Introducing the Hydropedological assessment guidelines: Theory and application in a South African wetland management context

Citation:
Van Tol, J.J., Job, N., Chetty S., Bouwer, D. & Le Roux, P.A.L. Integrating hydropedology in wetland delineation and management guidelines. Water Research Commission WRC Report No. TT 925/23.

https://www.researchgate.net/publication/379840160_HYDROPEDOLOGICAL_ASSESSMENT_GUIDELINES_Theory_and_application_in-a_South_African_wetland_management_context_Report_to_the_Water_Research_Commission

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Background

DSA’s directors, along with collaborators from the University of the Free State, the Institute of Natural Resources, and the South African Biodiversity Institute, recently published new hydropedological assessment guidelines. These guidelines were developed under a Water Research Commission-funded project, initiated in response to a request from the Department of Water and Sanitation (DWS) to include hydropedological assessments in Water Use License Applications (WULAs) for projects involving significant land-use changes like open-cast mining, infrastructure development, and large residential projects.

Previously, there has been a lack of clear and comprehensive guidelines for conducting hydropedological studies within wetland management contexts. Existing literature on hydropedological theory is scattered across various sources, lacking a cohesive methodological framework. This report aims to consolidate and refine previous guidelines into a comprehensive document, providing both theoretical insights and practical guidance on conducting hydropedological assessments, with a specific focus on wetland management.

The guidelines are structured into two main sections: Section A establishes a theoretical foundation covering hydrological processes, interpretation of soil properties, and hydropedology of hillslopes, serving as educational material for scholars interested in hydropedology. Section B offers practical guidance on conducting hydropedological surveys for wetland assessments and interpreting the findings. It aims to assist consultants in determining when and how hydropedological assessments should be conducted, and by whom, within the framework of wetland management.

Section A: Theoretical background

At the plot scale, the soil water balance involves inputs like rainfall, infiltration, and lateral flows, and outputs such as evaporation, root water uptake, and drainage, all influenced by soil hydraulic properties and their distribution in the profile. Hillslope or catchment-scale water flow occurs through specific flowpaths determined by topography, vegetation, and soil distribution, including overland flow, subsurface lateral flow, and bedrock flow. The dominant flowpath affects water residence times, impacting water quality and availability during dry periods.

Soil properties, like color and morphology, serve as indicators of hydrological behavior. Soil color correlates with conditions like saturation (grey colours), varying water regimes (mottles), or well-drained conditions (oximorphic colors). Other properties, such as macropores and texture, influence water movement and storage capacity, affecting rates of evapotranspiration and biochemical reactions.

Understanding hydrological behavior involves studying diagnostic horizons, which significantly influenced revisions to the South African Soil Classification System (SASCS) in 2018. Different soil horizons perform varying hydrological functions based on their sequence and position in the landscape. South Africa’s soil forms and families were regrouped into nine hydrological soil types, encompassing 1,657 families, including anthropogenic groups. The main groups are briefly summarized in Table A.

Table A. Key characteristics of hydrological soil types.

Hydrological soil type Key characteristics
Recharge (deep) Deep, freely drained soils overlaying impermeable bedrock or weathered saprolite exhibit no signs of saturation. Evapotranspiration-excess downward flow dominant.
Recharge (shallow) Shallow, freely drained topsoil horizons overlying fractured rock or saprolite. Limited contribution to transpiration.
Recharge (slow) Slow vertical movement is the dominant flowpath. Typically clay rich (luviated) subsoil horizons which act as a store, rather than a conduit of water. Evapotranspiration is dominant.
Interflow (soil/bedrock) Lateral flow is generated, either due to low permeability of the bedrock which restricts vertical drainage or due to return flow from the bedrock flowpath to the soils. Flow maintained on seasonal basis.
Interflow (shallow) Marked by vertical anisotropy in hydraulic conductivity where a permeable topsoil overlies a restricting subsoil layer. Flow generated by specific rain events and duration of lateral flow is short.
Interflow (slow) High clay content low conductivity subsoil horizon at soil/bedrock interface. Serve as a store rather than conduit of water.
Responsive (shallow) Soils with limited depth and hence small storage capacity. Underlying rocks are slowly permeable and rapid recharge of bedrock flowpaths not likely. Overland flow generated as normal rainfall events will the exceed the storage capacity.
Responsive (wet) Saturation close to the surface layers for extended periods, especially during the wet season. Additional precipitation will not infiltrate but overland flow will be generated (saturation excess).
Responsive (Hortonian) Vertic horizons will swell close during wet periods with very low hydraulic conductivity/infiltration rates. Degraded soils with surface crusting or sodic soils. Overland flow created due to infiltration excess (Hortonian flow).

