SHORT- AND MEDIUM-TERM TRENDS IN THE HYDROMETEOROLOGY OF THE CENTRAL SOUTHERN ALPS, NEW ZEALAND

INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 16, 1267- 1279 ( 1996)

SHORT- AND MEDIUM-TERM TRENDS IN THE HYDROMETEOROLOGY OF THE CENTRAL SOUTHERN ALPS, NEW ZEALAND

HAMISH A. MCGOWAN AND ANDREW I? STURMAN

Department of Geography, Universi@ of Canterbury. Private Bag 4800, Christchurch, New Zealand email: [email protected]

Received 22 May 1995 Accepted 30 January 1996

ABSTRACT

Short- and-medium term trends in the hydrometeorology of the central Southern Alps are examined with the aim of developing monthly and seasonal inflow forecasting models for alpine hydro lakes. Both ambient air temperature and precipitation are considered principal components in the generation of inflows. These two meteorological variables display both strong annual cycles and longer term fluctuations which are linked to the influence of different synoptic circulation systems and their regional impact, through such mechanisms as the ENSO phenomenon. Recent investigations have indicated that negative phases of ENSO tend to delay the onset of the spring melt season and to result in drought conditions within the central Southern Alps. Such conditions during 1991 and 1992 were compounded by a global increase in atmospheric turbidity following two large volcanic eruptions, and contributed to a national energy crisis in New Zealand during the 1992 winter. Both seasonal and monthly empirically derived inflow models are assessed, incorporating temperature, precipitation and circulation parameters to predict inflows. Although inflows for these time-scales correlate strongly with precipitation, temperature and atmospheric circulation for the same period, predicted inflows one season or month in advance did not. This supports the need for much shorter term monitoring of meteorological conditions within such alpine catchments. In addition, there also appears to be a need for quantifying inflows generated from the present long-term retreat of major eastern alpine glaciers, which also make a significant contribution to lake inflows.

KEY WORDS: Southem Alps; inflows; ENSO; precipitation; snowmelt; alpine; Lake Tekapo; New Zealand.

INTRODUCTION

The Southern Alps of New Zealand present a formidable barrier to the mid-latitude westerlies extending from approximately 42" to 45"s. They are orientated north-east-south-west and rise steeply from a narrow western coastal plain to a mean ridge height of approximately 2300-2500 m, with peaks exceeding 3500 m. As a result, the dominant onshore westerly airstream is forced to ascend this mountain barrier, or to flow around its northern and southern extensions. The forced ascent of the onshore maritime westerlies results in classic orographic processes, including precipitation enhancement. During moderate to strong westerly quarter airflow, spillover of precipitation occurs which is typically limited to the first 1&15 km downwind of the main axial range of the Southern Alps (McGowan, 1994). This rain and snowfall provides valuable inflows into the South Island's hydroelectric reservoirs which provide approximately 60 per cent of New Zealand's electric power generation capacity. However, in recent years, reduced inflows into these eastern alpine lakes have resulted in the drawdown of some hydro-lakes to their minimum operating levels. This has often resulted in the exposure of vast areas of lake shoreline to aerial processes, which is thought to have contributed to severe dust storms being experienced at Lake Tekapo during the spring of 1989 (McGowan, 1994), and a national energy crisis throughout the winter of 1992. These two events have been major catalysts in the initiation of scientific studies into the weather and climate of the Southern Alps, and inflows into the hydro-lakes, such as SALPEX (Southern Alps Experiment, Wratt et al. (1 995)), and the Lake Tekapo dust storm genesis investigation (McGowan, 1994). Results from the second study form the basis of this paper.

The influence of orography on precipitation has been the focus of numerous scientific studies for more than a century, as reviewed by Barry (1 992). In general, the forced ascent of a moist airstream by a topographic obstacle,

