Rd (right Scale) p

-80 -45 (m) -m l e -85 -50 lev

ater -90 -55 ndw

ou -95 -60 r g

-100 -65

-105 -70 1996 1997 1998 1999 2000 2001 2002 2003 2004 Date

Figure 2.14 Groundwater levels in Te Pirita bores (L36/1157, L36/1226) compared to longer term record for bore L36/0092, Courtenay Road

2.4.8 Rakaia - Ashburton Plains

Rainfall recharge as measured at Winchmore has been slightly above average for the year to date (Table 1.1). Flows in the North Ashburton River have been sustained over the winter of 2003 with improved levels in adjacent aquifers when compared with winter 2002. Groundwater observed in many long-term monitoring bores (Figure 2.15) has risen to average levels with the exceptions being the aquifers of the Chertsey Zone (the area to the north of the Ashburton – Lyndhurst Irrigation Scheme and those downstream areas affected by this scheme).

26 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

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Long-term monitoring bores on the coastal side of State Highway 1 responded to irrigation turnoff following heavy rainfall at the end of March and early April and then continued to rise to the present time (L37/0022). Levels are generally the same or a little higher than those in 2002, and are continuing to rise in the absence of irrigation due to September and October rainfall. In 2002 these levels were already starting to decline in response to early September irrigation.

The exceptions are the deeper aquifers of the Chertsey Zone which have been subject to a significant amount of new groundwater development for irrigation over the last few years and appear to be attaining new equilibrium levels as a result. Although water levels in these aquifers have also been rising the response has been more sluggish than in previous years and levels are between 1 and 2 m below the equivalent 2002 levels (e.g. L36/1338). Again levels continue to rise while at the same time last year they were falling so the difference should decrease until irrigation commences. It is for these aquifers that most concerns are held with respect to summer reliability.

Mid plains levels further inland are generally average (L36/0948) or a little higher. Water levels in some bores have been trending down since June as is typical. Many of these started rising to high levels from January onwards in response to irrigation under the Ashburton - Lyndhurst Scheme and then autumn rainfall. Water levels around Ashburton town are also about average for this time of year, following good September flows in the Ashburton River.

Inland bores that are dependent on annual rainfall recharge on the plains (e.g. shallow bores near Methven and the North Ashburton River), are currently at average levels for this time of year. Most of these are continuing to trend upwards in response to recent rainfall (K36/0045).

Environment Canterbury Technical Report 27 State of the Canterbury region water resource, October 2002

2.4.9 Ashburton – Rangitata Plains

Most wells are normal for the time of years except three wells closer to the Rangitata which are below normal (figure 2.16). These wells however have higher levels in summer because of borderdyke recharge. There are no long-term water levels available for deeper aquifers due to monitoring only recently beginning. The water levels of the short series available indicate a much lower level then last year so presumably the situation is similar to other deeper aquifer areas such as the Central Plains.

S OU TH B N OR R T AN H ®

CH B R A A N S C H H B U A R S T H O B MAYFIELD N U

R R T 1 Y H I O V A IN N W E D R H R I IG S V K37/0200 E H R E T A R T S IV E

R

7 ASHBURTON

K37/0136

R A N HINDS G 7 IT AT A R I Y 1 V WA E GH R HI TE STA

K38/0100 LOWCLIFFE

7 0 3 6 9 12 Kilometers

RJRE 18/10/2002 Ashrangi 021018.mxd Figure 2.16 Ashburton - Rangitata Plains location map showing selected monitoring sites

2.4.10 Rangitata – Opihi

2.4.10.1 Rangitata River Groundwater levels to the south of the Rangitata River (Figure 2.17) have been at average to low levels for much of the 2003 year, and remain in the average to low range.

28 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

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Figure 2.17 Rangitata – Opihi location map showing selected monitoring sites

2.4.10.2 Rangitata-Orari Deep Aquifer Water levels in this aquifer were at the lowest ever in March 2003, due to increasing irrigation demand. Level quickly recovered and stabilised to normal levels following the cessation of pumping, and are currently average, and higher than that recorded in 1998, due to the later beginning of the irrigation season.

2.4.10.3 Orari River System Groundwater levels in the upper Orari River shallow aquifer system were at the lowest recorded in February 2003, rose in April, and again recently in August as a result of increased river flow. Levels have since fallen slightly (with lower river flow) and are presently average over the whole aquifer.

