4.1. A Practical and Applicable Typology of Catchment Controls for Waterbodies in England and Wales
Selected catchment controls have been used in previous applied typologies to delineate homogenous river sections [2
], but the associations between catchment controls, and the response of river reaches to their combined effects, is often not considered. The typology presented here is less focused on classifying reach processes for local management than previous typologies. Instead, the typology was designed to capture multiple catchment controls and their associations for identifying natural boundaries in catchment functioning for strategic management at the national level.
The typology of catchment controls, developed using the SOM approach for waterbodies in England and Wales, was successful at differentiating between key features of the landscape including national reserves, topographical and geological features, major rivers and urban centres (Figure 2
). The approach incorporates multiple catchment characteristics that have a functional control on river reaches (Table 2
) rather than being limited to only characteristics that are not correlated with one another. Furthermore, the typology boundaries are based on naturally occurring thresholds in the data identified by the clustering algorithm rather than arbitrary boundaries.
These factors likely explain why this waterbody typology differentiates habitat features between types better than the current WFD System A typology. When evaluated against flow type, substrate size and geomorphic activity indices derived from seminatural RHS sites, 0% of WFD System A types were statistically different to all the other types (at a significance level of p
< 0.05, [22
]). However, in this typology, using the same level of significance, up to 100% of types produced statistical differences in habitat indices between all other types (Figure 3
), including 42–57% for the summary indices used to assess the quality of reaches. This indicates that this typology has relevance for river managers and conceptually improves upon the current WFD System A typology, which is based solely on elevation, catchment area and geology (Table 1
) and has arbitrary boundaries between categories [20
The strength of this typology is the range of catchment characteristics included that often showed cross-correlations (Figure 1
d). Cross-correlation makes it difficult to isolate individual effects from catchment controls as they interact [26
]. This is because catchment controls are not independent [21
]; therefore, grouping waterbodies with similar controls is beneficial rather than relying on a single control to describe all catchment influences.
The inclusion of multiple characteristics was possible because the SOM method was adopted. This and other machine learning techniques are becoming more prevalent in multivariate analysis as they can deal with natural artefacts of many environmental datasets, which often make multivariate environmental analyses challenging [26
]. The heatmap outputs from the SOM (Figure 1
d) also allow for easy visualisation of variable distributions, positive and negative correlations between variables such as the upland-lowland gradient, and anomalies such as the higher drainage density anomaly in the large urban type [28
4.2. Critique of the Typology
Whilst the waterbody typology shows promising differentiation between landscape (Figure 2
) and reach features (Figure 3
), its limitations must be understood to ensure it is not applied for management in ways that are inappropriate given its design. The most obvious example of limitations is the wide ranges of habitat index values within each waterbody type, despite overall significant differences between most types (Figure 3
). As the aim of this paper was to create a waterbody typology that can be applied widely, this is expected, but reasons for these variations are discussed below to highlight limitations of the typology.
The variation in characteristics within waterbody types was greatest in aquifer, large urban and both upland types (Figure 1
b). Creating more types may capture more variation, and the selection of the number of types in any typology is ultimately subjective [15
] but is aided by statistical measures and expert opinion (for the methods used here, see Appendix A
). An interpretable classification will never capture the whole range of variation of its population, nor is it expected to, but it must capture enough variation to be fit for purpose. As discussed above, we believe that seven types are appropriate to capture the variation in catchment controls at this national level, evidenced by evaluating the types against survey data (Figure 3
The limitations of the RHS dataset, used here to represent reach features, should also be noted. The RHS was not designed as a geomorphological survey to capture dynamic process [73
] but does include the presence/absence of features that are useful to estimate dominant channel habitat conditions over a standardised 500 m reach. The identification of dominant features present at each transect in the survey means that the diverse conditions of the reach may be underestimated, which may mute more extreme differences between waterbody types. However, although the RHS is not detailed, it does provide a wide spatial coverage with a consistent methodology that makes it a valuable tool for use in national typologies [19
The waterbodies used as the unit for the typology developed here are much larger than reach or subreach units employed by bottom-up typologies (e.g., [2
]), which has practical benefits. For example, the resolution of the GIS-derived datasets used to build the typology can be relatively coarse, and there are numerous RHS surveys available within each waterbody type to effectively evaluate the typology. The waterbody unit also reflects policy units that are widely applied in river management in Europe [12
] providing a continuous typology across the landscape not possible if relying on survey data alone. However, the use of waterbodies as subunits of the wider catchment means that controls from upstream of the waterbody are not considered. Only the cumulative catchment area characteristic indicates the position of the waterbody within the wider catchment, which contributed to the large urban waterbody type, separating waterbodies at the downstream end of catchments from other waterbody types. The use of a relatively large study unit also means that variation will be present within types because each waterbody contains a range of processes and local pressures, such as sediment mining, dams and channelization, that are not included in the typology, which is a limitation of this methodology. The aim of this typology, however, was to capture the catchment controls that influence the reach, rather than directly classifying reach processes and features such as channel stream power, slope and planform, which have been the focus of previous top-down and bottom-up typologies (e.g., [2
]). For increased utility of this typology for operational river management at a more local level, data on controls and characteristics at the reach-level should be integrated into the waterbody typology.
The typology also is a temporary snapshot of catchment controls, which is often a critique of river typologies [3
]. While many catchment characteristics change over long timescales, such as morphometry or geology (~102
years), some characteristics are more temporally dynamic such as land cover and rainfall patterns (~101
]). This is addressed to some extent by taking a long-term average of rainfall (from 1961 to 2016) and a land cover map for the time period most relevant to the validation surveys (2007). While this is not ideal, the top-down nature of this approach means the typology can easily be updated at a relatively inexpensive cost to the user as, and when, major landscape alterations are made or when new data become available. The typology is also evaluated with RHS surveys occurring over a long time period (1994 to 2015) each providing a snapshot of river features that change ~10−1
years rather than the long-term changes of the catchment controls. Although the link between catchment changes and channel features is complex, the fact the typology performs well when evaluated against over twenty years’ worth of surveys suggests that the typology is relevant over long time periods.
Whilst there are limitations, primarily as a result of the selection of the top-down approach, the validation of the waterbody typology with reach-level data not only creates a useful typology tool with distinctive classes but enhances understanding catchment controls on reach habitats. The top-down method means that this approach can be applied to any waterbody with available data, without expensive and systematically biased surveys. However, the broad distribution of habitat features within each type (despite statistically significant differences; Figure 3
) emphasises that this typology is not substitute for detailed surveys and monitoring, but a means of assessing the spatial distribution of catchment controls at a national level. Future work should compare different datasets that reflect other aspects of the geomorphology or ecology of the channel to this typology.