By Femke van Woesik (GOPA), Hugo de Boer (Utrecht University) & Stefan Dekker (NIOO-KNAW), Frank van Steenbergen, Francesco Sambalino (GOPA)
Reflections from the 3rd Restoring the Small Water Cycle Roundtable, 2 September 2026
Microclimates and small water cycles at work. Two fields, the same soil, and also the same August afternoon. One field is covered with a cover crop, the other has been ploughed and left bare, and between them the topsoil temperature differs not less than 25°C to 30°C. Hanspeter Liniger, founder of WOCAT and Sustainable Land Management expert, has been measuring this in five countries. Nobody planned to build a heating plate out of farmland, and yet in many places soil is left bare and exposed to high temperatures season after season. More on Hanspeter’s session is shared below.
Things should be different and require a shift. That is what this Roundtable group is trying to say: land management determines how much solar energy becomes heat and how much becomes evapotranspiration and biomass, and you can see the difference with a thermometer.
Forty people joined the third Small Water Cycle roundtable, hosted by Utrecht University, NIOO-KNAW and GOPA MetaMeta. The second roundtable ended with an agreement to build a roadmap. This third gathering was about deciding what to document, who would do it, and where to share the narrative.
Towards restoration of forest patches in Aruba
Maarten Eppinga (University of Zurich) works on restoring dry tropical forest in Aruba, with the University of Aruba, the volunteer groups Ban Lanta y Planta and Trees for Aruba, the Aruba Conservation Foundation, Camille Delavaux and Stefan Dekker at NIOO-KNAW, and theory from Jim Bever and Theo Michaels at the University of Kansas. Caribbean dry forest is among the most threatened forest types in the world, with less than 10% of the original areas left. The degradation started 500 years ago: colonial deforestation, introduced grazers, and drought during the Little Ice Age. Restoration today yields some success, but mixed, and the question is why.
The answer Maarten hypothesizes is the system of forest feedback loops. A forest modifies its own environment through shading, wind, temperature, and nutrient availability. Under low environmental pressure, a forest maintains itself whatever the starting point; under very high pressure, it degrades regardless, and in between the same feedbacks can drive either recovery or collapse depending on where you start. Restoring the dry forest in Aruba has been challenging – it appears it is not only about restoring forest cover, but also about reviving the feedback loops. Here the nucleation theory offers an interesting perspective: if there is a small forest area where the feedback loops are working well, can this trigger healthy forest restoration in the area around it? And what is the critical patch size, and the species composition, and how will the process go from a nucleus to a larger forest area? Above a certain size a patch grows on its own; below it, the patch shrinks and disappears.
The plan is to test this theory, using Sentinel-2 imagery to find high- and low-resilience patches, field measurements in coupled pairs, greenhouse and field experiments, and a model parameterized for local conditions. So far, the nucleation theory has been tested in grasslands, not in Caribbean systems. What makes it worth testing well beyond the Aruba testbed is the form of the answer. Knowing that a system has a tipping point and knowing how big a patch has to be is critical knowledge for restoration projects. That is what this research work delivers: the size of the patch to restore, calculated for local conditions and weighed against the effort of improving those conditions by other means. If the feedback holds, a patch above that starts expanding on its own.
Soil moisture, the next step in water management
Tessa van Hateren (Acacia Water) came with a pitch: Water management has concentrated historically on surface water and groundwater. The zone in between is where the small water cycle is actually critical. Soil moisture is the water held in the unsaturated zone above the groundwater, and it is what connects the two ends of the cycle: it determines how much rain infiltrates instead of running off, how much reaches the groundwater as recharge, and how much leaves again through plants as transpiration/biomass rather than straight off as surface runoff or evaporation. It is a critical component of the small water cycle. It also influences soil chemistry, water availability and soil strength, and it decides how agricultural and natural systems function.
Her examples showed how wide that reach is, and how little of it is managed. The Dutch winter surplus and summer deficit are bridged by aquifer storage and recovery, moving water to the season that needs it. In Morocco, irrigation measurements showed something uncomfortable: when a lot of water was applied, groundwater levels rose, meaning the water was passing through the root zone rather than staying where the crop and the atmosphere could use it. Storing water is not the same as keeping it in the part of the profile that does the work.
