The hydrological cycle in the era of the Anthropocene – does the water cycle scheme need revision?

“The rain glistened on rooftops and turrets, not realizing what a swarming, hateful world it was falling on. The more fortunate rains fell on sheep in the mountains, rustled quietly among the forests or – a bit incestuously – splashed the surface of the sea with drops. The rain falling on Ankh-Morpork was bringing trouble on its own. In Ankh-Morpork they did terrible things with water. Drinking was just the beginning of her problems.”
Terry Pratchett Guard! Guard! (translated by Piotr W. Cholewa)

Do you remember from your geography lessons the diagram of the water cycle in nature? Basically, it was a fairly simple diagram, showing the evaporation of water from the ocean into the atmosphere, its condensation, precipitation as rain, and underground and surface runoff back into the ocean. Water in its various states of aggregation moves through the environment as a result of physical processes such as evapotranspiration (field evaporation, which includes transpiration and evaporation), condensation, precipitation, infiltration, surface runoff or groundwater outflow. In extreme terms, the essence of the hydrological cycle is the evaporation of moisture in one place and its precipitation in another.

However, the hydrological cycle in reality is a much more complex system, and proper recognition of the relationships that condition it is essential to understanding the challenges of environmental change (including climate) and human interference with the Earth’s water resources.

How much water where?

The amount of water on Earth is relatively constant at 1.386 billion ^km3 [1], but its share in different states of matter (liquid, vapor, ice) and forms (ice reservoirs, fresh and salt water reservoirs, atmospheric or soil water) varies depending on environmental conditions.

Classical diagrams of the Earth’s water resources indicate that approx. 97% of the resource is salt water, stored in the seas and oceans. Other approx. 3% (2.5 million ^km3) is freshwater, which is 68% stored in glaciers, ice sheets and permafrost, and 30% is underground. Surface freshwater resources in rivers, lakes, swamps and marshes are about 93,000. ^km3, which is only a paltry fraction of the Earth’s total water resources. Nevertheless, rivers and lakes are the primary source of water in people’s daily lives [2].

Available estimates say that only about 577,000 are involved in the hydrological cycle per year. km3 of^water , i.e. 0.04% of the total volume of the hydrosphere. This means that only part of the water resources circulate in the hydrological cycle, while deeper groundwater, part of the ice sheets or the deep zones of the All-ocean may not enter the cycle at all. A key role is played by the ocean, from the surface of which 86% of global evaporation occurs and which takes in approx. 80% of the precipitation (so it has a negative water balance). From the evaporation of all water on Earth, 90% of water vapor enters the atmosphere. The remaining 10% is the result of evaporation (evaporation from the ground) and transpiration (drainage of water by plants).

Man modifies the water cycle

The water cycle is driven by two natural forces: solar energy, which causes water to evaporate into the atmosphere, and the force of gravity, which allows water to fall from clouds as precipitation and flow down the drainage basin over land, under the surface and downstream. Human use also has a significant impact on where and in what quantities water is stored and how it circulates. Man regulates and changes the course of rivers, builds dams, drains wetlands.

About_{three-quarters} of the Earth’s ice-free surface has been transformed by activities such as agriculture, deforestation and drainage, significantly affecting evapotranspiration, groundwater recharge, changes in river flow rates and precipitation at the continental scale. Man draws water from rivers, lakes, surface and groundwater bodies for domestic supply, agricultural irrigation, livestock maintenance and industrial activities such as mining, power generation and aquaculture.

A very important influence on the hydrological cycle is climate change. Changing precipitation patterns (change in frequency and intensity), which determine the occurrence of extreme events such as droughts and floods, as well as rising sea and ocean levels, modify the amount of water and its circulation time. The primary reason for the intensification of the water cycle is the increased amount of greenhouse gases that lead to the heating of the atmosphere. According to a physical law, described by the Clausius-Clapeyron equation, the saturated vapor pressure increases by 7% when the temperature increases by 1°C. An increase in atmospheric temperature is therefore associated with a higher proportion of water vapor in the atmosphere, which affects evaporation and precipitation.

Humans also affect water quality. In agricultural and urban areas, irrigation and rainfall cause fertilizers and pesticides to leach into rivers and groundwater. Surface runoff carries chemicals, sediment and other pollutants that enter rivers and lakes. Power plants and factories return heated and polluted water to rivers. Climate change is also contributing to the deterioration of water quality. For the water cycle itself, its quality may not matter much, but for its availability for specific human needs, it does.

Hydrological cycle to be revised

Human interference in the water cycle has made the regional and global water cycle models used so far obsolete and no longer authoritative. This thesis was verified by researchers from an international team led by Benjamin W. Abbott of Brigham Young University, USA. They reviewed nearly half a thousand publications on the global water cycle diagram and analyzed more than a hundred English-language diagrams from textbooks, peer-reviewed articles, government materials and online sources.

For each, they quantified detailed metrics, including biome, scientific field, and the number, size and proportion of water resources and their flow. They also compared approx. 350 water cycle diagrams from 12 countries (available in national languages). They published their results in 2019. In the pages of Nature Geoscience [3]. They also included many intervening observations and thoughts in a comprehensive commentary that appeared in Hydrological Processes that same year [4].

