Climate change impacts on Lake Ellesmere (Te Waihora)

Transcription

1 Climate change impacts on Lake Ellesmere (Te Waihora) NIWA Client Report: WLG September 2010 NIWA Project: ENC10301

2 Climate change impacts on Lake Ellesmere (Te Waihora) James Renwick Graeme Horrell Alistair McKerchar Piet Verburg Murray Hicks Einar Örn Hreinsson NIWA contact/corresponding author James Renwick Prepared for Environment Canterbury NIWA Client Report: September 2010 WLG NIWA Project: ENC10301 National Institute of Water & Atmospheric Research Ltd 301 Evans Bay Parade, Greta Point, Wellington Private Bag 14901, Kilbirnie, Wellington, New Zealand Phone , Fax All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system.

3 Contents Executive Summary iv 1. Background Objectives New Zealand climate Lake Ellesmere (Te Waihora) 5 2. Climate Change scenarios General information New Zealand Christchurch/Canterbury Implications for Lake Ellesmere (Te Waihora) Climate variables Temperature Rainfall Evaporation Tributary inflows Wave regime and sea levels Uncertainty in climate projections Water balance modelling Lake level and lake opening Possible effects of climate change on the ecosystem in Lake Ellesmere Discussion and summary References 29 Reviewed by: Approved for release by: Andrew Tait Principal Scientist, Climate David Wratt Chief Scientist, Climate

4 Executive Summary This report describes an initial study into the effects of climate change upon Lake Ellesmere (Te Waihora), in particular the effects of changes in the hydrological cycle. The study was designed to provide a set of future scenarios for lake conditions, presented in the context of current observed variability. Scenarios cover the effect of changes in water inflow and evaporation on lake water balance, and the effects of changes in air temperature, solar radiation and wind on mixing in the lake. Information is provided in terms of surface climate variables (temperature, rainfall, etc) and in terms of water balance, lake hydrology/ecology, and impacts on lake opening regimes. Lake Ellesmere (Te Waihora) is artificially opened to the sea via a cut through the beach barrier to lower the lake level when surrounding farmland is threatened by flooding. The artificial opening management regime for the beach barrier has been modelled using 21 years of data for , to assist with future management scenarios. Scenarios for lake inflow and evaporation could feed into a future project to develop a lake water balance model to assess the impact of various lake opening regimes on lake levels. The main components of natural variability that affect Lake Ellesmere are the El Niño-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO). El Niño events tend to be associated with drier than normal conditions near the lake (especially during summer) while La Niña events are often associated with near normal rainfalls near Lake Ellesmere. The IPO modulates the ENSO cycle (with a tendency for more El Niño events during the positive IPO) but has only a weak effect overall on rainfall in the vicinity of Lake Ellesmere. ENSO and IPO variability is expected to continue as the climate warms. The projected effects of climate change upon the lake were assessed for three climate change scenarios, and for two time slices, one centred on 2040 and the other on The main influence is that with elevated temperatures (between 0.7 and 2.7 C), evaporation loss from the lake increases (between 6 and 129%), leading generally to slightly lowered lake levels, and a reduced frequency and duration of lake openings (up to ~10 days per year). Note that in this study, annual rainfall onto the lake, and freshwater and groundwater inflows, are assessed as only changing to a minor degree (~1% on an annual basis, but up to ±10% seasonally). However it is likely that inflows will be affected by future, but as yet unknown, developments in irrigation on the Canterbury Plains between the Rakaia and Waimakariri Rivers. Further studies will be necessary as the developments occur. It is assumed that the beach barrier height above mean level of the sea will rise as sea level rises. Possible changes in storm frequency building the gravel beach barrier and closing the lake after an opening have not been included, largely because the present guidance available is minimal. Projected rising sea levels suggest that the threshold rules for lake opening will need to be reconsidered in future. Climate change impacts on Te Waihora/Lake Ellesmere iv

5 1. Background 1.1 Objectives Published broad-scale projections of climate change in New Zealand over 50- and 100-year time spans (to the end of the 21st Century, Mullan et al. 2008, MfE 2008b) show the potential for significant changes in the hydrological cycle as it affects Lake Ellesmere. Any consideration of long-term changes in the management of Lake Ellesmere should be evaluated in the light of scenarios for future climate change. This report is designed to provide underpinning information to better inform strategic management of the lake, including the lake opening regime. Periodic artificial openings (currently achieved by bulldozer) have been made for over 100 years to lower the lake levels when surrounding farmland is threatened by flooding. Records of the dates of lake openings and closings and corresponding levels are available over that time. Since Pakeha settlement, these occur typically two or three times a year when the still-water level reaches 1.05 m AMSL in summer, or 1.13 m in winter. The lake area varies with the level, but is typically about 180 km 2, which rates it as the fourth or fifth largest lake in the country. A first step in understanding climate change effects on the lake levels and the frequency of artificial openings is to undertake a water balance assessment of the lake to quantify the fluxes of water entering and exiting the lake. The picture is complex (Horrell, 1992). Water enters the lake from rivers and streams, from groundwater, from rainfall directly on the lake, from the sea by barrier overtopping during storms, from seepage through the barrier when the lake level is below the sea level and through artificial openings. Water leaves the lake by evaporation, seepage through the gravel beach barrier and through artificial openings. The most significant climate-driven changes to consider are (i) effect of changes in rainfall, water inflow, and evaporation on lake water balance (ii) effect of changes in air temperature, solar radiation and wind on mixing in the lake. Another significant likely impact of climate change is on groundwater use, and the consequential impact of groundwater use on stream flow. However, the effects of climate variability and change upon groundwater are as yet poorly understood and is the subject of current research. Hence, although it is recognised as an important issue, groundwater change is not discussed here. Climate change is projected to produce gradual trends in addition to the significant and on-going interannual variability (including the El Niño-Southern Oscillation) and Climate change impacts on Te Waihora/Lake Ellesmere 1

