Soil and water: towards a larger-scale perspective of their relations

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Courtesy of European Commission, Environment DG

Land use changes over time have altered relations between soils and water cycles throughout Europe. There are regions where forests were cut for agriculture or herding, or for industrial, mining, and/or railroad use. Soils were lost, through mud floods, and the water cycles changed so that their present status is one of badlands and/or desert-like areas. Early stages in the path to degradation can now be observed in other parts of Europe, while there are also indications that land use changes in one place can propagate their effects within its own river basin, and further, to affect drought and floods at larger scales.

Consequently, the closely interlinked areas of soil and water have become an urgent issue for European policymakers; highlighted by the EC’s 7th Environment Action Programme (7EAP) that calls upon the European Union and the Member States to address soil quality issues as soon as possible. This Thematic Issue aims to provide a review of new research into the links between soil and water issues in Europe to help inform policymakers, including a subliminal message that the soil-water links must be considered at their proper spatial scales.

Most of the articles included in this issue can be broadly divided into two types: papers from waterabundant regions concerned with retaining the soil in their lands, and papers from water-scarce areas, worried about retaining the water in their soils. In both cases, they explore methods to avoid problems caused by intense precipitations, such as mud-flows and/or floods, and how to minimise the leaching of nutrients to aquifers. They also consider how to convince the powers that be (governments and people) to do something about their problems, in their own locations, at reasonable cost.

Soil-water interactions affect a number of European Directives, including the Water Framework Directive (WFD, 2000/60/EC)1, the Flood Directive (2007/60/EC)2, the Blueprint to Safeguard Europe’s Waters (EC, 2012)3, as well as other actions derived from the Common Agricultural Policy. Some of the articles mention that certain proposed actions are difficult to apply, or are rarely applied in practice. And here, again, a gap appears between issues and solutions in water-abundant and water-scarce areas of Europe. Finally, if we consider that, to be effective, policies require well-defined (engineering-like), and applicable (cost-efficient) operational procedures, we can conclude that there remains a no-man’s land involving processes affecting the soil-water relations which have not yet been duly considered.

Soil retains water: a truism. And some of that water is returned to the atmosphere via the soil itself, or via its vegetation–forests, evapo-transpirating interface. That water vapour can then become part of precipitation somewhere downwind and, thus, contribute to water recycling. Solar heating during the day, and radiative cooling at night: and one has all the ingredients that drive meteorological processes upwards, from the micro-scale, through the meso‑, to the larger macro-continental scales. These are the same meteorological processes that govern the amount of water precipitated (downwards) to the soil. Thus, an important question is: at what scales do the soil-vegetation-water cycles become closed?

A second issue concerns soil-water user links, which will necessarily affect policy development. The point here is that the disciplines that deal with water-and-soil related issues evolved along different paths over history. And, correspondingly, they also developed conflicting needs, and noncomplementary operational procedures, from way back. Some consider the land-soil used for housing and infrastructures as simply a resource you pay for once, without due regard for other soil-related environmental services. For example, soil’s role in precipitation and, thus, in the water resource itself, as well as in other feedback processes in the local-toglobal hydrological cycles.

Probably the most common assumption regarding the hydrological cycle is that the ‘water resource’ is universally provided by precipitation from the large weather systems, from water evaporated over the oceans. In the northern hemisphere this holds true only on the western seaboard of the continents (e.g. European-Atlantic), above ≈ 30º North Latitude. Thus, the notion that the water resource is just ‘there’, a “heritage” (WFD, 2000/60/EC), and all that is required is to manage it properly, is perhaps the most widely extended fallacy regarding the water cycle. In principle, this single detail underlies some of the difficulties experienced when trying to deal with soil and water issues in Europe.

The geographic fact is rather that Europe straddles two of Earth’s catchment basins (Figure 1), the Atlantic and the Mediterranean — each with different precipitation regimes — located at either side of the European Continental-Water Divide (EC-WD), which stretches from Gibraltar to western Russia, and which separates the water-abundant from the water-scarce parts of Europe. In the more western parts of the Atlantic Basin, up to 100% of precipitation is frontal, from water directly evaporated over the Atlantic Ocean (e.g., Ireland, UK, Portugal, western France). Soil — and land-use — properties on either side of the divide will affect the capacity of the surface to retain water (and, hence, to avoid flooding). But further inland, moving towards the EC-WD, evapo-transpiration from the surface can also contribute to the development of convective summer storms, and thus to a second precipitation component involving water recycling.

