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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - OPTWET (Finding optimal size and location for wetland restoration sites for best nutrient removal performance using spatial analysis and modelling)

Teaser

Summary

A growing economy and population in the world is causing large-scale land use and landscape changes and an increasing pressure is put on water resources all over the world. Diffuse water pollution that is mainly driven by agricultural food production, is considered to be one of the major problems for water quality in many countries. Poor water quality has consequences for the health of aquatic ecosystems, biodiversity, human health, the use of water in industry and as a resource for public water supply and recreation. Wetlands play a key role in controlling flooding and diffuse source pollution. Most of the former wetland area in many areas of the world has been lost due to draining for agricultural production, and the trend is continuing particularly for small wetlands in agricultural landscapes. There is a need for detailed identification and assessment of the wetlands remaining, and also of potential locations for wetland rehabilitation and creation to help manage diffuse contaminant losses that originate from continuously intensifying agriculture.
The main prerequisite for combatting excessive nutrient losses to water bodies is to study how the pollution sources can be traced and reduced. Compared to point pollution, diffuse water pollution is more complex and difficult to control due to its numerous and dispersed sources, and the difficulties in tracing its pathways. The capacity of wetlands to improve water quality is dependent on a large number of parameters that have been widely studied, such as vegetation cover or type, water retention time, climatic variables, and also their size and spatial arrangement in the watershed. However, the question where wetlands should be located in agricultural catchments to achieve the most effective nutrient removal at the catchment level has not been clearly resolved. This project aims to identify wetlands in the landscape and determine the optimal sizing and location for wetlands in agricultural catchments to reduce nutrient (nitrogen and phosphorus) loads in catchments.

Work performed

Fusion of high resolution orthophotos (0.4 m), Landsat imagery (30 m), ALOS2-PALSAR2 SAR image (6m) and LiDAR based terrain indices (Topographic Wetness Index) were used to identify small-scale wetlands in the Waituna catchment in the Southland region, New Zealand. Supervised and unsupervised image classification was used to obtain land cover data that together with LiDAR data enabled to identify existing small-scale wetlands. To identify feasible locations for wetland restoration, in addition, flow accumulation was calculated. By applying a threshold value to the results of the Flow Accumulation, a virtual stream network was delineated. These inferred surface drainage networks were then used to estimate catchment areas above the prospective wetland sites and to calculate the appropriate wetland drainage areas. Combined wetland probability maps gave reliable results for identifying feasible areas to wetlands. Most suitable areas were identified downstream and close to stream convergence points (up to 80% of probability) which usually are the best places to reconstruct wetlands. A common issue was that most of these suitable areas were quite recently been wetlands and farmers had drained these areas where wetland construction was most feasible. This emphasises the need for early intervention and detection of those wetland areas before they are drained. Moving away from the streams the probability decreased quickly which was due to the fact that we assigned a higher weight to the distance to the streams. This gave better results than equal weights with elevation data.

For modelling the efficiency of wetland at removing nutrient loads a simple dynamic model operating on an hourly time step was used to explore potential wetland nitrogen removal performance of a wetland in Waikato region, New Zealand. In comparison to constructed wetlands receiving defined surface flows (e.g. tile drainage), estimating the nutrient removal effectiveness of natural wetlands receiving a variable mix of seepage and overland flows is challenging. Hourly measurements of inflow, outflow, rainfall and Penman evapotranspiration estimates were used to calculate a dynamic water balance for the wetland. In addition, nitrogen concentration measurements at inflow, outflow and piezometers installed in the wetland were used as input data for the model. A dynamic nitrogen-N mass balance was calculated by coupling influent concentrations to the dynamic water balance and applying a first order areal removal coefficient (k20) adjusted to the ambient temperature. Storm events above a certain threshold were assumed to always result in surface runoff or overland flow and were assigned higher nitrate based on surface run-off measurements. The removal efficiency was estimated for all nitrogen forms. The results show that small seepage wetlands can be very effective at removing nitrogen loads. As their nutrient removal efficiency per unit land area is high, farmers might be more willing to maintain these natural wetlands undrained and fence them to prevent stock access.

Final results

Agricultural pollution loads are increasing all over the world and there is need to find most effective ways to reduce those loads but at the same time retain the agricultural production. Wetlands are one of the most effective practices that can remove nutrients from surface waters. Due to intensifying agriculture, wetlands have been drained and converted for centuries all around the world. The remaining wetlands are fragmented and often in a degraded state. Ground-based surveys of small wetlands are time consuming and analysis of aerial photographs and satellite images is affected by trees. The use of terrain analysis to predict â€œpotentialâ€ wetlands has the advantage of being systematic and avoids problems caused by trees. New monitoring techniques with high temporal and spatial resolution offer improved opportunities to analyse flow paths and determine suitable locations for wetland restoration.

The developed method for identifying existing wetlands and wetland restoration sites helps to speed up finding and pre-processing the input data needed for catchment management. The method can be used either at farm-scale or at a catchment scale to identify wetland restoration sites which is a pre-condition for modelling their effectiveness in nutrient removal. As it is using remote sensing data it is possible to use always up-to-date input which is an important aspect in the quickly changing world.

For estimating the removal efficiency of wetlands, a model was developed that helps to assess nutrient removal performance of wetlands and is designed to be potentially used for any wetland. Estimating wetland nutrient removal efficiency also enables to evaluate their cost-effectiveness which is an important factor for planning their restoration.

The study also helps to raise awareness of the functions and values of wetlands in the eyes of the farmers and in the eyes of the society in general. As wetlandsâ€™ nutrient removal efficiency per unit land area can be very high, farmers might be incentivised to not drain these natural wetlands and fence them off to prevent stock access in order to maintain their nutrient removal functions.