Abstract:The level of Lake Tana, Ethiopia, fluctuates annually and seasonally following the patterns of changes in precipitation. In this study, a mass balance approach is used to estimate the hydrological balance of the lake. Water influx from four major rivers, subsurface inflow from the floodplains, precipitation, outflow from the lake constituting river discharge and evapotranspiration from the lake are analysed on monthly and annual bases. Spatial interpolation of precipitation using rain gauge data was conducted using kriging. Outflow from the lake was identified as the evaporation from the lake's surface as well as discharge at the outlet where the Blue Nile commences. Groundwater inflow is estimated using MODular three-dimensional finitedifference ground-water FLOW model software that showed an aligned flow pattern to the river channels. The groundwater outflow is considered negligible based on the secondary sources that confirmed the absence of lake water geochemical mixing outside of the basin. Evaporation is estimated using Penman's, Meyer's and Thornwaite's methods to compare the mass balance and energy balance approaches. Meteorological data, satellite images and temperature perturbation simulations from Global Historical Climate Network of National Oceanographic and Atmospheric Administration are employed for estimation of evaporation input parameters. The difference of the inflow and outflow was taken as storage in depth and compared with the measured water level fluctuations. The study has shown that the monthly and annually calculated lake level replicates the observed values with root mean square error value of 0Ð17 and 0Ð15 m, respectively.
The Watershed Assessment Model was used to simulate the runoff volume, peak flows, and non-point source phosphorus loadings from the 5870 km(2) Lake Okeechobee watershed as a case study. The results were compared to on-site monitoring to verify the accuracy of the method and to estimate the observed/simulated error. In 2008, the total simulated phosphorus contribution was 9634, 6524 and 3908 kg (P) y(-1) from sod farms, citrus farms and row crop farmlands, respectively. Although the dairies represent less than 1% of the total area of Kissimmee basin, the simulated P load from the dairies (9283 kg (P) y(-1) in 2008) made up 5.4% of the total P load during 2008. On average, the modeled P yield rates from dairies, sod farms and row crop farmlands are 3.85, 2.01 and 0.86 kg (P) ha(-1) y(-1), respectively. The maximum sediment simulated phosphorus yield rate is about 2 kg (P) ha(-1) and the particulate simulated phosphorus contribution from urban, improved pastures and dairies to the total phosphorus load was estimated at 9%, 3.5%, and 1%, respectively. Land parcels with P oversaturated soil as well as the land parcels with high phosphorus assimilation and high total phosphorus contribution were located. The most critical sub-basin was identified for eventual targeting by enforced agricultural best management practices. Phosphorus load, including stream assimilation, incoming to Lake Okeechobee from two selected dairies was also determined.
Abstract:The groundwater flow contribution into Lake Tana was simulated using Gumera River sub-basin as a case study. Draining into Lake Tana, the Gumera sub-basin boundary was delineated from a 90-m Shuttle Radar Terrain Mapping (SRTM) digital elevation model (DEM) using ArcHydro tools. The sub-basin boundary served as the divide line of the groundwater flow while stream networks were used as internal drainage lines. Input parameters such as transmissivity and hydraulic conductivity were obtained from past studies and experts' knowledge. Based on the geological information of the sub-basin, unconfined subsurface flow condition was considered and simulated using MODular 3D finite difference ground-water FLOW model (MODFLOW). Analytical solutions were also used for comparison purposes. The result indicates that head contours are aligned to the streams showing their relationship as a subdued form of the surface water flow, which are dictated by the morphology of the basin. This suggested the need to account groundwater contribution of the sub-basins to Lake Tana as baseflow, and to estimate groundwater inflow from the floodplain separately. The contribution from the floodplain was estimated using a finite differences method which showed an inflow of 0Ð15 billion cubic meter (BCM) from rivers into the floodplain out of which 0Ð09 BCM is flowing out to the Lake Tana and 0Ð06 BCM stored in the aquifers of the floodplain. The stored quantity could meet irrigation demands as well as contribute to the preservation of the ecological structure and function of the area. The study suggests further research on validation of parameters through inverse modeling approaches and optimization of water allocations.
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