Yass Valley
Please note: Text or data highlighted in this colour represent an update to this indicator for the period 2008/09. All temperature figures have been updated on this page. Evaporation and Soil Moisture have also been included in this update.
Temperature Analysis
Temperature observations are taken at Yass Hostel in Linton Street (Bureau of Meteorology Station No. 070091). While the temperature record for Yass is generally good, there are four years in which missing monthly data have resulted in there being no annual temperature figures.
Note that in the following analyses temperatures for individual months are compared to the long-term average (or climatology) from 1975-2004. A 30-year period is used to calculate climatological averages to conform with World Meteorological Organisation practice; this takes into account year-to-year variations and less frequent events such as El Niño or La Niña. This particular 30-year period is used in this analysis because it is the period with the best temperature data availability across the Australian Capital Region.
The annual averages of maximum temperatures for the period 2005-2008 (Table 1 and Figure 1) were on or above the long-term mean, with 2006 having the highest average maximum temperatures since 1971. The daily maximum temperature is usually measured in the early afternoon, while the minimum temperature is usually recorded just before sunrise. Mean minimum temperatures were close to average in 2005 and 2008, above in 2007 and marginally below in 2006. Minimum temperatures in 2007 were the warmest on record since 1971. Overall mean daily temperatures, calculated as the average of the maximum and minimum temperature each day, were above average in all four years.
2005 |
2006 |
2007 |
2008 |
|
|---|---|---|---|---|
| Mean Daily Maximum Temperature | 21.5 |
22.4 |
21.7 |
20.8 |
| Anomaly (oC) | 0.8 |
1.7 |
1.0 |
0.1 |
| Anomaly (%) | 4% |
8% |
5% |
0% |
| Mean (1975-2004) | 20.7 |
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| Mean Daily Minimum Temperature | 7.5 |
7.1 |
8.6 |
7.5 |
| Anomaly (oC) | 0.2 |
-0.2 |
1.3 |
0.2 |
| Anomaly (%) | 3% |
-2% |
18% |
3% |
| Mean (1975-2004) | 7.3 |
|||
| Mean Daily Mean Temperature | 14.5 |
14.7 |
15.2 |
14.2 |
| Anomaly (oC) | 0.5 |
0.7 |
1.2 |
0.2 |
| Anomaly (%) | 4% |
5% |
9% |
1% |
| Mean (1975-2004) | 14.0 |
|||
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In the 54-month period January 2005- June 2009 there were 41 months (76%) in which the monthly maximum temperatures were above the long-term average (Figure 2). This four year period had generally warmer monthly average daytime temperatures than the long-term mean, particularly in 2005/2007. Maximum temperature anomalies were highest in October 2006 (4.5oC above the monthly average), whereas in February 2008 the average daily maximum temperature was 2.8oC below the monthly average.
For minimum temperatures, there were 36 months (67%) with temperatures above the long-term average (Figure 2), particularly between June 2005 and March 2006, from October 2006 to January 2008 and from June 2008-June 2009. Minimum temperature anomalies were highest in June 2008 (3.3oC above the monthly average), whereas in May 2006 the average daily minimum temperature was 2.5oC below the monthly mean.
Overall, the daily mean temperatures show a warm period with fourteen months below the long-term average. Note that an El Niño event occurred in 2006/2007; temperatures tend to be above average during these events, particularly during spring and summer.
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Long Term Climate Trend
It is useful to compare recent climate observations with a longer record in order to assess how unusual recent events may be. The temperature record at Yass is generally good, with only four years of missing data (Figure 3). Both maximum and minimum temperatures have increased since 1971, with maximum temperatures increasing slightly more than minimum temperatures.
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Evaporation and Soil Moisture
Evaporation and soil moisture are important components of the moisture balance in the environment. Potential evaporation (the amount of moisture evaporated from an open water surface under prevailing conditions) occurs as a consequence of a combination of factors including incoming solar radiation, temperature, wind and the humidity of the air. The balance among the factors controls the amount of potential evaporation, so that an increase in solar radiation or temperature (which would cause evaporation to increase if all other factors remained unchanged) might be offset by a decrease in wind (which would cause evaporation to decrease). Soil moisture reflects the interaction between rainfall and evaporation, and provides an indication of the amount of moisture available for plant growth.
The variation in potential evaporation at Yass for the period 2000-2008 is illustrated in Figure 4. There is strong seasonal variation between summer and winter, with low values in winter and high values in summer. Potential evaporation over this period shows little overall trend or change from year to year, except during the 2008-9 summer when potential evaporation increased.
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Soil moisture variations at Yass in the upper and lower layers in the period 2000-2008 are illustrated in Figure 5. The impact of rainfall on the upper level soil moisture profile is evident in the relatively large and rapid fluctuations within the time period, which closely mirror the variability of rainfall at Yass. The lower level soil moisture shows the effects of the extended dry period since 2000; there is an overall decline in the lower soil moisture, with any increases in moisture being relatively abrupt (following larger rainfall events) followed by longer periods of drying. The time lag in changes to the lower level soil moisture following rain events is also evident.
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About the data
Evaporation and soil moisture data were extracted from the Australian Water Availability Project (AWAP) dataset, a jointly funded project developed by CSIRO Marine and Atmospheric Research (CMAR), the Bureau of Meteorology (BoM) and the Bureau of Rural Science (BRS) to monitor the terrestrial water balance of Australia. Upper level soil depth is considered to be to a depth of 0.2m while lower level soil depth is considered to be to between 0.2m and 1.5 m.
References
M.R. Raupach, P.R. Briggs, V. Haverd, E.A. King, M. Paget and C.M. Trudinger, 2009 Australian Water Availability Project (AWAP): CSIRO Marine and Atmospheric Research Component: Final Report for Phase 3. CAWCR Technical Report No. 013. 67 pp.