6 interdecadal variability (including the Interdecadal Pacific Oscillation). One aim of this report is to ensure that climate change impacts are presented in the context of current observed variability. The scenarios for lake inflow and evaporation presented here feed into a lake water balance model that is used to assess the impact of climate change on lake opening regimes and lake levels. Scenarios for changes to air temperature, solar radiation and wind could potentially feed into further development of a hydrodynamic model of the lake. 1.2 New Zealand climate New Zealand lies in the middle latitudes of the southern hemisphere (34 to 47 S). The climate is affected all year round by the band of mid-latitude westerly winds and by the subtropical high pressure belt. Both major circulation features move north and south with the march of the seasons. The westerlies are farthest north in winter and spring and the influence of the subtropical high is strongest in summer and autumn (Figure 1). The windiest season of the year is spring, as the subtropical high begins to migrate southwards and the surface pressure gradient across New Zealand strengthens. Much of the country s weather is influenced by the passage of fronts and depressions in the westerlies, which cross New Zealand longitudes every 4-5 days at all times of the year (Maunder 1971, Sturman & Tapper 2006). Figure 1. outhern hemisphere mean wind circulation for winter (JJA, left) and summer (DJF, right), on the 850 hpa pressure surface (approximately 1 km above ground level). Data are an average over from NCEP/NCAR reanalyses (Kistler et al. 2001). Climate change impacts on Te Waihora/Lake Ellesmere 2

7 The main New Zealand mountain chains, particularly the Southern Alps, are aligned almost at right angles to the prevailing westerly wind flow and provide a significant barrier to that flow. Much of the rich regional detail in New Zealand climate comes from complex interactions between the large-scale atmospheric circulation and the rugged topography. The most notable effect is the east-west gradient in rainfall, ranging from 3 to 4 m per year in Westland to 12 m or more in the Alps, but less than 500 to 700 mm in Otago and Canterbury (Wratt et al. 1996) (Figure 2). Figure 2. New Zealand median annual rainfall (mm). Climate change impacts on Te Waihora/Lake Ellesmere 3

8 Year to year variability in New Zealand climate is influenced by a number of components of the large-scale climate system, primarily through their influence on the mid-latitude westerly circulation that defines much of the country s climate. The most notable are the El Niño-Southern Oscillation (ENSO) cycle (Gordon 1986, Mullan 1996) and the Interdecadal Pacific Oscillation (IPO) (Salinger et al. 2001). The ENSO cycle involves an irregular exchange of heat in the upper layers of the ocean between the western and eastern Equatorial Pacific. The normally cool region off the coast of South America warms up in an El Niño, associated with a weakening of the trade winds and changes in tropical rainfall patterns. Such tropical shifts have flow-on effects across the Pacific, resulting, on average, in cooler conditions with stronger westerly winds over New Zealand. Rainfall tends to be enhanced in western regions, with an increased risk of dry conditions in the east and north. In a La Niña, the eastern Equatorial Pacific cools and the trade winds increase in strength. The net effect on New Zealand is on average reduced westerly winds, with warmer conditions, especially over the summer months. Rainfall is on average enhanced in the north and east, and reduced in the west and south. Figure 3: Median seasonal rainfall anomalies (percent departure from normal) for ten moderate-strong El Niño events (left) and ten moderate-strong La Niña events (right). The years listed are associated with the end of each summer period (i.e is Dec 1963-Feb 1964, and so on.) Climate change impacts on Te Waihora/Lake Ellesmere 4

9 Figure 3 shows anomalies in the median summer rainfall (percent of normal) around New Zealand observed for El Niño conditions (left) and La Niña conditions (right). During El Niño events, all of Canterbury tends to experience less rain than normal, while during La Niña, the pattern of response is much less coherent spatially. In the region of Lake Ellesmere, El Niño summers are associated with below-median rainfall in around two thirds of cases, while La Niña summers are evenly distributed between below- and above-median seasonal rainfall. The IPO is essentially a long-term modulation of the ENSO cycle, bringing year periods of stronger and more frequent El Niño events, alternating with periods of weaker El Niño and stronger La Niña conditions. The IPO is manifested as a change in the background state of the Pacific Ocean, moving towards an El Niño state during the positive phase (e.g., late 1920s to mid 1940s, late 1970s to late 1990s), and towards a La Niña state during the negative phase (e.g., late 1940s to mid 1970s, and since 2000). During the positive IPO, with a predominance of El Niño events, New Zealand tends to experience generally stronger westerly wind flow, with higher mean rainfalls in western and alpine regions. During the negative IPO, the mean westerly circulation over New Zealand slackens, and rainfalls tend to reduce in western regions, while increasing somewhat in the northeast of the country. The average effect of the IPO upon rainfall around Lake Ellesmere is weak. 1.3 Lake Ellesmere (Te Waihora) Lake Ellesmere (Te Waihora) is a large shallow lake formed behind a coastal beach barrier to the southwest of Banks Peninsula in Canterbury (Figure 4). The sheltering effect of the Southern Alps, combined with the prevailing westerly wind flow over New Zealand (as discussed above) results in a relatively dry climate, with a high frequency of warm days (in north-westerly wind conditions). Annual mean rainfall in the general vicinity of the lake is between 500 and 750 mm (Fig. 2), which falls relatively evenly through the year. Climate change impacts on Te Waihora/Lake Ellesmere 5