Hot earth is not cool and the Small Water Cycle
Hanspeter Liniger, formerly of the University of Bern and the initiator and director of WOCAT until 2020, presented work done with Jovana Askrabic. Ten years in Kenya left him asking why fields sit bare through the dry season, so he started measuring. They measure surface temperature to take a picture of differing land management practices
The numbers are blunt. Forest floor peaks around 30°C to 37°C on a hot summer afternoon. A half-dry meadow goes over 40°C, and cover crops and mulch stay around 40°C. Bare tilled soil sat at the top of the measured range at 55°C, fluctuating 41°C between daily minimum and maximum. Between land management practices on the same soil, topsoil temperature differs by 25°C to 30°C. With a thermal drone the hottest readings came from row crops before the canopy closes, where there is no air circulation between the rows: up to 70°C on a light soil and over 75°C on a dark one. In an olive plantation in Puglia the ploughed strip peaked at 75°C and averaged close to 60°C, while the tarmac road nearby stayed below 60°C.
Hanspeter also showed that topsoil aggregate stability deteriorates when soil is left bare and exposed to high temperatures, and that aggregate stability plays a key role in the small water cycle. Poor aggregate stability reduces infiltration and increases surface runoff and direct soil evaporation, instead of allowing more water to be used productively through plant transpiration. He argues that aggregate stability is therefore an important factor influencing both flood risk and the severity of drought impacts. He concludes that the high surface temperatures of tilled, bare soils have so far received insufficient attention, including their consequences for the small water cycle and the local microclimate. Hanspeter also presented the outline of a watershed tool that he considers highly relevant for demonstrating how changes in the small water cycle affect small catchments. The tool has already been developed and applied in several contexts. It is simpler than the SWAT model, while still being sensitive to land-management practices because it uses the same Curve Number approach. Additional project support and funding would be needed to further develop the tool into a user-friendly application that could be applied by practitioners, researchers, and other stakeholders.
The evidence comes from a simple soil aggregation test, the slake test, carried out with farmers in the field. Forest and pasture aggregates held together for a day or two, while soil exposed to heat or tillage fell apart. No bare summer soil in the study had good aggregate stability, so groundwater recharge is lost along with the runoff. High soil temperature also suppresses dew formation, a major source of agricultural water.
What leaves the field also matters to the neighbours. About 80% of annual precipitation in Switzerland arrives from the ocean, and 20% is recycled from land, but in summer every second drop is recycled, and mapping shows where those proportions rise to 70 or 80%. Moisture recycling matters not only at field level but at country level too[1].
Hanspeter also named what they cannot yet show. The effect of land management practices on rainfall is the hard one to prove, and the thresholds at which soil organisms lose function remain uncertain. A paper is in preparation on the correlations between surface temperature, soil cover, deeper soil temperature, organic matter, soil respiration and aggregate stability.
Then a photograph from Spain. Below a ploughed area, runoff had collected and concentrated; a gully four to five metres deep and wide had cut through the entire field. Two months later the same area flooded. In contrast, an adjacent area with adequate cover faced no problem. What struck Hanspeter was the media coverage: the documentaries and reports on the drought and floods he saw attributed to the drought, the erosion, and the floods to climate change, and none of them raised land management as a factor at all.
His message is one sentence long: cover the soil, with vegetation if possible, with mulch at the minimum. The Hot Earth campaign video shows the Swiss situation. A second video, focused on the water cycle and drawing on footage already filmed at three sites in Spain and in Puglia, is ready to be made, and the funding fell away just before signature. Around EUR 20,000 would complete it and smaller contributions would be enough to start.
Stockholm World Water Week, and the knowledge gap
How far is the discussion on small water cycles travelling? Henk Ovink has been a pathfinder on this topic and has spent years successfully pushing green water onto agendas where it had no place before. During this roundtable, Henk shared an update on where that work now stands. Green water was very visible across Stockholm World Water Week this year, in sessions on restoring land, on forest and land management, on the governance of green water, and in the first outlines of a green water knowledge agenda with equity, finance and advocacy attached to it.
With UNESCO and the WUR, a group of institutes and individuals has proposed a Green Water Flagship Initiative for the UN Water Conference. The term comes from the conference co-hosts, who want to identify initiatives capable of moving the global agenda. The flagship would work as a light coordination platform that makes full hydrological cycle thinking routine in development, environmental governance and investment decisions, bringing soil moisture, ecosystems, evapotranspiration and terrestrial moisture recycling into water, climate, land, food, biodiversity and development frameworks.
That means common metrics and monitoring, so countries and financiers work from the same evidence, model language for planning and investment documents, and a finance pipeline through existing channels rather than new funds. UNESCO, WUR, the Global Environment Facility, FAO, IWMI, the Stockholm Resilience Centre, Deltares and KTH are among those involved, with support from the World Bank and the Green Climate Fund.