2.4.10.4 Levels Plains Groundwater levels in the upper Levels Plains were at the lowest ever recorded in August 2003, but following rainfall, have risen to normal levels. The lower plains are primarily influenced by recharge from the Levels Plain Irrigation Scheme in summer months, hence fall in the winter. Levels here have risen in response to recent rainfall, and are in the low to normal range.

2.4.10.5 Fairlie-Ashwick Flats There are only four years of groundwater data available, thus trends are difficult to establish. Groundwater levels have been average over the winter period, and have all risen in September, in response to increase rainfall and river flow, and subsequently fallen slightly in October. Levels are currently average to high for the time of year.

Environment Canterbury Technical Report 29 State of the Canterbury region water resource, October 2002

2.4.11 Pareora – Waitaki

The Pareora-Waitaki groundwater systems are dependent on sustained flows in the foothill rivers (Pareora, Otaio, Makikihi, Hook, Waihao, Figure 2.18) throughout the summer, as the aquifers have little storage. These rivers experienced good recharge over the 2003 autumn, however less recharge from late June led to falling groundwater levels. Increasing flows in September have recharged these groundwater systems, leaving groundwater adjacent to rivers at average levels. In areas more distant, or with greater reliance on rainfall, levels are currently at low for the time of year.

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Figure 2.18 Pareora - Waitaki location map showing selected monitoring sites

2.4.11.1 Pareora Levels have been generally low over late summer to winter 2003, but with increased recent river flow have risen to average levels. Levels on the south side of the river have not experienced the same rise in levels as the north side since August.

2.4.11.2 Otaio There is only four years of data is available for comparison in this catchment. Levels in the upper catchment have been low over much of the last year, but increase river flow since August has lead to a large rise in groundwater levels. Levels nearer the coast have been average over the 2003 winter, and are currently high.

2.4.11.3 Waihao Levels are average to high in the areas near to rivers, such as the Waihao and Hook, which have had recent increased flows. The ‘recharge pulse’ from the river flow has not yet propagated out to wells in the lower Waihao-Wainono lagoon area, where levels are still in the low range.

30 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

3 Prognosis

3.1 Climate

3.1.1 El Niño Southern Oscillation as a climate indicator

Wind, rainfall and temperature values are determined by the passage of different weather systems over New Zealand. These systems are linked to larger global circulations that fluctuate between seasons and between years. Water temperatures in the Pacific Ocean determine one such circulation, the El Niño Southern Oscillation (ENSO).

ENSO fluctuates between warm (El Niño) and cool (La Niña) phases, and these phases have a direct bearing on climatic conditions in New Zealand. El Niño produces a dominant westerly flow, which commonly produces dry or drought conditions in eastern areas. Conversely La Niña circulation is characterised by the reduced frequency and intensity of westerly airflows, and thus is often associated with airflows from the south-east and north- east quarters. As a result rainfall in the south and west is often below average during La Niña

Pressure differentials between Tahiti and Darwin are used as a measure of El Niño/La Niña occurrence and magnitude. The pressure differential is referred to as the Southern Oscillation Index (SOI), and as a measure of the current state of ENSO has become a useful tool in seasonal and long-term climate forecasting. However, climatic conditions associated with each ENSO phase (broadly referred to above) may not transpire at the exact time that the phase is at its strongest. Rather there may be a lag time, so that associated conditions actually transpire as the phase in question is diminishing. Also, the ENSO phase (as indicated by the SOI) is superimposed on other phenomena, which together determine the exact nature of climatic conditions. Climate predictions provided by weather experts below should therefore be interpreted only as likely conditions, especially when the indicators used in long-term forecasting are at a weak or undefined stage.

3.1.2 Sea surface temperatures and El Niño Southern Oscillation as at mid October 2003

The Meteorological Service and Blue Skies Weather report that for the past six months sea surface temperatures in the central Pacific Ocean have been near normal. Consequently, for the past three months the Southern Oscillation Index (SOI) has been close to zero (Figure 3.1). These neutral conditions mean that we are not currently in strong El Nino or strong La Nina phase; and so ENSO is likely to have little influence on our weather over the next few months. NIWA predict that this neutral state will remain through to the end of summer 2003– 04.

Information on the current state and development of the El Niño conditions can be found on the World Meteorological Organisation website at www.wmo.ch.

The Meteorological Service has also reported warmer than normal sea surface temperatures in the Tasman Sea at this time.

