Modeling surface water potential and allocating water demand in the Nashe River watershed using water evaluation and planning

Water scarcity is a pressing concern for development, livelihood, and ecosystems, as a result of global water demand continuing to rise with population growth. However, the available water resources are not projected to meet future demands (Gervas et al. 2019). Straatsma et al. (2020) conducted a study revealing that the global water gap, which represents the difference between water demand and supply, is expected to expand throughout the 21st century, adversely affecting agriculture, industry, and households. Furthermore, Abu-Zeid & Shiklomanov (2004) emphasized that the primary driver of rising water consumption is population and commercial growth. Without changing current behaviors and water utilization patterns, global water scarcity and deteriorating water quality will persist in many parts of the world.

Water resource management and allocation in Ethiopia suffer from inadequate planning and coordination, with significant gaps in research related to assessing surface water potential and addressing water demand allocation, as existing studies have primarily focused on technical aspects such as streamflow modeling, irrigation potential, and soil erosion while neglecting integrated approaches to water demand management (Dawit et al. 2020; Tufa & Sime 2021). Although the Nashe sub-basin in Ethiopia holds significant agricultural and hydroelectric power potential, there is a lack of comprehensive development and consideration for the future availability and allocation of these resources (Adeba et al. 2015; Adgolign et al. 2016).

Water potential analyses are essential for assessing available water resources and maintaining the water balance at the river basin scale (Goyal & Surampalli 2018; Hirbo et al. 2022). These analyses take into account various factors such as land use, soil characteristics, climate patterns, and hydrological processes to identify areas susceptible to drought or water scarcity, facilitating the development of effective water management strategies (Bodner et al. 2015; Noori & Singh 2023). Water potential analyses also enable the identification of inefficiencies in water resources management systems, providing decision-makers with valuable insights to optimize water utilization and maximize the use of available water resources (Hajkowicz & Collins 2007; Candido et al. 2022). Overall, the application of water potential analyses at the river basin scale is crucial for informed decision-making, sustainable water management practices, and the long-term preservation of water resources.

The optimal allocation of water resources should be guided by the reasonable demands of regional water users and allocated through scientifically informed water supply systems (Grouillet et al. 2015; Nel et al. 2022). The allocation of water resources has garnered significant attention in research in recent decades (Janjua & Hassan 2020). In developed countries, water demand allocation has evolved from considering a single water source to multiple sources, from single to multiple objectives, from temporal to spatial considerations, from demand-focused to supply-oriented models, and from solely assessing water quantity to also incorporating water quality and their interrelationship (Fawen et al. 2012; Li et al. 2022; Luo et al. 2022). This demonstrates the growing complexity and comprehensive nature of water resource allocation, taking into account various factors to ensure efficient and sustainable management of water resources. Jayantari et al. (2019) highlighted the importance of adopting mathematical and software-based models like the water evaluation and planning (WEAP) system to enable integrated and sustainable water resources management planning.

Water resources assessment involves quantifying both surface and groundwater resources and assessing their suitability for different uses in terms of quality. Planning and allocating water supplies to various users within a watershed is a crucial task in water resources management. McCartney & Menker Girma (2012) emphasized the challenge of limited knowledge about water supplies and the implications of different investment options in the Abbay Basin, which includes the study area. By utilizing software-based management models and bridging knowledge gaps, an integrated approach can be adopted to effectively plan and allocate water supplies, promoting sustainable and comprehensive water resources management.

The greater understanding and implementation of integrated water resources management (IWRM) have led to significant transformations in water resources management (Dinsa & Nurhusein 2023). In support of IWRM, water resources modeling projects have adopted an integrated approach (Banda et al. 2022; Nagata et al. 2022). An important aspect of this approach involves considering both water supply and demand in demand forecasts and hydrologic modeling to some extent (Wang et al. 2019). This integrated approach recognizes the interdependencies between water availability and the varying needs and demands for water resources. By incorporating both supply and demand factors into water resource models, IWRM initiatives can better assess and plan for sustainable water management practices (Islam et al. 2023; Zegait et al. 2023).

The study utilized a physically based, conceptual, computationally efficient, and semi-distributed WEAP model to assess the hydrology of the Nashe River watershed. This model was employed to estimate the present quantity of surface water and predict future water demands, taking into account the potential impacts of newly planned water resource projects through the creation of demand scenarios. The findings of this study hold great importance for water resources management by enabling planners and decision-makers to optimize water allocation, ensure equitable distribution, and secure the long-term sustainability of surface water resources. This information is crucial for the development of water management strategies that effectively address competing demands while maximizing the utilization of available water resources in the Nashe River watershed, Ethiopia.

