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SLIP, SLIDING AWAY

Municipal authorities in Vejle Amt, Denmark, are waging a county-wide campaign to prevent soil erosion and safeguard water supplies from contamination. Rodney Leek reports on the pivotal role of spatial modelling in devising effective countermeasures

Often described as ‘Denmark in miniature’, the county of Vejle Amt in Eastern Jutland is a green and pleasant land with a sizeable of population and thriving commercial sector (see thumbnail sketch facing page).
Striking a balance that safeguards the environment and maintains biodiversity without stifling progress is a top priority for the county administration in Vejle. Central to its strategy is the need to ensure supplies of clean, fresh water for future generations, something high on the political agenda of the European Union as a whole. With this in mind, Vejle Amt has formulated a comprehensive "Water Area Plan" that describes the current situation, what needs to be done, and the means to achieve it.

 

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Addressing the threat
Currently, the main threat is the widespread loss of nutrients from agricultural land due to soil erosion, the latter prompted by rainwater runoff, melting snow or flood.
Modern intensive agriculture relies on huge inputs of fertilisers, pesticides and herbicides to achieve high levels of production, but the leaching of chemicals such as phosphates into freshwater encourages algal growth and leads to changes in water chemistry. When coupled with soil erosion, this ultimately causes a massive deterioration in water quality, leading to pollution and eutrofication. Almost inevitably, this has a negative impact on water supplies for human consumption.
The traditional method of addressing the issue is for agronomists to conduct field inspections on foot and then issue advice to farmers. This is costly, time-consuming work and subject to various errors. With the present-day emphasis on operating more rationally, the field work must be targeted and the use of agronomists more focussed and cost effective. The first step, therefore, is to identify potential problem areas using GIS methods. The next is to convince farmers that their land is threatened by erosion and that it is in their interest to do something about it.

County-wide analysis
While Vejle Amt had a plan and much data on observed problems, as well as digital data on land use and parcel registration, its initial efforts to predict the risk of erosion in pilot areas with GIS tools met with no real success. The need, it seemed, was for an analysis of the entire county from which risk maps could be generated for the benefit of planners, managers and farmers.
Geo Graphics AS of Oslo was contacted because of its previous work in this field and its links with Danish research institutes. The company agreed to help Vejle Amt achieve its objectives and proceeded to develop a system combining spatial and process models that generated the required information. This was then imported into a standard GIS to produce maps and other products that could not have been delivered by other methods. The system allowed the county’s environmental officers to identify areas within fields that held the risk of erosion and also those linear elements where gullies would form in a storm situation. Such events are of particular concern as the deposition of large volumes of sediment can, in extreme conditions, lead to blocked roads and streams.
With this information at their disposal, officers could then enter into dialogue with farmers and agree countermeasures that supported the county’s environmental plan. Input data included:

  • Digital point data on terrain heights: airborne LIDAR-scanner, 25 meter grid, and a digitised contour map
  • Standard digitised overlays of vector data
  • Land parcels and land use in single polygons for each landowner/use, also registered in a database
  • A digital soil type map, classified in the 11 main soil types, based on an interpretation of quarternary geological mapping of this part of Denmark
  • Field observations with a number of detailed soil and soil profile investigations.

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Modelling
Predictions of erosion risk are developed through a complex modelling process.
During storm conditions, soil particles dislodged by raindrop impact are transported as sediment by flowing water. The same action carries nutrients from the soil and any surplus phosphorus is washed out together with the sediment. When the latter reaches fresh water, the phosphorus encourages algal bloom and initiates a chain of events leading to a deterioration in water quality and, finally, pollution with its attendant health risks. Remedial measures can be extremely difficult, time-consuming and costly, as exemplified in the campaign to clean up Norway’s Lake Mjøsa some 25 years ago.
Because of the complexity of these natural processes, computer modelling is essential for predictive purposes. Account must be taken of such variables as climate, raindrop impact energy, plant growth, tillage, field slope, slope length and soil erosion resistance.
The process models employed include an adapted version of the standard USLE (Universal Soil Loss Equation - a well documented model) where erosion is computed for regular polygons. This was combined with a spatial hydrological model that predicts the movement of sediment through fields to identify probable gullies and establish degrees of severity.

Countermeasures
Where land is on an incline, erosion is more pronounced and it is possible to establish sediment loss as a function of slope degree and slope length. However, plant residue and roots bind the soil, use nutrients (thus making them non-available) and slow water flow. So it is important to establish and maintain plant cover on areas where erosion can be expected, especially during seasons of high probable rainfall.
Contour ploughing is one possible answer, but poses a safety hazard for operators in most fields in Europe and is seldom advisable. Reducing the input of nutrients is another, perhaps more desirable solution, provided it strikes the right balance between erosion loss and plant needs.
Using computer models, one can ascertain those fields where erosion will occur; which routes the run-off, gullies and waterways will take; their severity classes and, finally, the degree of sediment loss. As explained earlier, these findings are imported into Vejle Amt's general GIS for the creation of maps and other products. This approach also lends itself to e-government: on Vejle Amt's home page the public can now see detailed maps and choose the themes. They can examine the relevant laws, the reasons why changes are being made, the comprehensive "water plan" for the county, and advice on reducing erosion risk erosion. See the link (water resources theme pictured left) at http://gis.vejleamt.dk/ArealInfo/ai_mapsimple.asp [in Danish only – Ed].
In conclusion, one can say that the described project has rationalised a hitherto costly, time-consuming process and, in so doing, made the county’s environmental efforts more effective.

Rodney Leek is responsible for GIS and modelling development at Geo Graphics AS, Oslo, Norway ( www.erosion.geographics.no) and can be contacted by email at: ermod.rl@geographics.no

Vejle: a thumbnail sketch
With almost 350,000 people living in an area of 3,000 km2 , Vejle County is one of the most densely populated regions in Denmark outside the Greater Copenhagen Area. 40 per cent of the workforce has a professional qualification and the county - one of 13 across the nation - hosts a large number of businesses in the fields of comestibles, foodstuffs, electronics and mechanics.
Two thirds of the population live in the four large east coast municipalities of Fredericia, Horsens, Kolding and Vejle. The western part of the county is more sparsely populated, consisting primarily of farms and small village communities. The county administration has overall responsibility for nature conservation and environmental protection and, as part of its remit, runs an emergency environmental response service that can be called out 24 hours a day in the event of the pollution of lakes, watercourses or coastal areas. More information at: www.vejleamt.dk/sw153.asp (in Danish, German and English)