Environment Canterbury Technical Report 31 State of the Canterbury region water resource, October 2002

30

La Nina El Nino Moving mean 20

10

0 1995 1996 1997 1998 1999 2000 2001 2002 2003

Southern Oscillation Index -10

-20

-30

Figure 3.1 El Niño Southern Oscillation Index, 1995 to September 2003

3.1.3 Implications for weather in Canterbury (until end 2003)

The absence of strong signals in the SOI make forecasting more difficult. Generally the Meteorological Service foresees an increased variety of weather patterns and a widening of extremes, with no particular pattern dominating. However, they go on to say that indications from past similar years (1968, 1984 and 1995) suggest that westerly-quarter winds, at times quite strong, will be dominant in October and November. This is usual for the time of year.

Blue Skies Weather predicts sustained periods of westerly airflow will develop in mid to late October and continue through November, with anticyclones becoming more dominant over the country by late November and into December. These regional weather patterns tend to bring differing weather conditions to the eastern and western parts of the region, so specific conditions within these two respective areas are considered below.

Above average sea surface temperatures in the Tasman Sea, such as they are now, result in enhanced humidity and the creation of low-pressure systems and associated cold fronts. This is likely to result in above average rainfall for the West Coast and Canterbury high country over this spring. Both temperature and sunshine are expected to be below normal in western areas of the region. The confidence in these predictions, provided by the Meteorological Service, is low to moderate. Predictions by Blue Skies Weather are in line with those provided by the Meteorological Service, with the additional comment that drier and warmer conditions are expected in alpine areas in the New Year.

A wide variety of weather patterns is likely over the next few months for eastern parts of the region. The Meteorological Service forecast that the wet easterly winds of early spring are to subside, and rainfall is to be near normal. The northwest winds typical of spring are expected to be stronger than normal, bringing several periods of dry weather and associated soil moisture loss. Similarly, Blue Skies Weather predicts drier conditions for eastern Canterbury from late October through to early December, with rain falling in a few heavy falls or wet

32 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

weeks with dry periods in between. They state that long periods of settled weather are likely during December with some rain and cooler days associated with southwest airflows.

Both forecasting agencies predict temperatures to be normal and sunshine hours to be normal to above normal until the end of 2003. They both give low to moderate confidence in these predictions.

3.1.4 Sources for climate prediction

The Meteorological Service provides a two month forecast after the 10th of each month in fax form. ‘Metfax’ provides a summary of current indicators, and separate forecasts for climatically similar regions within New Zealand. The Metfax number is 0900 999 88.

Blue Skies Weather provides three month forecasts every month for the Canterbury region which include a brief summary of conditions that occurred over the last month and predictions of weather patterns and rainfall, temperature and sunshine parameters. Forecasts can be obtained from Blue Skies Weather and Climate Services Ltd, PO Box 22, Amberely, telephone 03 314 7318 or email [email protected].

Also, NIWA provides a ‘Climate Update’ every month, which reflects on climate over the last month and offers predictions for the three months in advance. The ‘Climate Update’ can be received by mail or viewed on the Internet at www.niwa.co.nz/ncc/current.html.

All these avenues provide ongoing climate predictions, and it is recommended that water users employ such tools to help in their management of the Canterbury water resource.

3.2 River flows Above average rainfall over September and October has served to recharge river flows to above average rates, despite dry conditions over the winter recharge period. Rainfall was especially high in north Canterbury and in the vicinity of the Hunter Hills in North Canterbury.

However, the rainfall has had variable effect in terms of recharging soil moisture. In Kaikoura and North Canterbury the rain has been sufficient to raise soil moisture levels to above normal for the time of year. In mid Canterbury soil moisture is now about average for the time of year. In South Canterbury areas of below average soil moisture remain east and south of the Hunter Hills. This is also the area where flows currently deviate above normal by the smallest amount – compared to other rivers in the region.

Climatic indicators are currently weak and most forecasting experts are predicting generally average rainfall, sunshine and temperatures until the end of 2003. This includes a period of dominant westerly-quarter air-flow, which brings warm dry conditions to eastern Canterbury – which is typical for the time of year.