CCC 0899-8418/96/111267-13 0 1996 by the Royal Meteorological Society

1268 H. A. McGOWAN AND A. P. STURMAN

such as the Southern Alps, results in the adiabatic cooling of the ascending air until condensation occurs, after which the air continues to cool at the saturated adiabatic lapse rate until it crosses the obstacle and begins to descend. Condensation droplets within the cloud layer grow until their settling velocity exceeds the vertical velocity of the ascending air. As the droplets fall through the cloud they washout other droplets resulting in the classic seeder-feeder mechanism, as discussed by Carruthers and Choularton (1983). At higher altitudes, precipitation will occur as snow, which will remain in either the seasonal or perennial snowpack until the spring/ summer melt season, or until it is transported to lower altitudes in glaciers. Within the Southern Alps, most precipitation below 1000 m falls as rain, although during warm north-westerly rainfall events the freezing level may exceed 3000 m. Above this altitude, rain is rare, and most precipitation occurs as snow (Fitzharris, 1992). Fitzharris (1992) estimated that annual snow accumulation near the main divide exceeds 4000 mm water equivalent, although usually less than 1000 mm is recorded along the eastern South Island ranges.

Historical precipitation records from monitoring sites located along the western slopes of the Southern Alps display a spring maximum and winter minimum. This seasonal precipitation cycle is also observed in records from monitoring sites situated in the immediate lee of the alpine divide due to the spillover of precipitation during westerly quarter precipitation events. This is the dominant direction from which the rain- or snow-bearing weather systems that affect this region originate. Observations from rain-gauges situated further east within the inner montane basins show a more uniform seasonal distribution, reflecting the sheltering effect of the surrounding mountain ranges. However, changing synoptic circulation patterns due to such large scale influences as the El Niiio-Southern Oscillation phenomenon (referred to as ENSO) have been identified to have a significant effect on the weather and climate of the Southern Alps, and much of New Zealand in general. For example, negative phases of ENSO are synonymous with prolonged periods of south-westerly airflow over southern New Zealand as noted by Gordon (1985, 1986). Such conditions result in below average ambient air temperatures across much of the South Island and less frequent rain-bearing west to north-westerly airflows. Colder ambient air temperatures typically result in more precipitation falling as snow and delay the onset of the spring melt season (McGowan, 1994). During 1991-1992 the impact of negative ENSO conditions was compounded by the cooling effect of volcanic dust, which had been ejected into the stratosphere by the eruptions of Mount Pinatubo in the Philippines, and Mount Hudson in Chile during 1991. This led to a prolonged cold and dry period within the Southern Alps, resulting in drought conditions in many alpine lake basins, which culminated in the 1992 hydroelectric power crisis.

Only limited scientific studies into the spatial and temporal nature of precipitation within the Southern Alps have been published, such as Sturman and Soons (1984) who analysed monthly precipitation totals recorded at Chilton Valley in the Cass Basin, approximately 20 km south-east of the alpine divide. They identified a spring and autumn precipitation maximum and winter minimum, which exhibited a statistically significant link with cyclonicity and westerly synoptic airflow. Similar findings were presented by Salinger (1 980) and Fitzhanis (1992), who found that a higher than usual incidence of west to south-westerly airllow caused very low winter inflows into the South Island hydro-lakes, and anomalous summertime easterly circulations produced little rainfall in the central alpine region. McKercher and Pearson (1994) identified a weak but statistically significant link between inflows into the Clutha Lakes catchment and the Southern Oscillation Index (SOI). They also identified a negative correlation between summer Clutha Lakes inflows and the spring SOI, which they suggested resulted from enhanced spring snow accumulation during negative phases of the SOI, leading to additional snowmelt in the following summer. However, they did not use precipitation or temperature observations from the Clutha Lakes area to support their findings.

The aim of this paper therefore is to examine the linkages between different synoptic circulations and meteorological variables within the Southern Alps, with a view to developing a predictive model for inflows into one of New Zealand’s most important hydro-lakes, Lake Tekapo. Short- and medium-term trends in the hydrometeorology of the central Southern Alps are also presented.

PHYSICAL SETTING

Precipitation and inflow observations presented in this paper were obtained from monitoring sites within, and adjacent to the central Southern Alps (Figure 1). This area is characterized by high peaks, many of which exceed

HYDROMETEOROLOGY OF NEW ZEALAND 1269

2500 m, including Mount Cook (3764 m), New Zealand's highest mountain. The main axial range of the central Southern Alps is asymmetric, rising steeply from a narrow western coastal plain, but decreasing through a series of subsidiary ranges and basins to the east (Whitehouse, 1988). East of the alpine divide, Lakes Ohau, Pukaki and Tekapo (Figure 1) occupy glacially excavated rock basins and are the principle hydroelectric reservoirs within the Waitaki Catchment. Coalescing Pleistocene outwash surfaces between and to the south of these three lakes merge to form the lowland area of the MacKenzie Basin, which dips in a southerly direction (Figure 1).

