Ethiopian Journal of Water Science and Technology

Optimizing Agricultural Water Use: A Comparative Analysis of Soil Moisture and Evapotranspiration-Based Irrigation Scheduling for Carrot Crop (Daucus Carota)

Melkamu Ateka Derebe — 2025-10-28

Crop production in Ethiopia is limited owing to water scarcity. Various technologies and management options are being used for efficient use of the available water resources in crop production. This study evaluated the performance of Soil Moisture (SM) and Evapotranspiration (ET) based irrigation scheduling methods on carrot yield and Water Use Efficiency (WUE), Water Productivity (WP) and field water use efficiency at water scarce areas of Arba Minch for two consecutive years/seasons of 2021 and 2022. The experimental design was a randomized complete block with three replicates. The treatments combined two scheduling techniques (soil moisture, SM, and evapotranspiration, ET). Water was delivered to furrows using an RBC flume, and data was analyzed with ANOVA at 5% significance level using SAS software and the graphs were drawn by Python. Across 2021 and 2022 seasons, SM-based irrigation consistently required less water than ET-based scheduling, achieving 5–5.4% water savings at full irrigation levels without reducing yields. Under moderate deficit irrigation (50–75%), both methods sustained comparable yields (42–43 t/ha), but SM-based treatments showed higher WUE, FWUE, and WP, with peak values at SM50% (WUE = 31.4 kg/m³; FWUE = 22.7 kg/m³; WP = 1.78 kg/m³) compared to ET50% (WUE = 25.8 kg/m³; FWUE = 18.1 kg/m³; WP = 1.78 kg/m³). Severe deficit irrigation (25%) drastically reduced yield and all efficiency indices in both methods. Economic analysis indicated that moderate irrigation levels (50%) maximized net benefits and cost-benefit ratios. Overall, SM-based irrigation was more efficient in water use, improved yield stability, and enhanced WUE, FWUE, and WP across irrigation levels. Thus, it demonstrated its suitability for sustainable carrot production under limited water resources. This saving is particularly relevant for Ethiopia where water scarcity limits crop production. It demonstrates a practical strategy for farmers to grow more food with less water while supporting sustainable resource management.

Keywords: Soil moisture; Furrow Irrigation; Crop evapotranspiration; ET-Based irrigation; SM-based irrigation

Evaluation of Koka Reservoir Operation under Sedimentation: Implications for Irrigation Water Demands in Upper Awash Valley, Awash River Basin

Habtamu Nemera — 2025-11-10

Water scarcity has been a major issue in the Awash River Basin of Ethiopia. The Koka Reservoir is the sole water storage source supporting extensive irrigation activities in the Upper Awash Valley, a key part of the basin. However, sedimentation has reduced the reservoir’s active storage capacity, posing serious risks to its ability to meet irrigation demands in the valley. This study aims to evaluate the reservoir’s operational performance by assessing irrigation water demand and analyzing the water supply-demand balance under recent sedimentation conditions. Key datasets include crops, census, digital elevation model, hydro-meteorological, and reservoir operation data. The primary software tools applied were CROPWAT, ArcMap, and HEC-ResSim. Water resource system performance evaluation criteria such as reliability, vulnerability, and resilience were employed. The annual estimated gross irrigation water requirement for the study region was 1,525 Mm³. Reservoir operation has been simulated using three decades of daily inflow from 1985 to 2015 by considering different alternatives of the irrigation water demands. When the model was simulated for Alternative 1, the reservoir met full demand less than half the time (time reliability 42.7%), supplied most of the total demand volume despite shortages (volumetric reliability 73.7%), experienced severe deficits when failures occurred (mean vulnerability 25.4 m³/s, maximum 80.0 m³/s), and had limited ability to recover (resilience 27.7%); even in Alternative 2, reliability and resilience improved and vulnerability was moderate, but full demand was still not consistently met. The results indicate that the reservoir capacity under recent sedimentation conditions is insufficient to store high inflows and meet high demand during rainy and dry seasons, respectively. This study does not account for the potential impacts of land use/land cover and climate change, which could influence the hydrological balance and reservoir performance.