On the basis of these forecasts we should expect to see river flows return to around normal rates over the months leading up till Christmas. The most significant decline in flows is likely to be in areas south of the Hunter Hills in South Canterbury, where soil moisture is currently slightly below average, and where rivers flows are currently about normal. In their October “Climate Update” NIWA predict that river flows will be normal to below normal for the east of the South Island. On the basis of current soil moisture and flow patterns within Canterbury, below normal river flows are most likely to occur in South Canterbury and least likely to occur in North Canterbury.

Overall flows in spring-fed streams in the region are expected to be average to slightly above average, as a result of aquifer recharge from rainfall in early spring 2003.

Environment Canterbury Technical Report 33 State of the Canterbury region water resource, October 2002

3.3 Groundwater

3.3.1 Kaikoura Plains

Despite below average autumn/winter rainfall, groundwater levels have recovered from a late summer low and are within their normal range having benefited from above average rainfall in September. No major difficulties are expected with groundwater supply.

3.3.2 Waipara/Omihi/Amberley

Again, despite below average autumn/winter rainfall, groundwater levels are currently within their normal range and generally continue to rise following the above average September rainfall. Groundwater levels over this summer are not expected to fall below previously recorded lows.

3.3.3 Ashley - Waimakariri Plains

Groundwater supplies should be sustained through the summer and there are currently no areas of concern.

However, as usual, if flow in the Ashley River in the Rangiora/Fernside area is not maintained over the summer then the associated groundwater could be expected to decline to low levels. This is unlikely based on the climatic forecast, but the consequence of such an occurrence could be that associated springfed streams (such as the Cam River, Taranaki Creek and Waikuku Stream) would reach minimum flows. The springfed streams away from the Ashley River (such as Ohoka Stream and the Kaiapoi River) are likely to be reliable this season.

3.3.4 Christchurch - West Melton

Groundwater levels in the unconfined aquifer area are currently below average and some water users within the West Melton/Yaldhurst Groundwater Management Zone (particularly those within the M35/1100 zone) may experience restrictions over the coming summer. No difficulties are expected in the confined aquifer area.

3.3.5 Central Plains

With the shallow groundwater levels recharged by the Selwyn River above normal levels, no problems are expected for the upper and lower Selwyn area. The confined aquifers in the lower zone have also benefited from this river recharge event and therefore no problems are expected this year. However, the deep aquifers in the Central Plains are still below normal and the combination of low levels and increasing demand might cause some problems in some specific area’s especially higher up the plains.

3.3.6 Rakaia - Ashburton Plains

It is anticipated that for most of the Ashburton - Rakaia Plains groundwater will be at levels sufficient to sustain pumping demand. However, recovery of levels in the Chertsey Zone has been slow, and there has been increased allocation in this area over the last year which could cause levels to drop one or two metres below the lowest recorded levels from the start of 2003 if a high demand irrigation season is experienced.

34 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

3.3.7 Ashburton - Rangitata Plains

All the wells below SH1 have all benefited from the latest rain and therefore no problems are expected this summer, especially not for the areas below borderdyke irrigation schemes. Some new deep aquifers wells higher in the plains have still low water levels compared to previous year because of a lack of winter rainfall and this might cause some problems with supply.

3.3.8 Rangitata - Opihi

Groundwater levels in shallow aquifers for the 2003/04 irrigation season are dependent on the flow in the Orari River. Levels are currently higher than those leading into the 2002/03 season, and should maintain aquifer levels for the first half of the season. If the river experiences a low flow summer then groundwater levels, may fall in the later part of the season.

Deep groundwater levels around Rangitata are currently average, and higher than those leading into the 2002/03 season. This is due to the later commencement of irrigation this season. High demand on the aquifer due to increasing allocation may lead to increased lows this summer.

3.3.9 Pareora - Waitaki

Groundwater levels in the Pareora - Waitaki area are currently average, and have benefited from recent river recharge, and are currently trending upwards. These aquifers, however, have little storage, and with no significant rainfall predicted until December, river flows may drop, and hence lead to quick declines in groundwater levels are dependent on river recharge and with surface water flows already low and predicted to decline, those with restrictions associated with river flows can expect continuing restrictions over summer.

Environment Canterbury Technical Report 35 State of the Canterbury region water resource, October 2002

4 Acknowledgements

Environment Canterbury would like to thank the following agencies for the provision of data and advice used in this report:

Flow data Meridian Energy National Institute of Water and Atmosphere

Climate Forecasts New Zealand Meteorological Service National Institute of Water and Atmosphere Blue Skies Weather

36 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

Appendix A: Calculating rainfall trends 1994 to 2003

Below average rainfall for the winter of 2003 was identified in section 1.1 as being part of a longer cycle of below average rainfall throughout much of the Canterbury region. An analysis of rainfall patterns over the last ten years is presented in this appendix.

