Browsing by Author "Okedi, John"

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    Open Access
    An Experimental Study on the Infiltration Potential of Stormwater Ponds in Zeekoe Catchment, Cape Town, South Africa
    (2022) Mavundla, Kgomoangwato; Kalumba, Denis; Armitage, Neil; Okedi, John
    In early 2018, the city of Cape Town, with a population of approximately 3.8 million, was at risk of running out of water from the six large reservoirs to the east of the city. This was due to the worst drought in almost a century, which occurred between 2015 2017, causing the city to be declared a disaster area. Although alternative water resources had been identified, they had not been developed. This has now become imperative as droughts are expected to recur in the future. This study investigated the prospect of using existing stormwater ponds in the Zeekoe catchment area as infiltration cells transferring detained stormwater into the underlying aquifer storage zone to enhance the available groundwater resource. The Zeekoe catchment is a 89 km2 area within the 630 km2 Cape Flats Aquifer (CFA). Based on hydrogeological data and aquifer parameter interpretation, it is considered to have good storage characteristics that can support groundwater development for water supply. Bouwer (2002) highlights how infiltration tests in the field can be useful for estimating desired volumetric recharge rates within a certain area. The hydraulic conductivity of the unsaturated layer is an essential non-linear function of soil-water content and has been generally recognised as the most important transport property to describe the ability of soil to permit water movement. A series of in-situ infiltration experiments were conducted at three representative stormwater ponds using a Double Ring Infiltrometer (DRI) to determine the rate of water recharge. Infiltration data was interpreted using both the Green-Ampt and Horton methods to determine the hydraulic conductivity and infiltration decay constants. A total of 18 core samples retrieved from the in-situ infiltration test locations were analysed in the laboratory to determine the ed hydraulic conductivity through constant-head permeability (CHP) tests. The physical and hydraulic soil parameters gathered from field and laboratory tests were used as inputs for a finite element numerical modelling software (HYDRUS 2-D) to estimate the range of recharge rates for the study area. Based on field infiltration test results, the hydraulic conductivity was found to be 0.3 19.9 cm/hr; typical for silty sands to fine sands. Hydraulic conductivities estimated in the laboratory were greater than the field hydraulic conductivity by 103%. This could be attributed to entrapped air under field conditions which reduces the effective cross-sectional area available for water to flow. From the HYDRUS 2-D simulations, the period required for the wetting front movement from the pond surfaces to the water table ( 5.5 m below the surface) was 15 140 hours. Hydraulic conductivities estimated using the pedotransfer function (PTF) of the built-in software, Rosetta-Lite, were also greater than the field hydraulic conductivity values by 118%. For an actual test pond, the infiltration rates would be expected to be slower, and recharge times would be greater because the HYDRUS 2-D simulations did not consider layers of low permeability suggestion that the field saturated hydraulic conductivity could be taken as roughly 0.5 times the laboratory hydraulic conductivity was thus considered reasonable. This means that ponds in the central area of the catchment would be suitable for artificial recharge with an estimated infiltration rate of around 20.6 cm/hr which could provide a mean annual groundwater yield of 29 33 Mm3 . A more extensive survey would aid in assessing local conditions that may impede groundwater flow.