The study uses 2019 as the base year and 2050 as the projection year. The choice of the base year 2019 and the projection year 2050 in this study focused on water management strategies in the Nashe River watershed, Ethiopia, holds significant importance. By establishing 2019 as the starting point, the study provides a snapshot of the existing conditions and dynamics within the watershed, offering a solid foundation for analysis. This baseline data serves as a crucial reference for evaluating changes and trends over time. Looking ahead to 2050 allows the study to project how various factors influencing water availability and demand may evolve, enabling the development of forward-looking, sustainable water management strategies. Given the likely increasing pressures on water resources due to factors such as population growth and climate change, understanding these trends and planning accordingly is essential for ensuring that the available water is utilized efficiently and equitably to meet the needs of various stakeholders in the Nashe River watershed.

The Nashe River watershed includes grassland, wetland, cultivated land, forest, and areas with shrubs, trees, and crops (Figure 2). The upper watershed is mainly dense woodland with some cultivation, while the lower part is shrub-grassland. Administratively, it spans three districts in the Horro-Guduru Wallaga Zone: Horro, Abbay Chomen, and Jarte Jardaga. Agriculture, particularly mixed farming (crop-livestock), is the main economic activity.

To conduct water potential and demand allocation analyses using the WEAP model, various input data are required. These include hydrological data (such as streamflow), weather data (including rainfall, temperature, relative humidity, sunshine hours, and wind speed), digital elevation model data, land use data, water supply data (including population numbers, growth rates, and per-capita water consumption), livestock data (daily water consumption, livestock types, and numbers), irrigation data (including agricultural land area, monthly variation in demands, and annual water requirements per hectare of crops), hydropower data (storage capacity, initial storage, volume elevation curve, net evaporation from the reservoir, reservoir zoning, maximum turbine flow, tailwater elevation, plant factor, and generating efficiency), and instream flow data. These inputs are crucial to accurately assess water potential and allocate water demands using the WEAP model.

Streamflow data is an important part of water resource modeling because it helps to explain how it functions under various hydrologic conditions. Daily river discharge data were obtained from the Ethiopian Ministry of Irrigation, Addis Ababa.

The meteorological data, including rainfall and temperature recorded over a period of 31 years (1989–2019), were obtained from the National Meteorological Service Agency (NMSA) in Addis Ababa, Ethiopia. The weather data were used for the determination of crop water requirements and used as an input to the WEAP model.

A 12.5 m by 12.5 m digital elevation model (DEM) was downloaded from https://vertex.daac.asf.alaska.edu/. The DEM data was used as a basic input for the watershed delineation and also uploaded into the schematic view of the WEAP model to orient and construct the system and refine area boundaries.

The WEAP model, developed by the Stockholm Environmental Institute (SEI), is a user-friendly software designed for integrated water resource planning (Sieber & Purkey 2007). This model encompasses several variables, including streamflow, base flow, groundwater potential, sectoral water demand, water allocation priorities, reservoir operations, hydroelectric power generation, financial planning, water quality, and environmental standards. The model allows for the simulation of various hydrological processes (such as infiltration, runoff, and evapotranspiration) and water demand sectors (such as domestic, environmental flow, irrigation, livestock, and hydropower) using five different approaches. In this study, the simplified coefficient method (known as the rainfall-runoff method) of the WEAP21 model was employed to assess the surface water resources of the watershed by delineating the catchment area (Abdi & Ayenew 2021). By using the WEAP21 model, water managers could analyze and plan for different water resource scenarios and explore the potential impacts of water management strategies on the overall system.

To assess the surface water resources of the watershed, a simplified coefficient method of the WEAP21 model was used. To perform the rainfall-runoff simulation method, land use data, crop coefficients (K_c), effective precipitation, and ET_o were used. The design document of the irrigation projects on the Abbay River basin (MWR 1999), obtained from the Minister of Irrigation is used as a reference.

According to Yates et al. (2005), demand analysis is central to integrated water planning analysis with WEAP21 since all supply and resource calculations are driven by the allocation routine. The allocation routine determines the final delivery to each demand node, based on the priorities specified by the user.

