Ethiopian Journal of Water Science and Technology

Remote sensing approach to evaluate the effect of short-term land cover change on flood inundation and impact, Lower Awash Basin, Ethiopia.

2025-10-18T14:06:07+03:00

Flood risk management has been severely constrained by limited information on the causes and impacts of flooding. In this study, we evaluated the contribution of short-term land cover change of Logiya catchment to flood inundation and impact on Dubti town and it’s surrounding in the Lower Awash River Basin, Ethiopia. The land cover maps were generated by applying a machine learning algorithm on the Sentinel-2 optical satellite images. Land cover and soil data were used to generate the Curve Number (CN) map of the study area for the period stretching from 2017 and 2023. Sentinel-1 based flood maps show that roads and irrigation canals were washed away by the 2020 extreme flood which led to the inundation and abandoning of the Tendaho irrigation scheme. The runoff generating potential (CN) significantly increased over 27% of the Logiya catchment between 2017 and 2023 contributing to enhance flooding. The remote sensing analysis showed that overflow of the Logiya River in 2020 was intercepted and conveyed by the main irrigation canal of the Tendaho scheme resulting in inundation of the Dubti and surrounding. Availability of earth observation data (e.g Sentinel-1 SAR on the study area every 6 days at 10m resolution for 7 years) enabled detail characterization of the cause, dynamics and impacts of the historical flood events. This study’s results can guide flood risk management in the study area and serve as a reference for future studies in flood affected area.

Effectiveness of alternative furrow irrigation methods on cabbage yield and water productivity in Angacha District, Central Ethiopia Region

2025-10-18T14:06:52+03:00

This study aimed at investigating the effectiveness of alternative furrow irrigation methods on the head yield of cabbage and water productivity at Guder Irrigation Scheme in Angacha district Kembata Zone Centeral Ethiopia Region. Three levels of treatment were replicated five times, in Randomized Complete Block Design (RCBD). Farmers were used as replication from farmers’ research extension groups. The maximum head yield (52.8t/ha) was obtained from conventional furrow, following the second maximum head yield (49.07t/ha) at alternate furrow irrigation method. The minimum yield (38.112t/ha) was obtained from farmers’ practice of irrigation method. Their yield advantages (26.8%) and (22.3%) were recorded from conventional furrow, and alternate furrow irrigation, respectively. The highest water productivity (22.23kg/m³) was obtained from alternate furrow irrigation, following the second maximum water productivity form conventional furrow irrigation (11.96kg/m³) and minimum water productivity (8.15kg/m³). Farmers obtained a net income of (1,278,300 ETB/ha), (1,190,730 ETB/ha) and (909,903.3ETB/ha) from conventional furrow, alternate furrow, and farmers’ practices, respectively. Highest benefit cost ratio (18.63) was recorded form alternative furrow irrigation method :( 17.83) from every furrow and (14.15) from farmers’ practice. This implies that alternative furrow irrigation saves half of irrigation water when assimilated with conventional furrow and doubles net income on the farm gate. Therefore alternative furrow irrigation method should be recommended as a best agricultural water management technology without limiting fresh head yield of cabbage, and water productivity. The technology should be scaled up in to other irrigation schemes with the same agro ecology.

Key Words: - Alternate Furrow Irrigation, Head Cabbage, Yield, Water Productivity, Farmer Practice, Fertilizer, Guder

Analysis of Precipitation Variability in the Kulfo River Watershed: Insights from Wavelet Analysis and Back-Trajectory Approaches

2025-10-18T14:07:17+03:00

Analysis of precipitation time series is a fundamental undertaking in meteorology and hydrology. The heavy precipitation scenario of the Kulfo River watershed is studied using wavelet analysis and the advanced atmospheric transport model. Daily rainfall data for the 1991-2020 study periods are collected from the office of the Ethiopian Meteorology Institute. Meteorological fields on a three-dimensional grid at 1^o x 1^o spatial resolution and daily temporal resolution are also obtained from the Global Data Assimilation System (GDAS). Wavelet analysis of the daily precipitation processed with the lag-1 coefficient reveals some high power recurred once every 38 to 60 days with greater than 95% confidence for red noise. The analysis also identified inter-annual periodicity in the periods 2002 - 2005 and 2017 - 2019. Back trajectory analysis for 3-day periods up to the heavy precipitation day during the main and short rain seasons indicates the Indian Ocean and Gulf of Eden source; trajectories crossed the southern and eastern African escarpment to arrive at the Kulfo watershed. Atmospheric flows associated with the Western Indian monsoon redirected by the low-level Somali winds and Arabian ridge are responsible for the moisture supply. The spatial distribution of relative humidity (RH) during heavy precipitation events ranges from 50% to 88%. The findings indicated that the time-localization of the wavelet power spectrum yields valuable hydrological information and the back-trajectory approaches provide useful characterization of air mass sources and pathways.