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    Open Access
    Evaluation of water quality in urban drinking water distribution networks: a case study in Johannesburg, South Africa
    (2024) Luyaba, Lubabalo; Okedi, John; van Zyl, Jakobus
    In the Republic of South Africa (RSA) access to drinking water is a constitutionally guaranteed human right. The supply of safe drinking water to consumers is a legal requirement and the numerical limits for drinking water quality are described in SANS 241:2015. However, in 2023 over 30% of RSAs Water Services Providers (WSPs) supplied water that was of poor quality, with breakouts of cholera becoming more frequent. Poor drinking water quality is a result of a complex combination of factors requiring different interventions, one of the complex factors is drinking water quality management in a distribution network. What makes drinking water quality management in a distribution network complex is inter alia the fact that the water quality deteriorates within the distribution network. This presents a need for tools that can assist WSPs to better understand and manage drinking water quality in a distribution network. The evaluation of widely used, credible and freely available modelling tools, that can assist WSPs with limited resources (financial and human capital) for drinking water quality management in their distribution networks, was considered worth exploring. The main objective of the study was therefore to evaluate water quality in an urban drinking water distribution network, considering a case study in RSA, utilising a widely used and freely available modelling tool. The study calibrated (hydraulic and water quality) and validated (water quality) the distribution network following international best-practice, prior to commencing with the evaluation. To ensure practical relevance for WSPs, the study focused on the key sources of uncertainty in drinking water quality modelling in water distribution networks. Drinking water quality (considering various specific determinants) was the dependent variable, with the independent variables being: hydraulic definition, level of calibration, pipe age, pipe material, water demand pattern, load shedding and tank (reservoir) mixing model. The study also considered the practical usefulness of a free and widely used tool (EPANET 2.0) in optimising a network drinking water quality sampling programme. To optimally evaluate the sources of uncertainty two independent models were developed through skeletonisation and reduction. These were the Medium-Level Detail Model – MLDM (reduced all pipes model) and the Low-Level Detail System – LLDS (significantly reduced and skeletonised). It is reasonable to assume that utilities will not always have all the distribution network hydraulic data, but they may still need to model the water quality of these networks with incomplete data (in the interest of protecting public health). A consideration of various water quality determinants(physical, chemical and biochemical), modelled on the MLDM and the LLDS showed that distribution network simplification (through reduction and skeletonisation) does not compromise water quality modelling accuracy. However, the MLDM proved accurate for more determinants (dissolved oxygen, total chlorine, chloramine and dissolved organic carbon) than those of the LLDS (free chlorine, biodegradable organic carbon). It was therefore concluded that there is some benefit in investing in additional hydraulic detail (hydraulic definition) as the returns are higher levels of water quality modelling accuracy, for certain determinants. The study then considered the impacts of the level of calibration on water quality modelling accuracy for both the LLDS and the MLDM. As expected, the level of calibration was shown to correlate directly with water quality modelling accuracy. However, the investigation showed that hydraulic definition was more important than the level of calibration, in extended period simulation. This was because the second best calibrated MLDM (known control status), produced more accurate results than the most calibrated LLDS (variable control status) when considering total chlorine. This further highlighted the need to prioritise hydraulic definition as far as practically possible. Distribution network pipes are generally underground as such their material, age and or condition is often not reliably known, resulting in a need to estimate the roughness coefficient (C-value). For both models (LLDS and MLDM) it was clear that the pipe material and age were significant parameters as there was notable variance in total chlorine concentrations (up to 34% for the MLDM and up to 95% for the LLDS) with changes in the roughness coefficient (C-value).When considering the same C-value, the MLDM was generally more accurate (range was between 10% and 20%) than the LLDS, as observed from the lower errors (difference between model outputs and field measurements). Both models followed the same pattern of water quality deterioration, meaning they were both useful for modelling purposes. For both models, the most significant adverse impacts (on water quality) were for badly corroded steel, iron and clay pipes. This meant that special care to determine pipe age (condition) must be taken when modelling networks with these materials, otherwise the model will produce errors that render the model useless. Pipe age, condition and material were found to have the most significant influence on water quality modelling accuracy. Therefore, the most effort should be expanded on this source of uncertainty, as incorrect C-values can render the water quality model outputs useless, thus undermining the entire exercise. Water demand patterns are another source of uncertainty in practise, as they are not always exhaustively known. In this study demand patterns were found to have a very minimal impact on water quality, with an absolute maximum difference of 4% when considering total chlorine. However, demand patterns are critical for hydraulic and operational considerations. South Africa has been struggling with rolling electricity blackouts (load-shedding) for well over a decade. Load-shedding has a direct impact on water supply as the study area relied on pumps (additionally, the absence of electricity influences water demand patterns). Load-shedding was shown to have the most pronounced adverse water quality impact during stages 5 (five) to 8 (eight), with water quality precautions (additional dosing or boil notices) needed during stages 7 (seven) and 8 (eight); this was because water ages increased to a point (well above 70 days) where free chlorine was well below 0.2mg/l. Tank (reservoir) mixing models were shown to influence water quality most significantly under conditions of short-circuiting (First-In-First-Out), presenting a need to understand under what conditions tank short-circuiting was likely and how it could be prevented. The identification of optimal critical sampling points is a key facet of drinking water quality management (adhering to a preventive risk management philosophy), and the selected modelling software was shown to be one of the options that can be considered for this exercise. The importance of both hydraulic and water quality parameters was observed in all scenarios, underscoring the need to understand both independently and jointly (the impact one has on the other); as the end objective is ensuring safe drinking water is delivered to consumers, to preserve public health. It was concluded that EPANET 2.0 was a useful software, that could add significant practical value to WSPs in managing drinking water quality in a distribution network to protect and preserve public health.
