Summary

The objective of the InVEST nutrient delivery model is to map nutrient sources from watersheds and their transport to the stream. This spatial information can be used to assess the service of nutrient retention by natural vegetation. The retention service is of particular interest for surface water quality issues and can be valued in economic or social terms, such as avoided treatment costs or improved water security through access to clean drinking water.

Introduction

Land use change, and in particular the conversion to agricultural lands, dramatically modifies the natural nutrient cycle. Anthropogenic nutrient sources include point sources, e.g. industrial effluent or water treatment plant discharges, and non-point sources, e.g. fertilizer used in agriculture and residential areas. When it rains or snows, water flows over the landscape carrying pollutants from these surfaces into streams, rivers, lakes, and the ocean. This has consequences for people, directly affecting their health or well-being (Keeler et al., 2012), and for aquatic ecosystems that have a limited capacity to adapt to these nutrient loads.

One way to reduce non-point source pollution is to reduce the amount of anthropogenic inputs (i.e. fertilizer management). When this option fails, ecosystems can provide a purification service by retaining or degrading pollutants before they enter the stream. For instance, vegetation can remove pollutants by storing them in tissue or releasing them back to the environment in another form. Soils can also store and trap some soluble pollutants. Wetlands can slow flow long enough for pollutants to be taken up by vegetation. Riparian vegetation is particularly important in this regard, often serving as a last barrier before pollutants enter a stream.

Land-use planners from government agencies to environmental groups need information regarding the contribution of ecosystems to mitigating water pollution. Specifically, they require spatial information on nutrient export and areas with highest filtration. The nutrient delivery and retention model provides this information for non-point source pollutants. The model was designed for nutrients (nitrogen and phosphorous), but its structure can be used for other contaminants (persistent organics, pathogens etc.) if data are available on the loading rates and filtration rates of the pollutant of interest.

The Model

Overview

The model uses a simple mass balance approach, describing the movement of a mass of nutrient through space. Unlike more sophisticated nutrient models, the model does not represent the details of the nutrient cycle but rather represents the long-term, steady-state flow of nutrients through empirical relationships. Sources of nutrient across the landscape, also called nutrient loads, are determined based on a land use/land cover (LULC) map and associated loading rates. Nutrient loads can then be divided into sediment-bound and dissolved parts, which will be transported through surface and subsurface flow, respectively, stopping when they reach a stream. Note that modeling subsurface flow is optional; the user can choose to model surface flow only. In a second step, delivery factors are computed for each pixel based on the properties of pixels belonging to the same flow path (in particular their slope and retention efficiency of the land use). At the watershed/subwatershed outlet, the nutrient export is computed as the sum of the pixel-level contributions.

 

Figure 1: Conceptual representation of the NDR model. Each pixel i is characterized by its nutrient load, load_i, and its nutrient delivery ratio (NDR), which is a function of the upslope area, and downslope flow path (in particular the retention efficiencies of LULC types on the downslope flow path). Pixel-level export is computed based on these two factors, and the sediment export at the watershed level is the sum of pixel-level nutrient exports.