10 Figure 4: Location map of Lake Ellesmere (Te Waihora) and the surrounding area of eastern Canterbury/Banks Peninsula. (Source: Google Maps) The Waihora outlet occurs at a narrow, low section at the southern end of the Kaitorete Spit barrier complex. The divide separating lake from ocean is a single beach ridge, the height of which changes often in response to ocean waves (that may either cut-back or build-up the ridge), lake opening events (where the lake is artificially opened to the sea via a cut through the beach barrier, to lower the lake level when surrounding farmland is threatened by flooding), and sand-dune formation from onshore winds. To the north, the barrier widens into a succession of accretionary beach ridges that have built out over recent millennia. To the south, the barrier continues as a single beach ridge that separates the ocean from the low-lying hinterland. The ridge is most fragile at the outlet for two reasons. The first is a direct consequence of opening events, when gravel is scoured from the outlet channel and its banks. Indeed, at times in recent millennia when the Waimakariri River has avulsed into the Waihora catchment, much larger floods have drained through the outlet and will have pre-cut a wider weak point (Hemmingsen, 2001). The second reason relates to ongoing adjustment of the northern end of the Canterbury Bight shoreline to the current sea-level stand (e.g., Leckie, 2003). In brief, because Climate change impacts on Te Waihora/Lake Ellesmere 6

11 Banks Peninsula traps the coarse sand and gravel being driven north along this shore by the prevailing southerly wave climate, the Kaitorete Spit ocean-shore has been accreting and pivoting clockwise to align more with the wave approach direction. South of Kaitorete Spit, the shore is retreating due to ongoing erosion of the Pleistocene alluvial fan of the Rakaia River and stormwave-driven rollover of the barrier ridge immediately south of the lake. This retreat, along with gravel and sand from the modern Rakaia River, feeds the beach to the north, albeit with losses to abrasion. As time advances, the shoreline pivoting means that the hinge point between coastal retreat and advance has been migrating northwards, and currently it appears to have passed the outlet location hence that too is now undergoing longterm retreat, even if the spit further north is stable/accreting. This type of autocannibalistic behaviour with the downdrift (northern) end of the barrier feeding in part off material eroded from its updrift (southern) end, is a not uncommon situation for gravel coasts (e.g., Orford et al., 2002). The expectation is, therefore, that even without any future climate change effects, the narrow neck at the lake outlet will gradually widen northwards. The lake supports eel and flounder fisheries and has important cultural and recreational values. In the past the lower Selwyn River has been an excellent trout fishery, but this is much diminished in recent years. Reasons for this decline are still unclear, but are possibly related to the generally poor quality of the lake water associated with nitrate leaching (Larned and Schallenberg 2006, Verburg et al. 2010). Climate change impacts on Te Waihora/Lake Ellesmere 7

12 2. Climate Change scenarios 2.1 General information The extent of future climate change depends in large part on future concentrations of atmospheric greenhouse gases and aerosols. In the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), the range of global mean temperature increase during the 21st century was given as C, over the six illustrative scenarios (Figure 5). Projections based upon the A1B scenario result in mid-range changes in global mean temperatures and other parameters of the climate system. Using the A1B scenario, global mean warming is projected to average a little under 3 C by the end of this century, relative to average temperatures in the late 20th century (or about 3.5 C relative to pre-industrial temperatures). Figure 5. Global mean surface warming for a range of emissions scenarios and climate models. Solid lines are multi-model global averages of surface warming (relative to ) for the scenarios A2, A1B, and B1, shown as continuations of the 20th century simulations. Shading denotes the ±1 standard deviation range of individual model annual averages. The orange line (bottom line) is for the experiment where concentrations were held constant at year 2000 values. The grey bars at right indicate the best estimate (solid coloured line within each bar) and the likely range of warming by 2100 for the six SRES 1 marker scenarios. (Source: figure SPM.5, IPCC (2007)). 1 SRES: Special Report on Emissions Scenarios, Nakicenovic et al. (2000). Climate change impacts on Te Waihora/Lake Ellesmere 8