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    Open Access
    Groundwater quantity and quality assessment for aquifer recharge in Ohangwena region, Namibia
    (2022) Ndakola, Hilja; Okedi, John
    The high population growth rate and changing climatic conditions for the Ohangwena region create increasing pressure on the local water balance. As groundwater demands increase and availability declines, sustainable groundwater management is required for the Ohangwena region. This thesis assessed the suitability of using runoff for aquifer recharge to augment groundwater in the Ohangwena region and minimize the lowering of water table. The Geographical Information System (GIS) and groundwater flow models were used in this study to assess the water resources in the study area. A multicriteria approach using a weighted rating was used in this study to generate a map showing areas suitable for groundwater recharge. The resultant groundwater potential recharge zones map was produced based on the overall weights of seven influential factors for groundwater recharge namely lithology, land use/land cover, lineaments, drainage, slope, geology, and soil type. The results of the assessment indicated that 85% of the Ohangwena region is characterized by high groundwater recharge zones, while 25% is characterized by very high groundwater recharge zones. The very high groundwater recharge zones are mostly found in the central part of the region, on the far upper western side and the eastern side of the region. The recorded recharge rates for the region in the range of 40 – 60 mm/year were introduced to an established model using the MODFLOW software, to assess the impact of Aquifer Recharge on the groundwater levels. The impact was assessed with both the steady-state model to approximate aquifer recharge under controlled conditions, and the transient state model for close representation of recharge in reality at four wells namely, WW201045, WW201637, WW201634, and WW20267, where it was evaluated by the hydraulic heads and water budget analysis. The steady state model results indicated a change in groundwater levels in the range of -0.10 - 0.70 m. In the same manner, the transient state model results show a gradual increase in groundwater levels in the range of 9.2 - 12.10 meters for the 350 m deep Ohangwena aquifer. The groundwater levels can be improved locally by infiltrating runoff into the KOH-II aquifer via the injection wells or infiltration basins during the rainy season (November to April) when there is plenty of runoff and flood in the region. The high soil infiltration rates in the region make runoff to be suitable for Aquifer Recharge implementation in the region. Transferring runoff to the aquifer aims at making use of the large aquifer storage space and limiting evaporation loss. The study also employed particle tracking and MT3DMS to assess the transport of contaminants associated with runoff within the aquifer. This was assessed at two wells thatserved as injection wells in the model, and four contaminants namely chloride, Electrical Conductivity, Total Dissolved Solids, and E-coli were studied. The outcomes of this assessment indicated that both Chloride, Electrical conductivity, Total Dissolved Solids, and E-coli concentrations decrease from 1000 mg/l to 0.01 mg/l, 532 mS/s to 0.1, 478.56 mg/l to 0.01 mg/l, and 9.58 to 0.01 respectively as timesteps increase, and it takes 20 timesteps (94672800 seconds) for them to disperse further into the aquifer. The dispersion of Chloride, EC, TDS, and E-coli within the aquifer covers a maximum distance of 12.1, 9.6, 8.7, and 6.7 km respectively.
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    Internet of Things (IoT) application for hydrological measurements: measuring the Urban Heat Island effect
    (2025) Alexander, Samuel; Okedi, John
    This study investigated the feasibility of employing Internet of Things (IoT) technology as an alternative data collection method for studying the Urban Heat Island Effect (UHIE). Urban Heat Islands (UHIs) are localised and typically built-up areas, that experience significantly higher temperatures than the surrounding undeveloped areas. This temperature difference is primarily due to increased heat absorption and reduced cooling from construction materials like concrete and asphalt, as well as the removal of shaded green spaces. An IoT Wireless Sensor Network (WSN) comprising 14 sensor nodes were implemented using readily available, ‘off-the-shelf' products in South Africa, resulting in a competitive build cost of R1523.14 per node. The sensor nodes were deployed at the University of Cape Town (UCT) campus in both shaded green spaces and unshaded paved areas to monitor temperature and humidity differences. Over the course of 116 days, from 7 September to 31 December 2023, the IoT WSN provided real-time temperature and humidity data, yielding 84 148 transmissions with only a 0.1% transmission error rate. The data was stored and managed using the MongoDB database. The investigation found that urban shaded green spaces were consistently cooler than unshaded paved areas; peak temperatures on the warmest days of each month reduced by 4°C on 28 September and by 2°C on 19 October, 15 November, and 27 December. This study demonstrates that IoT technology is highly capable of monitoring UHIE whilst remaining economically feasible to deploy.