The climate of the study area is varied and complex, with many microclimates existing within the mountains, valleys and lake basins. In general, areas west of the main divide may be classified as temperate and wet with approximately 180 rain-days per year (days on which 1 mm of precipitation or more fell). Hessell(l982) notes that the average annual precipitation near the west coast is approximately 3000 mm, which increases to 5000 mm near the western foothills. Sunshine hours are low at approximately 1770 year-' (Garr and Fitzharris 1991) and the region experiences il mrm temperature uf 10°C. The wind repimp. of thk area is dnminated by Iight thermally driven circulations at low elevation, such as the local sea-breeze and valley wind circulations, as discussed by Russell (1987), whereas at higher altitudes strong westerlies predominate in all seasons (Coulter, 1965).

Above the vegetation line, high peaks and vast snow and icefields exist, forming the neve region for many of New Zealand's largest glaciers including the Tasman, Fox, and Franz Josef Glaciers. Meteorological observations from this region are limited, with only short-term or seasonal records available for some sites. This region experiences very strong winds that predominantly originate from the west and north-west and exhibit a spring maximum and winter minimum. Observations from the Sealy Range in Mount Cook National Park and the Upper Godley River Valley indicate that mean wind speeds of 30 to 40 m s-' are common in this area, and that wind gusts in excess of 55 to 70 m s-' regularly occur during fohn north-westerly windstorms (Chinn, 1980; McGowan, 1994). Precipitation may vary considerably within this mountainous region owing to changes in atmospheric circulation associated with such phenomena as El Niiio and La Niiia. Both rain and snow may fall, with annual precipitation estimates in excess of 14 000-1 5 000 mm (Department of Lands and Survey, 1985). In the last decade, the advance of the Fox and Franz Josef Glaciers has been linked to increased snowfalls within the central Southern Alps as a result of more frequent west to south-westerly conditions during the prolonged and intense El Niiio events experienced over the past 12 to 15 years.

East of the alpine divide, rain-shadow effects dominate the local climate in the absence of any relative maritime moderating influence. A strong west to east precipitation gradient exists within this region, with annual precipitation totals exceeding 5000 mm adjacent to the lee slopes of the Southern Alps, but at sites only 30 to 40 km further east, yearly precipitation may average only 600 mm (New Zealand Meteorological Service, 1983). Throughout winter numerous snowfalls may occur over the entire inner montane region, which are typically associated with the advance of cold polar south to south-easterly airflows over the central South Island. This region experiences a wide temperature range, with winter minimums of - 12°C or colder, and summer maximums that may exceed 30°C. The wind regime of this area is dominated by local thermally generated circulations, such as lakeAand breezes and valley/mountain wind circulations (McGowan et al., 1995). Warm, dry and gusty fdhn north- westerly winds are the most frequently monitored surface level synoptic airfiow observed within this area, and are usually followed by short periods of south-westerly airflow and clearing weather.

Vegetation cover in this region consists mostly of tussock grassland which is subject to pastoralism. The quality of the eastern alpine and inner montane tussocklands is directly related to the grazing pressures placed on them and the strong west to east precipitation gradient. As a result, well vegetated areas in the upper catchments of Lakes Pukaki and Tekapo give way to severely degraded tussocklands further east, which are prone to severe wind erosion (McGowan et al., 1995).

SHORT- AND MEDIUM TERM-TRENDS IN ENSO, PRECIPITATION AND TEMPERATURE

The ENSO phenomenon has been shown to strongly influence the weather and climate of New Zealand, most importantly precipitation and temperature (Gordon 1985, 1986; West and Healy, 1993). This large-scale teleconnection exhibits two different time-scales of variability, namely a biennial mode (Southern Oscillation Index) and a lower frequency 4-5-year cycle (El Niiio) (Rasmusson et al., 1990). The Southern Oscillation Index (SOI) is a measure of the difference in standardized sea-level atmospheric pressure between Tahiti and Darwin. During negative phases of the SO1 the Humboldt current along the Chilean and Peruvian coast may break down as

1270 H. A. McGOWAN AND A. I? STURMAN

Figure 1. Location map of the study area, including Lakes Ohau, h k a k i and Tekapo.

coastal upwelling of cold water is suppressed due to warmer tropical waters being blown eastward by stronger mid- latitude westerlies, resulting in the El Niiio phenomenon.

