Our Great (but Tricky) Benefactor – the Hydrologic Cycle

By Dr. Leon Bren

Most of us don’t think about water resources in terms of an abstract concept of the hydrologic cycle; rather we think in terms of tapping a big river or water flooding our house or town. But these are really subsets of what the hydrologic cycle does.

What Is the Hydrologic Cycle?

For most, the hydrologic cycle is a diagram (Figure 1). It shows water evaporating from a benevolent, tranquil ocean to form pretty, fluffy, cumulus clouds. These pass inland and, by some action, cool, causing rain to fall. This then runs down the hillside to form rivers and streams. Usually, they show grass and trees busy evaporating. All in all, they are a pleasant, tranquil scene of hydrology at work. The concept goes back to veteran hydrologist Robert Horton in the 1930’s. Hydrologists Langbein and Hoyt in 1959 noted that “the hydrologic cycle is one of nature’s grand plans”.1

It is only recently that the hydrologic cycle concept has come under some scrutiny. Sociologist Jamie Linton has noted that the hydrologic cycle is better viewed as a hydro-social cycle, since each component has large implications about how we live.2 He also observed a “temperate bias,” since Figure 1 could just as logically show deserts instead of forest and pasture. Societies spend a lot of time trying to modify the hydrologic cycle to gain some local advantage. For example, in the latter part of the last century “cloud-seeding” was the favoured way to make it rain on a local catchment, probably at the expense of rainfall in some distant and unknown place. Historians of colonialism have noted that often one of the first acts of colonial powers was to modify the hydrologic cycle to their advantage by building channels and dams.

Figure 1: The hydrologic cycle as viewed by the US Geological Survey. Source: USGS (Public Domain)
The Role of Science in Hydrologic Cycle Management

Historically, the development of water resources around the world was viewed as a triumph of applied science. This involved measurement of volumes and flow rates, developments of concepts of water energy and water quality, and applying these to natural systems. Under the leadership of engineer Ronald East, Victoria had a large program of dam building from about 1936 to 1975. These dams were viewed as temples of applied science. This enthusiasm then dimmed – possibly because we had a run of benign seasons until the 1990’s. Economist Bruce Davidson in “Australia Wet or Dry” pointed out that we had gained a degree of hydrologic protection but argued that this wasn’t worth the cost.3 And there was some “counting the cost” of unique environments lost to dams – of which Lake Pedder (Tasmania) is an example.

More recently, the hydrologic cycle is being viewed as a major part of the Earth’s radiation balance and is being incorporated into Global Circulation Models. These have proven to be difficult to formulate and program, particularly at local levels. The advent of imagery and thermal measurement from satellites is creating new opportunities at local, national, and international levels. Controversial Russian work has suggested that volume changes of water in the hydrologic cycle creates winds which transport water far inland (“flying rivers”, “rivers of rain”).4, 5, 6 The “bottom up” approach of scientists last century (based on observation and modelling of ground-based flows) has been replaced by a literally “top-down approach” involving global circulation and modelling. Difficulties include the training and equipment to participate, going from a global approach to what it means for a particular town or catchment, and issues of political water management that may emerge involving water vapour crossing international boundaries.

Our Cities Buffer Us from the Hydrologic Cycle

In 1854, Dr John Snow was trying to work out why cholera was killing people in London. Through inspired research he realised that the water in a main London public tap was contaminated by sewage, and that this was responsible for the outbreak. It is hard to underestimate the importance of this in an age where theories of infection were vague at best. Cities around the world took note that a good water supply and sewerage system were necessary for good health. In Melbourne, this realisation ultimately led to the construction of the Yan Yean water supply in 1857, which would provide the city with an adequate water supply for the next few decades.

Melbourne’s growth has hardly slowed down. From an 1857 population of 400,000 to a 2023 population of five million, the city has kept growing – and so too did the water supply projects. Having an inadequate water supply was not (and is not) an option for towns and cities. The large storage buffer created has substantially (but not entirely) made the vagaries of the hydrologic cycle irrelevant for most of us.

Do We Manage the Hydrologic Cycle?

The short answer is probably no, but we do manage aspects of it to gain a degree of protection. In particular:

  • We put a lot of work into storing water so that, should it choose not to rain for a short or long period we have enough to get us through
  • The volume of water stored may “take the peak” off high flows, thereby mitigating flooding.

Our larger river channels are usually capable of transmitting large flows. Such “bankers” can be very uncommon – so uncommon that they are often forgotten about in the planning process. This then gives a “blame game” when people build houses or businesses on this land within what is ultimately the high flow channel. We use mathematics to make errors of frequency (e.g., 1 in 100 years) but the error bars in such estimates are huge and often the assumptions implicit are tenuous. We often build earthen walls (“levees”) to restrict the widths of the channels.

All things considered, the hydrologic cycle is something we still work around, not manage.

But the Hydrologic Cycle Still Bites Us

Most Victorian residents can claim a few “degrees of separation” from the hydrologic cycle, but the daily news is as filled with “hydrocycle adventures” as ever. There is a certain inevitability of “once in a hundred-years” events, earned by the fact that they have occurred, on average, once every hundred years for a certain period of time.