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    Open Access
    Risk Assessment tool for water reuse - University of Cape Town as a case study
    (2025) Tendo, Zindzi Sekyana Nabassagi; Okedi, John; Carden, Kirsty
    The 2015 – 2018 Cape Town drought led to investigations into alternative water sources such as greywater, rainwater, and stormwater. To implement these alternative sources effectively, the associated risks must be understood. The World Health Organisation's Quantitative Microbial Risk Assessment (QMRA) already exists but can be complex, and relevant pathogen data is limited in South Africa. The existing risk assessment framework for irrigation with greywater in South Africa also lacks a detailed risk analysis. Lastly, no tools assess combined risk for the integrated use of greywater, rainwater and stormwater for a context such as Cape Town. This study developed a simplified qualitative risk assessment tool to provide risk management strategies for greywater, rainwater and stormwater. The tool's questions and answers were informed by the QMRA framework and the Irrigation with Greywater framework. Tool users were categorized based on whether they knew what alternative water source they intended to use. The user's answers were then assigned weights reflecting both the probability of exposure and the severity of consequence, with certain answers prioritized due to their substantial impact on elevating risk. From the weightings, a risk profile with an accompanying score ranging from ‘Low-1' to ‘Very High-25' – where higher numbers represent higher risks within a risk category, was assigned based on a risk matrix informed by risk matrices from Nel et al. (2022), World Health Organization (2016), and NRMCC et al. (2006). The tool was tested on four buildings in the study area, i.e., the University of Cape Town: The Hasso Plattner School of Design Thinking Afrika (d-school), the New Lecture Theatre (NLT), the Tugwell residence and the Liesbeeck Gardens residence. The results indicated a high-15 (the high scale was from high-4 to high-15) risk when using untreated rainwater for toilet flushing at the d-school and a very high-25 (highest risk on the very high scale and overall) risk for any alternative water use at the NLT. The Tugwell student residence also exhibited a very high-25 risk for toilet flushing with greywater and stormwater. In contrast, Liesbeeck Gardens showed a moderate-10 (highest risk on the moderate scale of moderate-6 to moderate-10) risk of using rainwater in washing machines across the whole residence, and a very high-25 risk for greywater and stormwater and a moderate-10 risk for rainwater, used in toilet flushing within a flat at the residence. Common risk factors included the presence of tall trees and roof inclination angle for roof-harvested rainwater, proximity to uphill parking lots and motorways for stormwater, and the presence of individuals with low immunity in buildings utilizing greywater. Additionally, the season in which the alternative water source was used significantly influenced the risk profile. The study concluded that the tool can be used to support users in identifying areas of high risk when using these alternative water sources, and therefore aid decision-makers in prioritising resource allocation. The tool can also be used as an educational tool in living labs.