The ENS0 events may change amplitude from cycle to cycle and may sometimes change phase, which makes their longer term forecasting difficult, although regular monitoring of their progress is relatively straight forward. Figure 2 shows a 40-year time series (1953-1993) of monthly SO1 values overlaid with the 5-month moving average values. In agreement with results presented by Enfield (1 988) and West and Healy (1 993), no significant trend is identifiable in the SO1 as may be expected by its irregular oscillating nature (Figure 2). Strong negative periods of the index are followed by a rapid change to strong positive periods without warning, although the length of either negative or positive index conditions may vary considerably fiom event to event.

Figure 3 presents monthly precipitation observations for three central South Island sites within the study area for which complete observation records are available. These are Fox Glacier township, Mount Cook (Hermitage) and Tekapo Village (Figure I), for the 1967-1993 period. Mean monthly precipitation records from both Fox Glacier township and Mount Cook (Hermitage) exhibit similar trends, with a spring maximum and winter minimum. Both


1953 1958 1963 1968 1973 1978 1983 1988 1993

Year

Figure 2. A 40-year time series of the monthly Southern Oscillation Index, 1953-1993. The solid line is the 5-month moving average.

November and February are months when recorded precipitation is notably less than for the months either side, which may be indicative of changing seasonal synoptic circulation patterns and the dominance of anticyclones during February, as noted by Sturman et al. (1 984). A similar pattern is also evident in maximum and minimum monthly precipitation observations. At Tekapo Village, approximately 40-50 km east of the main divide, mean monthly precipitation totals are almost an order of magnitude less than those presented for the two other sites. This highlights the considerable west-to-east precipitation gradient that exists across the study area and the Southern Alps in general, as discussed by Griffiths and McSaveney (1983). Mean monthly precipitation totals for Tekapo Village are also more uniform with much less variance between months than observed at Fox Glacier or Mount Cook, with an August maximum and February minimum.

To identify longer term trends in precipitation within the study area, an integral-difference curve was computed for total monthly precipitation recorded at Mount Cook (Hermitage) for the period 1930 to 1992 inclusive. This method, also referred to as curves of total deviationfiom the long-term average value, does not move boundaries between cyclic fluctuations in the variable examined as do many other methods, such as average sliding curves (Battalov, 1971). As a result, it allows for the identification of wet and dry periods. For example, where the integral- difference curve displays positive slope more precipitation was recorded than the long-term average (1 930-1 992), and where it has negative slope, less precipitation was recorded.

In Figure 4 longer term precipitation trends are clearly identifiable which have superimposed on them a strong seasonal component. From 1930 to 1939 precipitation recorded at Mount Cook was near average (for the 1930- 1992 period), after which there was a declining trend until 1942. This was followed by a period of increasing precipitation, before declining sharply again up to 1948. A slight rise to 1950 was followed by a more gradual decline in precipitation until 1956-1957. A sudden increase in 1958 was followed by a continued fall below average until 1967, when two wet years were followed by a drier interval. From 1978 to 1992, precipitation recorded at Mount Cook was above the long-term average (1930-1992) indicating that the central Southern Alps had entered into a progressively wetter than average climatic cycle (Figure 4). A similar pattern was observed in the precipitation record fiom Fox Glacier township by McGowan (1994).

The long term trends in precipitation monitored at Mount Cook are mirrored by the inflow record for Lake Tekapo. No long-term precipitation observations are available for this lake catchment, although short-term precipitation monitoring by McGowan (1994) in the headwaters of this lake catchment did correlate with


Fox Glacier Township 12001

(a)

lo00 ""1 T m Y € 8 0 0

.-

'8 400 & 300

200 100 0 4 . . . I . I . I . . . I

Month Mt Cook (Hermitage)

Jan Feb Mar Apr May Jun Jut Aug Sep Od Nov Dec

1500 1400 1300

(b)

(a Tekapo Village

I I 8 160 140

.- *= 120

0-60 *g g

40 20 0 . .

