Development of evaluation framework for the selection of run-of-river hydropower potential sites to be included in the Zambian Hydropower Atlas

Hydropower is a vital source of renewable energy, which provides electricity around the world. Other renewable energy resources include biomass, biogas, solar radiation, and wind power (Department of Energy 2017). Hydropower became a source of electricity in the late 19th century, a few decades after the British-American Engineer James Francis developed the first modern water turbine (Nunez 2019). Before that, more than 2,000 years ago, hydropower was used as a source of mechanical energy by the Greeks for grinding wheat into flour using water wheels. In the 18th century, hydropower was used extensively for the milling of grain and pumping of irrigation water; however, recent studies proved the advantage of using the same equipment to harness energy in water, thereby providing a renewable and decentralized form of energy (Angelakis et al. 2022).

Zambia's small energy requirements have always largely been supplied by thermal power plants, but with the development of the mining industry, the first 18.4 kW hydropower plant was developed in Mulungushi to supply the zinc–lead mining complex in the Kabwe district of the central province of Zambia (Mihalyi 1977). Currently, 80.5% of the country's electricity requirements are supplied by hydropower plants, which are managed by ZESCO (ERB 2020). Hydropower is dependent on the topographic, climatic, and hydrometerological conditions of an area. Thus, the availability of this data is important in hydropower estimation studies. A study conducted by Mohammad et al. (2020) emphasized that accurate hydrometeorological records and observations with different timelines are crucial to assess climate evolution and weather forecast. The study further reviewed that although the science of hydrometeorology has significantly improved recently, there is still a lack of adequate knowledge to accurately forecast extreme hydrometeorological events (Mohammad et al. 2020). Thus, innovations and processes such as the development of evaluation frameworks for easy and accurate estimation of hydrologic data at river systems are important to enable the determination of the available hydropower potential.

Climate change continues to be a major threat to African countries. This is evident in the study conducted by Almazroui et al. (2020), which projected a continuous increase in annual temperature over all of Africa in the 21st century. Together with the world's focus on renewable energy goals as set out in the Paris Agreement, which encourages the accelerated stationing of renewable energy measures to meet the 90% decarbonization goal by 2050, emphasis has been placed on harvesting energy from renewable sources. With the signing of the Paris Agreement on climate change during the United Nations treaty signing in September 2016, the government of Zambia committed itself to focus on scaling up the use of renewable energy sources and energy efficiency in Zambia (GRZ 2016). Previous studies conducted by JICA (2009) in Zambia indicated that the country has the potential of generating about 6,000 MW of hydropower from the river systems; however, only 2,398.5 MW has been tapped largely from a few large hydroelectric power stations and only 0.7% from small-scale hydropower plants (ERB 2020). Small-scale hydropower plants can boost the country's electrification by providing electricity to isolated rural households, streets, clinics, schools or industries, and buildings. Existing facilities like weirs, barrages, canals, waterfalls, dams, or pipelines can be optimized by installing small turbines for electricity generation (Kumar et al. 2011). These small installations can generate power ranging from pico (<20 kW), micro (up to 100 kW), to mini (up to 1MW), to possibly supply a school or clinic, a cultural village centre, or even a whole community (Klunne 2009). Therefore, various potential sources of small-scale hydropower potential must be assessed. The identified potential sources and sites could be compiled and added to a hydropower atlas. A hydropower atlas is a tool that is used to showcase a region or country's hydropower potential to the local stakeholders including the private sector, financial sector, and government entities. Furthermore, the hydropower atlas makes us aware of the opportunities that small-scale hydropower technologies bring, and the efforts required to get this technology to be successfully implemented.

Hydropower atlases have been developed and implemented for some African countries. These include Madagascar, Tanzania, Rwanda and the 14 Economic Community of West African States (ECOWAS) (Pöyry & ECREEE 2017b; World Bank 2017b, 2018; Rwanda Water Portal 2019). The hydropower atlases for these countries have been developed mainly for run-of-river types of hydropower. The evaluation frameworks followed in the development of these existing hydropower atlases were developed specifically for the respective countries and regions, in which the topological, terrain, climatic, and hydrologic parameters were analysed. Since these parameters vary from place to place, the frameworks could not be applied directly to the Zambian situation. However, these existing atlases provide good examples of successfully developed hydropower atlases and therefore provide applicable information regarding hydropower potential and the data selection process. This paper presents the evaluation framework that is specific to the Zambian topology, climatic, terrain, and hydrologic conditions and the currently available information. The main objective of this study was to develop the data selection criteria and evaluation frameworks for the selection of run-of-river sites with hydropower potential to be included in the Zambian Hydropower Atlas (ZHA). This was similarly done by other researchers as a first step in the development of the existing hydropower atlases.