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    Open Access
    Sustainable management of water resources through Real Time Control with University of Cape Town dam as a case study
    (2023) Mogano, Malesela Michael; Okedi, John
    There is a growing interest in South Africa to supplement water demands by harvesting stormwater as concerns over the security of the country's water supply increase. Studies have demonstrated the potential for stormwater harvesting (SWH) to simultaneously provide water to meet non-potable water demand and mitigate flooding by minimising stormwater flows to downstream locations of urban catchments. To determine pathways to enhance these benefits, application of Real-Time Control (RTC) system to operate a dam outlet could potentially be used to store stormwater. To investigate the economic viability of harvesting stormwater through RTC from an existing dam, a case study was performed on a representative urban catchment – the UCT watershed, located in Cape Town, South Africa. RTC procedures were applied to the UCT dam operations to initiate pre-storm releases in real time based on rainfall forecast. Four different stormwater harvesting configurations that modelled non-potable water demands were developed. A catchment stormwater model and a Life Cycle Cost Analysis (LCCA) were used to model the four configurations. The study identified benefit in application of RTC linked to increase in harvested stormwater and reduction of water loss through overflow. Continuous simulation was employed at the UCT dam to determine the prospects of enhancing SWH to deliver non-potable water for irrigation of sports fields. The study compared performance of Static control approaches to SWH with application of RTC. The dynamic management of the UCT dam with RTC approaches increased yield and volumetric reliability whilst maintaining the required level of service of a stormwater harvesting system. Static control approaches result in water savings of approximately 9% in comparison to RTC. In addition, Static configurations harvested stormwater at a relatively low unit cost in comparison to RTC configurations. Hence, RTC approaches increase yield and volumetric reliability with relatively low-cost implications. In addition, RTC approaches has the potential meet about 6.4% to 10.9% of the residences potable water demand respectively whilst satisfying irrigation demands if stormwater could be fully treated. It was found that SWH with RTC required special design as it provides an active operation which, across varying climatic conditions optimizes the performance of the system. It was concluded that the SWH system with RTC technology exhibits great potential; the ability of an RTC system to provide centralised control and failure detection, which can be readily adapted to variation of climate and local conditions over both the short and long term provides a system that is more stable and reliable.
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    Open Access
    The prospects for stormwater harvesting in Cape Town, South Africa using the Zeekoe Catchment as a case study
    (2019) Okedi, John; Armitage, Neil P.
    The City of Cape Town in South Africa faced the possibility of taps running dry in 2018 because of a prolonged drought that commenced in 2015. With such droughts expected to reoccur frequently in future, this study investigated the prospect for stormwater harvesting (SWH) and use as an alternative water resource - selecting the 89 km2 Zeekoe Catchment situated on the southern part of Cape Town as a case study. The study assessed potential to supply partially treated stormwater to non-potable water needs or fully treated water to potable water demands. The study determined that there was temporal mismatch between the identified non-potable water demands and the stormwater that is seasonally available from winter rainfall. Due to the mismatch, balancing storage was required in the range of 20 - 30% of the mean annual stormwater volume estimated at 18 Mm3 . The 61 stormwater ponds in the Zeekoe Catchment were assessed to determine the capacity to provide the required storage. Since the stormwater ponds were mainly for flood-control, the opportunity for multi-use was investigated using RealTime-Control (RTC). RTC would allow for extended storage in the stormwater ponds for water supply and pre-emptive draining before storm events for flood control. The application of RTC on the stormwater ponds provided a capacity of 1 Mm3 (about 5.5% of the mean annual stormwater volume). The capacity was inadequate as the stormwater supplied from the storage would only meet 44 - 60% of the demands, with a spill (water lost as overflow) of 35 - 51%. To provide additional capacity, an assessment was undertaken in stepwise increments of 1 Mm3 to determine the optimal storage required from the shallow lakes (vleis) in the study area i.e. Zeekoevlei and Rondevlei. It was determined that after 4 Mm3 , there was limited increase in demand met and reduction in spill. Stormwater could also be abstracted, fully treated to potable water standards and injected into the local potable water distribution system. Alternatively, the abstracted water from the vleis could be pre-treated at a new proposed WTP in the study area and then pumped to one of the existing water treatment plants. The assessment to determine influence of storage on quantity of stormwater supplied as discussed for non-potable water was undertaken for potable water. It was determined that for potable water that is required all the year round, the supply was not sensitive to changes in storage volume. Since the influence of balancing storage was limited, optimisation of SWH system was based on plant capacity to maximise supply and minimise spillage. The available surface water storage i.e. stormwater ponds and vleis are currently used for other purposes such as flood control, recreation - including sailing - and to maintain ecology that require a permanent pool of water. The sensitivity