Month

Figure 3. Mean monthly precipitation, minimum, maximum and one standard deviation for (a) Fox Glacier township, (b) Mount Cook (Hermitage), (c) Tekapo Village.

precipitation monitored at both Fox Glacier and Mount Cook (Hermitage) over the same observation period, resulting in values of 0.63 and 0.67 respectively, in both cases at the 0.05 significance level. These observations therefore support the use of the long-term Mount Cook precipitation record as a suitable proxy for precipitation within the upper reaches of the Lake Tekapo catchment. Regression analysis performed on integral difference


;;$ 10

: q l , , I , , I , , I , , I , , I , , I , , I , , I , , , -14 1930 1936 1942 1948 1954 1960 1966 1972 1978 1984 1990

Year

Figure 4. An integral difference curve for total monthly Precipitation monitored at Mount Cook (Hermitage) for 1930 to 1992. Cy(K - 1)is the algebraic sum of the deviations from the average, where K equals the individual observation divided by the long-term average, and C, is the

coefficient of variation.

values for total monthly precipitation at Mount Cook against integral difference values for mean monthly inflows into Lake Tekapo for the period 1952-1992 produced an R2 value of 0.68, at the 0-05 significance level. This 40- year interval was chosen because it represents the most accurate monitoring of inflows into Lake Tekapo following the construction of a lake-level control structure and power station adjacent to Tekapo Village (Figure 1) in the early 1950s. This analysis confirmed that wetter periods at Mount Cook did result in increased inflows into Lake Tekapo, and that dry conditions resulted in reduced inflows.

Within the alpine environment, ambient air temperature is a principal control of snow and ice melt, and largely determines whether or not precipitation falls as snow or rain. Consequently, air temperature has a significant influence on run-off and inflows into alpine lakes, such as Lake Tekapo. The only long-term alpine air temperature record available for a site within the study area was from the Mount Cook (Hermitage) climate station. Monthly mean air temperatures for this site are presented in Figure 5(a) with plots of the estimated monthly mean air temperature for 1500 m and 2000 m also indicated assuming an environmental lapse rate of 0.6"C per 100 m altitude gained. Run-off from snow and ice melt may therefore occur throughout the year at Mount Cook (Hermitage) and presumably within the alpine catchment of Lake Tekapo for locations below 740 m, whereas at 1500 m and 2000 m run-off is confined to the months of September-May and December-February, respectively during favourable conditions. That is, when the temperature of the snowpack reaches an isothermal state at O'C, and when the liquid water holding capacity of the snowpack is exceeded. Such conditions are realized due to the input of energy fluxes, especially from net radiation, sensible and latent heat and heat from precipitation. Within the study area the most rapid snow and ice melt typically occurs during fdhn conditions, when warm ambient air temperatures and rainfall enhance the melt process.

In Figure 5(b) an integral difference curve of mean monthly air temperatures for Mount Cook (Hermitage) is presented for 1931-1992. This plot identifies periods of warming and cooling (positive and negative graph slope respectively) compared with the 193 1-1992 average, and a strong annual temperature oscillation is also displayed. From 193 1 to the mid-1950s a cooling of the climate was experienced, which was followed by a slight warming until 1963, when below average temperatures were again monitored (Figure 5(b)). Between 1968 and 1973 air temperatures recorded at Mount Cook were above the record average, after which near-average temperatures were monitored until the early 1980s. From 1983 to 1990 a warming of the climate was experienced that was also recorded by numerous climate monitoring sites around the world and was interpreted by many researchers as proof of global warming. Cooler conditions in 1991 and 1992 were linked to increased atmospheric turbidity following the eruptions of Mount Pinatubo and Mount Hudson and negative ENS0 conditions, as previously discussed.

1274 H. A. McCOWAN AND A. P. STURMAN

9 c. f h E F

:::I , , , , , , , , , , , , -16

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nw Dec Month

(b) 94 I

Figure 5. (a) Monthly mean air temperatures for the Mount Cook (Hermitage) climate station (1930-1980), and predicted mean monthly air temperatures for 1500 m and 2000 m assuming an environmental lapse rate of 0.6"C per 100 m elevation. (b) An integral difference curve for

monthly mean air temperatures for Mount Cook (Hermitage) for 1931-1992.

The temperature trends shown in Figure 5@) have also been identified in air temperature records from other South Island alpine monitoring sites (McGowan, 1994). As a result, periods of cooling and warming highlighted in Figure 5 @ ) were regional in nature, reflecting the influence of meso- and macroscale phenomena on the weather and climate of the Southern Alps.

