New erosion-mapping model in Andalusian olive groves more accurately predicts gully formation
In Andalusian olive groves, where at first glance the landscape seems calm and shaped over centuries, water is increasingly leaving traces that can no longer be ploughed over and “levelled” within a single season. Gully erosion – deep, permanent channels carved by torrents through the soil – turns some plots into a hard-to-cross mosaic, raises production costs and ultimately reduces the agricultural potential of a region that underpins European olive-oil production. When such channels branch out, scenes emerge that resemble miniature canyons, and the damage is no longer only local: the washed-away soil ends up in watercourses and reservoirs, increasing turbidity and burdening drinking-water supply and irrigation systems.
Early 2026 further underscored how sensitive Mediterranean catchments are to episodes of heavy rainfall. After storms Leonardo and Marta, which brought intense rain and flooding to parts of Spain and Portugal, experts once again warned that extreme downpours on bare or poorly protected slopes can easily trigger surface runoff and landslides, followed by deep incision of the soil. That is precisely why an accurate map of where gullies form – and a clear picture of which gullies are “dormant” and which are still advancing – is becoming one of the key tools for prevention.
In that context, a team of researchers from the University of Córdoba and partner institutions presented a regional approach that combines detailed mapping and modelling, resulting in a markedly better ability to predict where gullies will open and what activity already existing gullies show. The paper, published in the journal
Catena (February 2026), also provides figures that for the first time offer a comparable picture of gullies across representative types of olive-growing landscapes in the Guadalquivir basin.
Why gullies are a problem that goes beyond a single plot
Unlike shallow erosion rills that can be removed with annual operations in conventional tillage, gullies are permanent geomorphological structures. They cut across routes used by agricultural machinery, create occupational-safety risks and can “swallow” trees along the channel margins, especially when the upper edge (the so-called gully head) actively migrates upslope. In practice, that means loss of arable area, additional remediation costs and the need for specific interventions that differ from standard anti-erosion measures.
The problem does not stop at the property line. Water carrying finer soil particles downslope increases turbidity and sediment transport in rivers, and over time in reservoirs it can contribute to siltation and a reduction of useful storage volume. That is why gully erosion becomes a catchment-management issue: it affects water quality, infrastructure maintenance and irrigation planning – especially in the Guadalquivir basin, where agriculture and the water regime are in constant tension between needs and available resources.
A regional map and an index that brings multiple factors together
The research team led by Paula González, together with Adolfo Peña and Tom Vanwalleghem and collaborators from Belgium, starts from the insight that previous approaches were often limited to local studies. The most commonly used “topographic threshold” relies primarily on the relationship between slope and catchment area (drainage area) to estimate where gullies may appear. Such an approach can be useful in small areas, but is less reliable when covering a diverse region, and especially when it is important to distinguish active gullies from those that have stabilised.
The new approach is based on the
Gully Head Initiation index (GHI), a process-oriented indicator that combines multiple factors into a single value. Its calculation combines slope, drainage area, rainfall, hydrological characteristics that affect runoff generation (including the so-called
curve number), soil type and clay content. The idea is simple: a gully is more likely to open where torrential runoff can generate enough “erosive power” to overcome the soil’s resistance. GHI was developed at KU Leuven, previously tested on examples from Ethiopia, and this is the first time it has been systematically applied to Andalusia’s olive-growing landscapes, with a direct comparison to the traditional topographic threshold.
How the data were collected: orthophoto series and four landscape types
One of the study’s key innovations is the time dimension. Instead of a one-off snapshot, the researchers analysed a series of orthophotos and mapped changes from 2008 to 2019. This allowed them to determine whether a gully appears “suddenly” in a given period, remains in the same place without further expansion, or whether the margins and gully head migrate over the years, indicating active dynamics.
The analysis covered four representative study areas, each 25 square kilometres in size, within the Guadalquivir basin. These areas were chosen to represent the main types of olive-growing landscape: rolling agricultural lowlands (campiña) with different landforms, transitional foothill zones, mid-mountain areas and lowland valley plains. In each of these types, gully-head locations and associated forms were mapped, and activity was then determined based on changes over time.
In total,
475 gully heads were identified. Of these,
261 were classified as active,
76 as newly formed (recently formed) and
138 as stable. This breakdown is not just statistics: it shows that some gullies are dormant or stabilised, but a large number are still advancing, meaning the window for preventive measures remains open – but it requires precise targeting.
Where gullies most often open and what that says about soil management
Comparing landscape types revealed clear differences. The highest gully density and the largest share of active features were recorded in olive groves of rolling agricultural lowlands, where cultivated surfaces often extend over gentle to moderate slopes and the length of slope segments favours runoff accumulation. Under such conditions, a series of small changes – for example, removing vegetation cover during the period when downpours are most intense, or directing water along tracks and tractor ruts – can be the trigger that “focuses” surface flow and breaks into the soil.
Clay content and soil type matter because they affect infiltration and particle cohesion. Soils with more clay can, depending on structure and compaction, have lower infiltration during intense rain, which increases surface runoff. At the same time, once water enters a cut, it can accelerate lateral erosion and destabilisation of the margins. That is why a model that also includes soil texture, not only relief, provides a more realistic picture of risk.
The practical message for management is clear: it is not enough to know that a plot is “on a slope”. It is necessary to understand where water accumulates, which routes it follows and what the soil is like at critical points – because gullies often begin where small flows converge and create a sudden jump in runoff energy.