The hydrology of hillslopes, influenced by topography, soils, climate, and vegetation, plays a critical role in catchment-scale hydrological response. In this report, 12 hillslope-wetland response classes, reflecting dominant water delivery mechanisms and considering factors influencing water movement rates were identified (Figure A). Ongoing efforts aim to refine and improve these classes over time. Each of these hillslope classes have their own sensitivity linked to different kinds of development (Table B)

Figure A. Graphical summary of hillslope hydropedological response classes.

Table B. Summary of risk of local activities impacting on the wetlands’ functions based on hydropedological hillslope types.

Wetland group Wetland-hillslope class (Figure A) High-impact activities Risk zone Risk to wetland Potential contribution of hydropedological assessments1
Hydrological impact Activities
Groundwater-dependent, permanent 1: Recharge deep soils dominant Abstraction (broadscale lowering of regional aquifer from groundwater abstraction) boreholes Anywhere within the wetland catchment Moderate Low: Groundwater driven
Reduction (regionally extensive water-reduction activities) commercial plantations Anywhere within the wetland catchment Moderate Low: Groundwater driven
Groundwater-dependent, permanent 2: Recharge deep soils dominant Abstraction (broadscale lowering of regional aquifer from groundwater abstraction) boreholes Anywhere within the wetland catchment Moderate Low: Groundwater driven
Groundwater-dependent, permanent 3: Recharge shallow soils dominant Surface sealing and diversion (prevents infiltration of recharge water, changes recharge infiltration rates, diverts and converts water into peak flows and concentrated runoff) urbanisation: buildings, roofs, roads Anywhere within the wetland catchment Low Low: Groundwater driven
Reduction (interception and extraction of available recharge volumes) water-intensive woody alien invasive species Anywhere within the wetland catchment Low Low: Groundwater driven
Hillslope—dependent, permanent 4: Recharge shallow soils dominant Surface sealing and diversion (prevents infiltration of recharge water, changes recharge infiltration rates, diverts and converts water into peak flows and concentrated runoff) urbanisation: buildings, roofs, roads Anywhere within the wetland catchment Moderate High: Identifying and characterising recharge zones
Hillslope—dependent, temporary to permanent 5: Recharge shallow, to fractured aquifer Surface sealing and diversion (prevents infiltration of recharge water) urbanisation: buildings, roofs, roads Midslope, fractured rock recharge as well as soil return flow areas Moderate High: Identifying and characterising recharge zones
Hillslope—dependent, seasonal to permanent 6: Recharge deep soils dominant Surface sealing and diversion (prevents infiltration of recharge water, changes recharge infiltration rates, diverts and converts water into peak flows and concentrated runoff) urbanisation: buildings, roofs, roads; land use conversion Recharge area, typically at hillslope crest Moderate to high High: Identifying and characterising recharge zones
Reduction (interception and extraction of available recharge volumes) alien invasive plants Recharge area, typically at hillslope crest Moderate to high High: Identifying and characterising recharge zones
Wetlands rare or absent 7: Recharge slow soils dominant Not applicable Not applicable Low Low: Limited lateral landscape connectivity
Wetlands rare or absent 8: Recharge deep soils dominant Not applicable Not applicable Low Low: Limited lateral landscape connectivity
Hillslope—dependent, temporary 9: Responsive shallow soils dominant Surface sealing and diversion (increased, concentrated overland flow can change hydroperiod of a naturally seasonal or temporary wetland to more permanent) urbanisation: buildings, roofs, roads; mining Anywhere within the wetland catchment Low Low: Overland flow dominant
Hillslope—dependent, seasonal to permanent 10: Recharge deep soils to interflow soils Surface sealing and diversion (prevents infiltration of recharge water, changes recharge infiltration rates, diverts and converts water into peak flows and concentrated runoff) urbanisation: buildings, roofs, roads Fractured rock recharge area, areas of return flow from interflow High High: Identify and characterising recharge and interflow zones
Hillslope—dependent, seasonal 11: Interflow (slow) soils dominated by evapotranspiration Limited risk Hillslope has limited flow generation Moderate to low Low: Lateral connectivity limited
Hillslope—dependent, seasonal 12: Interflow soils dominant Surface sealing and diversion (interception or disruption of shallow flowpaths, diverting flows away from the wetland) urbanisation: buildings, roofs, roads; mining Bedrock interflow areas High High: Identifying and characterising interflow zones

1The contribution of hydropedological assessments to understand the hydrological behaviour of the landscape and contribute to protecting the wetland and manage the water resources more sustainably.