HYDROLOGY OF THE LAKE TEKAPO CATCHMENT

The Lake Tekapo catchment occupies approximately 1440 km2 of the central Southern Alps (Figure 1) and consists mostly of steep mountainous terrain, including large perennial snowfields and approximately 44 km2 of glaciers. No direct monitoring of inflows into this hydro-lake is conducted. Instead inflows are determined from the sum of machine flow and spill flow through the Tekapo power station and lake-level control structure respectively, and lake storage based on lake-level monitoring. The highest mean inflows are observed during summer and the lowest in winter (Figure 6). A similar inflow regime is identified for mean monthly minimum inflows, whereas mean


Figure 6. Monthly inflows into Lake Tekapo.

monthly maximum inflows display more variance. The majority of inflows are derived from seasonal snow and ice melt and north-westerly rainfall events, which result in the northwestern extremities of the catchment receiving substantially more precipitation than sites located further to the south-east. For example, from 1 August 1992 to 28 February 1993, 1 182 mm of precipitation was recorded by a rain-gauge located only 10 km from the main divide, whereas 28 km down-valley at the northern lakeshore only 465 mm was recorded over the same period. Such strong precipitation gradients are common along the eastern Southern Alps where spillover of precipitation during strong synoptic north-westerly conditions dominates the hydrological input regime of many eastern alpine catchments.

Flood events may be experienced within the lake catchment throughout the year, although they are most common in spring and summer during north-westerly rainfall events. Exceptional lake inflow (flood) events within the catchment, as reported on 25 December 1979 (1 170 m3 s-l) and 2 1 December 1984 (1 161 m3 s-') often result from high-intensity warm north-westerly rain-on-snow events that initiate rapid snow melt, as discussed by Fitzharris et al. (1 980). Minimum inflows are recorded during late winter prior to the commencement of the spring melt season when inflows may fall to as little as 6 m3 s-' .

The retreat of major glaciers within the lake catchment over the last 130 years, and most notably within the last 60 years has also contributed to the inflow regime of Lake Tekapo. For example, the Godley Glacier has retreated over 6 km up-valley since 1940 and thinned by approximately 60 m (Gellatly, 1985), and some tributary glaciers have thinned in excess of 200 m (McSaveney, 1992) and no longer reach the valley floor. The continued retreat of these glaciers presently provides important base inflows into Lake Tekapo, particularly during late summer and autumn after the seasonal snowpack has melted.

PREDICTING INFLOWS INTO LAKE TEKAPO

As Lake Tekapo provides storage for approximately 21 per cent of New Zealand's hydroelectric generation capacity, the ability to forecast inflows into this alpine lake is highly desirable. McKercher and Pearson (1 994), as


discussed previously, used the SO1 to forecast seasonal inflows into the Clutha Lakes, which is a similar alpine catchment to the Waitaki system, but located approximately 20 km south of the study area. They did identifjl a statistically significant link between seasonal inflows and the SOI, although the strength of the relationship was too weak for practical applications, such as hydroelectric operations. Bowden (1 994) used standard meteorological observations to initialize runs of the Martinec Model (Martinec, 1975) to estimate snowmelt and daily run-off from seasonal snow cover of the nearby Lake F'ukaki catchment (Figure 1). Although Bowden (1994) experienced limited success with initial model runs, the inclusion of a precipitation factor that simulated the precipitation gradient monitored within the catchment significantly improved model results. As a result, an R2 value of 0.63 was obtained for regression analysis between predicted and estimated run-off over a 7-month period from May to November 1993, with a volumetric difference between estimated and actual run-off of only 2-1 per cent. Bowden concluded that model runs of predicted inflows into alpine lake catchments such as Lakes Pukaki and Tekapo could improve only if the installation of meteorological and snow monitoring stations was undertaken. However, this has not yet been done and the only reliable long term meteorological monitoring site within the central Southern Alps is at Mount Cook (Hermitage) (Figure 1).

