Low Flow Trends and Frequency Analysis in the Blue Nile Basin, Ethiopia

Abstract

Low flow analysis provides crucial information for the planning and design water resource development, risk assessment and environmental flow management. Understanding the low flow regimes and evaluating the magnitudes for incorporating in water resources management is vital for the countries like Ethiopia where demand for water is increasing. However, there were hardly enough studies in understanding the trends of low flow and frequency analysis. Therefore, this study focuses on evaluation of the trends in low flows and regional low flow analysis in the Blue Nile Basin, Ethiopia. In order to carry out the study, 15 river sub-basins in the Blue Nile Basin were selected based on the long term data availability and presence of quality of data. The 3-day sustained low flow (3d-slf), the 7-day sustained low flow (7d-slf) and the 14-day sustained low flow (14d-slf) models were used to extract the data from the daily time series stream data obtained from MoWIE. Trends in low flow were analyzed separately by using Mann-Kendall (MK) trend test. Low flow frequency analysis was used to estimate the long term low flow quantiles. In addition, regional analysis for estimating the quantiles for ungaged catchments was also developed based on the regional growth curve and catchment characteristic of drainage basins. The results indicated that 3d-slf, 7d-slf and 14d-slf models of low flow series indicated no significant difference for each station at 95% CI. Out of the 15 selected stations, 12 of stations have indicated decreasing; two stations indicated increasing and remaining one station with no trend. Mainly decreasing trend was associated with the land cover and climate change which results in increasing runoff and evapotranspiration respectively. Weibull distribution-GEV and LGN was found best fit based on the L-Moment Ratio Diagram (L-MRD). Hence quantile estimations have indicated diminishing magnitudes of low flow quintiles for 2 -500 years return Where area (A) and rainfall (R) followed by slope were found sensitive to compute in developing the regional region equations between mean low flows and the physiographic data. This study indicated that there needs to be a new water management scenario and adaptation mechanism of climate change and land use land cover dynamics for utilizing water resource in the Blue Nile Basin.

Changes in Runoff and Sediment Load and Potential Causes in the Malian River Basin on the Loess Plateau

“…The Mann-Kendall non-parametric statistical test (M-K test) can effectively detect variations in hydrological data over time, and has been widely employed in the fields of hydrology and climate [34][35][36]. The method assumes a stable time series, and a random independence of the time series is required.…”

Spatial and Temporal Evolution of Low Water Flows in the Sassandra River Catchment Area in Cote d’Ivoire

“…For instance, the change of the forest to agricultural land could largely increase runoff in the rainy season and recovery of heavy rainfall after the year 1990 in West Africa. This has been indicated in different studies such as (Descroix et al, 2013;Sighomnou et al, 2013;Cissé et al, 2014;Panthou et al, 2014;Yao, 2015;Kouamé, 2017;Assefa & Moges, 2018). In addition, analyses using the Mann-Kendall statistical test showed that 50 to 67% of low-flow series do not show significant trends in variance.…”

“…Evaluating the presence and absence of trends in low flow for the selected stations were carried out by using the Mann and Kendall (MK) trend test (Kendall, 1975 presence of the monotonic increasing/decreasing trend. This statistical test has a variety of applications for trend analysis (Temesgen et al, 2014;Assefa & Moges, 2018;Thomas et al, 2019). The non-parametric Mann-Kendall statistical test has been affected and the most used to assess the significance of the trend in low-flow indices.…”

“…It is therefore important to be able to know these different indices at any point in the watercourse and to be able to assess the different impacts that can influence their values (Lang, 2011). The low flow analysis provides crucial information for the planning and design water resource development, risk assessment and environmental flow management (Assefa & Moges, 2018). Understanding the low flow regimes and evaluating the magnitudes for incorporating in water resources management are vital for countries like Cote d'Ivoire.…”

Homogeneity Analysis of Rainfall Series in the Upper Blue Nile River Basin, Ethiopia

“…The (UBNRB) is the part of the watershed of the Blue Nile River Basin which is the source of water for major irrigation schemes in Sudan and contributes about 60-69% of the Nile's annual flow (Conway, 2000;Tabari et al , 2015). The area of the basin is represented 199,812 km 2 and lies in three national regional states where 46% of the basin area is Amhara, 32% in Oromia and 22% in Binishangul-Gumuz (Assefa and Moges, 2018). The UBNRB represents an essential water resource for Ethiopia and for the downstream countries Sudan and Egypt as well.…”