The development process of the data selection criteria and evaluation frameworks included conducting a detailed literature review on existing hydropower atlases, existing data selection criteria, and the evaluation of hydropower potential. The methodology followed in the studies conducted by Bekker et al. (2020, 2021) and van Dijk et al. (2021) was reviewed and modified to fit the Zambian context. The methodology also included the use of an online Google forms questionnaire as a tool for the further development of the Zambian Hydropower Atlas. Institutions in charge of water infrastructures such as Lusaka Water and Sewerage Company and the Mulonga Water and Sewerage Company were visited to obtain data and reports, which were used in the development of the data selection criteria. Through these steps, the data and formulas required to evaluate hydropower potential were identified and evaluated to develop the evaluation process in the Zambian context.

The law of conservation of energy states that energy can never be created nor destroyed, but it can change from one form to another. In generating electricity, no new energy is created, but energy is converted from one form to another. Fundamentally, water moves by gravity from a high elevation point to a lower elevation point (Subhro et al. 2015). The available energy of this flowing water is given by the product of its weight and the height so-called effective head through which the water drops. Therefore, the hydropower potential of a water resource is the function of the head and the water discharge and is given by Equation (1). This equation was the basis for data evaluation and criteria development for the run-of-river hydropower.

Table 2 outlines the evaluation criteria used in the data selection process for the calculations of the hydropower potential of the sites included in the existing atlases for other countries. The table also includes data criteria from other studies on the run-of-river hydropower potential evaluation. These existing criteria were used in the analysis of datasets and the development of the selection criteria in the Zambian context.

The hydropower potential at a run-of-river hydropower potential site can be computed using Equation (1). Therefore, the evaluation of the potential depends on the data availability associated with the parameters in the equation. The selection criteria for datasets associated with each variable of the parameters in Equation (1) were developed as discussed below. It should be noted that this largely depended on the data availability associated with existing run-of-river hydropower setups, terrain, and hydrological data in Zambia.

According to the existing criteria presented in Table 2, the effective head at a run-of-river hydropower potential site depends on the riverbed slope, available gross head, and hydraulic head loss. These datasets related to the head and their selection criteria process are presented below.

• (a)

  River slope criterion

The existing slope selection criterion from the hydropower potential studies conducted by Vincenzo et al. (2019) and Kusre et al. (2010) entails selecting hydropower potential sites with a minimum slope of 5 and 2%, respectively. Applying Vincenzo et al. (2019)'s slope criterion to the six-existing run-of-river hydropower plants in Zambia with slopes shown in Table 3 would imply that only one site (Victoria falls) would be considered. The criterion discards the five sites with significant hydropower potential ranging from 0.75 to 14 MW. Similarly, applying Kusre et al. (2010)'s criterion discards the Zengamina hydropower plant that has a significant capacity of 750 kW with a slope of 1.2%. Therefore, there is no clear relationship between the river's slope and hydropower potential in Zambia. As can be seen in Table 3, some sites with steeper slopes have lower hydropower potential than some sites with less steep slopes. For these reasons, the slope criterion was not considered in the selection of hydropower potential sites to be included in the Zambian Hydropower Atlas. This was seen as a way of reducing the probability of leaving out sites with significant potential. It should, however, be noted that steeper riverbed slopes still indicate the presence of a potential head and therefore should be considered when identifying the location of hydropower potential sites, especially when using GIS-based methods (Kusre et al. 2010).