Previous studies therefore suggest that inflows into an alpine lake such as Lake Tekapo should be influenced, perhaps strongly, by ambient air temperature, precipitation and the SOI, which has been shown to influence the weather and climate of the region (Gordon, 1985, 1986) and alpine lake inflows (McKercher and Pearson, 1994). Therefore an attempt was made to develop a simple yet effective model to determine inflows into Lake Tekapo using standard meteorological observations of ambient air temperature and precipitation from Mount Cook (Hermitage), and the SO1 on a seasonal basis (Summer, December-February; Autumn, March-May; Winter, June- August; Spring, September-November) using observations from DJF 1952 to SON 1992.

Mean seasonal air temperatures were calculated from daily mean values, and a monthly average of precipitation was determined for each season in association with mean seasonal SO1 values. Using multiple regression analysis, equation (1) was calculated (forcing the y intercept through zero) to estimate mean seasonal inflows into Lake Tekapo, which returned an RZ value of 0.80 and a standard error in Q of only 14.7 m3 s-' at the 0.05 significance level.

Q 1 0 . 1 3 P + 4.3T + 0.06 SO1 (1)

where Q is the mean seasonal inflow into Lake Tekapo in m3 s-', P is the monthly average precipitation for the season at Mount Cook (Hermitage) in mm month-', T is mean seasonal air temperature at Mount Cook (Hermitage) in "C, and SO1 is the mean seasonal SO1 value.

The incorporation of both temperature and precipitation coefficients produced a significant improvement of the capabilities of such empirically derived inflow equations compared with using the SO1 index only, as attempted by McKercher and Pearson (1 994). However, when a one season lag was introduced in an attempt to predict inflows one season in advance, no useful relationship was identified. It was therefore felt that an improved model could be developed to predict future lake inflows by increasing the temporal resolution of the indices used and by incorporating a more localized circulation index that better represented the nature of synoptic circulation patterns over the study area than the SO1 index.

The synoptic index presented in Table I was derived on a daily basis from synoptic maps for the period MAM 1961 to SON 1990 using a subjective method described by Sturman et al. (1984). Multiple regression analysis was subsequently performed on the monthly sum of each daily circulation index, mean monthly air temperature and total monthly precipitation monitored at Mount Cook (Hermitage), and the monthly mean SOL The best expression derived from this procedure for estimating mean monthly inflows into Lake Tekapo with no lag returned an R2 value of 0.74 and a standard error in Q of 20.1 m3 s-' at the 0.05 significance level (equation 2). This empirically derived equation uses only the zonal circulation index, which appears to be the most significant circulation index to influence inflows on a monthly basis. The zonal index (Table I) accentuates westerly airflow, which typically is associated with precipitation or the warm fdhn conditions that initiate snow and ice melt within the study region.

Q = 0.13P + 4.4T - 0.5Z - 0.35 (2)


Table I. Daily circulation index values based on the South Island synoptic classification described by Sturman et al. (1984)

Northerly Index Northerly 2 Southerly Index Southerly 2

Westerly Index Westerly 2 Easterly Index Easterly 2

Cyclonicity Index Non-directional anticyclonic -2 Zonal Index Westerly 2

All unspecified 0 Northerly, southerly, easterly, 0

Non-directional cyclonic 2 All unspecified -2

North-westerly or north-easterly 1 South-westerly or south-easterly 1 All other classifications 0 All other classifications 0

South-westerly or north-westerly 1 South-easterly or north-easterly 1 All other classifications 0 All other classifications 0

Directional anticyclonic -1 North-westerly or south-westerly 1

Directional cyclonic 1 south-easterly and north-easterly - 1

where Q is the mean monthly i d o w into Lake Tekapo in m3 s-', P is the total monthly precipitation at Mount Cook (Hermitage) in mm, T is the mean monthly air temperature at Mount Cook (Hermitage) in "C, and 2 is the monthly zonal circulation index.

Using the monthly data,,including the circulation index, an attempt was subsequently made to predict monthly inflows into Lake Tekapo using the previous month's meteorological observations. All circulation indices except for the zonal index were incorporated into this analysis so that the flow-on effect of precipitation events associated with the previous month's synoptic circulations would be included in the analysis, including westerly conditions. This method met with most success using August and September observations to forecast September and October inflows, namely the two months when reduced inflows can have the greatest impact on the commercial operation of the Lake Tekapo hydroelectric resource. When August observations were used to forecast September inflows an R2 value of 0.45 was obtained and a standard error in Q of 3 m3 s-' (equation 3), and September observations returned an R2 value of 0.35 and a standard error in Q of 3 1 m3 s-' at the 0.05 significance level (equation 4) for predicting October inflows.

