Establishing Stage–Discharge Rating Curves in Developing Countries: Lake Tana Basin, Ethiopia

A significant constraint in water resource development in developing countries is the lack of accurate river discharge data. Stage–discharge measurements are infrequent, and rating curves are not updated after major storms. Therefore, the objective is to develop accurate stage–discharge rating curves with limited measurements. The Lake Tana basin in the upper reaches of the Blue Nile in the Ethiopian Highlands is typical for the lack of reliable streamflow data in Africa. On average, one stage–discharge measurement per year is available for the 21 gaging stations over 60 years or less. To obtain accurate and unique stage–discharge curves, the discharge was expressed as a function of the water level and a time-dependent offset from zero. The offset was expressed as polynomial functions of time (up to order 4). The rating curve constants and the coefficients for the polynomial were found by minimizing the errors between observed and predicted fluxes for the available stage–discharge data. It resulted in unique rating curves with R2 > 0.85 for the four main rivers. One of the river bottoms of the alluvial channels increased in height by up to 3 m in 60 years. In the upland channels, most offsets changed by less than 50 cm. The unique rating curves that account for temporal riverbed changes can aid civil engineers in the design of reservoirs, water managers in improving reservoir management, programmers in calibration and validation of hydrology models and scientists in ecological research.

Spatial and temporal simulation of groundwater recharge and cross-validation with point measurements in volcanic aquifers with variable topography

A physically distributed water balance model called WetSpass is applied to estimate the recharge for the semi-humid Lake Tana basin in northwest Ethiopia. Lake Tana basin is one of the growth corridors of the country, where huge waterworks infrastructure is developing. Estimating groundwater recharge at required spatial and temporal scales is a challenge in groundwater management, sustainability and pollution studies. In this study, the WetSpass model is developed at 90 m grid resolution. The spatial recharge map by WetSpass is cross-validated with water table fluctuation (WTF) and chloride mass balance (CMB) methods. The mean annual recharge, surface runoff, and evapotranspiration over the whole basin using WetSpass are estimated at 315 mm, 416 mm, and 770 mm of rainfall, respectively. The mean annual recharge ranges from 0 mm to 1085 mm (0 % to 57 % of the rainfall): 0 mm at water bodies and highest on flat, sandy loam soil and bush land cover. Similarly, a high range of recharge is also noted using WTF and CMB methods showing the strong heterogeneous nature of the hydro(meteoro)logical characteristics of the area. Generally, the recharge is found higher in southern and eastern catchments and lower in the northern catchments, primarily due to higher rainfall amounts in the former parts. A fair general correlation between the recharge by WTF and WetSpass is found. WetSpass is effective in aquifers where diffuse recharging mechanism is the predominant type and recharge is controlled by rainfall. It is less effective in the storage-controlled flat floodplain alluvial and fractured rock aquifer areas. In these areas, the point estimates by WTF and CMB are effective and can be considered as reliable values. The land use change from 1986 to 2014 brought a relatively small hydrological change in recharge although the land use has changed significantly.

Evaluating the potential impact of climate change on the hydrology of Ribb catchment, Lake Tana Basin, Ethiopia

Scientific findings indicated there is climate change that affects given hydrology and, hence, water availability worldwide. To quantify its impact on a specific catchment scale, since spatial and temporal variability of climate change impact, this study was carried out at Ribb catchment, Lake Tana basin, Ethiopia. The catchment hydrology was represented by the Soil and Water Analysis Tool (SWAT) through using historical observed data. Regional Climate Model (RCM) projection data set for Nile Basin studies at Representative Concentration Pathway (RCPs) (RCP4.5 and RCP8.5) were used for future streamflow generation on three-time horizons; 2020s (2011–2040), 2050s (2041–2070), and 2080s (2071–2098). A baseline period (1976–2005) was used as a reference. SWAT was calibrated (R2 = 0.83 and NSE = 0.74) and validated (R2 = 0.72 and NSE = 0.71). The analysis was done based on the changes from the baseline period to the 2080s. Temperature showed an increasing trend but rainfall is decreasing. The mean annual streamflow could potentially reduce from 42.78 m3/s to 40.24 m3/s and from 42.78 m3/s to 37.58 m3/s based on RCP4.5 and RCP8.5 scenarios, respectively. On a monthly time scale, decreases in streamflow were found from March to August whereas a slight increase from September to February. Concerning individual months, June flows were found to have maximum impact in both scenarios (63.3% at RCP8.5 and 55.45% at RCP4.5 scenarios). The least impacted month was August based on the RCP8.5 scenario which is decreased by 6.64% and April based on the RCP4.5 scenario which is reduced by 1.21%. Looking at total volume, July showed a maximum decrease in both scenarios which is reduced by 21.08 m3/s at the RCP4.5 scenario and 51.22 m3/s at the RCP8.5 scenario. The maximum increase was found in October with 10.31 m3/s and 11.26 m3/s at RCP4.5 and RCP8.5 scenarios respectively. The future streamflow of Ribb River has decreased annually and monthly due to increasing temperature and reduction of rainfall.