13 The distribution of future warming is likely to be non-uniform, with the largest warming expected over the Arctic and the high-latitude continents of the Northern Hemisphere (Hansen et al 2006; 2007). The least surface warming is expected over the southern oceans. As the world warms, the tropical regions expand (as has already been observed, e.g. Seidel et al. 2007), leading to significant large-scale changes in rainfall patterns globally. Broadly, increased rainfall is expected in regions near the Equator and over middle and high latitudes of both hemispheres, while subtropical regions (mostly between 30 and 40 latitude) are expected to see decreased rainfall as the subtropical high pressure belt spreads poleward (IPCC 2007). The global hydrological cycle is expected to accelerate with climate change (Wentz et al. 2007) and global evaporation has been predicted to increase (Ramanathan 2001). While the information presented here represents our best understanding, climate change science continues to evolve. The projected magnitude and scope of future climate change is continually being adjusted to account for new findings such as the effect of terrestrial biogeochemical feedbacks in the climate system (Arneth et al. 2010), and for refinements in climate models. 2.2 New Zealand Global warming of around 3 C this century, under the A1B emissions scenario, would translate to a mean warming over New Zealand of around 2.1 C (Mullan et al. 2008), a warming rate of about 70% of the global mean. A consistent signal from global climate model projections is for an increase in the westerly wind circulation over New Zealand, especially in winter and spring. During summer, there may be a tendency for reduced westerly winds over New Zealand, but this is less certain (Mullan et al. 2008). Hence, evaporation may increase more (at least in relative terms) in winter than in summer. As a result of expected wind changes, there is likely to be an increase in annual mean precipitation in western regions of New Zealand, and a decrease in rainfall in the east of the country. Such changes are likely to be most pronounced in winter and spring. During summer, there may be a reversal of this trend, with somewhat increased rainfall in the east of the country, and decreases in the west (Mullan et al. 2008). Such changes in the mean climate would result in many changes in extremes of climate: reduced frost frequency and increased risk of heat waves over the whole country, reduced soil moisture and increased risk of drought in the east of the country, increased risk of forest fires in many eastern and northern regions, and increased risk of heavy rainfalls in most places. Climate change impacts on Te Waihora/Lake Ellesmere 9

14 As a result of temperature and rainfall changes, reductions in snow pack are likely in the Southern Alps. Recent modelling (Mullan et al. 2008, Hendrikx et al. 2008) projects decreases in seasonal snow cover and depth throughout the country, particularly in the South Island. Under a middle of the road scenario (IPCC A1B), there are likely to be significant decreases in snowpack right through the 21st century. Heavy rainfall events are projected to be more frequent and intense across New Zealand. Modelling by Pall et al. (2007) suggests that extreme rainfall events with a return period of greater than 30 years will have on average about 8% more rain for every 1 C rise in air temperature. Carey-Smith et al. (2010) demonstrate that increases in extreme rainfall amounts may be even higher, through a combination of thermal and dynamical effects. Gray et al. (2005) modelled three storm events for the west coast Buller catchment of New Zealand for the current climate and for three scenarios of temperature increase (0.5 C, 1.0 C, and 2.7 C). Modelled rainfall increased on average 3%, 5%, and 33%, respectively. Analyses of this kind for other New Zealand catchments have not yet been carried out. A substantial increase is projected for the number of warm days (days with maximum temperatures above 25 C), particularly at already warm sites. For the current climate, Christchurch has around 31 warm days per year. Projected changes for the region near Lake Ellesmere indicate an increase of around 25 to 40 more warm days, approximately a doubling in frequency compared to the current climate (Mullan et al. 2008). There are indications of an increased risk of extreme wind events with climate change. As the westerly circulation increases over the country in winter and spring, the frequency of strong winds above a given threshold will almost inevitably increase. Research is ongoing in this area, but work to date suggests that up to a 10% increase in wind speeds above the current 99th percentile (top 1% of wind speeds) is possible by the end of the century (Mullan et al. 2008). The sequence of climate experienced in New Zealand over coming decades will be a combination of natural variability (ENSO, IPO, and other components of the climate system) and anthropogenic effects as discussed above. Different components can work together or can cancel out: the negative phase of the IPO is associated with wind and rainfall changes opposite in form to what is expected from multi-decadal climate change, implying that an extended negative excursion in the IPO (such as that which began around ) would act to damp the climate change signal over New Zealand. Conversely, a switch to the positive IPO is expected to enhance anthropogenic climate change trends. Climate change impacts on Te Waihora/Lake Ellesmere 10

15 2.3 Christchurch/Canterbury Projected climate changes for Canterbury are taken from the relevant tables and discussion in section 2.2 of Mullan et al. (2008) and in Part 1 of MfE (2008b). Results reported here are based on a statistical downscaling of the output of 12 global climate models (GCMs) run for future climate under the A1B emissions scenario. For the 2008 study, the downscaled values for A1B were scaled according to the relative global mean temperature change to estimate figures for the other five marker scenarios used here. See Mullan et al. (2008) for more detail. Note however that for the Ellesmere projections discussed in section 3, values for the B1, A1B, and A2 scenarios were all downscaled independently. Downscaled projections of changes in air temperature for New Zealand show quite smooth geographic variations. Hence, projections are presented as averages over the whole Regional Council area, as there is little variation within the region. Table 1 shows 50- and 100-year projected changes in seasonal and annual average mean temperature (in C, from ). Figures shown are the average change, and the lower and upper limits (in brackets), over six illustrative scenarios (B1, A1T, B2, A1B, A2 and A1FI). Table 1: 50- and 100-year projected changes in seasonal and annual average mean temperature (in C, from ) averaged over the Canterbury regional council area. Figures shown are the average change, and the lower and upper limits (in brackets), over six illustrative scenarios (B1, A1T, B2, A1B, A2 and A1FI). Period Summer Autumn Winter Spring Annual (50 y) 0.9 [ 0.1, 2.2] 0.9 [ 0.2, 2.2] 1.0 [ 0.4, 2.0] 0.8 [ 0.2, 1.8] 0.9 [ 0.2, 1.9] (100 y) 2.1 [ 0.8, 5.2] 2.1 [ 0.7, 4.9] 2.2 [ 0.8, 5.1] 1.8 [ 0.4, 4.7] 2.0 [ 0.7, 5.0] Rainfall typically varies much more rapidly in space (and in time) than does temperature, and projected rainfall changes tend not to be spatially homogeneous across regional council regions. For example, projections for Canterbury show rainfall increases in the west of the region and decreases in the east. Hence, rainfall projections have been tabulated for specific places. Moreover, Environment Canterbury may need to carefully consider regional gradients in rainfall changes, when considering issues related to river levels and groundwater. In coastal Canterbury, rainfall is projected to decrease, but large alpine-fed rivers could have increased flows on average because of greater rainfall in the headwaters. Climate change impacts on Te Waihora/Lake Ellesmere 11