Demand analysis in the WEAP21 model employs a disaggregated, end-use-based approach to model water consumption requirements within the watershed. By incorporating economic, demographic, and water use data, alternative scenarios can be constructed to analyze the evolution of total and disaggregated water consumption in various sectors of the economy (SEI 2012). Regular evaluation and monitoring of water demand are crucial to ensure that water resources are allocated according to current and future needs.

The Abbay River basin integrated development master plan, established in 1999 with a 50-year time horizon, served as the foundation for constructing the scenarios in this study (MWR 1999). For this research, scenario analysis was conducted considering two main factors: irrigation expansion and natural climate variation. The scenarios for irrigation expansion were developed utilizing the master plan of the study area basin, namely the Abbay River basin, along with the irrigation expansion plan of the Horro-Guduru Wallaga Irrigation Development Authority. These scenarios provided a framework for examining the potential impacts of expanding irrigation activities and accounting for variations in natural climatic conditions within the study area. By considering these scenarios, the study aimed to offer insights into the implications and outcomes of different development paths related to irrigation expansion and climate variations. The irrigation expansion scenarios investigated the future trends of water demands in the watershed by considering the annual increase in irrigated land proposed in the master plan of the Abbay River basin, which was set at a constant rate of 3.9%. This scenario aimed to assess the potential impacts on water demands as a result of the expansion of irrigated areas over time.

In the second scenario, variations in climate data, specifically streamflow and rainfall, were taken into account. This involved defining different climate regimes (such as very dry, dry, normal, wet, and very wet) relative to a normal year, which was assigned a value of 1. Dry years were assigned relative values less than 1, while very wet years were assigned relative values greater than 1. This approach allowed for an evaluation of how water demands might be affected by natural climate variations.

For this study, the current year and the last year of the scenarios were selected based on available data. Specifically, the year 2019 was chosen as the reference year for the current conditions, while the year 2050 was selected as the current accounts year and the last year of scenarios, respectively.

The performance of the WEAP21 model was evaluated based on the monthly average streamflow recorded at the gauging stations of the Nashe watershed for a period from 1989 to 2019 years. The calibration results using the model statistical performance analysis techniques result in the R² value of 0.9558, the NSE value of 0.952, and the PBIAS value of 0.96 being observed. These values show that the simulated and observed flows were comparable, and the monthly average streamflow of the gauge and runoff from the watershed values have a good match.

In some months (Jan, Feb, Jul, Nov, and Dec), the simulated values are very close to the observed values, with only a small difference. In other months (Mar, Apr, May, Jun, Aug, Sep, and Oct), the simulated values are a bit further from the observed values, but they still follow a similar trend. The largest discrepancies between observed and simulated values are in May and June, where the observed values are significantly higher than the simulated values. This implies that the simulation may have overlooked factors or variables.

These findings demonstrate the importance of incorporating seasonal variations in runoff patterns in water resources management decision-making. For example, water managers may need to plan for increased storage capacities during the summer season to retain excess surface water, which can be used during the dry season to support water needs. Additionally, a more in-depth understanding of seasonal runoff patterns can help identify potential water shortages, facilitate the implementation of water demand management programs, and ensure the long-term sustainability of water resources in the watershed.

The current situation of water demand for the selected demand sites was allocated before any scenario was developed to know the situation of water demands in the watershed. Accordingly, monthly water demands in the base year for domestic and irrigation demand sites such as Nashe Large-scale Irrigation (NLSIP), Abuna Small-scale Irrigation (ASSIP), Gaber Small-scale Irrigation (GSSIP), and Cunquli Small-scale Irrigation (CSSIP) water demand, as well as livestock water demand, were considered, as shown in Table 1. The monthly water demand for domestic and irrigation for the base year (2019) was allocated initially, and scenarios were developed based on it.

The average monthly water demand for livestock and irrigation projects was 0.633 MCM (Table 1). Livestock water demand tends to remain relatively consistent throughout the year. The NLSIP shows a decrease in demand from May to July, followed by an increase. ASSIP, CSSIP, and GSSIP exhibit varying demand, with some months having higher demand than others. Livestock water demand remains fairly constant, indicating that it is not significantly influenced by seasonal changes. The large-scale irrigation demands show some seasonal variation, with lower demand from June to September and higher demand during the remaining months.