Keywords: Back trajectory; Extreme Precipitation Events; Kulfo Watershed; Power Spectrum

Assessment and Upgrading of Scour Depth Estimation Equation for Bridge Piers in Kombolcha-Weldia Highway, Ethiopia

2025-10-18T14:07:38+03:00

Frequent scouring caused by turbulent river flow has significantly impacted bridge stability along the Kombolcha-Weldia highway in Ethiopia’s Amhara region, leading to foundation erosion over successive years. To address this, six established scour depth estimation equations—Colorado State University (CSU), Bruisers, Jain and Fischer, Froehlich, HEC-18/Muller, and Laursen’s empirical equation—were evaluated for their accuracy in predicting bridge pier scour depths. A comprehensive field study was conducted, including topographic surveys, assessment of pier alignment and shape, riverbed material sampling, and direct scour depth measurements at five selected bridges. Peak flood discharge was calculated using the Rational Method for catchments under 50 km² and the Soil Conservation Service (SCS) unit hydrograph method for larger catchments. Total scour depth was determined by considering three key components: long-term degradation, constriction scour, and local scour. Critical variables such as pier width, shape, and flow alignment were found to significantly influence scour magnitude. The predicted scour depths from the selected formulae were compared with field-measured values. Comparative analysis revealed that the equations proposed by Jain and Fischer, Laursen, and Froehlich provided more accurate scour depth estimates than the other methods. Among these, the Jain and Fischer equation was identified as the most suitable for predicting local scour in both sand-bed and gravel-bed rivers within the study area. Based on these findings, it is recommended that the Jain and Fischer equation be prioritized for scouring depth estimation in similar hydrological and geomorphic conditions, ensuring better assessment and mitigation of bridge foundation risks in the region.

Keywords: Bridge Pier Scour; Design return period; Jain and Fischer equation; Kombolcha-Weldia; Scour depth estimation.

Morphodynamic alterations around the bridge opening and the process of scouring under the influence of hydraulic flow parameters on selected bridges in the south Ethiopia regional state, Sodo to Konso highway

2025-10-18T14:08:00+03:00

Numerous bridges in southern Ethiopia are greatly affected by Morphodynamic changes in the alluvial channel caused by hydraulic flow parameters. These bridges are vital for enabling traffic flows between communities in the region. In this study, Morphodynamic alterations around bridge openings were conducted using the hydraulic flow parameters based on the field and laboratory investigations were conducted in selected bridge sites between the Sodo and Konso highway routes. The Morphodynamic analysis of 1984 to 2021 years 1,276 Landsat images of 5 selected scour susceptible bridge sites have been analyzed using RivMap toolbox in MATLAB after rapid assessment of the channel stability of 15 bridges based on the field observation. The 38 years of channel geomorphological changes show that lateral widening of the alluvial valley is dominant in selected bridge sites. This channel widening also aggravates the contraction, local scour potential, and increased flow velocity induced by narrow bridge opening. The HEC RAS result confirms that the three of the selected bridge sites namely Kulfo, Alge, and Wajifo are low risk whereas Sile and Sego Bridge sites are the most scour critical. Moreover, Sego Bridge is severe for 50, 100, and 500-year return period floods due to the extreme narrowing of the valley at the bridge site and continuous upstream channel lateral widening of the floodplain through time. This indicates that this bridge needs a new relief culvert/countermeasure for safe operation. Thus, the design and analysis of the bridge structure in the lower alluvial reaches of the river requires a thorough understanding of the geomorphology and provision of adequate openings for the potential floodplain widening.