There are a handful of rainfall sites with very long historic records in Canterbury, which are suitable for studying long-term rainfall trends. For geographical representation rainfall trends over the last 10 years were considered for the Waipara (North Canterbury), Lincoln (mid Canterbury), Arthur’s Pass (alpine) and Duntroon (South Canterbury) rain gauges. All of these sites have at least 80 years of rainfall record. See Figure 1.1 for locations of these gauges.

Precipitation patterns over the last 10 years were analysed for these sites using the Standardised Precipitation Index1 (SPI). The SPI measures rainfall deviations from long-term rainfall normals, for any specified time scale. In the case of the one month time scale, a given month’s rainfall (e.g. January 2003) is compared with long-term mean rainfall for that month (all Januarys on record). Likewise, for the case of, say, the 4 month time scale, rainfall over the previous 4 months (e.g. October 2002 to January 2003) is compared with long-term rainfall for all past equivalent 4 month accumulation periods (all October through to January periods on record).This comparison is carried out for every month in the year.

The SPI allows easy identification of anomalously wet or dry periods, by providing a quantitative measure of rainfall surplus or deficit for the period in question. The SPI scale, from “extremely wet’ to “extremely dry”, is based on the extent of the deviation from average, and can be colour coded so that patterns are easily deciphered (Figure A.1).

Different components of the water resources system are sensitive to different time scales – soil moisture conditions may reflect precipitation anomalies measured at a month time scale whereas groundwater levels may reflect anomalies measured at much longer time scales. Hence SPI indices were calculated for time scales ranging from one month through to 12 months to disclose rainfall patterns over the last ten years. Figure A.1 is a colour coded presentation of SPI indices for Waipara, Arthur’s Pass, Lincoln and Duntroon. The plots provide a temporal and spatial description of the severity and persistence of any rainfall anomalies for these sites. The description is temporal if comparing the patterns of rainfall deviations from “normal” at a given site over time. The description is spatial where the variation is in severity and persistence of a given event from one site to another.

The use of the SPI plots can be explained by comparing the dry conditions shown for Lincoln in 1998 with a subsequent dry period in 2001 (a temporal comparison). The 1998 event was significant in terms of its persistence from late 1997 to mid 1999 and in its severity for the 8 to 12 month accumulation periods ending between June and December 1998. On the other hand, the 2001 event was less significant in terms of its persistence, with the dry trend ending abruptly with a “very wet” month in January 2002. The 2001 event was as significant as the 1998 event in terms of its intensity (deviation from the mean), but for time scales of 1 to 10 months instead of 8 to 12 months. The 2 events, and their impacts, are differentiated by persistence and the time scales over which rainfall deviated significantly from normal.

Looking at all 4 plots in Figure A.1 one can see that the overall pattern over the last 10 years is for periods of higher than average precipitation for alpine areas and lower than average

1 . McKee TB, Doesken NJ, Kleist J,. 1993. The relationship of drought frequency and duration to time scales. In Proceedings of the 8th Conference in Applied Climatology, 17-22 January, Anaheim, CA. American Meteorological Society: Boston, MA: 179-184. For more information on the Standardised Precipitation Index see www.drought.unl.edu/whatis/indices.htm

Environment Canterbury Technical Report 37 State of the Canterbury region water resource, October 2002

precipitation for eastern areas. The most noticeable anomaly among all four sites is that of the 1997/98 drought. Persistent westerly-quarter winds produced higher than average rainfall from November 1997 to October 1998 in alpine areas. However, for the rest of the region warm dry conditions prevailed at this time. This was followed by stable anticyclonic (dry) conditions over the summer of 1998/99. The accumulated effect was a drought in eastern parts of the region that was severe in terms of both magnitude and persistence. Also evident is an anomaly in autumn of 2001, which was “extremely dry” but short in duration.