• (b)

  Gross head criterion

The power output of a hydropower potential site depends on the available head and discharge. According to CETC (2004), there is a minimum head below which there may be no economic advantage for undertaking the hydropower project. This is quite difficult to specify because the desired minimum power output can be obtained by a combination of high values of the head with low values of the discharge and vice versa. The micro-hydropower system buyer's guide by the CETC (2004) recommends a minimum head of 1 m. In the Zambian case, the existing information from the study for the development of the rural electrification master plan in Zambia by JICA (2009) entails considering run-of-river hydropower potential sites with a minimum gross head of 3 m. This criterion was adopted in this study. It is recommended that the economic advantage of run-of-river hydropower potential sites having a gross head of below 3 m should be assessed in future research in Zambia.

• (c)

  Effective head criterion

where

• = the effective head (m), and

• = the available gross head (m).

The following two scenarios were considered in the evaluation of discharge data in this study based on the available datasets:

• (a)

  Gauged sites

The Water Resources Management Authority (WARMA) and the Zambezi River Authority (ZRA) have river gauging stations across Zambia to monitor the daily river water levels. A site in this study is assumed to be gauged if it is located within the catchment area of a river gauging station. Various factors affect the degree of completeness of hydrological data. In this study, two factors were considered: (1) length of dataset time series and (ii) presence of missing data. The former involved the minimum number of years of gauging station data to accept in giving a reliable estimate of discharge. The study for the development of the rural electrification master plan in Zambia by JICA (2009) considered gauging stations with discharge data records of 10 years and above; otherwise, a different method was used to compute the discharge. This criterion was adopted in this study because some hydropower plants that were identified during the study have been implemented and some are in the implementation phase (Gauri et al. 2013).

Regarding the latter factor, some gauging stations may happen to contain missing data. The existing criterion from the development of the Tanzania Hydropower Atlas study entails using gauging stations with no more than 5% of missing data (World Bank 2015). This is because having more than 5% of missing data affects the statistical analysis of hydrological data. This is also supported by Osman et al. (2018). This criterion was adopted to apply to the selection of the river gauging station data to consider in Zambia. It is, however, recommended that methods of filling in missing data in the flow datasets having more than 5% missing data such as the one presented by Osman et al. (2018) should be explored in future studies.

• (b)

  Ungauged sites or sites with incomplete hydrological data

When the potential site is situated in an ungauged location, it was assumed that the discharge should be estimated using the annual average discharge dataset available in the HydroATLAS of Zambia (HAZ) provided by WWF-Zambia. The HAZ provides hydrological data of Zambian Rivers categorized based on the annual average discharge with the following categories.

• (a)

  0.1–1 m³/s

• (b)

  1–10 m³/s

• (c)

  10–100 m³/s

• (d)

  100–1,000 m³/s

• (e)

  1,000–2,000 m³/s

To evaluate the mentioned criteria, the discharge was compared with the known design discharge values at three existing run-of-river hydropower plants in Zambia as shown in Table 6. As can be seen in Table 6, the criteria estimated the available discharge for hydropower generation to be 33.33, 51.52, and 29.43% higher than the design discharge at the existing Shiwanga'ndu, Zengamina and Victoria fall hydropower plants, respectively. This could reasonably imply that the river discharge at these existing sites has not been fully utilized. Therefore, the criteria were adopted in this study.

The existing criterion from the development of the Madagascar Hydropower Atlas study entails selecting run-of-river hydropower potential sites with capacities of 50 kW and above (World Bank 2017a). In the Zambian case, the study for the development of the rural electrification master plan for Zambia by JICA (2009) considered hydropower potential sites with capacities of 30 kW and above to be suitable for non-electrified rural areas in Zambia. Therefore, this was adopted in this study to give a first-order selection of sites to be included in the Zambian Hydropower Atlas. It is recommended that future studies should investigate the economic benefits of including run-of-river sites with capacities of no more than 30 kW.

The application process and calculations, together with the data parameters, are shown in Table 8. The step number in the table corresponds to the number indicated in the evaluation framework. The Zengamina site was selected as a case study because of its uniqueness in terms of capacity (smallest run-of-river plant in Zambia), location (isolated), and purpose (rural electrification). Therefore, the success of the framework in evaluating hydropower potential at this site implies that the framework could also be used to evaluate similar sites in Zambia.