16 Table 2 shows the estimated range in precipitation change over the six illustrative SRES scenarios, for two sites in Canterbury: Christchurch (near lake Ellesmere) and Tekapo (indicative of changes in the headwaters of major rivers). The average change over all 12 models and six scenarios is also given. Table 2: 50- and 100-year projected changes in seasonal and annual average precipitation (in %, from ) for two selected sites in Canterbury. Figures shown are the average change, and the lower and upper limits (in brackets), over six illustrative scenarios (B1, A1T, B2, A1B, A2 and A1FI). Period/location Summer Autumn Winter Spring Annual (50 y) Christchurch 2 [ 15, 22] 5 [ 10, 30] 8 [ 30, 7] 1 [ 8, 9] 1 [ 10, 9] Tekapo 1 [ 16, 16] 2 [ 12, 10] 8 [ 1, 19] 6 [ 3, 17] 4 [ 0, 13] (100 y) Christchurch 3 [ 17, 25] 6 [ 6, 20] 11 [ 41, 10] 2 [ 15, 25] 2 [ 14, 16] Tekapo 2 [ 30, 31] 0 [ 16, 17] 18 [ 5, 41] 10 [ 6, 47] 8 [ 0, 29] The annual average rainfall change has a pattern of increases in the west (up to 5% by 2040 and 10% by 2090) and decreases in the east and north (exceeding 5% in places by 2090) (Mullan et al. 2008). The annual pattern of being wetter in the west and drier in the east is driven by that pattern occurring in the winter and spring seasons, when westerly winds are likely to be stronger than at present across New Zealand. In summer and autumn, the pattern is quite different, especially for the North Island where the pattern is reversed (although winter/spring changes dominate in the annual mean). There is still a lot of variability between models, although most models agree on the sign of the projected precipitation change across Canterbury (see Mullan et al. 2008, Appendix 3 for further discussion). Climate change impacts on Te Waihora/Lake Ellesmere 12

17 3. Implications for Lake Ellesmere (Te Waihora) 3.1 Climate variables Over recent years, NIWA scientists have developed gridded data sets of daily climate parameters, on a 0.05 latitude by 0.05 longitude grid (approximately 5 km by 5 km) covering the whole country (a total of approximately 11,500 grid-points). The Virtual Climate Station (VCS) data set begins for most variables in 1972 (1960 for rainfall and 1997 for wind speed), and is based on interpolated data from approximately 150 climate stations and approximately 500 rain gauges using a sophisticated interpolation technique developed at the Australian National University in Canberra (Tait et al. 2006; Tait and Woods 2007; Tait 2008). Here, we have averaged VCS data for grid points over Lake Ellesmere to derive time series of daily climate variability. Synthetic future climate time series were generated by applying offsets to the present-day data, consistent with regional-scale projected changes discussed in Mullan et al. (2008). The 21 year period was adopted to study climate change influences on the lake level and opening regime. The projections and the projections are used for the B1, A1B and A2 emission scenarios detailed above, giving a total of six cases. Seasonal averages of the mean adjustments applied are summarised in Table 3. The percentage changes for rainfall and potential evapotranspiration for the six cases are applied to the data and the results, in terms of the lake water balance and opening regime, are compared with those for the recorded data. Climate change impacts on Te Waihora/Lake Ellesmere 13

18 Table 3: 50- and 100-year offsets in seasonal and annual average mean temperature ( C), precipitation, and PET (%) for Lake Ellesmere, used in scenario calculations for lake water balance. The scenario names (B1, A1B, and A2) are shown on the right MAM JJA SON DJF Ann MAM JJA SON DJF Ann Temperature ( C) B1 Rainfall (%) PET (%) Temperature ( C) A1B Rainfall (%) PET (%) Temperature ( C) A2 Rainfall (%) PET (%) Temperature Although the temperature projections are important because of their influence on evaporation (as described below), they are not used directly in the water balance study of the lake. The evaporation data are considered below Rainfall Monthly adjustments to the rainfalls estimated to occur over the lake for each of the six cases were applied. In addition to mean monthly adjustments, the rainfall distribution was changed to dry out and decrease the number of low rain days and to increase the high rain days (as described in Woods et al. (2009)). As seen in the upper panel of Figure 6, these factors resulted in only minor (less than 5 mm/yr) changes in the annual rainfalls, a result consistent with the changes in annual rainfall for Christchurch noted in Table Evaporation Changes in potential evapotranspiration (PET, Penman 1963) were estimated from temperature data as follows. Monthly average temperature changes for each climate change scenario were calculated by averaging across the 10 VCSN grid point over the lake. The associated changes in monthly total PET (mm) were estimated by scaling the temperature changes by (based on a regression between historic mean monthly temperature and PET at Christchurch Airport). Percentage changes in monthly PET Climate change impacts on Te Waihora/Lake Ellesmere 14

