Water cycle

Technical articles

Water management Soil regeneration Regenerative hydrology

River in the Amazon rainforest. Jlwad

The amount of water on Earth is constant. It is distributed among several water reservoirs such as the atmosphere, seas and oceans, glaciers and ice caps, rivers, lakes, groundwater, and vegetation. Water circulates between these different reservoirs in the form of liquid water or water vapor gas. All these flows between reservoirs constitute the water cycle.

How the water cycle works

The water cycle can be summarized as follows :

Simplified diagram of the water cycle in a watershed where the water cycle functions optimally (e.g., deciduous forest).

Accumulation in the atmosphere

Rainwater forms by condensation of water vapor present in the atmosphere as liquid water. On continents, two-thirds of rainfall is caused by the vegetation cover. Precipitation comes from the mixture of water vapor from the sea and evapotranspiration from the land, distributed as follows :

• 60% of water is evapotranspired by vegetation in continental areas (especially deciduous trees).
• 30% of water comes from evaporation of seawater. Solar energy enables evapotranspiration of water by vegetation through photosynthesis. Solar energy is the engine of the water cycle.
• 10% evaporates from the soil.

Return to the ocean (rivers and surface runoff)

• Part of the rainwater evaporates immediately (≈10%).
• Another part of the rainwater infiltrates into the soil. Water infiltrated into the soil joins the groundwater and aquifers by gravity. Some groundwater feeds the rivers and streams that flow into the ocean (≈30%). Soil and groundwater can also be absorbed by plant roots.
• Another part of the rainwater can flow on the surface as runoff. Surface runoff directly feeds rivers and streams that flow into the ocean without prior infiltration into the soil. In a deciduous forest, surface runoff is nonexistent, and all non-evaporated rainwater infiltrates the soil. The portion of rainwater returning to the ocean then does not exceed 30%. In an ecosystem where vegetation is less abundant or in urbanized areas, surface runoff occurs. Rainwater is then excessively discharged into the ocean via rivers (>30%).

Complete water cycle (USGS)

Withdrawal vs resource

Water withdrawals represent only 2 to 3% of the water resource, that is, the volume of annual precipitation. Water resources are not lacking, but the challenge is to better capture this resource and better manage its uses collectively.

Diagram of the origin and uses of water withdrawals.

Once withdrawn, water is used for :

• Human use (34%) - including drinking water but also sanitation and washing.
• Agricultural use (46%) - including irrigation and use in agricultural processes. Water used for irrigation either returns to aquifers or to the atmosphere by evapotranspiration.
• Industrial and energy production use (20%).

Once used, water undergoes treatment before ending up in rivers or infiltrating into soils and possibly joining aquifers. Ultimately, used water is returned to the sea. Discharges into rivers cause pollution and increase watershed drainage. Recycling water means not discharging it into the sea via rivers, thus re-infiltrating it into the soil after treatment or recycling it for non-domestic uses such as irrigation.

NB : Water is used and not consumed, because it does not disappear - in all cases, it eventually returns to the cycle! However, its use is accompanied by more or less significant pollution.

Vegetation in the water cycle

Vegetation plays a central role in regulating the water cycle. At the interface between soil and atmosphere, it is necessary to maintain the balance between evaporation, transpiration, infiltration, and runoff of rainwater.

It reduces evaporation by :

• an windbreak effect
• creating shaded and humid areas through foliage,
• lowering soil temperature (about 20°C less).

It limits runoff and promotes infiltration by :

• slowing rainwater by foliage,
• structuring the soil through roots that ensure good soil porosity and stability (useful reserve),
• protecting against Erosion thanks to the humus layer (decomposed organic matter and plant material).

Evapotranspiration phenomenon

Vegetation locally contributes to precipitation formation through leaf transpiration. Transpiration is conditioned by several factors :

• Plant species : not all plants have the same transpiration capacity. A deciduous tree transpires twice as much as a conifer and thus causes twice as much rain as a conifer. Ideally, a forest should not consist of more than one-third conifers. The same applies to corn which has a much higher evapotranspiration capacity than wheat, for example.
• Climatic conditions : humidity and air temperature, wind, and solar radiation impact plants' evapotranspiration capacity.