Flooding in the River Murray system is still being cleaned up,7 towns in western NSW have run out of drinking water,8 the town of Lismore in NSW is contemplating a very-expensive relocation,9 the city of Brisbane’s future is clouded by its flood vulnerability and the discovery that upstream dams can’t guarantee immunity.10 Victoria discovered that once flood-prone sites along the Maribyrnong River are still flood-prone,11 and development along the river may have enhanced this tendency.12 Flood insurance for houses in some areas have reached five figures and are viewed as “unaffordable”.13

In the public comment there is at times an undertone of, “how can such things be allowed to occur in the modern age?”. Some of us are not separated at all, and, arguably, the gap between comfort and various levels of disaster is not always great and, possibly, diminishing. We probably have an overstated view of our society’s “cleverness”.

Our Future Living with the Hydrologic Cycle

Overall, we have done OK out of the hydrologic cycle. In Victoria we’ve been well fed (thanks partly to irrigated crops) and housed (mostly undisturbed by floods) and, as individuals, hardly need to pay it attention. It has created pretty landscapes and a valued biota well-adopted to coping with its short and long-term vagaries.

The question is, can this continue for the indefinite future? If we looked into the past as far as our records go back, we’d be tempted to say “Sure – why not? It has worked (in its hiccupping way) for recorded history; why should it falter in the next century?”. Science shows us that the world climate is changing, and the experiences of the past are no longer the guaranteed guide to the future that they once might have been. We have a greater knowledge of the processes involved in managing the Earth’s climate but still require greater understanding of the inherent instabilities. And if the issues of managing rivers that cross State or National boundaries have been complex, imagine trying to manage “flying rivers” of water vapour across these boundaries.

We have benefitted from the application of science to its water resources. Commonly this involved increasing the amount of storage in some particular stream component.

We now have a more mature appreciation of the hydrologic cycle, its vagaries in statistical terms, and at least some of the social implications of it, but we are a long way from “understanding” the stop-go nature of it. This will be a major research area in the future with exciting but “difficult-to-use” tools becoming available. Full consideration of it involves the nature of the relationship between humans and the natural environment. For most of us we are fortunately remote from these difficult questions and the hydrologic cycle should continue to provide us with clean, safe places to live.

Dr. Leon Bren trained as a forester at the Victorian School of Forestry. After some years working in the field, he did a doctorate in the hydrology of small, forested streams. He worked as a lecturer in forestry, specialising in forest harvesting and forest hydrology at The University of Melbourne. Since then he has been a consultant and author of books on forest hydrology. He is currently writing an environmental history of Lake Corangamite in south-western Victoria.


  1. Langbein, W. B., and W. G. Hoyt (1959). Water Facts for the Nation’s Future: Uses and Benefits of Hydrologic Data Programs (Conservation Foundation Studies in Water Resources). Ronald Press Co.
  2. Linton, J. (2010). What Is Water?: The History of a Modern Abstraction. UBC Press.
  3. Davidson, B. R. (1969). Australia Wet or Dry?: The Physical and Economic Limits to the Expansion of Irrigation. Melbourne University Press.
  4. Makarieva, A. M., & Gorshkov, V. G. (2007). Biotic pump of atmospheric moisture as driver of the hydrological cycle on land. Hydrology and Earth System Sciences, 11(2), 1013–1033. doi.org/10.5194/hess-11-1013-2007
  5. Makarieva, A. M., Gorshkov, V. G., et al. (2013). Where do winds come from? A new theory on how water vapor condensation influences atmospheric pressure and dynamics. Atmospheric Chemistry and Physics, 13(2), 1039–1056. doi.org/10.5194/acp-13-1039-2013
  6. Pearce, F. (2020). Weather makers. Science, 368(6497), 1302–1305. doi.org/10.1126/science.368.6497.1302
  7. National Emergency Management Agency (2023). River Murray flood clean up in full swing nema.gov.au/media-centre/about-us/River-Murray-clean-up
  8. Williams, C. (2023). The Australian town where water insecurity is felt more than some communities in Bangladesh. ABC News. abc.net.au/news/2023-04-13/walgett-nsw-water-insecurity-worse-than-bangladesh/102212784
  9. Chenery, S. (2022). Land swaps, relocations or rebuilds: Lismore community grapples with its future. The Guardian. theguardian.com/australia-news/2022/apr/09/land-swaps-relocations-or-rebuilds-lismore-community-grapples-with-its-future
  10. Cook, M. (2018). “It Will Never Happen Again”: The Myth of Flood Immunity in Brisbane. Journal of Australian Studies, 42(3), 328–342. doi.org/10.1080/14443058.2018.1487871
  11. Barraclough, A. and Paul, M. (2022). Residents along Maribyrnong River still out of homes after flooding in Melbourne’s west. ABC News. abc.net.au/news/2022-12-23/residents-flooding-melbourne-west-maribyrnong-river-homes/10158601
  12. Bucci, N. (2022). Did the wall that saved the Melbourne Cup racetrack contribute to the flooding of 245 homes? The Guardian. theguardian.com/australia-news/2022/oct/21/did-the-wall-that-saved-the-melbourne-cup-racetrack-contribute-to-the-flooding-of-245-home
  13. Climate Council. (2022). Uninsurable Nation: Australia’s most climate-vulnerable places. climatecouncil.org.au/resources/uninsurable-nation-australias-most-climate-vulnerable-places/