Effect of irrigation regime on yield and water productivity of maize (Zea mays) in the Lake Tana basin, North West Ethiopia

Background Proper scheduling gave water to the crop at the right time in the right quantity to optimize production and minimize adverse environmental impact. Therefore, the objective of this study is to quantify the effects of irrigation regimes on yield and yield components of Maize in the Lake Tana basin during 2016–2018. Methods CROPWAT 8.0 model was used to determine the crop water requirement. Almost all parameters were adopted the default value of CROPWAT 8.0. Field data including; field capacity (FC), permanent wilting point (PWP), initial soil moisture depletion (%), available water holding capacity (mm/meter), infiltration rates (mm/day), and local climate data were determined in the study area. The treatments were arranged in factorial combinations with five irrigation depths (50, 75, 100, 125 and 150% of ETc) and two irrigation intervals (14 and 21 days) laid out in a randomized complete block design with three replications. Results The result was analyzed using SAS 9 software and significant treatment means separated using least significant difference at 5%. The result showed that the interaction of irrigation depth and irrigation frequency has no significant effect on the average grain yield and water use efficiency of maize. At koga, the highest grain yield (7.3 t ha− 1) and water use efficiency (0.9 kg m-3) obtained from 100% ETc. while, at Ribb the highest grain yield (10.97 t ha− 1) and water use efficiency (1.9 kg m− 3) obtained from 21 days irrigation interval. Conclusion Therefore, for Koga and similar agro ecologies maize can irrigated with 562 mm net irrigation depth and 21-day irrigation interval and at Rib and similar agro ecologies maize can irrigated with 446.8 mm net irrigation depth and 21- days irrigation interval.

Spatial and temporal simulation of groundwater recharge amount and cross-validation with point measurement-based estimations on a tropical basin comprising volcanic aquifers: case study of Lake Tana basin, northwestern Ethiopia

<p>A physically distributed water balance model called WetSpass is applied to estimate the recharge for the semi-humid Lake Tana basin in northwest Ethiopia. Lake Tana basin, one of the major sub-basins of the Upper Blue Nile River basin, covers 15,077 km<sup>2</sup> of which 3,156<sup></sup>km<sup>2</sup> is the lake water body. The basin is regarded as one of the growth corridors of the country, where huge waterworks infrastructure is being developed. The basin has complex volcanic aquifer systems due to the multi-stage volcanism of the Cenozoic and Quaternary eras comprising many dikes, extended volcanic necks, and centers. Hence, estimating hydrological terms such as groundwater recharge considering the high basin physical heterogeneities is difficult, though highly important. In this study, the WetSpass model is developed, and recharge surface, surface runoff, and evapotranspiration at 90 m grid resolution have been developed. The spatial recharge map is cross-validated with water table fluctuation (WTF) and chloride mass balance (CMB) methods. The mean annual recharge, surface runoff, and evapotranspiration over the whole basin using WetSpass are estimated at 315 mm, 416 mm, and 770 mm, respectively. The mean annual recharge ranges from 0 mm to 1085 mm: 0 mm at water bodies and highest on highly fractured Quaternary basalt. Similarly, a high range of recharge is also noted using WTF and CMB methods showing the strongly heterogeneous nature of the hydro(meteoro)logical characteristics of the area. Generally, the recharge is found higher in the southern and eastern catchments and lower in the northern catchments, primarily due to higher rainfall amounts and highly permeable geological formations in the former parts. A fair general correlation between the recharge by WTF and WetSpass is found. However, WetSpass is more effective in the highland areas where the recharge is controlled by rainfall, while the WTF method is more effective in the storage controlled flat floodplain area. CMB is applied in a less spatially distributed way, and hence, the spatial performance of the method is not well evaluated. However, logged water infiltration in the floodplains, and transpiration from the groundwater in shallow water table areas have disturbed the estimated recharge by the CMB method. The land-use change from 1986-2014 brought relatively small hydrological change, although the land use has changed significantly.</p>