Highest recorded surface atmospheric pressure in the last week was around 1032 hPa (following a ferocious wind change on the 6th). This pressure was greater than or equal to only two other pressures (2-year period), in October last year, or April 2010. It clearly represents significant departure (change in conditions) from late January to the end of March, during which the pressure did not exceed 1020.

Indications are that it [pressure] may be the highest recorded pressure in the last two years, and is quite possibly contributing to low rainfall/streamflow.

What little rainfall there is, is soaking straight into soil – major sign of deficit in water availability! .

Not expecting any falls to impact on that deficit within at least a week. I have concerns we may be in for a prolonged period of frosts in the coming weeks, but I hope [if so] they are not too severe.

The most likely possibilities at the moment are that:

(a) We’re entering into a prolonged period of weekly frost conditions (~20/0 – 15/0), with intermittent ~25/5-20/5;
(b) Winter is approaching earlier than expected. Whether this means more regular or coherent pressure-troughs (intermittently accompanying high-pressure systems, a sign of significant rain) is open to question.

Streamflow is continuing on downward trend (getting quite dusty)… winds on the 6th were ferocious from the west and southwest. Debris strewn everywhere – massive change in wind speed (bending trees) synonymous with change in pressure and specific humidity (1032 on 11th). Initial wind change clearly visible mid-morning, after which probability of [any] rain fell.

Current conditions for runoff are concerning!

Apologies with any timing of the above post, but I'm staying on topic.

Where would others suggest I put information concerning “impending drought conditions without significant rainfall in the Adelaide Hills, April 2012” (without reference to climate change)…or could I start a new thread?

If it was intended as SA specific - then maybe the SA forum section. If you're thinking wider than just SA a new topic in General Weather may be the place.

Naz, I am mulling over a similar situation at my place. Very low humidity may lead to frosts.
You will have seen my graphs on recent anomalously low values of monthly Relative Humidity.

This month (April) has a low average morning Dew Point of 6.3 to date, and I expect it to average about 5.5 with all data in. That is about 3.5 below normal for this month. In terms of Relative Humidity, that will be about 77% instead of the usual 87%. I had such low RH values last July and in September 2009, but they are very rare. They don't occur in my earlier record except in in the peak of our last drought in October 2002.
In my data, even recent months with high rainfall values have also had low humidity values.
I wonder if this has happened at other stations.

About streamflow...
My local stream, Greenhatch Creek, with a catchment of 70 square kilometres, flows only a few times in a decade. Extreme rainfalls in November and February have left some ponds up to now. There must be flow under the bed.
There is no convenient point to observe surface flow.

I think, under the assumption of localised conditions, it would have to be SA specific because the assumption of impending conditions is based observations within the Adelaide Hills, even though, as Surly has indicated, it could be more widespread. I don’t think this idea of “impending drought” can be attributed to one or two sole causes, and the hydrometeorology (in this case) is very dynamic, hence my no-climate change reference. Taking all into consideration, I would also like to see what others have to say from other regions/localities, so perhaps General Weather is a better place .

Sound like it’s ephemeral (flow based on rainfall only). I think our local river is meant to be perennial (flow all year round), but only in recent years (particularly the end of 2010 and now) has moderate-to-significantly low flow become more evident. It’s very dusty underfoot and the river has actually not been flowing (as in visible, non-groundwater surface flow) for weeks to a month or more. There was a period when groundwater “spring” contribution was evident, but it was intermittent at best. All that is left at the moment are some large 0.5-1 metre deep areas, the rest is dry, as in puddles I did not expect to dry up completely (although I can be certain they wouldn’t have) did. The river environment also provides much support and habitat to local fauna and flora, including (at one point earlier this year) small fish, wild ducks (quite common), rodents, ibis and probably some other things that live in the depths. So if one removes water from the equation, one removes a hell of a lot of natural activity.

I thought the puddles that dried up back in May 2010 probably wouldn’t dry up again for some time – the puddle are drying up (if they haven’t already), despite the green tinge in the pastures, a contradiction if one could call it that. Very dry!