The authors found that as many as 95% of the diagrams depicted a single catchment area without considering the interrelationships of different river basin areas, only 15% took into account the human impact on the hydrological cycle, and only 2% referred to links to climate change, water pollution or land cover changes.

According to the authors, the information appearing in the public space about the availability of fresh water for people is far overestimated, which has three main reasons. First, in terms of surface water, the diagrams do not distinguish between the division of lakes into saline and fresh (while half of their global volume is saline) and the division of groundwater resources into renewable and non-renewable (97% of groundwater should be considered unusable due to its insufficient rate of renewal or high salinity). Quantitative diagrams usually gave the total volume of these reservoirs (for example, 0.19 million ^km3 of water in lakes and 22.6 million ^km3 of groundwater), which grossly overstates the actual freshwater resources.

Second, no diagrams indicated freshwater resources actually available for human use, when in fact they account for less than 10% of annual land precipitation and 25% of annual river runoff. Only 5% of fresh groundwater can be extracted in a way that does not upset the hydrological balance. This means that globally available and sustainable blue water resources are only from 5,000. Up to 9,000. ^km3 per year, which is alarmingly close to current estimates of global consumption, remaining at between 3,800 and 3,800. Up to 5,000. ^km3 per year.

Third, ignoring man’s consumption of the so-called “green”. gray water (water needed to dilute the pollutant load to background concentrations and the required quality standard), the diagrams not only depicted less than the actual human impact on the hydrological cycle, but also overestimated potentially available resources by 30 to 50%.

Why are water cycle diagrams important?

Someone might ask what all the fuss is about. Well, water cycle diagrams not only present our knowledge and understanding of the hydrological cycle, but also shape society’s attitude to water and the problem of its availability. Since they have been available and widely used since at least the 1930s, why do they contain so many basic errors, and do these errors contribute to poor water management?

The synthesis performed by Abbott and co-authors revealed the need for a significant revision of many water resource and flow estimates. Such an update is possible thanks to the development of survey and analytical techniques, such as remote sensing and modeling, which enable more precise calculations.

The latest estimates of human consumption of blue water (captured from surface and groundwater reservoirs), green water (stored in topsoil and plant tissues, including crops) and gray water indicate about 24,000. ^km3 per year, which means a redistribution equivalent to half the global river runoff or double the global annual groundwater recharge.

The omission of human activities from the water cycle diagrams may suggest that they have no impact on one of the Earth’s most important and endangered resources. This misrepresents the actual causes of today’s most significant social and environmental crises, such as threats to water security and access to drinking water, loss of biodiversity, climate change and eutrophication of surface waters. Given the enormous scale of human drama and ecological disruption associated with the global water crisis, we must use all of our scientific and cultural resources to better understand the hydrological cycle and accelerate the implementation of sustainable water management.

As early as 200 years ago…

The brilliant German naturalist and traveler Alexander von Humboldt, during an expedition to Venezuela in 1800, observing the anthropogenic transformation of the landscape in the Aragui Valley and its destructive impact on Lake Valencia, was one of the first to formulate the problem of the effects of human-induced environmental change. In his diaries he wrote:

“When forests are destroyed, as everywhere in America, by European planters with reckless rashness, the springs dry up completely or become less productive. Riverbeds, dry for part of the year, turn into streams only when heavy rain falls in the mountains. Grass and moss disappear along with scrub from the slopes of the mountains, rainwater no longer has obstacles in its path, and instead of slowly recharging the level of the rivers by gradual seepage, it rushes up the slopes during rainstorms, pushing down the loosened soil and causing unexpected flooding that destroys the country.”
(The excerpt is from A. Wulf’s The Man Who Understood Nature, translated by P. Chojnacki and K. Bazynska-Chojnacka; based on AH Personal Narrative 1814-1829, t. 4, s. 143-144).

It seems that modern man, more than two centuries later, is rediscovering the same truths.

In the article, I used, among others. From the works:

[1] Shiklomanov I. 1993. World fresh water resources. [w:] Peter H. Gleick (ed.), Water in Crisis: A Guide to the World’s Fresh Water Resources. Oxford University Press
[2] https://water.usgs.gov/edu/watercyclepolish.html
[3] Abbott B.W., Bishop K., Zarnetske J.P., Minaudo C., Chapin F.S., Krause S., Hannah D.M., Conner L., Ellison D., Godsey S.E., Plont S., Marçais J., Kolbe T., Huebner A., Frei R.J., Hampton T., Gu S., Buhman M., Sayedi S.S., Ursache O., Chapin M., Henderson K.D., Pinay G. 2019. Human domination of the global water cycle absent from depictions and perceptions. Nature Geoscience, 12.7: 533-540. https://doi.org/10.1038/s41561-019-0374-y
[4] Abbott B.W., Bishop K., Zarnetske J.P., et al. 2019. A water cycle for the Anthropocene. Hydrological Processes.33: 3046-3052. https://doi.org/10.1002/hyp.13544