Nutrient Export Risk Matrix

The Nutrient Export Risk Matrix (NERM) is a decision support tool to allow farmers and land use planners to assess the risk of nutrient loss from their land and to explore options to reduce nutrient loss whilst maintaining farmer income.

If we assume that the soil type within your field is the same then we only need to use a 2-dimensional matrix such as the Phosphorus Export Risk Matrix (PERM).

The goal is to allow farmers to compare their current land use practice within the wider context of alternative land management options.

The NERM helps to identify where nutrient source and hydrological connectivity meet creating critical sources areas.

Axis 1: P availability - is a synthesis of all possible P application forms, crop covers, tillage and husbandry regimes into a single estimate of how much of the P is actually available for direct mobilisation by overland, drain or subsurface flow. This is essentially an estimate the P surplus per unit area.

Axis 2: Flow connectivity - assesses surface topography and  landscape features, such as tyre tracks and land drains, and also the potential benefits of environmental features such as buffers strips and wetlands, both natural and human influences are assessed together. This axis also points towards many possible cheap land management options that will reduce P. For example, lowering surplus P in hydrological active zones or managing runoff at key locations within the landscape.

Axis 3: Soil type - reflects the behavior of a certain soils, under certain cultivation to lose P due to overland flow , to retain P or to lose P.

A demonstration of how the NERM could be applied in conjunction with flow connectivity modelling is shown in the diagram on the left. This is a scenario based on Field A which shows a potential reduction in P export from critical source areas achieved using farm dams. Using spatially targeted farm ponds a 60% reduction in surface runoff from the field to the ditch is achieved by ponding in 1% of the field area. This results in a reduction of the hydrological connectivity of the field.

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