In the Mediterranean Catchment Basin, there are three precipitation components which change as we move from the EC-WD towards the centre of the Basin. In Spain, the Atlantic fronts contribute some 20% of precipitation right along the mountains that form the EC-WD. The other 80% of precipitation within the same area is provided by summer storms (15 %) and Mediterranean cyclogenesis events, i.e., ‘Levanters’ to UK sailors (65 %). These percentages change as we move towards Greece and Turkey where nearly 100% of precipitation (summer storms and cyclogeneses) is from water evaporated right within the Basin itself, both from the land surface and from the sea. The right percentages in each basin still need to be determined by isotopic studies of rainwater. The studies that have led to these conclusions are in the paper summarised in ‘Land use changes in the Mediterranean may be triggering large weather shifts’, which is included here to provide some larger-scale context for this Thematic Issue.

This work relates the loss of summer storms around the Mediterranean Sea to land-use changes along the coasts in the last century, including draining of coastal marches, urbanisations and water saving measures. It then tracks the path of the non-precipitated (storm) water vapour to torrential rains in Central-Eastern Europe in summer. And further, through larger-scale feedback processes, to possible causes of intense precipitations over the Mediterranean coasts in autumn-winter, and to more intense rains and floods over Atlantic Europe in summer-autumn.

‘Flood risk from modern agricultural practices can be mitigated with interventions’ is a highly readable and descriptive text that reviews changes over the past 50 years in UK land use and management practices, driven by UK and EU agricultural policies. The UK has suffered from severe coastal and inland flooding in recent years, particularly in the winter of 2013-14. There is substantial evidence that modern land-use management practices have enhanced surface runoff generation at the local scale, frequently creating impacts through ‘muddy floods’. It also raises fundamental questions about the propagation of soil-land-use perturbations to larger scales.

Small field wetlands, constructed along runoff pathways, offer one option for slowing down and storing runoff in order to allow more time for sedimentation, for nutrients to be taken up by plants or micro-organisms, and for keeping soil out of rivers. Additionally, field wetlands represent a promising option that may contribute to maintaining local hydrological cycles, including summer convective storms, in water-abundant regions – as summarised in ‘Artificial wetlands on farmland help to prevent soil loss and recapture agricultural by-products’.

The following article, ‘More than one-third of soils studied in southwest England are highly degraded’ describes field investigations between 2002 and 2011 which identified soil structural degradation to be widespread in SW England, with 38% of the 3,243 surveyed sites having sufficiently degraded soil structure to produce observable features of enhanced surface-water runoff within the landscape. Soil under arable crops often had high or severe levels of structural degradation. Late-harvested crops such as maize had the most damaged soil where 75% of sites were found to have degraded structure, generating enhanced surface-water runoff. Soil erosion in these crops was found at over one- in- five sites.

Land Use and Cover Changes (LUCCs) significantly increase the frequency of mudflows in the silty areas of north-western Europe. Predicting the effects of a range of possible LUCCs helps local authorities choose policies that can help to mitigate the risks to which local populations are exposed, e.g., mud flows. The researchers for ‘Who should pay for best management practices to reduce soil erosion?’ find that practical solutions in France respond to both internal (farmers) and external drivers (e.g., CAP requirements), and may require subsidies from a higher level (Europe). Finally, the assessment of the policy effects at the local scale should use spatial databases, including the boundaries of farm areas.

Protection measures are needed to control nutrient leaching from agriculture to the Baltic Sea. Ecological Recycling Agriculture (ERA), as described in the article ‘Integrating animal and crop production can reduce nutrient leaching from agricultural fields’, is based on local nutrient resources, integrating animal and crop production on farms or in their proximity. In Finland, three agricultural study catchments were chosen to demonstrate environmental impacts of ERA, in a work that combines experimental measurements and modelling.

Soil erosion by water affects soil quality and productivity, its water-holding capacity, nutrients, organic matter, soil biota and soil depth. It also impacts on ecosystem services such as water quality and quantity, biodiversity, agricultural productivity and recreationalactivities. The article ‘New data on soil erosion by water reveals Mediterranean at highest flood risk’ evaluates rainfall erosivity at the European level using models and the best available datasets.

Drought is a predominant cause of low yields worldwide. There is an urgent need for more water efficient cropping systems to deal with large water consumption of irrigated agriculture and high unproductive losses via runoff and evaporation. Identification of yield-limiting constraints in the plant-soil-atmosphere continuum is the key to improved management of plant water stress. However, in the context of this Thematic Issue, the article, ‘Research into root systems: key for long-term crop management’, about European farming practices to increase water efficiency, should alert us that, unless the whole water cycle is considered at its proper scale, ‘good water management’ (i.e., evaporation savings) in one place may disrupt precipitation elsewhere, e.g., at the headwaters of the watershed, and diminish the water resource (precipitation) in the river basin.

