Assessment of Pitman Model Capabilities in Modelling Surface Water-Groundwater Interactions in the Lake Sibaya Catchment, South Africa, 2020-2022

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Difficulties arising from data scarcity, input data error or uncertainty, heterogeneous environments, lack of process understanding, and model structural uncertainty frequently constrain hydrological assessments of South African catchments. This research aimed to assess the usefulness of a “simpler” conceptual model for the conjunctive management of surface water and groundwater. The idea is that, to leverage the limited available data and information, a compromise between model complexity and data availability is required, which improves the use of models to produce reliable hydrological systems assessments. The research methodology focused on catchment-scale lake-groundwater dynamics to explore the limits of the groundwater components of the modified Pitman model in this type of environment, thus, determining the potential for using this model for integrated water assessments in South Africa. The Pitman model is one of the most widely accepted models regarding surface water hydrology in South Africa; however, the newly incorporated groundwater components have not been applied as extensively as the surface water components. There remains uncertainty regarding their capability to adequately simulate groundwater processes and accurately represent surface and groundwater interactions in some environments. The model was assessed based on how well simulated water balance variables accurately reflected available evidence and expected catchment response (objective 1). Secondly, the research identified and addressed uncertainties as regards the structure and application of the model’s groundwater interaction components (objective 2). The model was set up for the Lake Sibaya catchment, which is a predominantly groundwater-driven system and, thus, provides an important opportunity to interrogate different aspects of uncertainty in both the conceptualizing and quantifying interaction processes. The study’s overall conclusion is that the model performed satisfactorily as it was able to simulate the lake’s water balance correctly enough such that the influences of dominating components were sensibly reflected in variations in streamflow and lake volumes. The following key findings were noted; (i) the lake volume shows a continuous decline, (ii) the lake volume decreased with increasing development (forestry and abstractions) in the lake catchment, (iii) there is significant rainfall uncertainty in the study area and the model showed high sensitivity to rainfall differences, (iv) robust conceptual knowledge of local catchment conditions was valuable for reducing some of the data related uncertainty in the study area and for producing realistic model simulations, (v) the Pitman model updated GW components can provide a valuable tool for modelling integrated hydrological processes; nevertheless, when applying the model to specific environments, implicit approaches may be necessary to account for processes that are not fully represented in the model.

Sustainable water resource development remains elusive because development has largely externalized costs to the environment and vulnerable people. There is a need for novel research theory, methodologies & practice in order to meet the UN SDGs and realise the Africa Water Vision 2025. We propose to launch an innovative research approach: the Adaptive Systemic Approach (ASA). Our aim is to apply transformative, transdisciplinary, community-engaged research, to shift water development outcomes towards achieving the SDGs. We focus on continental water development priorities: water supply and pollution. This collaboration brings together the ARUA Water Centre of Excellence (CoE) and UK partner, the University of Sheffield (UoS). The 8 CoE nodes are: i) Addis Ababa U, Ethiopia; U Rwanda, Rwanda; U Cheikh Anta Diop, Senegal; Dar es Salaam U, Tanzania, Makerere U, Uganda (DAC least developed); ii) U Lagos, Nigeria (DAC lower-middle income); and iii) U Cape Town, Rhodes U (CoE Hub), South Africa (DAC upper-middle income). We propose a country-based Case Study structure to support local research development and pathways to local impact (Figure 1 in Case for Support). We use an SDG6 (water and sanitation) centred model, that links SDGs related to landscape water resources with SDGs related to water services. (This model underpins the successful UKRI:GCRF Capability Grant:"Water for African SDGs"). We raise three research questions (RQ) related to water development priorities. Three catchment-based Case Studies address RQ1: HOW IS WATER USED, TO WHOSE BENEFIT? (Rufigi R Tanzania, Senegal R Senegal, and Blue Nile R Ethiopia). Two Case Studies focus on urban water pollution (Kampala City Uganda and Lagos City Nigeria), addressing RQ2: WHAT ARE THE SOURCES, PATHWAYS AND IMPACT OF POLLUTION IN URBAN WATER SYSTEMS? A cross-cutting Case Study addresses water resource protection and biodiversity in all CSs, and a biodiversity site in Rwanda. By the completion of the project we commit to leaving local people effectively linked with institutions making decisions about water that affect them. Therefore all Case Studies address the question RQ3: HOW CAN LOCAL CAPACITY TO ENGAGE IN PARTICIPATORY GOVERNANCE BE DEVELOPED FOR: I) EQUITABLE WATER SHARING, II) COMMUNITY POLLUTION RESILIENCE, AND III) ECOSYSTEM PROTECTION AND RESTORATION? The novel Adaptive Systemic Approach (ASA) provides a coherent methodological framework that will support Case Study comparisons, changed water development practice, and will embed pathways to impact throughout the project. The ASA requires engaged research, and draws on three core theoretical concepts, with associated methods: Complex Social-Ecological Systems, Transdisciplinarity, and Transformative Social Learning (Elaborated in Case for Support). These concepts underpin four ASA steps, followed in each Case Study: 1. BOUND: Researchers engage with a full range of stakeholders to identify a relevant, local, water-development issue, and scope the Case Study. 2. ADAPTIVE PLANNING PROCESS: Stakeholders co-create a contextually informed vision of the future state of their selected local issue, and co-develop an objectives hierarchy to move towards resolving the issue. 3. CONCURRENT ACTIVITIES 3.1 RESEARCH Each Case study team addresses the specific research questions, delivering data for resolving the problem. 3.2 PARTICIPATORY GOVERNANCE DEVELOPMENT Local people, formal, and traditional, water governance institutions together move towards local people being part of land and water decision-making. 3.3 STRATEGIC ADAPTIVE MANAGEMENT (SAM) - stakeholders will be trained in a process for systemic, responsive, contextual, co-management. 4. PARTICIPATORY MONITORING AND EVALUATION OF REFLEXIVE LEARNING Researchers and stakeholders co-develop indicators, co-monitor, co-reflect on progress, co-learn and adapt, using SAM. Following the ASA in the case studies embeds the theory of change, and the pathways to impact.