Three moments follow in quick succession: the first IPCC expert workshop on water and climate in the history of the panel; the biodiversity COP in October with its day on water; and the UN Water Conference in December, which brings the three Rio conventions together under Water for Planet.
The Small Water Cycle Restoration narrative
We discussed a road map. The narrative needs to exist on paper in three forms. A peer-reviewed paper establishes the evidence and makes the claims citable by everyone else. A practitioner’s note turns that evidence into something NGOs and service providers can use in the field. A policy brief carries it to ministries and funders. They depend on each other: the field cases give the paper its evidence, the paper gives the policy brief its authority, and the policy brief opens the space practitioners then work in.
Some of it is already moving. Åse Johannessen (Deltares) and team have a paper of close to 200 pages nearly finished, pulling together earlier work. Hanspeter Liniger and colleagues are preparing a paper on soil processes, covering what soil temperature and organic matter do to aggregation, infiltration capacity and microbial life. Stefan Dekker proposed a paper that brings the separate pieces into one narrative and clear framework. Illustrated with practical examples, and he and Åse were asked to funnel the discussion into that overview. GOPA volunteered to lead the practitioners note. On timing, Hanspeter made the case that the window for policy debate is open now. The fires, the extended droughts and the floods of the past year are all connected to disturbed water cycles – and this needs to be told. It is no longer a Mediterranean story: also, Switzerland had Mediterranean conditions this summer[1]. He has already collected substantial video material and submitted a proposal for funding a follow-up video focusing on the effects of hot, bare soils on the small water cycle, including their implications for droughts and floods. Unfortunately, the proposal was not funded. However, he considers such a video a very effective and attractive way to present the experiences of different land users in Spain and Italy, supported by measurements collected during the 2023 and 2024 field campaigns. He would therefore welcome suggestions for possible funding opportunities, as well as support for video production, launch, and dissemination.
Cases arrived in the chat while the discussion ran: a report connecting the Valencia floods to land management, which shows that soil-based interventions cost around 5% of large concrete structures, coverage of regenerative farmers in Spain whose land burned less and the work of Commonland, a widely shared visual on why rehydrated landscapes do not burn, Millan Millan’s 2014 paper on extreme hydrometeorological events in Europe, and Zoetwaterboeren, where rainwater is harvested at farm scale and stored in deeper groundwater for dry periods. Much of this already carries the small water cycle message and has not yet been connected to it.
Where do we tell it
The IWA World Water Congress runs in Glasgow from 4 to 8 October, followed on 6 October by an IAIA webinar on integrating land, water and atmosphere processes into environmental assessment. European Water Technology Week in Leeuwarden, 26 to 28 October, carries two sessions on understanding and restoring local water cycles organised by Sarah Warnau and Fransje van Oorschot: a scientific session on where rain comes from and how to bring it back, and a poster-based workshop mapping what we know and where the gaps are. COP31 is in Antalya from 9 to 20 November, the UN Water Conference in Abu Dhabi from 8 to 10 December, the 26th ICID Congress, and Cairo Water Week.
A home online
The Small Water Cycles and Local Climates dossier on TheWaterChannel is open to gather the material coming out of this group and beyond: pilots, publications, videos, work that carries the small water cycle message without having been connected to it yet.
A fourth roundtable will follow. To join the community of practice or the next roundtable, contact Femke van Woesik (femke.vanwoesik@gopa.eu).
The full slide deck of the third roundtable is here.
The recap of the second roundtable is here: https://thewaterchannel.tv/thewaterblog/restoring-small-water-cycles-local-climate-action-with-measurable-results/
References
[1] van der Ent, R. J., Savenije, H. H. G., Schaefli, B., & Steele-Dunne, S. C. (2010). Origin and fate of atmospheric moisture over continents. Water Resources Research, 46(9), Article W09525. https://doi.org/10.1029/2010WR009127
[2] Hanspeter Liniger has already collected substantial video material and submitted a proposal for funding a follow-up video focusing on the effects of hot, bare soils on the small water cycle, including their implications for droughts and floods. Unfortunately, the proposal was not funded. However, he considers such a video a very effective and attractive way to present the experiences of different land users in Spain and Italy, supported by measurements collected during the 2023 and 2024 field campaigns. He would therefore welcome suggestions for possible funding opportunities, as well as support for video production, launch, and dissemination.