19 were calculated as a fraction of the mean monthly PET at Christchurch Airport climate station and were used to scale observed monthly PET values from the Lincoln site (Lincoln, and Lincoln Broadfield) near Lake Ellesmere. It should be noted that this method of estimating future changes in PET based on an historic relationship to air temperature and projected temperature change is simplistic. NIWA are currently updating their climate change projection statistical downscaling methodology which will include directly downscaling PET from Global Climate Models. The effects of using different methods of calculating PET are discussed in the report on drought recently completed for the Ministry of Agricluture and Forestry (Clark et al. 2011). The mean annual potential evapotranspiration (PET, Penman 1963) estimates for the lake for and for the six future scenarios are plotted in the middle panel of Figure 6. In each of the six cases, the PET estimates show increases, with values ranging from 110 to 374 mm/yr. The mean estimates of the annual balance of PET less rainfall are shown in the lower panel of Figure 6. These differences with the present state of PET-rain range from 16% to 60% and are important because they will have a major influence on the lake water balance. Climate change impacts on Te Waihora/Lake Ellesmere 15

20 550 Mean rain (mm/yr) Present B1-40 B1_90 A1B_40 A1B_90 A2_40 A2_ Mean PET (mm/yr) Present B1-40 B1_90 A1B_40 A1B_90 A2_40 A2_90 Mean PET-Rain (mm/yr) Present B1-40 B1_90 A1B_40 A1B_90 A2_40 A2_90 Figure 6: Present ( ) mean annual rainfall (top panel), potential evapotranspiration (middle panel) and the difference between potential evapotranspiration and rainfall (lower panel), together with the mean annual values for the six climate change cases. Data were taken from a selection of VCS grid points over the region of Lake Ellesmere. Future scenario values are calculated for B1, A1B and A2 SRES emissions scenarios (Nakicenovic et al., 2000), for future periods centred on 2040 (e.g. A1B_40) and 2090 (e.g. A1B_90). Climate change impacts on Te Waihora/Lake Ellesmere 16

21 3.1.4 Tributary inflows Freshwater inflows to the lake are from groundwater and a large number of streams, many of which are sustained by groundwater from the Canterbury Plains. On an annual basis, the total mean inflow into the lake is approximately 12 m 3 /s and the two major inflows are the Selwyn and L2 rivers (mean flows both about 2 m 3 /s); other important tributaries (mean flows 1 to 2 m 3 /s) are the Irwell River and Harts Creek. The inflows are sustained mainly by groundwater on the lowland parts of the Canterbury Plains, but also by perennial streams such as the Kaituna which drains from the Banks Peninsula and by the Selwyn River which during periods of high flows conveys water along its channel from the Canterbury foothills. The groundwater itself is sustained by channel losses from the Selwyn River, and the Rakaia River to the south and the Waimakariri River to the north, as well as from recharge on rain on the plains, mainly during winter months, when soil moisture levels are at field capacity. There is minimal guidance with which to assess the climate change effects on these inflows. The approach adopted here is to use factors derived for rainfall responses for the plains for each of the six cases. These factors produce reductions ranging from 3 to 21 percent in the mean inflows for B1_40 (which is the B1 emission scenario-based projection for 2040; likewise for the following scenarios), B1_90, A1B_40, A1B_90, A2_40 scenarios while the A2_90 increases by 1 percent (Table 4). Table 4: Lake Ellesmere plains-area rainfall used for stream inflow and groundwater estimation for the six scenarios. Scenario B1_40 B1_90 AIB_40 AIB_90 A2_40 A2_90 Rainfall change (%) Wave regime and sea levels Closures after openings of the beach barrier to lower the lake level typically occur during southerly storms which supply gravel to the beach barrier. In this study no account is taken of possible changes in storm conditions, mainly because of limited knowledge on the topic. Rising sea levels are another issue. Ministry for the Environment (2008a,c) gives detail and recommends that while a base value of 0.5 m sea level rise should be used, Climate change impacts on Te Waihora/Lake Ellesmere 17

