This graph shows the total rainfall shortage in this drought at Manilla NSW. It uses the same rainfall figures as in the October Rainfall Status graph, that is, the rainfall totals for up to 360 months. In this case, I have subtracted from them the normal rainfall totals. That shows the rainfall deficit: how much less rainfall we have had than we normally would.

Generally a 1-year deficit

In many cases, the shortage now is about 650 mm, or one year of normal rainfall. This is true when calculated for 24-months and for several other durations including 360-months. In broad terms, this drought has left Manilla about one year short of rain.

Deficits are smaller at short durations.

A one-year deficit could occur at a duration of 12-months only if there were no rain. A curve on the graph shows “No rain at all”. A second curve is labelled “50% of normal Rain”. These October data show that the rainfall totals for durations below 24-months are around 30% of normal.

Deficits larger than 650 mm occur at some long durations

There are three dips in the curve where deficits are greater than one year of rainfall.

1. The 3-year deficit is now 820 mm, due to very low rainfall in the last six months. Such values may yet appear at other durations.
2. The 7-year deficit is now 920 mm. This deficit is due to the inclusion of severe rainfall shortages of 12-month to 30-month duration in 2013 and 2014. These appear on this drought duration graph, but were scarcely noticed at the time.
3. The 20-year deficit is now 1100 mm. The 650 mm deficit due to the current drought is supplemented by deficits of some 200 mm carried forward from both the 2013-14 shortages (above) and the extreme 12-month shortage of 2002.

What the deficits mean

In times of normal or excess rainfall, the whole terrain is kept supplied with water. It goes to aquifers deep underground, to shallow aquifers near streams, to stream-flow, and to the soil and plants. In general, the longer the duration of normal or excess rainfall, the larger the reservoirs, both underground and on the surface, that can be filled.
The present deficit of one year of rainfall must have drawn down all the reserves of water in the terrain. The empty state of the Keepit Dam (0.8%) is an obvious result, but all other reserves must have fallen.