WATER QUALITY
What makes water quality different
Water quality is the case in which the benefit of a Nature Based Solution is most often the primary objective rather than a co-benefit. Nutrient retention, sediment trapping, pathogen reduction and temperature moderation are all delivered by the same interventions that reduce flood peaks, and in many catchments the water quality obligation is the one with a legal deadline attached.
The assessment problem is that the benefit is usually not a damaged avoided but a concentration reduced, and the thing that has to be compared with the cost is a compliance target rather than a monetary loss. That changes the form of the appraisal: the question becomes what it costs to reach the standard, and whether a natural system reaches it at lower cost or with fewer unwanted effects than the treatment alternative.
How it is assessed
The modelling requirement is a representation of the transport and transformation of the substance concerned, coupled to the hydrology, so that a change in the flow path or in the residence time produced by an intervention is reflected in the load delivered. This is the point at which the integrative modelling framework earns its place, because the water quantity model and the water quality model have to share the same representation of the catchment or the results will not reconcile.
Valuation follows the co-benefits methodology. Where the benefit is compliance, it is expressed as the cost of the alternative means of achieving it, which is a market-based comparison and requires no stated-preference work. Where the benefit is recreational, ecological or amenity-related, the valuation methods for non-market goods are used, together with the physical quantities in cases where no credible monetary value exists.
The multi-criteria toolkit is frequently the right instrument where water quality is one of several objectives, since a compliance obligation can be treated as a constraint while the other criteria are traded against one another.
Where it is used at NAIAD
Water quality objectives are present in several of the demonstration areas, most explicitly where the intervention is intended to reduce the load reaching a water body while also moderating flow. The Lez and Brague basins provide the project's Mediterranean cases, where low flow and quality are connected, and the Lower Danube provides the large agricultural catchment case.