22 rises of at least 0.8 m need to be considered over the rest of this century. The rising sea levels may be expected to build the beach barrier height, but the current rule for lake opening which is based on lake level above mean sea level will need to be reconsidered, since higher mean levels of the sea will mean that successful lake openings will be less likely Uncertainty in climate projections Estimates of future values of climate variables used in this report vary strongly with the choice of emissions scenario, as seen in the tables above. For each of the scenarios, offsets applied to climate variables were based on median values form Mullan et al. (2008). However, as shown by Mullan et al. (2008), for a given emissions scenario, there is significant variation between different climate model projections of future changes in regional climate (see Appendix 3 of Mullan et al. (2008) for a fuller discussion). The level of agreement between models is generally highest for temperature change, where almost all models evaluated agree on the seasonality of changes (generally least warming in Spring) and most agree broadly on the magnitude of change and its regional pattern around New Zealand. There is somewhat less agreement for rainfall change (and therefore for associated hydrological variables such as river flow), where for some regions, several models project rainfall decreases while others project increases. This is especially obvious in summer (Mullan et al. (2008) Figure A3.8), where changes to rainfall patterns are especially sensitive to how each model represents changes to the regional wind circulation over the country. Projections for derived variables such as PET are subject to uncertainties in the underlying variables, and to those associated with the calculation method (Clark et al. 2011). For the Canterbury region, and for the area around Lake Ellesmere (Te Waihora), there is relatively good agreement between models that winter rainfall is likely to decrease by the late 21 st century, while summer rainfall is likely to stay near present levels or to increase (Mullan et al. (2008) Figure A3.8). In the annual mean, the majority of models project a rainfall decrease for the Canterbury region, although a minority project rainfall increases (Mullan et al. (2008) Figure A3.10). Uncertainties also exist around future changes to climate variability. The assumptions used here are designed to represent well the expected changes to the mean climate, rather than to variability and extremes. Again, this is likely to be more of an issue for rainfall than for temperature, as rainfall extremes are very sensitive to temperature, and to changes in atmospheric circulation (Carey-Smith et al. 2010). Climate change impacts on Te Waihora/Lake Ellesmere 18

23 3.2 Water balance modelling The water balance model used in the study is described in Horrell (1992). It uses estimates of daily water fluxes into and out of the lake, as well as the times that the lake has been mechanically opened to the sea to drain it when the level exceeds 1.05 m AMSL between August and March, and 1.13 m between April and July. Sea conditions directly influence the success of the lake opening operation and lake closure time and wave rider buoy data are used to give information on sea conditions. The water balance is stated as: S = (I as + I g + I hs + I r + I t ) (O a + O e + O s ) where: S is increase in lake volume; I as is seawater incursion during artificial openings; I g is groundwater inflow to the lake; I hs is seawater spilling over the beach barrier into the lake during storms when seas are high. I r is water added to the lake directly by rainfall on the lake surface; I t is total tributary inflow; O a is outflow through artificial opening; O e is water loss through evaporation; O s is net seepage through the Kaitorete Spit. Average components of this water balance for the period from a water balance completed for this study are shown in Figure 7. The feature of note in this figure is that the main input into the lake is the inflowing streams and the main output is through the lake openings. The evaporation from the lake surface (which is approximated with the potential evapotranspiration: i.e. the Penman (1963) PET is similar in magnitude to evaporation over open water) is approximately twice the rainfall on the lake. Climate change impacts on Te Waihora/Lake Ellesmere 19

24 Average rate (m 3 /s) 0-5 Sea inflows through cut Groundwater inflow High seas overtopping Rainfall on lake Tributary inflows Outflows through cut Evaporation from lake Barrier seepage Figure 7: Average components of input and output to Lake Ellesmere (Te Waihora), Lake level and lake opening For a mean sea-level rise of the order of m over 100 years (and m over 50 years), the simplest expectation is that the beach system along Kaitorete barrier will rise, more or less in pace with sea level. This will occur as swell and storm waves ride in on progressively higher sea-levels. Since the beach ridge will build-up from its existing foundation, more sediment will be needed to raise the profile. It is expected that this sediment will come in part from the lower foreshore. Also, since there is no backshore ridge at the outlet area, the beach ridge will also roll landward through storm-wave overwashing processes. Hence, some retreat of the shore is expected. This type of response is widely predicted for gravel barrier shores (Ministry for the Environment, 2008). Predictions of the retreat are made with simple geometric models that relate the retreat to the foreshore slope and the seal-level rise. Since the foreshore is relatively steep on these shores, the retreat will be relatively small (order 10 m for a 0.8 m sea-level rise). The shoreline response is likely to be more complex than this simple cross-shore adjustment, however, due to the broader coastal setting. As outlined in section 1.3, the present shoreline stability pattern reflects mainly a northward reduction in the wavedriven longshore transport, due to the eastward pivoting of the shoreline from a drift- Climate change impacts on Te Waihora/Lake Ellesmere 20

25 aligned to a swash-aligned configuration. Climate change will affect this system if it causes changes in the supplies of gravel and sand from the Rakaia River and cliff erosion and/or causes a change in the wave climate. Projected increases in rainfall about the main divide will likely increase the gravel and sand supply from the Rakaia River, while the retreat-rate of the eroding Pleistocene fan cliffs and hence the gravel supply from that source will likely also increase slightly with a higher sea-level. However, if the gravel supplies are diverted into building-up the beach barrier to the south of the outlet, then there is the possibility that the whole Kaitorete barrier will go into retreat mode. Changes in wave climate are currently too hard to project, but could lead to a change in the net rates and spatial gradients of longshore drift, and therefore changes in the pattern of shoreline retreat and advance. There will also likely be time lags as morphological responses propagate alongshore. Evaluating such effects on future shoreline position at the Waihora outlet is well beyond the scope of this study. However, whichever scenario may develop, the likely outcome is for a higher, narrower, landward-migrating beach ridge spanning the outlet zone. Left unmanaged, the lake level would rise to match sea-level and the higher beach ridge. The regime of lake levels modelled with the recorded data (termed present climate ) is presented in Figure 8, along with the modelled level regimes for the six climate change cases. Note that the values shown here derive from the adjustments made to climate variables that are listed in Table 3. Figure 9 presents four years of the modelled data ( ) in more detail. The levels for the six cases generally follow the present climate, but occasional departures occur when the rules for lake opening or closing are invoked by a modelled level reaching a threshold, or by storms occurring. The variables with estimated climate changes are groundwater inflows, rainfall on the lake, tributary inflows and evaporation from the lake (Figure 7), other changes to variables such as sea inflows through cut, outflows through cut and barrier seepage are a consequence of those changes (see also Table 3). Climate change impacts on Te Waihora/Lake Ellesmere 21

