Wetlands have been constructed to reduce nutrient loads (nitrogen and phosphorus) from agricultural land, increase biodiversity, increase water storage and manage climate change. In addition, they add a beautiful environment. Although a lot of money has been spent on constructing wetlands, there is hardly any follow-up on how they fulfill their purposes.
"There is an obvious knowledge gap here as few measurements have been made", says Pia Geranmayeh, researcher at the Department of Aquatic Sciences and Assessment, Swedish University of Agriculture, SLU. Together with Faruk Djodjic, Dennis Collentine, Hampus Markensten, and Martyn Futter who are also affiliated with the same department, she has participated in a study that evaluated the optimal size and location of constructed wetlands in relation to how well they perform as a tool for reduction of nutrient losses.
So far, no follow-up has been carried out on the effectiveness of the wetlands at the landscape level. A lot of resources are used for the construction of wetlands, which should make it justified to investigate whether the money is used properly. If we don't know how effective the existing wetlands are, why do we continue to construct them and subsidize them?
- Wetlands are a positive thing, they are multifunctional, says Pia and continues: Today there are many individual landowners who construct wetlands for recreational purposes or as irrigation ponds. Our study shows that wetlands are a good and cost-effective measure when done correctly.
When the researchers studied the cost-effectiveness of the wetlands, it was in comparison with other nutrient retention measures such as planting catch crops.
The cost-effectiveness of wetlands
In the study, the researchers examine how cost-effective it is to construct wetlands in agricultural areas for the purpose of nutrient retention, with the measure of Swedish krona per kilogram of nitrogen and phosphorus respectively.
- The model we developed in order to estimate water and nutrients flow to a wetland, is based on high-resolution elevation curves that show how water flows in the landscape, says Pia. This is in combination with calculations of how much phosphorus and nitrogen different types of soil and land use release depending on, among other things, cultivation measures and climate.
Pia continues:
- When we know the amount of nutrients the wetland receives, based on data from previous studies that show how efficient wetlands are when it comes to purification, we can calculate how much nitrogen and phosphorus the wetland is assumed to mitigate.
The wetlands included in the study
The researchers examined two regions to be able to cover, among other things, different land use and soil types. Both regions consist of three adjacent catchment areas and are located partly in northwestern Skåne and partly in Södermanland. Not only land use and soil characteristics differentiate them but also differences in temperature, precipitation, and emission patterns.
Prerequisites for effectiveness in wetlands nutrient retention
The local conditions affect how nutrients are received in the wetland. Since wetlands clean more if the water contains a lot of nutrients, the location becomes extremely important. Much more nutrients are released from arable land compared to forest land. Therefore, wetlands should be placed in areas with a lot of agricultural land in the catchment area.
The study showed that there are wetlands that are very efficient in taking care of nutrient loads. However, most constructed wetlands do not receive much nutrition because the proportion of agricultural land in their catchment area is low.
How much water the wetland receives is also important for its efficiency. The wetland cannot clean excessive volumes of water because flows become too high. In addition, the water does not stay long enough for the purification processes to take effect. On the other hand, very low water volumes also mean low nutrient amounts.
The wetlands in the future?
In order for the future wetlands to be cost-effective, the researchers want requirements to be in place. The wetland's main purpose needs to be clear and acted upon - they must have the right size, design, and location.
- We hope that our tool can help landowners, wetland consultants, and wetland administrators to make sure to place and dimension correctly. If new requirements are set, it should also be possible to pay more to landowners based on the ecosystem service that the wetland provides.