Keywords: Awash River; CROPWAT; HEC-ResSim; Performance Evaluation; Reservoir Operation; Water Scarcity.

Evaluation of Bias Correction Techniques for RCA4 Model of CORDEX-Africa Precipitation and Temperature Data in Case of Omo Gibe River Basin, Ethiopia

Ayano Hirbo Gelebo — 2025-12-22

The accurate assessment of climate change that impacts on water resources is fundamental to sustainable development. While Regional Climate Models (RCMs) are essential tools for this task due to their high resolution, their outputs contain significant biases that must be corrected. This is particularly critical in data-scarce regions like East Africa where the selection of optimal Bias Correction Methods (BCMs) remains largely unexplored and often relies on generalized recommendations. This study addresses this research gap by evaluating and identifying the most suitable BCM for the Omo Gibe River Basin in Ethiopia. The observed climate data (1990–2020) from 35 stations and precipitation and temperature variables from the Rossby Center regional Atmospheric Mode for African domain (RCA4) RCMs of the CORDEX-Africa project were utilized. The finding of the study demonstrated that the delta-change method outperformed other techniques, achieving exceptional performance metrics for bias-corrected historical data: for temperature Root-mean-square error (R²) is 0.95, Nash Sutcliffe efficiency (NSE) is 0.97, Coefficient of determination (RMSE) is 0.0028 and for precipitation (R² = 0.9, NSE = 0.95, RMSE = 0.0025). The study concluded that the delta-change method is the most robust approach for correcting both historical and future climate projections in the basin. Its application is therefore highly recommended for subsequent climate change impact studies on hydrology in the Omo Gibe Basin and similar regions in East Africa.

Keywords: RCM, bias correction; climate variables; Omo Gibe River Basin.

Comparative Evaluation of Model Sensitivity, Calibration, and Parameter Uncertainty in Streamflow Simulation Using SWAT and HBV Light in the Geba Watershed, Ethiopia

Abebe Temesgen Ayalew — 2025-12-22

Evaluating and simulating streamflow is extremely useful for managing water resources in almost all regions, particularly in arid ones. This study focuses on the performance of two hydrological models, HBV Light and SWAT, in streamflow simulation in northern Ethiopia. The models were evaluated using an ensemble modeling approach, which integrated Monte Carlo simulations for HBV Light and SWAT, while SWAT-CUP was used for calibration and validation. During the sensitivity analysis, key parameters controlling the model outputs were identified. For HBV Light, the parameters, K2, MAXBAS, and BETA, reflect subsurface processes, whereas SWAT, CN2, GWQMN, and SOL_AWC were used to control surface runoff. During calibration and validation, SWAT demonstrated statistically superior performance in modeling streamflow (R²=0.73, NSE=0.81) and (R² =0.72, NSE=0.72) respectively. While HBV Light recorded a performance of (R²=0.71, NSE=0.70) during calibration and (R²= 0.71, NSE= 0.71) during validation,which was closer to the observed streamflow. This indicates during the validation phase, SWAT still performed better but HBV Light demonstrated narrower predictive uncertainties at 95% along with more identifiability of the parameters that reduced the problem of equifinality. The bottom line of this case was that SWAT was statistically better, while HBV Light was more transparent and reliable with uncertainties. All things considered, both models could simulate streamflow, but their differences suggested that context-based choice would be optimal. While predictive consistency and uncertainty portrayal were most important, HBV Light was advantageous, and SWAT was better suited for use in cases that rank calibration accuracy above all else. The hydrology of the catchment could be better understood if streamline decision-making for the effective and sustainable management of water resources in the Geba Catchment and similar semi-arid areas were combined, or if multi-model ensembles were utilized.

Keywords: Streamflow simulation, Hydrological modeling, HBV Light model, SWAT model, Uncertainty analysis, Parameter sensitivity