Based on photos taken of the river on the 17th, breaking winter rains (or rains of any sort) would need to be substantial (say at least 20-30 mm or more over 2-3 days) to get it flowing properly again. For the water level to reach median conditions again – i.e. at least to the water mark seen on the side of the riverbank in places – would likely require many inches of rain, over an extended period of time. Causes of recent trends in river conditions are unknown (aside from periods of light to moderate intermittent rain or showers), but low-to-very-low minimums may be playing a part – last night 2 degrees. An increase in the minimums brought on by an increase in precipitable water and humidity might also be worth consideration.

See here for latest on rainfall prospects.

For more information on conditions of the last couple of months, see post #1099642 - 15-04-2012 08:32 PM, possibility "a".

30 mm in the 20 hours 15 mins to 11.15 am this morning. River is flowing upstream. Water level downstream has probably risen 75 cm.

River level is steady at 75 cm to 1 metre rise (and visibly flowing) – 64.6 mm in the 5 days 10 hours to 10 am today.

Note: temperature was 6 degrees at 4 pm on Thursday the 24th. The following overnight low was just 1 degree. The local (sfc.) pressure dropped to 1003 hPa on the 23rd.

This event had been a possibility for at least a week, but not until the last few days before the onset did the confidence appear increase sufficiently. For this reason I was not going to over- or under-dramatise how things would pan out (or how much of an impact rainfall would have on run-off), and decided to wait and see what happened for a change .

http://www.bom.gov.au/climate/ahead/rain_ahead.shtml?link=1

Not looking that flash for Central SA for the coming winter…nor Southwest WA .

Possibility of significant change in soil moisture, run-off circa 4th June.

Probability of significant change in soil moisture, run-off.

BoM - 95% chance of 10 to 20 mm tomorrow (I suspect more atm); latest forecast.

8 pm stats: 1014 hPa (sfc.), 2 deg C, 87% RH.

Current rain rate approx. 3.4 per hour. 30.2 mm system to date (3rd into 4th). River up probably another 25 to 50 cm. Higher gaugings to the east and south.

Biggest estimated increase in soil moisture since 24.7 mm on the 29th of February, when it rose from ~0.185 to 0.213 (that is the fraction of soil containing water). Soil moisture (est.) was ~0.202 on the 19th (of May), rose to ~0.229 by the 26th, and is now around 0.226 following 33.9 mm over the last 3 days (most falling overnight on the 3rd-4th and 4th-5th). River obs. indicate streamflow peaked sometime overnight close to midnight (4th-5th). Vegetation and other grasses along the riverbed show clear signs (contours) of water travelling down the river. The water is also quite murky and there is some debris strewn in patches which may indicate just how high it got (perhaps another 10-15 cm).

While this has not been a major change in runoff (e.g. like September 2010), there has been enough rain over a long-enough period of time to replenish surface and sub-surface water significantly.

The average winter maxima and minima June 1st to date are 14.6 degrees and 4.2 degrees respectively, with -1 recorded on the 31st (May).

Post: #1099642 – 15-4-2012:


2012: Sub-zero frost or near-frost conditions – Overnight Lows May-June to date.

*Indicates definite frost.

15th-16th-May: -4.*
16th-17th-May: -1.
20th-21st-May: -3.*
31st-May-1st-June: -1.
7th-8th-June: -2.*
9th-10th-June: -2.*
10th-11th-June: -6.*

May be an indicator of lower rainfall.

Or alternatively freezing rain.

Possible significant/major rainfall event starting 13th/14th...noted on forecasts 4th.

11.1 mm to approx. 6.45 pm yesterday; moderate rainfalls, probably barely significant – enough to contribute to soil moisture a bit.

Written 18th June 2012:

It seems likely there will some rain as of Thursday (update 19th: Wednesday-Thursday) the 21st coming through from a frontal system in the southern ocean. At a guess, I’d say maybe 10-20 mm over a 12-to-24-hour period from Thursday into Friday at the moment; based on that an estimate of soil water content might increase from its current 0.219 to between 0.221 and 0.226 as a fraction of total volume, and streamflow up to 0.0034 (as an index) from the current 0.0013, which is about 2.6 times the magnitude.

2.6 times the magnitude of current flows, given the last substantial rains raised the river level by up to 125-150 cm from relatively low levels prior to the 20th of May, would mean a rise in the order of another 2 to 2.5 metres (which sounds a bit exaggerated, maybe a metre or two, but who knows – we might get more rain). A rise of 2 to 2.5 metres would have the local flood plain close to or actually flooding. This doesn’t take into account a lack of concrete knowledge of underground systems but is basically based on an analysis of rainfall and temperature patterns, which may partially factor in evaporation and transpiration through temperature-dependence.

From the recent forecasts made available by weatherzone (today (18th), 12 noon), 20-40 mm is suggested for parts of north-eastern Victoria, with snow and sub-zero temperature forecast for the mountains to the east from about the 21st onwards for a few days. That may mean the penetration for the frontal system of this coming event will be much further north, or alternatively that it will peak in that region and clip (update 19th: most likely clip) the Adelaide Hills. A lot of rain is also forecast for the south-east as of about Wednesday, with 5-10 and 10-20 mm a common feature from then onwards for several days in Mt. Gambier.

30 mm to 10 am – estimated soil water content now 0.23.
The river another ~25 cm higher; Index ~0.005.

The figures might seem accurate, but the 25 cm rise is not 2.6 times the magnitude of 125-150 cm. It’s between 20 and 17% of the magnitude, which means the relationship may be more logarithmic than linear with the overnight rain rate.

The response of the river system to the breaking rains, and now these, is a little contrasting and perhaps puzzling. Over the period 22nd to 29th (May) we received about 67 mm, yet the index rose from 0.00032 to 0.00214 (0.001 difference), whereas with 30 mm in 17 hours is only 0.0013 to 0.005, or 0.0037 difference. This seems to indicate the river level is steady, in the sense that it’s neither rising nor fall dramatically.

A rough re-calculation based on these obs suggests to get a metre rise from levels a day ago would require at least 155 mm in the same period.

NB: Figures (estimates) to be taken as a rough guide only.

Also: http://www.bom.gov.au/cgi-bin/wrap_fwo.pl?IDS20368.html

Written 4.30 pm, 23rd.

After 61 mm over a period of 71 hours (with a few brief sunny periods after 12.30 pm and more after 3.30 pm, 22nd), the Upper Onkaparinga was beginning to resemble a raging torrent. While the river height had not changed much, its visible breadth had.

River beginning to peak:



(Shorter footage uploaded due to the size of longer video)

The ground was saturated. The rain at times in the 24-hours to 3 pm, 22nd, had been enough that when the sun did poke through the clouds, the surface water (still running off the paddocks from the last intense event) was a vast sight (of adjoining puddles).

The rain-soaked surroundings:





The river peaked moderately sometime between 3.30 and 4.30 pm, Friday.

Almost 140 mm for the month (pending the rest of the day)...the most since August 2010. Soil moisture estimates approaching the 0.25 threshold surpassed in that year (was 0.233 yesterday).

Post #930498 - 16-01-2011 09:41 PM:


The above still stands…I’m considering only mentioning extremes events/periods, if that, if no one else gives some sort of an indication or interest that they want information continuing in this thread. This thread is not just about my observations, anyone can post theirs in here too.