**1. Introduction**

Sub-Saharan Africa is a region that is very sensitive to, and is highly affected by recurrent droughts, flooding, and untimely weather conditions. Floods and droughts have affected water supplies and have set a challenge for water management. At the same time, water management practices in these developing regions are not adequate for dealing with the challenges of significant changes in climate [1–4]. Increasing pressure on land and water resources due to population growth and human activities have also resulted in the degradation of vulnerable ecosystems and in reduced biodiversity [4–6]. Moreover, this degradation of ecosystems hinders the potential use of ecosystem services [7].

In addition, climate change is a driver of many societal and environmental problems of the 21st century [8,9]. Together with the impacts of population growth, it puts pressure on the management of natural resources such as water resources [5,10]. It can also alter the hydrological cycle, resulting in large-scale impacts on water availability. These impacts could be temporal or become permanent. Climate change can also affect the temporal conditions of the water balances [11]. Water balances are components of the water cycle that exist at different scales and in different conditions in each locality. They are highly affected by the state of the environment and by the climate. Climate change highly affects the water balance conditions both spatially and temporally at the local or regional scale. For instance, Africa is vulnerable to inter-annual climate variations due to the El-Niño southern oscillations [12,13]. To evaluate the conditions of water resources in a basin or region, it is essential to know the water balance conditions under certain circumstances. The water balance components may vary due to different spatial and temporal aggregations, reference

**Citation:** Truneh, L.A.; Matula, S.; Bát'ková, K. Hydroclimate Impact Analyses and Water Management in the Central Rift Valley Basin in Ethiopia. *Water* **2023**, *15*, 18. https:// doi.org/10.3390/w15010018

Academic Editor: Adriana Bruggeman

Received: 12 October 2022 Revised: 14 December 2022 Accepted: 15 December 2022 Published: 21 December 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

periods, and climate change impacts, as well as the interventions of humans for the purpose of water use [14].

Climate change refers to changes in conditions such as temperature and rainfall over long periods of time in a region. It has been caused by the increasing concentration of greenhouse gases (GHGs) in the atmosphere since the pre-industrial era. The Intergovernmental Panel on Climate Change (IPCC) concluded that more than 90% of the accelerated warming of the past five decades has been caused by the industrial release of GHGs such as CO2 into the atmosphere [15].

In the CRVB, there are high levels of rainfall variability, water scarcity, and weather variability, and it is a place where water resources planning and management are greatly challenged by the impacts of climate change [16]. For example, an increase in temperature and variability in rainfall affected the seasonal and total water supply and led to the occurrence of extreme hydrological events [17]. It is therefore essential to know the trend of climate change over a long period of time to manage possible extreme hydrological events, either droughts or flooding, in the region [15,18–20].

A climate impact study can also provide a reliable basis for water resources planning [21]. Nowadays, long-term water resources planning studies need to take into consideration ongoing and future global climate changes in order to curb the uncertainties in the management of water resources [22]. In such studies, the effects of climate change must be quantified with high spatial and temporal resolutions at basin scale [1,23–25].

Various studies have been carried out on the water resources of the Central Rift Valley Basin (CRVB) in an attempt to describe and evaluate the impact of climate change on existing water resources [16,26–30]. However, only a few of these studies have been aimed at analyzing the impacts of climate change based on various regional concentration pathway (RCP) simulations in different climate scenarios to evaluate the conditions of the components of the water balance in the sub-basins. For example, in Ethiopia, Legesse et al. (2003) used the Precipitation Runoff Modeling System (PRMS) model to simulate runoff, and they predicted a 30% decrease in runoff in response to a 10% decrease in the amount of precipitation [26,31]. A 1.5 ◦C increase in temperature resulted in a 15% decrease in runoff [32]. Similarly, it was indicated that a higher temperature leads to an increase in evaporation rates, reductions in stream flow, and an increase in the frequency of droughts [28]. In addition, a vast number of studies have been conducted to analyze the impacts of climate change on crop productions [17,18,33–35]. However, very little consideration has been given to the potential impact of climate change on the current and future water balance components in the region and on their management methods. Therefore, a deep understanding of the effects of climate change on the components of the water balance for identifying site-specific climate-smart agricultural water management measures is necessary. In this context, the findings of this study can contribute the input information for the purpose of agricultural water management in the CRVB to adapt to the impacts of climate change.

An analysis of the impact of climate change on the components of the water balance involves hydrological models and projected plausible future climate change variables from global circulation models (GCMs) [23,36–38]. The GCMs determine the effects of changing concentrations of greenhouse gases on global climate variables such as temperature, rainfall, evapotranspiration, humidity, and wind speed [38]. Similarly, global circulation models that predict long-term climate trends (rainfall, temperature, and humidity) are often unsuitable for regional scale studies because of their coarse grid-size resolution. It is therefore essential to downscale GCM data to the region-specific climate impact through the use of statistical or dynamical downscaling techniques [38,39].

Various hydrological models can be applied to analyze the impacts of changes in the climate [10]. These models investigate the degree to which observed changes in climate may affect the resources due to natural variability, human activity, or a combination of both [40]. The results and projections produced by such models provide essential information for making decisions of local, regional, and national importance on matters such as water

resources management, agriculture, transportation, and urban planning [41]. However, hydrological models need to be calibrated to site-specific conditions before they are used for climate change impact analyses [22].