Table 8

Application of the run-of-river evaluation framework to the Zengamina site in Zambia

Name of site: Zengamina .		Coordinates: Latitude: 11°07′26′′ S Longitude: 24°11′32′′ E .
Step no. .	Framework step .	Explanation .	Result .
1	Consider the rivers within the six main river catchments of Zambia	The Zengamina site is located on the Upper Zambezi River.	The river is within the Zambezi River Catchment of Zambia.
2	Is the site within a protected area?	No. The site is not within the protected areas of Zambia shown in Figure 9.	Consider the site in the evaluation
Head evaluation
3	Determine the gross head:	From Google Earth Pro – the estimated gross head is 13 m (i.e., 1,222–1,209 m) over a horizontal distance of approximately 500 m.
	Is?	Yes. The gross head is 13 m.	Can proceed to compute the effective head
4	Record the value of
5	Calculate the effective head:	Effective head:
Discharge evaluation
6	Is the site gauged?	No discharge data records were available.	Consider the site to be ungauged
	Go to the HAZ	The HAZ contains the hydrological data for Zambian Rivers.	The upper Zambezi river discharge data are considered.
	Select the river discharge category (Figure 8)	The Upper Zambezi River in the Ikelengi district falls in the river category of 10–100 m3/s.	Discharge category = 10–100 m3/s
	Determine the median discharge of the river:
7	Compute the design flow:
Turbine selection and efficiency
8	Select turbine type	With and ⁠. A cross-flow turbine is selected.	From Table 1,
Calculation of hydropower potential
9	Calculate hydropower potential
Inclusion in the Zambian Hydropower Atlas
10	Is the hydropower potential ≥ 30 kW?	Yes. The hydropower potential is 1,629 kW.	Include the site in the Zambian Hydropower Atlas

Name of site: Zengamina .		Coordinates: Latitude: 11°07′26′′ S Longitude: 24°11′32′′ E .
Step no. .	Framework step .	Explanation .	Result .
1	Consider the rivers within the six main river catchments of Zambia	The Zengamina site is located on the Upper Zambezi River.	The river is within the Zambezi River Catchment of Zambia.
2	Is the site within a protected area?	No. The site is not within the protected areas of Zambia shown in Figure 9.	Consider the site in the evaluation
Head evaluation
3	Determine the gross head:	From Google Earth Pro – the estimated gross head is 13 m (i.e., 1,222–1,209 m) over a horizontal distance of approximately 500 m.
	Is?	Yes. The gross head is 13 m.	Can proceed to compute the effective head
4	Record the value of
5	Calculate the effective head:	Effective head:
Discharge evaluation
6	Is the site gauged?	No discharge data records were available.	Consider the site to be ungauged
	Go to the HAZ	The HAZ contains the hydrological data for Zambian Rivers.	The upper Zambezi river discharge data are considered.
	Select the river discharge category (Figure 8)	The Upper Zambezi River in the Ikelengi district falls in the river category of 10–100 m3/s.	Discharge category = 10–100 m3/s
	Determine the median discharge of the river:
7	Compute the design flow:
Turbine selection and efficiency
8	Select turbine type	With and ⁠. A cross-flow turbine is selected.	From Table 1,
Calculation of hydropower potential
9	Calculate hydropower potential
Inclusion in the Zambian Hydropower Atlas
10	Is the hydropower potential ≥ 30 kW?	Yes. The hydropower potential is 1,629 kW.	Include the site in the Zambian Hydropower Atlas

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The application of the framework at the Zengamina site estimated the hydropower potential at the site to be 1,629 kW and entails that the site could be included in the Zambian Hydropower Atlas since the potential is greater than 30 kW. However, from the feasibility studies conducted during the development of the project, the site is said to have a potential of 1,400 kW. The framework, therefore, overestimates the hydropower potential by 14%. This error can be attributed to the accuracy of both head and discharge data since hydropower potential depends on these two parameters as they are related in Equation (1). In terms of discharge data, the framework estimated the discharge to be 16.5 m³/s, while the available discharge at the existing site was estimated to be 14.9 m³/s from the feasibility studies (ERB 2015). This shows that the framework overestimates this discharge by 9.6%. This could be due to the use of the discharge data from the hydrological modelling results of the HydroATLAS developed by WWF-Zambia. This data could contain some uncertainties that were not evaluated in this study. Therefore, this framework should be considered as a first-order estimation in the evaluation of hydropower potential at Zambian river schemes. More accurate methods of obtaining hydrometerological data as outlined in the study conducted by Mohammad et al. (2020) should be followed to confirm the actual existing hydropower potential once the potential sites have been sifted.