26 Lake level (mm) Present climate B1_40 B1_90 A1B_40 A1B_90 A2_40 A2_ Figure 8: Regime of lake levels modelled with the recorded data, together with the modelled level regimes for the six climate change cases. Climate change impacts on Te Waihora/Lake Ellesmere 22

27 Present climate B1_40 B1_90 A1B_40 A1B_90 A2_40 A2_ Jan-91 Mar-91 May-91 Jul-91 Sep-91 Nov-91 Jan-92 Mar-92 May-92 Jul-92 Sep-92 Nov-92 Jan-93 Mar-93 May-93 Jul-93 Sep-93 Nov-93 Jan-94 Mar-94 May-94 Jul-94 Sep-94 Nov-94 Lake level (mm) Figure 9: Regime of lake levels modelled with the recorded data, together with the modelled level regimes for the six climate change cases. Climate change impacts on Te Waihora/Lake Ellesmere 23

28 The results in terms of average number of days open per year, and average number of openings per year are shown in Figure 10. These plots show that with the projected climate change effects on the lake water balance, the lake should be expected to be open for slightly fewer days per year, and that the number of openings is expected to fall slightly. There are some interesting variations within the scenarios, with the A1B_90 resulting in the least number of openings/days open due to the increase in lake evaporation and decrease in rainfall, while the rainfall upon the plains decreases by 14 % reducing the tributary inflows a large water balance variable for the lake (Figure 7). In contrast, the A2_90 scenario shows a smaller decline from the A1B_90 due to an increase in rainfall over the plains (1%) but a larger evaporative reduction upon the lake and increased reduction of rainfall over the lake cumulating in lake opening days/openings similar to the other four scenarios Days open/yr Present B1-40 B1_90 A1B_40 A1B_90 A2_40 A2_90 Number openings> Present B1-40 B1_90 A1B_40 A1B_90 A2_40 A2_90 Figure 10: Upper panel: average number of days per year that the lake is open; Lower panel: average number of openings per year. Climate change impacts on Te Waihora/Lake Ellesmere 24

29 3.4 Possible effects of climate change on the ecosystem in Lake Ellesmere. Lakes are warming in most parts of the world, from the tropics to Antarctica, as an effect of global warming (e.g. Quayle et al. 2002, Verberg and Hecky 2009). The effects of the warming on the ecosystems in lakes range from reduced productivity in deep stratified lakes by reduced vertical mixing and reduced internal renewal rates of nutrients from deep water (Verburg et al. 2003), to enhanced productivity by longer ice-off times, reduced light limitation, increased nutrient loading by melt water runoff (Quayle et al. 2002) and by internal loading. While actual effects of climate change on ecosystems have not yet been observed in New Zealand lakes it is useful to consider the possible effects of future warming on New Zealand lakes, and Ellesmere in particular, by comparing with findings elsewhere in the world. Change in duration of ice-off time does not play a role at Ellesmere, and reduced light limitation by reduced vertical mixing is not important either because the lake is shallow and productive. As a result of climate warming, the duration and strength of stratification in lakes increase because surface waters warm more than bottom waters (Winder and Schindler 2004; Verburg and Hecky 2009). Stratification of the water column limits the amount of vertical mixing that can occur. Minimum temperatures, which occur generally at night, are projected to increase more than maximum temperatures in a warming climate, in most parts of the world (Easterling et al. 1997, 2000; Meehl et al. 2007). Should this occur, mixing in the water column, which is usually most vigorous at night when cool air cools the water surface, is reduced. In relatively shallow and productive lakes like Lake Ellesmere (Verburg et al. 2010) reduced vertical mixing tends to reduce the replenishment of oxygen in bottom water and anoxia may occur or become more frequent in bottom water. Furthermore, increased runoff and external nutrient loading will compound such problems by increasing productivity with resulting increased oxygen consumption in bottom water. Oxygen depletion in bottom water generally results in a higher trophic state and the potential for blooms of algae, in particular of blue-green algae, by enhancing internal loading of nutrients from sediments. A proportion of the total nitrogen and phosphorus that enters lakes from their catchment is buried permanently in the sediment that builds up on the bottom of lakes (Vollenweider 1976), and thereby removed (sequestered) from the ecosystem. In addition, a portion of the nitrogen load is removed by denitrification at the sediment-water interface (Saunders & Kalff 2001; Windolf et al. 1996). Hypolimnetic oxygen consumption affects the efficiency of burial of nutrients in the sediment in lakes (Nurnberg 1984; Vant 1987). Anoxic bottom water enhances the release of ammonia from the sediment to the water column. Phosphorus is usually more in demand than nitrogen for algal growth in lakes. The burial efficiency of phosphorus in particular decreases with decreasing oxygen Climate change impacts on Te Waihora/Lake Ellesmere 25