of these activities was the driver for the study to consider alternative storage options such as groundwater aquifers. The study area had an unconfined aquifer with sandy soils that could support Managed Aquifer Recharge. The physical characteristics in the Zeekoe Catchment, i.e. largely flat terrain, pervious sandy soils and a relatively deep unconfined aquifer (20 - 50 m) would support typical borehole abstraction rates of 3.5 - 8.1 L/s per borehole from some 140 boreholes. This would provide a mean annual groundwater yield of 29 - 33 Mm3 (about 15% of Cape Town water demand in 2018). Overall, stormwater harvesting from groundwater storage was more advantageous than surface water as it provided larger water quantities and improved water quality. This study has contributed towards identification of an alternative water resource by considering the possibilities of SWH from surface and groundwater storage for supply to potable and non-potable demands at a catchment scale. The study determined that the optimal use of stormwater requires a shift in the use to potable water uses. Treatment to potable standards would also eliminate the potential public health risks from cross connections. It was also determined that the treatment to potable water standards is more cost-effective for SWH at a catchment scale (centralised system) than using the water for non-potable purposes as it eliminates the need for the costly dual reticulation. Accordingly, this study recommends SWH and reuse to be for potable water needs where the abstraction is from a single location and the distribution through the existing potable water system. The factors that were determined to be important for the efficacious application of SWH included inter alia the availability of storage (surface or groundwater), the catchment characteristics (terrain, soil types, level of development, population density), and seasonal availability of the stormwater resource (winter or all year rainfall). The study also assessed the impact of land use and climate changes on the quantity of the stormwater. In terms of wider application, the study has provided insight into opportunities for stormwater use with partial or full treatment for non-potable or potable water demands respectively. The study has also provided a useful understanding of the potential scale and magnitude of the available non-potable water needs. It was also noted that reliability of the SWH was a function of storage capacity especially for supply to non-potable demands and the local rainfall regime. These issues need to be assessed to determine the prospects and viability of SWH and reuse in the given area.
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    Transition to Novel Internet of Things Technology for Management of Groundwater Resources? Case of Cape Town, South Africa
    (2023) Arinaitwe, Miriam; Okedi, John
    Groundwater is significant for humans and nature due to its varying extent, prevalence and natural quality. Effective management and protection of groundwater resources is essential and requires detailed knowledge and quantitative/qualitative characterisation of the aquifer. It is vital to confirm the groundwater parameters during hydrogeological investigations as groundwater depends on factors like geology, climate, lithology, slope and many others. However, there needs to be sufficient data to conduct these investigations and develop models for decision support in managing groundwater resources. In this study, Internet of Things (IoT) technology was applied to collect groundwater level data from a borehole to solve the problem of insufficient hydrogeological data. IoT provided consistent data collection, appropriate capacity to deal with immense records and ensured speedy and accurate information analysis. The interval resolution of collecting the IoT-based data was changed from 15 minutes to 24 hours to increase the battery life of the water level sensor to have continuous communication between the gateway and sensor. A combination of MODFLOW Software, Geographical Information System (GIS) and Analytic Hierarchy Process (AHP) techniques was then adopted to assess the groundwater processes related to Managed Aquifer Recharge (MAR) in the study area. This research investigates the workability of using IoT technology to collect groundwater level data to assess the processes that occur in the Cape Flats Aquifer around a stormwater pond catchment area. The study area in the Mitchells Plain West Catchment comprises the catchment areas of two stormwater ponds and covers an area of 0.315 km2 . The AHP and GIS techniques were used to produce a groundwater recharge potential zone map of the study area to develop a numerical groundwater model in MODFLOW. Recharge rates, hydraulic conductivities and other groundwater paraments were assigned and varied with estimated boundary conditions during model development. It was carried out based on acceptable ranges for the study area from previous studies and water resources management agencies of South Africa. The model was run and calibrated using the IoT-based groundwater levels collected at the monitoring borehole at Green dolphins pond. The study determined that the stormwater ponds were located in areas with very high groundwater recharge potential that covered 10.6% of the total study area. They are located in a flat area with quaternary deposits, low drainage density and high lineament density, which increase water infiltration rate, thus promoting groundwater recharge. The impact of the groundwater recharge was evaluated using the water budget, as there was an increase of 0.86 m between the observed head value and simulated head value at the monitoring borehole. It was observed that varying the recharge rates and hydraulic conductivity influences the fluctuations in the water table and the outflows presented in the water budget tables. Therefore, the increase in the water table at the borehole shows the importance of stormwater ponds as a water source for MAR.