Delineating critical zones of riparian processes and setting effective buffer areas using spatial data

1. Introduction

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The riparian zone occupies the critical interface between land and watercourses where processes have great potential to influence stream and river biogeochemical and ecological conditions and is a key management location. Riparian influences on water quality may arise from (i) filtering and buffering capacities pollutant transfers from the land activities to water, and (ii) wider riparian functions that increase the resilience of the riparian-channel system to local or upstream pressures, such as coupling of terrestrial and aquatic ecosystems, or shading.

We make a distinction between the riparian zone and a buffer zone. The former implies a zone with a gradient of hydrological, habitat and geomorphic change from land to channel, the latter implies a management zone designed to separate potentially negative pressures of land activities from the aquatic ecosystem. There has been considerable debate over minimum riparian buffer widths for protection of stream ecosystems (e.g. Sweeney & Newbold, 2014). Hansen et al. (2015) found that studies collectively supported needs for variable buffer widths to protect watercourses depending on circumstances but the available body of riparian literature insufficiently or inconsistently reported the landscape contexts necessary to explain inter- and intra- study variability in outcomes. Hence, riparian structure and functions, both current and potential and informed strongly by landscape setting, are required to be considered alongside the environmental objectives (such as maintaining, or improving water quality and other multiple benefits).

Figure 1. Schematic of how assessment of pressures, riparian indicators of function and relationships with water quality parameters contributes to the two aims of the current review.

Figure 1 shows twofold aims that guide this current review. The first concerns the spatial targeting of understanding and management to maintain and restore riparian functions, to design and place effective riparian buffer zones. The second aim is to give catchment scientists improved methods for characterising important catchment riparian functions that relate to water quality to improve evidence on the connection between riparian condition and water quality to facilitate better management. The delineation of riparian zones is a key requirement of riparian management, forming a basis for the understanding the optimal zones to maintain or restore critical riparian functions. Yet delineation remains challenging and the literature suggests a variety of methods that have not been reviewed previously. This review examines:

      • Key riparian processes;
      • How spatial data is contributing to process understanding and how such data are being pooled;
      • How riparian areas are delineated from simple fixed width methods to variable width methods considering processes, catchment pressures and desired

        outcomes.

2. Processes and indicators relating to riparian functions
 

There are numerous schemes for assessment of riparian functions and key functions themselves concentrated on by regulatory actions. An example of a full description of a long-time implemented assessment scheme is given by Swanson et al. (2017) for the riparian Proper Functioning Condition (PFC) assessment that is integrated with, and assessed against, water quality monitoring in Nevada, U.S. The PFC assessment is based on scoring indicators of vegetation, hydrology and geomorphology against potential natural condition with regard to provision of functions: dissipation of stream energy, capturing sediment, increasing water recharge, rooting and bank stability, maintaining channel characteristics. Such core functions are common to many riparian assessment studies. The structure of section 2 here expands on this grouping according to: hydrological connectivity, water quality, shading and temperature regulation, resource transfers and terrestrial habitat. We describe indicators of the functions prior to assessing how these functions and their data are currently integrated into riparian delineation in section 3.
The dominant process groups are briefly presented below and key studies are indicated in Table 1:
 

       • Extreme flows and hydrological connectivity
                  o Surface roughness – the effects of riparian vegetation and buffer surface form on runoff hydrology, trapping eroded material from upslope sources

                     and the protection of erosion within the buffer itself.
                  o Groundwater-surface water interactions for water table and stream flow buffering - the hydrological regime (fluxes, water level, duration,

                     frequency, timing) is key to buffer functions and varies across scales and is damped by the presence of riparian wetlands.
       • Maintaining water quality - Buffers can act on three key pollutant source pathways, namely: surface runoff, stream bank erosion and shallow and deeper

          groundwater pathways.
                   o Surface runoff trapping and minimising bank erosion - The trapping efficiency per width is dependant on vegetation type and buffers intended

                      mainly for this trapping are often called filter strips.
                   o Groundwater-surface water interactions for water quality - the upslope-riparian-channel continuum’s hydrological regime constitute an

                      important interaction determining water quality in adjacent streams, with controls of water residence time factors (riparian slope, soil texture and

                      hydraulic conductivity) with water table depth and soil organic carbon.
        • Shading and temperature regulation - water temperature is a key aquatic habitat variable and riparian trees reduce heating of in-channel water by

           shading solar radiation and cooling by evapotranspiration of shallow groundwater and soil water.
        • Resource transfers – two-way resource exchanges occur e.g. aquatic insects provide food for terrestrial species and aquatic ecosystems receive multiple

           resources such as DOC, leaf litter, woody material and food from invertebrates that benefit a range of aquatic trophic levels.

        • Terrestrial habitat – water table variation affects riparian plant communities through water table as well as nutrient and acidity controls and local

           topographic variation and upwelling groundwaters resulted in ‘hot-spots’ of high plant species richness.