Setting up a Basin Study Scale of Application

Spatial scale

The Freshwater Health Index can accommodate a range of spatial scales: sub-basins, basins, regions of adjacent basins, nations and even global assessments. For greatest utility for management, we recommend that the Index be applied to basins represented as a network of connected sub-basins (see section 3 below). In some contexts, some indicators may be most appropriately considered at a different spatial scale than the scale of assessment. For instance, some governance indicators may be best considered at the national scale even when the Index is being evaluated at the basin scale. In such cases, relevant national information can be applied to the smaller scale if local data is unavailable or local governance is dictated by national processes. For transboundary basins, assessments may need to consider information across multiple nations to determine the best and most relevant information to use for a basin-scale assessment. In many cases data will be available and most relevant at a scale smaller than the assessment scale e.g., water quality data at point source locations. Such data will need to be aggregated to the basin scale for final numeric values, with data and/or indicator values represented at the disaggregated scale through maps. Moreover, scales finer than the sub-basin may need to be considered to detect any changes in the indicator over time. For instance, land cover naturalness may need to be evaluated at a 30 m scale to detect meaningful changes from one assessment period to the next.

Temporal scale

The indicators that comprise the Freshwater Health Index measure the status and/or trends of freshwater system attributes. When calculating an attributes status, the most recent year for which data is available should be used. For current status calculations, the dates of the most recent available data do not have to be consistent across all indicators, although care should be taken to use data sets that reflect current conditions. For instance, data collected on water quality 10 years prior to the assessment date is unlikely to be indicative of current water quality, but 10-year-old data might be reflective of current land cover if it is known that land-use change has been negligible in the intervening years. Hence, it is important to use the most current data wherever available. For calculations of trends in freshwater system attributes, the current value must be compared with an historic value. For the first iteration of trend calculations, we recommend using five years prior to the assessment date as the reference point to which current values are compared, or as close to this as possible. For example, a first assessment of decline in a species of concern undertaken in 2020 should use 2015 as the reference year to which 2020 population size is compared. All subsequent iterations of trend calculations should compare the current value with the previously calculated value. We recommend that the Freshwater Health Index be re-evaluated at least every five years.