12 September, 2011

Whether the weather

Every time I run round the river when it is windy, I could swear that I seem to be running into a head wind, no matter what direction I am running. This perception probably isn't helped by the fact that I often start my run in a westerly direction and the prevailing wind around here is yes, westerly.
Rain storm moving across Geelong, viewed from Tait's Point
Yesterday was no different. I hit the river at Landy Field and headed upriver, beating into a solid westerly. By the time I was on my way back, I could swear I was still running into the same head wind and not for the first time, it occurred to me to wonder what impact the weather has on the river and - conversely - what effect the river and its surrounding geographical features might have on the weather.
In previous blog posts I've mentioned the impact of day to day wind patterns on the wildlife which lives along it banks - fish and birds seem quite sensitive to wind levels. In geological terms, wind caused the build up of sand dunes at the mouth of the Barwon, helping to shape the estuary as we see it today.
However, the most obvious element of the weather to affect any river is rain - its lack or abundance determining water levels along the length of the watercourse. The Barwon River is no different in this respect, with the river and its associated ecosystem relying on regular flood cycles to maintain a healthy balance along its length.
Barwon at Geelong in flood, 16th January, 2011
Not surprisingly, rainfall levels vary from the headwaters in the Otways to the river mouth at Barwon Heads. Through Geelong, the average annual rainfall is 552.7mm, however the main ridge of the Otway Ranges averages almost four times this amount at 2000mm per annum. This occurs as a result of what is called a rain shadow which is caused by the Otways themselves. In short, warm, moist air flowing in from the west hits the mountains and is forced upwards, cooling and condensing as it rises. Any remaining cool, dry air flows over the mountains and descends to the plains below which are sheltered from the rain on the other side of the range. The rain shadow cast by the Otways extends as far as Geelong and Werribee, making this the driest area in the state, with an average rainfall only about 55mm higher than - for example - Barcaldine in outback Queensland and 16mm lower than Roma.
Little wonder then that, most of Geelong's water supply comes from catchments in the Otways, namely the West Barwon Dam which also plays a significant role in mitigating the effects of flooding on the lower reaches of the Barwon. It does not however, prevent it entirely as we saw earlier this year when moderate flooding was recorded.
Baum's Weir in flood, 16th January, 2011
Flooding occurs through Geelong after about 150mm of rain on a dry catchment or 75mm on a wet catchment over a 1-3 day period. Typical weather patterns which produce this kind of rainfall are either a slow moving low pressure system over Victoria and southern New south Wales, a low pressure trough moving down from the northern states - such as in the case of a cyclone as occurred in January - or persistent winter rainfall.
The three highest flood levels recorded through Geelong were in 1880 (4.59m), 1952 (5.47) and 1995 (5.23m). By comparison, the flood this January (reaching around 4m through Geelong) was not outstanding. Despite this, there was still quite a mess to be cleaned up and it is easy to see the impact the the force of a flooded river can have on the landscape.
Another factor which has a significant impact on river health is temperature, which along with rainfall can affect water levels through increased water storage use and through evaporation. This, along with issues of pollution can lead to increased salinity in the river system which in turn puts stress on the plants and animals which rely on the river. Likewise, a change in the water temperature of the river in combination with low water levels and high nutrient content can - and in summer often does - lead to an outbreak of blue-green algae. This is toxic to humans and animals, causing skin irritation, nausea and asthma/allergy symptoms.
Of course,wind is also an important aspect of weather conditions, however, I must say that despite a significant on-line search, I can find little information about wind patterns and the Barwon, unless it is to discuss short-term forecasts. I would imagine that at the very least, the shadow of the Otways would provide protection from wind for some surrounding areas or that at some points, the natural geography of the river would influence local wind movements. But I can find no on-line mention of such effects.
Queen's Park from Windmill Reserve
One final weather phenomenon which I have experienced around the river but so far had not until now photographed - although others have - is fog. The fog which forms in river valleys is generally of the type called radiation fog. It forms at night in clear weather with little wind. After the sun sets, radiant heat is lost from the Earth's surface and forms a layer of moist air close to ground level. Humidity rises and fog is formed. This fog can be quite dense and can hang around for quite some time. I live on higher ground above the river, so I can sometimes leave home on a bright, sunny morning, descend into the river valley and find myself in another world filled with damp, cool air. This was the case during a trip to Fyansford earlier in the year when it took a good several hours for the fog to clear to reveal the sun which I knew was just overhead.
I imagine that the natural depth of the river valley, the moist environment and the slightly cooler temperature closer to the water all play a part in preventing the dissipation of fog which occurs on higher ground - another example of the river influencing local weather patterns.

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