Predicting Water Availability

Natural water sources can be perennial (flow all year round) or non-perennial (intermittent). Whether a natural water source contains water depends on physical variables like temperature, rainfall and humidity. A bushwalking group looking for a water source has to decide on how likely a watercourse is to contain water, if the water is in sufficient quantity to collect, if the water is accessible (i.e. not down a steep embankment or with a wall of blackberries), and ideally, not needing treatment.

A crucial natural variable for the amount of water availability in the bush is the amount of recent rainfall, which varies with:

1. Location: Coastal and northern Australia and Tasmania experience higher levels of rainfall than central and western Australia.
   
   Therefore, walking the Overland Track in Tasmania is less likely to present water issues than the Larapinta Trail in the Northern Territory.
2. Season: Australia contains both temperate, tropical and arid regions that have different rainfall regimes. In general, the northern parts of the country have summer rains, and the Southern have winter rains. The east coast has moist temperate conditions with hot summers, but thunderstorms with rainfall are common during the summer months too. Cooler months mean less evaporation of potable water sources.
   
   In general, rainfall across Australia is low and seasonal with many water courses only appearing after rainfall.
3. El Niño and La Niña Phenomena: The air pressure index between Tahiti and Darwin is used to calculated the Southern Oscillation Index (SOI). El Niño is characterised by a strongly negative index, often leading to lower winter and spring rainfall in eastern Australia, and a weaker monsoon up north. By contrast, La Niña is characterised by a strongly positive index, with higher winter and spring rainfall in eastern Australia and higher rainfall during the tropical wet season.

Other important factors are:

1. Altitude: water flows downhill and is more likely to collect at lower altitudes than higher ones.
2. Area of the catchment: the larger the catchment area, the more likely the source is to contain water because it has a larger area feeding into it.
3. Soil type: soil absorbs water at different rates depending on the porosity and permeability of the ground. Hence, the total amount of groundwater available and where other water sources can surface in that environment is a function of the soil type there.

Global warming is leading to drier conditions, and availability is changing and decreasing in some places[note]Letcher, T.M., Climate change: observed impacts on planet Earth. 2015: Elsevier[/note].

Adequate planning before a bushwalking trip allows the group to devise ways to manage obstacles like sheer cliffs, blackberries, and thick scrub that may obstruct water sources. In general, going along the riverbank may lead to easier access. The group may find a secondary source that has less flow but is accessible.

In practical terms, predicting water accessibility and availability comes down to good knowledge of the land, experience with how local weather patterns affect flow, and an understanding of how clean that water source is. While this knowledge can take years to accumulate, a good starting point is talking to land managers and other people who have recently walked in that area.