Furthermore, in terms of the head at the Zengamina site, there is an underestimation of 5 m. With this developed framework, the gross head was found to be 13 m, while the estimated head from the feasibility study is 18 m (ERB 2015). This could be attributed to the use of Google Earth Pro in estimating the gross head that is not accurate due to its resolution; therefore, the user must be aware of its limitations. The result shows that the estimation of hydropower potential depends significantly on the accuracy of the head and discharge data parameters. With the availability of more of this data in future, the evaluation framework can be updated and improved. The developed evaluation framework, however, provides a reasonable first-order evaluation of run-of-river hydropower potential to be included in the Zambian Hydropower Atlas because, with its application, the ungauged sites can be easily evaluated without conducting detailed feasibility studies. The framework also includes the selection of the turbine, which makes it possible to take the turbine efficiency into account when evaluating the hydropower potential. The process followed above can similarly be applied to another river site in Zambia. If the estimated hydropower potential is found to be greater than or equal to 30 kW, the site will be included in the Zambian Hydropower Atlas. The potential sites identified using this method will need further investigation using field assessments during the detailed feasibility studies for design purposes.

Zambia has not developed a hydropower atlas and, therefore, its hydropower potential has not been quantified in detail. Thus, there is a need to develop a hydropower atlas for Zambia; however, there is little technical information and literature regarding the evaluation of hydropower potential and hydropower in the existing water infrastructure in Zambia. This study attempted to address this problem by developing the data selection criteria and evaluation frameworks to be followed in the development of the Zambia Hydropower Atlas. To achieve this objective, the data required to evaluate the run-of-river hydropower potential were identified and the various available sources of this data were also identified. These included online data sources such as USGS explorer, institutional reports, and institutions in charge of water resources in Zambia. These data were collected from the various identified sources to ensure easy accessibility of the data for the further development of the Zambian Hydropower Atlas.

The run-of-river hydropower evaluation framework and selection criteria, to which a specific river scheme should conform to, to be included in the Zambian Hydropower Atlas were developed. The developed framework was applied to the existing Zengamina run-of-river hydropower scheme located in the North-western Province of Zambia to demonstrate a step-by-step application process of the framework. The framework overestimated the hydropower potential at the Zengamina river scheme by 14% as compared to the estimation from the feasibility study report.

Finally, it can be concluded that depending on the available data on water infrastructure in Zambia, the data selection criteria and evaluation frameworks for hydropower potential can be developed. The developed criteria and evaluation framework can provide preliminary guidance in the evaluation of hydropower potential sites to be included in the Zambian Hydropower Atlas. The main challenge encountered in this study was the lack of data availability. This is somewhat commonly experienced by other researchers, especially where hydrological data are concerned. This challenge could also be attributed to the Covid-19 pandemic since most institutions were closed during country lockdowns.

Based on the findings of this study, the following recommendations have been made for further research: (i) future research should conduct detailed environmental, social, and economic studies associated with each site selected using the evaluation framework developed in this study. The framework could be updated to incorporate such information: (ii) the evaluation frameworks developed in this study should be considered only to give a first-order evaluation of hydropower potential. Future research should consider validating the frameworks and updating them. The frameworks can be updated with the availability of more data: (iii) future research should conduct cost–benefit assessments for each hydropower site identified using the developed evaluation framework.

This research was made possible by the financial and academic support of the German Academic Exchange Service (DAAD) and the University of Pretoria whose support is acknowledged with thankfulness. Many thanks to the following DAAD representatives: Mr Martin Schmitz (DAAD – Born), Dr Aceme (University of Pretoria), Ms Onalerona (University of Pretoria), Mrs Tshabalala (University of Pretoria) and Mr Liswaniso (University of Zambia). Finally, this research was complimentary to the University of Pretoria and the Water Research Commission of South Africa Project for developing a Hydropower Atlas for South Africa, and thus, their concept is acknowledged.

This research work was supported by the German Academic Exchange Service (DAAD) organization through the ‘DAAD In-Region Scholarship Programme, University of Pretoria 2020’ Cohort 3 project.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.