InVEST - Nutrient Supply Ratio:

The Nutrient Delivery Ratio model uses a simple mass balance approach, which describes the movement of a mass of nutrients (phosphorus or nitrogen) through space using empirical equations. The model considers the sources of nutrients throughout the landscape, making use of the so-called nutrient loads, which are a function of the uses and covers of the soil. The representation of nutrient loads can be divided into dissolved and sediment-bound parts (transport through surface and subsurface flow, respectively). The figure presents the diagram of the information requirements that the model needs, the processes it considers and the results it generates.

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