# What I didn't do on my holidays

Get into the Western Plains Zoo. We turned up yesterday afternoon, but admissions had already stopped for the day. There was a big storm overnight, and the zoo was closed for repairs today. So we drove back to Brisbane, seeing most of the northern catchments of the Murray-Darling Basin in the aftermath of excellent and widespread rain. My back of the envelope estimate suggested that several thousand GL of water had fallen in a single day, but I’m glad to be corrected on this.

I also missed the Senate Committee on Telstra which, by a surprising coincidence, held its hearings in Dubbo yesterday. I’m appearing tomorrow in Nambour, a little way north of Brisbane.

## 3 thoughts on “What I didn't do on my holidays”

1. Jim Longmire says:

John, The volume of water falling can be calculated from the following:
1 megalitre = the volume of a swimming pool of 50m x 10m x 2m
= the volume of 100 millimetres of water x 1 hectare.
If there are 100mm of rain on average over 1 million hectares, 1000 gigalitres fall. Cheers Jim

2. John says:

Jim, your envelope matches mine, excepted that I used 100 000 sq km = 10 million hectares and 1cm=10mm for each GL, and guessed several cm of rain had fallen.

Checking, I find that the total area of the basin is close to 100 million ha, but presumably not all of it got as much rain as the bits I saw.

Do you have an estimate of total annual rainfall for the Basin and the proportion that ends up in the river system?

3. Jim Longmire says:

For the Murray Darling Basin,total area is 1,061,000 sq kilometres (14% of total area of Aust). Mean annual runoff is 24,300 GL. Mean annual outflow is 12,200 GL. See http://www.mdbc.gov.au/education/encyclopedia/Surface_Water/Surface_Water_Resources.htm#Table%201
Using a Basin area of 1 million sq kilometres, or 100 million hectares, and an average annual rainfall of 400mm over the whole catchment (the wetter areas of NSW & Vic will exceed 1000mm p.a. while the vast inland areas will average about 300mm p.a.) the total volume falling as rain is 400,000GL p.a.. So runoff is only 24.3/400 or 6% of total rain falling. Makes sense for much of the Basin. In wetter areas runoff would be a much higher proportion of rainfall. As well there is the Snowy River water which delivers about 1000GL p.a. to the Basin as best I can interpret the science. see http://ea.gov.au/water/rivers/nrhp/snowy/hydrology.html
Main point here is that the Murray Darling river system is largely fed by tributaries east of the Newell Highway (and extensions), especially by those in the more mountainous catchment areas (the exception is in those big wet years when all of the inland gets well above average rainfall.) Cheers Jim