A plant under water stress no longer transpires (leaves dry out). To keep the cycle functioning, it is therefore necessary to provide water to plants.

The evapotranspiration cycle is necessary to trigger rainfall over the watershed. In other words, the absence of evapotranspiration will generate an absence of rain. Conversely, it rains almost continuously over jungles, and never over deserts, regardless of the distance to the ocean.

Key role of the tree

The tree plays a crucial role in the water cycle. 

• The depth of its roots is essential for communication between the soil surface and shallow aquifers (e.g., sand aquifers in the Landes or aquifers accompanying rivers).
• The mycorrhizal network promotes soil hydration and hydration of surrounding vegetation.

In response to local weather variations, the tree thus helps regulate access to deep water.

Disruption of the water cycle

Disruptions in the water cycle result in an imbalance in the annual distribution of rainfall, leading to alternating floods and drought episodes.

Temperature increase

Higher temperatures increase evaporation capacity. In practice, warm air can hold more water than cold air. This phenomenon is observed with dew : when cooling at the surface of plants, air releases its excess water.

Insufficient infiltration and excessive runoff

Simplified diagram of the altered water cycle in a watershed where water infiltration is insufficient (insufficient vegetation density and/or urbanized soils).

When the water cycle functions optimally, only 30% of rainwater is returned to the sea via rivers. When water infiltration into soils is insufficient, this proportion increases. :

• At the Garonne River, 50% of rainwater is currently discharged into the sea.
• In 2020, this proportion reached 75% at the Sèvre Niortaise in Nouvelle Aquitaine.

This excess water not stored in soils leaving the watershed leads to a decrease in the volume of water that can be transpired by vegetation, thus a decrease in precipitation volume. Floods are then followed by periods of drought.

Infiltration and soil quality

Water infiltration into the soil is closely linked to soil type and soil quality.

Soil texture obviously determines its infiltration capacity, but this infiltration rate can be improved by regenerating soil life : a living soil has a structure that promotes water infiltration and limits runoff.

As with infiltration capacity, soil structure directly impacts its water retention capacity (useful reserve).

Several mechanisms are at work :

• Roots (including mycorrhizal networks)
• Spaces left by soil fauna (microfauna, but also earthworms, etc.)

Conversely, soil tillage, compaction, and ploughing reduce infiltration and water retention in the soil. Even if it increases soil porosity, ploughing destroys mycorrhizal networks and micropores covered with organic matter necessary for soil stability.

Bare soil also generates crusting phenomena.

Soil sealing

Reduction of vegetation cover due to deforestation, urbanization, and certain practices in agriculture has degraded soil quality and disrupted the water cycle both locally and globally.

Reducing vegetation area results in :

• Reduced rainfall volume formed over land due to reduced evapotranspiration.
• Impaired communication between surface water and deeper water due to deforestation and absence of trees in some agricultural or urban areas.

Impermeable soil surfaces such as roads and sidewalks in urban areas block water infiltration. 

Similarly, many plots that used to serve as buffers during heavy rains (floodable plots, wetlands, etc.) have been dried and drained over time to favor agricultural production or building construction, increasing the runoff phenomenon.

In summary, like bare soils or those with low vegetation density, impermeable urbanized soils generate excess runoff that disrupts the water cycle, increases flooding, and reduces evapotranspiration, thus precipitation.

Comparison of a desert ecosystem and a deciduous forest

A deciduous forest corresponds to an ideal ecosystem where the small water cycle, also called the short cycle, functions optimally. In a desert ecosystem where vegetation is sparse and soil life is limited, the small water cycle is altered or even nonexistent. The table below summarizes various water cycle parameters in these two ecosystems.

	Desert	Vegetation (deciduous)
Precipitation	Irregular	Regular
Runoff	Maximum	Minimal
Evapotranspiration	None	Maximum
Water reserve	Minimal	Maximum (structured soil)
Water flow	Fast	Slow

Regulating the water cycle requires considering the structural links between vegetation and forest cover, biological activity and soil quality, and water flows that make up the water cycle.

What regulatory levers?

Water cycle disruption occurs when too large a portion (over 30%) of rainwater is discharged into the sea by runoff.