Q , y ~ p r = 0.13P +4.6T - 1.2C - 2.4s - 0.86W - 1.06E - 2N + 84.6 (3 1

Qocr = 0.2P + 4.8T - 1.03C - 0.85s - 0.44W - 0.16E - 2.1N + 54 (4)

where Q is the predicted mean monthly inflow into Lake Tekapo in m3 s-l, P is the total AugusdSeptember precipitation at Mount Cook (Hermitage) in mm, T is the mean AugusVSeptember air temperature at Mount Cook (Hermitage) in "C, C is the AugusVSeptember cyclonicity index, S is the AugusVSeptember southerly index, Wis the AugusdSeptember westerly index, E is the AugusdSeptember easterly index, and N is the AugusVSeptember northerly index. Although these empirically derived formulae do not explain all factors influencing inflows into Lake Tekapo, they are the most significant yet developed using easily obtainable daily meteorological observations.

SUMMARY AND CONCLUSION

This study has shown that precipitation and air temperature records from the central Southern Alps display strong annual cycles which are superimposed on longer term fluctuations that influence the hydrology of alpine lake catchments, such as Lake Tekapo. Both short- and medium-term trends in temperature and precipitation identified by this investigation and those presented by Salinger et al. (1 994) may result from the cyclic nature of ENSO, perturbations in the mid-latitude westerlies, volcanic eruptions or luni-solar cycles, as discussed by Currie (1 993). For example, westerly wind anomalies result in below average temperature and above average precipitation in western areas including the Southern Alps, whereas easterly wind anomalies produce above average temperatures and below average precipitation (Salinger et al., 1994). In 1992, the influence of negative ENS0 conditions on the


weather of the study area was compounded by the cooling affect of stratospheric dust from recent volcanic eruptions, culminating in one of the coldest winters on record. Although such phenomena can easily be monitored, they are often impossible to predict for inclusion into meteorological and hydrological forecasting models, such as the equations presented in this paper.

Results from this investigation have shown that on a seasonal basis, inflows into Lake Tekapo are mostly dependent on precipitation, air temperature and the SOI, which generally characterizes the meridional flow of southern Australasia. However, the inability to forecast inflows one season in advance using these variables highlights the need for further studies to be conducted on the importance of inAows generated by, for example, the long-term retreat of eastern alpine glaciers, and strengthens the need for continuous monitoring of the hydrometeorology of the Lake Tekapo catchment. With the availability of meteorological observations from sites situated within the catchment, the predictive capabilities of empirical models such as equation ( 1 ) can only improve.

Such observations would undoubtedly also improve the forecasting abilities of monthly inflow equations (2), (3) and (4), which incorporate the circulation indices outlined in Table I. This approach appears more promising for developing a practical inflow forecasting model to predict inflows for the following month based on the previous month’s total precipitation, mean temperature and summed circulation indices. By incorporating the circulation indices a more representative assessment of zonal flow over the study area is obtained than by using the SOL This is particularly evident for westerly flow, which is associated with the dominant precipitation bearing weather systems that affect the central Southern Alps.

Of notable interest in equations (2), (3) and (4) is the negative relationship between the circulation indices and inflows. This feature may result from the release of precipitation that fell in the previous month(s) and which remained in the snowpack until released during wanner conditions associated with the spring/summer melt season. This hypothesis is supported by McKercher and Pearson (1994) who suggest that a negative correlation between SON SO1 and inflows in DJF for the Clutha Lakes was probably related to enhanced spring snow accumulation by anomalous southerly conditions during negative SO1 phases. However, this does need to be tested through field studies such as the establishment of a snow monitoring system within the lake catchment. Until such research is undertaken the advancement of inflow forecasting models as presented in this paper will be limited.

ACKNOWLEDGEMENTS

The authors acknowledge the financial assistance of the Electricity Corporation of New Zealand in making this study possible as part of the much larger Lake Tekapo dust storm genesis investigation, and for providing inflow records for Lake Tekapo. The authors are also grateful to the Geography Department, University of Canterbury for its continued financial and technical support.

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SHORT- AND MEDIUM-TERM TRENDS IN THE HYDROMETEOROLOGY OF THE CENTRAL SOUTHERN ALPS, NEW ZEALAND