Time will tell whether this next “rain event and showers” circa 21st-25th (this month) will come to anything. GFS seems to indicate maybe 10-15 mm over 2-3 days. From my own estimates I have 10-20 mm, which is right on the edge of the possibly significant falls (i.e. 3-4 mm per hour for 5 days), with 25 mm at the upper extremity. I also have to factor in (in my estimate) the humidity which goes up and down with the diurnal cycle like a yo-yo; it basically means every day there is a very remote chance of rain, just that the right conditions (other variables) are highly unlikely to be sufficient most of the time.

Certainly, Greenhatch Creek is "ephemeral", but I don't think that means "flow based on rainfall only".
As I mentioned, water appeared in the channel some time during our deluges of November and February. I had not noticed water for some years, even though there had been rainfall events. I surmise that the water table had been too low recently for even heavy rain to cause surface flow. Even in this ephemeral stream, surface flow requires a saturated catchment.
I notice that you are near a place called "Chain of Ponds". This is a recognized geomorphological term, but Google search is now swamped by commercial entries. You have to add "geomorphology" as a search term to by-pass the spam.
One page that gives the sense and significance of "chain-of-ponds" is here:
http://www.springerlink.com/content/t1730878358772n6/
Stream channels in SE Australia, in certain climate conditions, can lose the ability to form a channel, which is replaced by a chain of separate ponds. This landform was commonly observed by early explorers.
I doubt that research has yet revealed how it happens.

Excuse the mistake I made earlier upon editing, it was 3-4 mm per day for 3-5 days, not 3-4 mm per hour .


By ephemeral, I meant flows intermittently, which can include artesian flow from groundwater; could have clarified a little better. By rainfall-only I meant that unless there are sources of water upstream, runoff from higher ground, whether above the surface or sub-surface, contributes to streamflow. But I don’t live in your area, and so am thankful to have input from on-the-ground from somewhere else.

Is now exceeded, approaching 30, last recording (~8.30 am) 26.3 mm.

32.7 mm to 4.30 pm yesterday (21st-24th)...no significant changes to flow, not that the river is visibly flowing.

The chance of flows being as low as they were in early-to-mid May 2010 (within the next week or two), has probably now diminished. However, 3 to 4 inches (~75 to 100 mm) over a few days to a week might be enough to have proper (mean?) flows again.

On another note, observations of rainfall over the last 2 years (relatively short period of time so far) suggest whether “breaking rains” occur may be affected by the capacity of the surface soil layer to retain moisture, and more importantly the connectivity of soil pores, which is affected by whether there is moisture or not, which can include condensation from frosts and high humidity (or fogs).

At the moment, from my own observations, it seems there might be the possibility of significant rain on or around the 8th and 9th. Things look fairly changeable as well, but will wait and see .

As an aside, 10 small (5-10 cm diameter) ant nests (mounds) have been observed in a cluster in an approx. 5-m square area near the river.

I guess not.

Basically it seems like there might have been some other factor I did not take into account as to why there was little chance of rain during that period – the 8th to 9th. The estimate I made was based only on precipitable water and pressure. Perhaps specific or relative humidity were more significant!?

Second sub-zero frost in as many days – possibility of major onset of rain in week’s time or so...

Do you think the stream has seeps into it or it relies only on surface drainage?

Not sure because it seems fairly dynamic, probably yes. Spring activity probably more certain (especially upstream).

I think we're on an alluvial plain.

Naz, may I suggest that, without learning a lot more about your landforms, you will be shooting in the dark.
The plain near the stream could be an erosional plain, not formed by a river, or it could be an alluvial plain, in which case it could be a flood-plain or a terrace. Floodplains are actively changed by channeled stream-flow or by over-bank flow in the present climate: terraces are too high to be affected. I am sure there is data on similar streams near you that you could use to assess what kind of a plain it is.
In Australia, floodplains are generally flooded in 2% of the years. In the old world it seems that floodplains are flooded almost once a year.
A technique for describing landforms to help to understand them is here.
http://books.google.com.au/books?printse...epage&q&f=false
Part of the technique is in this book sample, but you have to buy the book to get all of it, and you really have to think it through thoroughly. I recommend the Second edition.
A great tool for the work is a detailed topographical map. You can easily outline the alluvial land because it has exactly the same slope as the bed of the stream, and runs in the same direction. I am sure that maps at 1:25,000 scale exist, and probably maps at 1;10,000 or larger, with a 1 metre contour.
Best of luck.

Thank you for your insight and information, I will consider the material .