In ‘Rejuvenating arid badlands: from barren slopes to living forest in 80 years’, we see that badlands can be a major source of sediment, as observed in several European basins, and are witness to poor soil-land management in the past. This paper presents the history of badland generated in the Saldaña region, northern Spain, as well as the main responses after the start of restoration – in terms of vegetation, soil and erosive processes. Eight decades after the restoration project, forest vegetation has covered almost the entire area. This is a practical example that could benefit from a larger scope in terms of its impact on the local water cycle in (currently) semi-arid lands.

Mechanical tillage represents the most common technique of soil management in olive orchards within the semi-arid lands of the Mediterranean Basin. Such practice may result in soil structure degradation which can significantly reduce water infiltration causing runoff and erosion processes. In ‘No-tillage management of olive groves can improve soil structure while maintaining yield’, an alternative opportunity is introduced by using cover crops which eliminate most of the disadvantages of conventional tillage.

Small-scale farms in populated countries must produce sufficient quantities of food to meet the ever-growing population needs. In ‘Straw covering on soil can increase crop yields and improve the efficiency of water use’ Chinese researchers show how the integrated systems of wheat and maize relay-planting combined with straw mulching can decrease soil evaporation, reduce water consumption, and increase crop yield and water use efficiency significantly, compared to conventional monoplanting of wheat and maize.

All this information could be used to elaborate manual-type procedures, i.e., best practices, for climate change adaptation at the proper scale(s), while bearing in mind that some of the solutions are specific to their hydro-climatic areas, on their respective sides of the European Continental Divide. Thus, after ‘Land use changes in the Mediterranean may be triggering large weather shifts’, the contents of the next three articles (Artificial wetlands on farmland help prevent soil loss and recapture agricultural by-products’, ‘Notillage management of olive groves can improve soil structure while maintaining yield’ and ‘Flood risk from modern agricultural practices can be mitigated with interventions’) would be applicable to the “water-abundant” Atlantic part of Europe, while the contents of the last five articles in the issue are most applicable to “water-scarce” Mediterranean Europe. Information from the articles, ‘Who should pay for best management practices to reduce soil erosion?’ and ‘Integrating animal and crop production can reduce nutrient leaching from agricultural fields’ describe solutions that could be used in all areas. In the case of the Mediterranean Catchment Basin, however, these procedures are not enough to adapt to the ongoing climate change. This includes the anthropogenic, land-use-driven components of that change, which have affected the hydrological cycles in these lands over history. However, they could be further complemented with additional steps towards the recovery of local water cycles in selected watersheds or river basins.

When studying the loss of summer storms in the Mediterranean, I found the reduced precipitation signalled a change in the quality of land use in the catchment basin. Evapo-transpiration from the soil and vegetation on the surface used to provide the additional amount of water that triggered storm formation, and this change seems to signal the loss of vegetation within the catchment area, showing how a direct link exists between the dominant weather systems and local soil-land-use decisions. The messages here are: (1) that water evaporated mainly over the coastal plains and slopes, is (or can be) recovered at the headwaters some 60 to 100 km inland, and (2) that it establishes a direct relationship between soil-surface properties, including water content, and the water cycle at the river basin scale.

Knowledge of these phenomena opens the door to ‘cultivating summer storms’ or, really, to recovering part of the old precipitation regimes, i.e., before the land was altered. In the Mediterranean, the effects of coastal land-use changes (e.g. massive soil sealing by urbanisations) could be compensated by re-forestation along the airmass’ path to the storms’ focal areas, and similarly in other parts of Europe. This, however, requires disaggregating the precipitation by weather types, at a scale no smaller than the river basin, to find which components of precipitation respond to what changes in evapotranspiration from the soil-vegetation, when, and where within the same basin.

If we start with these premises, it becomes clear that the solutions and phenomena described within this Thematic Issue are not only aids for adapting to a changing climate, but also describe decisions that can and will affect weather systems on a macro scale.

In summary, soil and water are crucial environmental media that interact in many ways. For soil, water can be both a threat or a boost for fertility. For water, soil is a regulating agent, a buffer that prevents the consequences of weather peaks. It not only helps avoid flooding or droughts, but supports the water cycle.

The year 2015 has been designated the International Year of Soils by the UN – with one major aim being to promote awareness about the profound importance of soil for human life among society and decision makers. In time, this should help support decision makers to create effective policies for the sustainable management and protection of soil resources for the future.

Finally, I would like to acknowledge, with thanks, the help of the SfEP editorial team and the constructive comments of the EC scientific officer.

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