Research Document - 2000/054
Precipitation, discharge and temperature variability in Atlantic salmon rivers in Acid Rain Impacted Southern Uplands Area of Nova Scotia.)
By D. Caissie
Abstract
This paper provides analysis of long-term variation in precipitation, air temperature and discharge in the Southern Uplands area of Nova Scotia. Data at the Halifax Airport Meteorological station were used to analyze precipitation and air temperature trends on an annual and seasonal basis. Discharge data were used for two rivers in the study area, namely the LaHave and St. Marys rivers.
On an annual basis, air temperature in the study area varied 5.1 °C to 7.4 °C with a long-term average of 6.1°C. Annual precipitation ranged between 1048 mm and 1931 mm with an overall average of 1445 mm. No trends were observed in the mean annual air temperatures and annual precipitation. Variability in annual precipitation was also reflected in variability of mean annual discharge at the two studied rivers, LaHave and St. Marys River. The mean annual flow at LaHave River ranged between 17.6 m3/s and 49.6 m3/s with an overall average of 34.4 m3/s. The mean annual discharge at St. Marys River ranged between 28.5 m3/s and 63.9 m3/s with and overall mean of 43.0 m3/s. Annually, the variability in both precipitation and discharge were similar as reflected by the coefficient of variation in the order of 20%. No significant trends were detected in the annual discharge at both rivers.
On a seasonal basis, it was observed that the air temperature and precipitation were more variable than annually. On a seasonal basis, significant trends were observed in mean spring air temperatures (p < 0.005) and mean summer temperatures (p < 0.04). The mean spring air temperatures showed an increase of 0.4 °C per decade, while mean summer air temperature increased by 0.2 °C per decade. No significant trends were observed in mean seasonal precipitation at p < 0.05.
Also on a seasonal basis, discharge showed higher variability than air temperatures and precipitation with coefficient of variation between 27% and 58%. No significant trends were detected at LaHave River seasonally and St. Mary River showed a slight increase in mean winter discharge (p < 0.05) of 1.5 m3/s per decade. Highest variability for discharge was observed for annual extreme events such as high and low flows. For instance, minimum annual discharge showed a coefficient of variation of 83% (St. Marys River) and 89 % (LaHave River). No trends were detected in maximum or minimum annual discharge at both studied rivers.
Given the wide range of annual precipitation and discharge, it was expected that annual deposition of acid precipitation could also vary significantly. Also studies have shown that pH can be highly related to stream discharge and therefore any variability in discharge, either annually or seasonally, can affect annual or seasonal mean pH values.
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