More recently, it can be seen that below average rainfall has prevailed in eastern parts of the region from mid 2002 to August 2003. There are geographical variations within this. Rainfall deficits are greatest at the one to six month time scales, leading up to May, June and July, at Waipara and Lincoln. There is a lapse in the severity of the rainfall anomaly for several months in late 2002/early 2003 at these sites. On the other hand, rainfall deficits are greatest at the six to 12 month time scales, for the months of March through till August, at Duntroon, and the deficits are greater here than for Waipara and Lincoln. Thus the recent dry interval can be described as being more severe and more persistent in Duntroon than for other sites considered in mid and North Canterbury.

Figure A.1 therefore provides some context for the dry conditions that prevailed this winter, as detailed in section 1.1. In the southern part of the region the winter conditions were part of a more extended dry interval, that persisted in the area for 9 months. The dry interval is most significant at time scales that are greater than the 3 month winter period considered in section 1.1. This helps explain why, despite above average rainfall for September, below average soil moisture levels and river flows are currently reported in the area. Conversely, equally dry winter conditions in the middle and northern parts of the region were part of a shorter dry interval, and consequently these areas were quick to respond to September rainfall. Consequently, soil moisture levels and river flows now being above average for the time of year (section 2.2 and 2.3).

38 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

CATEGORY SPI VALUE Extremely wet 2.0+ Very wet 1.5 to 1.99 Moderately wet 1.0 to 1.49 Near normal -0.99 to 0.99 Moderately dry -1.0 to -1.49 Severely dry -1.5 to -1.99 Extremely dry -2 and less Data not available

NOTE: The SPI categories of 'wet' and 'dry' refer only to rainfall and do not take into account other factors that contribute to hydrological extemes.

Figure A 1 Standard Precipitation Index (SPI) analysis of four Canterbury long-term rainfall records

Environment Canterbury Technical Report 39 State of the Canterbury region water resource, October 2002

CATEGORY SPI VALUE Extremely wet 2.0+ Very wet 1.5 to 1.99 Moderately wet 1.0 to 1.49 Near normal -0.99 to 0.99 Moderately dry -1.0 to -1.49 Severely dry -1.5 to -1.99 Extremely dry -2 and less Data not available

NOTE: The SPI categories of 'wet' and 'dry' refer only to rainfall and do not take into account other factors that contribute to hydrological extemes.

Figure A 1 (continued) Standard Precipitation Index (SPI) analysis of four Canterbury long-term rainfall records

40 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

Appendix B: Groundwater Level Envelope Plots

Bore O31/0030, Chapmans Road Bore O31/0200, Mt Fyffe Road

Bore O31/0026, Kowhai Ford Road Bore O31/0197, Inland Kaikoura Road

Bore N34/0110, Omihi Bore N34/0049, Waipara

Bore M34/0670, Amberley Bore M34/0095, Glasnevin

Environment Canterbury Technical Report 41 State of the Canterbury region water resource, October 2002

Bore M35/0366, Rangiora Golf Course Bore M35/0008, Eyre River Zone, Horrelville

Bore M35/0205, Lower Eyre River Zone, Swannanoa Bore M35/1000, West Melton/Yaldhurst Trigger

Bore M35/1691, West Melton/Yaldhurst Trigger Bore M35/5696, West Melton/Yaldhurst Trigger

Bore M35/1110, West Melton/Yaldhurst Trigger Bore Bore M36/0217, West Melton/Yaldhurst Trigger Bore

42 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

Bore M35/5560, Ilam University, Avon River Spring Zone Bore M35/3691, Confined aquifer Burwood Gravel

Bore M35/3056, confined aquifer Riccarton Gravel Bore L36/0064, Upper Selwyn Zone

Bore M36/0354, Lower Selwyn Zone shallow groundwater Bore M36/0355, Lower Selwyn confined groundwater

Bore L37/0022, Langdons Road, Pendarves Bore L36/0948, Winchmore Research Farm

Environment Canterbury Technical Report 43 State of the Canterbury region water resource, October 2002

Bore K36/0045, Ashburton Forks Bore L36/1338, Chertsey

Bore K38/0100, Lowcliffe Bore K37/0136, Flemington

Bore K37/0200, Laghmor Bore K37/0501, Rangitata Island

Bore K38/0013, Semi-confined aquifer, Orton Bore K37/0300, Upper Coopers Creek

44 Environment Canterbury Technical Report State of the Canterbury region water resource, October 2002

Bore J37/0013, Ashwick Flat Bore J38/0122, Pleasant Point

Bore J39/0006, Pareora Bore J39/0255, Otaio

Bore J40/0071, Waihao

Environment Canterbury Technical Report 45