From the total surface runoff generated in the watershed, 1,197.94 MCM, irrigation and livestock demand together account for about 0.05%. The irrigation water demand is higher in months such as January, February, March, April, and December due to the lower amount of rainfall during these months, which typically represent the dry season. During this period, crops require additional water to meet their evapotranspiration needs. In contrast, during the monsoon season, which falls between June and September, irrigation water demand is lower as the increased rainfall is sufficient to meet crop needs (Table 1). Therefore, irrigation water is typically not required during this period. Understanding the seasonal variations in irrigation water demand is crucial for effective water resources management and planning, as it allows decision-makers to allocate water resources effectively, optimize crop production, and ensure the sustainability of water resources.

The average monthly sum of hydropower annual demand simulated in the watershed was 270 MCM. This shows that hydropower shares around 23% of the total available water demand.

The total amount of water required for all demand sites within the watershed, including domestic, irrigation, hydropower, livestock, and instream flow requirements, was 328.35 MCM (Table 2). Of the total available water, 82.2% was used for hydropower, 16.8% for instream flow, and the remaining 1% was used for domestic, irrigation, and livestock needs. The results indicate that the total annual water supply in the watershed was much greater than the demand. However, during dry months, there is a shortage of water as demand exceeds the available runoff.

Runoff exhibits notable variability, peaking in the latter months from July to November, suggesting a potential seasonal influence from precipitation or snowmelt. Domestic water demand remains relatively constant over the months, indicating a steady need for household water consumption. Livestock water demand sees its highest levels in January, gradually declining as the year progresses. In contrast, irrigation demand reaches its zenith in October, likely due to agricultural activities during that period. Hydropower generation maintains a consistent output throughout the year. Additionally, instream flow requirements display some fluctuations, with elevated values observed in the later months, underscoring the importance of maintaining ecological health in the river ecosystem.

This study area is suitable for irrigation purposes; however, irrigation in the watershed is not fully developed. According to the master plan of the river basin, the Horro-Guduru Wallaga Zone, the Oromiya Irrigation Development Authority, and the Ministry of Irrigation, there are many potential areas for future irrigation expansion. Therefore, this scenario addresses the question of what the water demands would be if irrigation were expanded. Among the parameters that affect water demand, such as annual activity level, annual water use rate, monthly variation, and consumption, changes in the annual activity level, and water use rate were considered in these scenarios.

Table 3 presents the water demand for the irrigation expansion scenario from 2020 to 2050. The scenarios were based on the projected water needs by 2050, assuming that the area of irrigation development increased by 3.9% of the current area (according to the basin master plan and the cultivated land area). The results of the scenario show that an irrigation expansion rate of 3.9% per year for each demand site leads to an increase in annual water demand over time. Specifically, irrigation water demand increased from 0.300 MCM in the base year (2019) to 984 MCM at the end of the scenario period (2050), indicating an increase of 683 MCM in water demand for irrigation. Despite the increase in irrigation water demand, there will be no annual unmet water demand. The water demand for the NLSIP at the end of 2050 is projected to be 962 MCM, which constitutes a significant portion of the overall demand compared with smaller-scale irrigation projects (Table 3).

Surface water resources of the Nashe River watershed were modeled to estimate the available supply with the demand in a sustainable manner for social, economic, and environmental benefits. Model performance efficiency parameters, such as NSE, R², and PBIAS values, were in the acceptable interval, showing that the WEAP model is successfully used to model the surface water resources of the watershed for optimum water allocation. The Nashe River watershed has an annual surface runoff of 1,197 MCM, which is much greater than the water needed for the base year, 328 MCM, for all considered demand sites. The study results show that the surface water potential of the Nashe River watershed can fulfill water demands in the base year among multiple water users and no unmet demands were encountered in the base year 2019. The result of the future irrigation expansion scenario indicates that there is an increment in water demands and no unmet water demands annually. It also shows an increment of irrigation water demands from 300 MCM from current accounts to 984.7 MCM for the last year of the scenario. The water year method scenario shows the fluctuation of available supply as climate sequence varied from year to year; also, the scenario shows a decrement of streamflow from 1,197 MCM for the normal water year to 267 MCM for the very dry water year, an increment of the available supply from 267 to 577 MCM for a dry year, and an increment from 577 to 1,197 MCM for the normal water year again, lastly increasing supply from 1,197 MCM of the normal year to 2,118 and 2,588 MCM for the wet and a very wet water year, respectively. The Nashe River watershed has sufficient surface water resources to meet current and future water needs, demonstrating its potential for sustainable water resources management, and offering social, economic, and environmental benefits.

This study received no outside funding.

This article does not contain any studies with human participants or animals performed by any of the authors.

All relevant data are included in the paper.

The authors declare there is no conflict.