How reliable the model is and where its limits are
Model performance was assessed with statistical measures of predictive accuracy. In distinguishing areas with gullies from those without, GHI achieved a very high AUC of
0.93, meaning this approach can reliably delineate space with an increased likelihood of gully initiation. By comparison, the traditional topographic threshold in the same analysis showed much lower accuracy (AUC around
0.64), confirming that relief alone is not sufficient in complex, human-shaped agrarian landscapes.
An added value of the GHI approach is the attempt to distinguish activity types. The model could differentiate active existing gullies from stable ones, as well as newly formed gullies from active existing ones, although differences between stable and newly formed were less pronounced. This is expected: a gully that appeared “recently” may look stable for some time until a new extreme event reactivates headward migration.
An important finding is also that applying the model “separately” by landscape type is not necessarily better than a single regional model. In other words, when the goal is a tool applicable in practice at the regional level, it is better to have a consistent approach that recognises common process drivers across different landforms than a set of local solutions that are harder to compare.
From map to field: measures already being tested in Andalusia
A risk map and an inventory of active gullies are only the first step; the second is what to do on the ground. In Andalusia’s campiña, measures have in recent years been implemented and evaluated that combine agronomic changes, small engineering works and nature-based solutions. The focus is on slowing and dispersing water before it acquires a “channelised” character, and stabilising already existing erosion cuts with vegetation and barriers.
- Vegetation cover between rows – maintaining or sowing cover crops between olive rows reduces raindrop impact, increases infiltration and shortens the time surface flow remains on the soil.
- Small check dams and “albarradas” – light modular barriers and similar structures placed in gully bottoms or on small channels can trap sediment and reduce water velocity in critical reaches.
- Vegetative palisades and revegetation of margins – stabilising gully banks with native or well-adapted species helps bind the soil and reduce lateral collapse.
- Management of tracks and drainage – diverting water from farm tracks and preventing flow concentration in wheel ruts is often cheaper than remediation after a gully opens.
Such measures are not universal and always depend on location, but that is precisely where the map and model can be decisive: they make it possible to act first where risk is highest and where gully activity is already visible in the field, instead of scattering resources across low-impact interventions.
Wider context: olive growing as a strategic sector under pressure from climate and soil
Olive growing in Andalusia is not only an agronomic issue, but also an economic and social one. Official statistical overviews by regional institutions state that the olive-grove area in Andalusia in the 2024/25 season was about
1.66 million hectares, representing the majority of Spain’s total olive area. In practice, this means that any soil degradation over large areas has a cumulative effect: it reduces production resilience to drought and heatwaves, increases maintenance costs and intensifies pressure on water resources.
At the same time, the meteorological context is changing. Episodes of extreme precipitation, such as those brought in early February 2026 by storms Leonardo and Marta, raise erosion risk precisely in periods when the soil is often sensitive due to seasonal operations or limited cover. In such situations, the debate about soil protection moves from “long-term” to “urgent”: after floods, the question is no longer whether gullies will appear, but where and how fast, and how much sediment will end up in water systems.
Scientific literature published in the first months of 2026 further confirms that Mediterranean olive groves are among the agroecosystems particularly exposed to erosion due to the combination of relief, rainfall intensity and soil management. In such an environment, approaches that link hydrology, pedology and spatial planning gain value because they allow soil-protection measures to be based on measurable risks rather than only general recommendations.
What could change in practice
The regional model and classification of gully activity open space for more concrete decisions. For farmers, this can mean a clearer answer to where to prioritise cover crops or where to adjust tillage to avoid runoff concentration. For catchment and infrastructure managers, it can be a starting point for planning protective buffer strips along watercourses, selecting locations for sediment trapping or assessing areas with higher turbidity risk after storms.
Equally important is the potential for monitoring change. Since the study showed the value of orthophoto time series, the logical next step is more regular updating of maps with newer imagery and linking them to extreme-rainfall data. After a season of heavy rains such as winter 2025/26, such a check could show whether “newly formed” gullies have become active, whether stable ones remained stable, or whether hotspots have shifted.
The final goal is not only to better understand olive-grove geomorphology, but to reduce the loss of soil as a resource that renews extremely slowly. Once erosion turns into a network of gullies, remediation costs rise and impacts spill over into production, water systems and local communities. That is why the combination of precise mapping and a process-explaining model – together with field measures – is increasingly a prerequisite for the “sea of olives” to remain productive in decades when weather extremes will be more pronounced.
Sources:- Catena (Elsevier) – scientific paper on a regional analysis of gully activity in olive groves of the Guadalquivir basin and testing the GHI index (link)
- Zenodo – open dataset and figures related to the research (orthophoto analysis series and landscape types) (link)
- Universidad de Córdoba (DAUCO, Department of Agronomy) – report on work on the problem of gully erosion and field stabilisation measures (link)
- KU Leuven – description of the development of the GHI index and projects related to modelling gully initiation (link)
- Junta de Andalucía – statistical overview and data on olive-grove area in Andalusia (campaign 2024/25) (link)
- Copernicus Soil – review scientific paper on erosion in Mediterranean olive groves and key process drivers (link)
- Euronews – reporting on storms and floods in Portugal and Spain in early February 2026, with warnings from meteorological services (link)
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