The general procedure for assessing the impacts of climate change on water resources and on watershed processes can be determined by physically-based distributed models. Due to its wider applicability and utility, different versions of SWAT have been used for several studies throughout the world [38]. SWAT has been used for hydrological modeling, soil erosion and sediment transport modeling, climate impact studies on stream flows, and modeling land use change and management impacts on sediment and stream flows. It can also be used for nutrient transport modeling in agricultural fields [38]. These studies have confirmed the successful use of the SWAT model across different watersheds on different scales and across different environmental, climatological, and hydrologic conditions [36,42,43].

The study presented here is therefore aimed at analyzing the impacts of climate change according to the regional RCP scenarios on the water balance components of the CRV sub-basins in Ethiopia. The results of the SWAT models integrating CMhyd, WGEN, and SWAT-CUP software packages, were used to identify possible sub-basin-wide water management options.

#### **2. Materials and Methods**

#### *2.1. Description of the Study Location*

The Central Rift Valley Basin (CRVB) is in Ethiopia between 38◦15 E and 39◦30 E longitude and 7◦10 N and 8◦30 N latitude, (Figure 1). It covers an area of approximately 9112.5 km2. It is a hydrologically closed lakes region with no known outlets for its total basin [27]. The study basin is a vast closed area and thus was divided into smaller subbasins with known outlets (Ketar, Meki, and Shalla sub-basins).

**Figure 1.** Location of Ethiopia in Africa, and the major river basins (bottom left); location of the study area within Ethiopia (top left), and the study sub-basins with their major stream outlets.

The mean annual rainfall of the study area varies between 600 mm near the lakes and 1200 mm–1600 mm in the highlands. The average minimum temperature is 10.5 ◦C, while the average maximum temperature is 24.3 ◦C [16]. CRVB comprises four major lakes: Ziway, Shalla, Abiyata, and Langano. It also has perennial rivers, which include the Meki, the Ketar, and the Jidu rivers [16].

The CRVB has diverse soil types. It has varying infiltrability and associated runoff potential. Coarse-textured soils (LT Leptosols) with high infiltrability are dominant in the eastern and western highlands and in the valley floor around the lakes. Mediumtextured soils (Euvertisols) with moderate infiltrability dominate the eastern and western mid-altitudes of the CRVB, whereas the lower reaches of the western highlands and some places in the central part of the eastern CRVB are dominated by fine-textured black soils (Vertisols) with lower infiltrability (Figure 2) [19].

**Figure 2.** Distributions of land use, soil, slope, and elevation ranges in the CRVB (Note: the land use and soil codes are according to the SWAT classification standard as indicated in Tables S1 and S3 in the Supplementary File).

#### *2.2. Sub-Basin Selection Methods (Boundary Delineation)*

The hydrologically closed CRVB comprises many sub-basins. It was delineated and subdivided into major sub-basins in GIS according to their river systems, using the outlet points [16] as indicated in Figures 1 and 3. The DEM data were delineated in Arc SWAT and with the spatial analyst tool in ArcGIS. The total area of CRVB was delineated based on the watershed boundaries or water divide lines obtained from the Ministry of Water Resources of Ethiopia. The CRVB is an endo hydrogenic basin [27]. Since there is no single outlet for the CRVB, this study aims to investigate the hydroclimatic impacts via its major sub-basins with monitored outlets (Ketar, Meki, and Shalla). The selected sub-basins form parts of the CRVB with different characteristics which, when summed up, can generally characterize the climate impact conditions of the CRVB. The sub-basins were selected based on differences in agroecology, microclimate, and socio-environmental interactions. The analyses were performed for each of the sub-basins separately. The outlet locations of each sub-basin are indicated in Figures 1 and 3.

**Figure 3.** Locations of meteorology stations (Meteorology STN), CORDEX grid point (CORDEX STN), and outlets (Discharge monitoring stations).

#### *2.3. Data Definition*

2.3.1. Spatial Data

The spatial data used for the modeling were analyzed step-by-step. Initially, the digital elevation model (DEM) data of the CRVB was delineated with GIS into Ketar, Meki, and Shalla. They were divided into sub-basins based on the topography and the river systems. Each sub-basin was consequently subdivided into hydraulic response units (HRUs) according to the land-use features, soil profile, and slope within SWAT. The major data inputs and their utilization are indicated in Table 1. The land uses characterize the hydrological process in the sub-basins. The land use map of the CRVB was obtained from the Ethiopian Geospatial and Information Institute (GSII).

The soil hydro-physical properties determine the existence and the quantity of each component of the water balance [44]. The soil physical properties and the area coverage of each of the soil types were classified based on the SWAT classification standards. The digitalized soil data for the study region with a resolution of 1ha was obtained from the Ministry of Agriculture and Natural Resources (MANR) of Ethiopia. The details are in the Supplementary File in Table S1. The spatial information maps of the study region including land use information, distribution of soil types, slope, and elevation information are indicated in Figure 2.

**Table 1.** Major input data used in the SWAT model.


Note: NMA—National Meteorological Agency, GSII—Geospatial and Information Institute, MANR—Ministry of Agriculture and Natural Resources, OBANR—Oromia Bureau of Agriculture and Natural Resources, MW—Ministry of Water Resources.

#### 2.3.2. Climate Data

Daily data on minimum and maximum temperature, hours of sunshine, relative humidity, wind speed, and precipitation from six meteorological stations, located in and near the sub-basins, were introduced into the model to simulate the water balances of the sub-basins (Table 1 and Figure 3). Hydrology data for stream flows were collected at the outlets indicated for each sub-basin. The CORDEX grid locations in the study area, based on which the climate data were downscaled and extracted, are also presented in Figure 3. The coordinate locations of the meteorology stations are indicated in Supplementary File in Table S2.