SECTION B: PRACTICAL GUIDELINES FOR PRACTITIONER AND DECISION MAKERS

Hydropedology of hillslopes in a wetland management context varies based on flowpaths, connectivity, and residence times, impacting wetland functioning differently across wetland types (Table B). A decision tree aids in determining the necessity of a hydropedological assessment, considering six land use change impacts and climate factors, distinguishing between no assessment needed, basic assessment required, or full assessment necessary based on risk and resource importance.

Norms and standards for hydropedological assessment reports are provided to ensure high-quality assessments and aid decision-makers in evaluating assessment quality.

Modelling approaches should focus on quantifying flowpath importance and the impact of land use changes on these pathways replenishing wetlands. Case studies highlighted critical issues, emphasizing the need for accurate soil/bedrock interface descriptions and advocating for direct measurement of relevant properties over excessive observation densities.

Steps for integrating hydropedology in wetland management include wetland delineation, catchment analysis, soil characterization, and conceptualizing hillslope hydrological responses. For full assessments, hydraulic measurements and flux quantifications are essential.

Steps for integrating hydropedology in wetland management

  • Step 1: Delineate wetland boundary on desktop.
  • Step 2: Delineate wetland catchment boundary on desktop.
  • Step 3: Identify influence of groundwater or rivers.
  • Step 4: Characterise the wetland catchment environment.
  • Step 5: Identify representative hillslopes.
  • Step 6: Delineate wetlands in the field.
  • Step 7: Conduct hydropedology transect survey.
  • Step 8: Conduct hydraulic measurements; in-situ and in lab.
  • Step 9: Regroup soil observations into hydropedological groups.
  • Step 10: Conceptualise hillslope hydrological responses.
    • Hillslope classes
    • Contribution to wetlands
  • Step 11: Describe impacts on processes and wetland responses.
  • Step 12: Quantify hydropedological fluxes.
  • Step 13: Develop mitigation and management plans to reduce or avoid impacts.

Note: Steps 8 and 12 are only for Full hydropedological assessments.

At a minimum, practitioners should be capable of classifying South African soils up to the family level, ideally supplemented with a short course on hydropedology in the context of wetland management.

Stakeholder engagement during guideline development highlighted the need for additional training for enforcement, spatial tools for survey identification, and expanded guidelines for various project types, ensuring practicality and effectiveness in South African water resource management.

Figure B. Decision tree for when and the type of hydropedological assessment required.

Notes:

  • Activities that are generally authorised for any person, institution and / or SOCs subject only to conditions of the General Authorisation for Section 21 (c) and (i) water uses do not require any level of hydropedological assessment (see examples in Table 8.1)
  • See triggering actions in listing notices in Appendix A
  • Linear in the context of the decision tree refers to belowground linear development (e.g. pipes and drains) and roads. Aboveground linear developments do not require hydropedological assessment as they do not significantly alter flow paths. However, authorities may request hydropedological assessment based on the specific development, method statement and expected impacts.
  • Regulated area as defined in the National Water Act (1998) – see definition under terms and definitions
  • Activities listed in Appendix A which will require basic or full Environmental Impact Assessment
  • Basic assessment focus only on conceptual description of pathways and connectivity (flow drivers) and the potential impact of the development on these
  • Risk/impact is based on risk matrix associated with different hillslope types (Section 6.4 & 6.5 and Table 7.4)
  • Full assessment: Include quantification of fluxes and loss/gain of different water balance components
  • Includes renewable solar energy projects. Wind farms typically excluded.
  • Agricultural activities include dams, planting in water source areas and changes from rainfed to irrigation agriculture. Water quality impacts (pesticides, herbicides and nutrients) should also be considered.
  • Changing of natural vegetation or cultivated agriculture to commercial plantations.