Solutions to address this disruption involve promoting water retention and limiting surface runoff to prevent excess rainwater from being discharged into the sea.

Slow down and regulate water flow

The longer rainwater takes to reach the sea, the easier it is for it to infiltrate the soil. Many techniques allow slowing water flow, and the more these techniques are implemented upstream in watersheds, the more effective they are.

• Swales, ditches, and passive hydrological infrastructures : swales and ditches retain water on slopes to allow infiltration time.
• Vegetation and forests : terrain structured by vegetation slows water flow.
• Beavers : beavers, through their dams, raise water levels in streams, irrigating a large area.

• Terraces : on sloped land, terrace farming breaks up runoff flow and slows water return to the sea. This architecture is notably found in Asian countries cultivating terraced rice paddies on steep terrain.

Water retention structures

There are various structures that allow water conservation :

• Dams : these constructions control river flow and/or store water. Dams may also be used for hydroelectric power generation.

• Hill reservoirs : surface runoff can be retained and temporarily stored by building dikes in valleys. Stored water can slowly infiltrate and/or be withdrawn for irrigation, human consumption, and industrial uses.

• Agricultural ponds : these are water reservoirs dug into the ground and covered with a waterproof film. Unlike hill reservoirs, water stored in agricultural ponds comes from withdrawals from aquifers during high water periods or from flooded rivers. This storage of deep water at the surface makes water easily accessible for irrigation in summer. Although controversial and raising social equity questions among farmers, they remain a means to secure water supply for crops and maintain living vegetation cover during summer, provided they are managed rigorously.

Agricultural practices promoting water infiltration

Water infiltration rate depends on soil quality and vegetation cover, so promoting water infiltration means favoring agricultural practices that improve soil structural stability and maximize vegetation density.

Conservation Agriculture

Conservation Agriculture rests on three pillars :

• Maximize vegetation cover year-round with cover crops or mulches.
• Diversify crop rotations and crop associations to extend soil exploration, soil water exploitation, and optimize restructuring.
• Reduce or eliminate tillage to maintain structural stability ensured by vegetation and soil biological activity. Agricultural machinery traffic should be limited to minimize soil compaction and production costs.

These practices provide several benefits :

• Increased soil organic matter content.
• Improved soil structure.
• Increased soil biological activity.

These benefits help promote water infiltration and retention in the soil and limit runoff.

Agroforestry

Agroforestry consists of combining trees and crops on the same agricultural plot. Given the central role of trees in regulating the water cycle, expanding agroforestry is essential to :

• Limit evaporation thanks to the windbreak effect of trees and hedges planted on farms.
• Protect soils and crops during heavy storms.
• Ensure recharging of aquifers and water supply to crops via tree root systems.
• Lower temperature ("natural air conditioning").

Not only does reintroducing trees and hedges on agricultural plots make crops more resilient to droughts and floods that are currently intensifying, but it also helps regulate the water cycle, thus mitigating droughts and floods.

Keyline design

Organizing plots according to keyline design principles allows directing, infiltrating, storing, distributing runoff water, and evacuating excess (towards swales or ponds), as well as infiltrating oxygen and nutrients, all aimed at promoting the development of a living, deep soil with high biological fertility. It is less costly to achieve this goal in a landscape organized for this purpose.

Do not confuse consumption and resource

Water does not disappear. It is a cycle. The only way to lose fresh water is to discharge it into the sea instead of recycling it on land. We do not lack water; we waste too much of it.

The more water we have, the more vegetation we have, and the more vegetation we have, the more water we have. During drought periods, there is a strong temptation to reduce irrigation, which accounts for half of water consumption. But consumption is not resource. Reducing irrigation amounts to amplifying the drought. Indeed, during drought periods, irrigation is necessary to maintain soil life and vegetation cover, which is itself necessary for the proper regulation of the water cycle. Maintaining irrigation when it is essential contributes to the proper functioning of the water cycle and ultimately helps limit drought periods.

The good hydric health of a watershed depends on its vegetation density (especially in summer), the ideal being the vegetation density of the forest ecosystem (deciduous trees).

• Hydrologie régénérative
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This page was written in partnership with Pleinchamp

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La version initiale de cet article a été rédigée par Laurent Denise.