Browsing by Author "Moges, Semu Ayalew"

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    Development and application of hydrological decision support tools Pangani River Basin in Tanzania.
    (University of Dar es Salaam, 2003) Moges, Semu Ayalew
    Various stand-alone Decision Support Tools were developed for describing and highlighting the major hydrological variables (decision issues) in Pangani River Basin. The tools consist of Spatial Rainfall (RAIN GEOSTAT), Evaporation (EVAP-DSS), Soil Moisture (D CSMB) and Integrated Water Resources Assessment Model (IWRAMP). The three spatial models were used to estimate spatial rainfall, evaporation and soil moisture variables in the entire Pangani basin for the benefit of hydrological and agricultural planning purposes. The spatial rainfall and evaporation estimates were further improved by incorporating digital elevation model (DEM) underscoring the influence of elevation in Pangani basin. The soil moisture model (D CSMB) was used to assess sustainability of rainfed agriculture in the basin. It was showed that it is practically impossible to depend entirely on rain fed agriculture without supplementary irrigation confirming the already existing situation of extensive irrigation practice in the basin. The IWRAMP model was developed to address the major management issues of the basin and highlighted three major findings 1). It was found that the NYM reservoir was not efficiently operated and alternative operation procedure was developed. The new procedure improves the long term release by as much as 4 m3/s, minimizes the spill and maintains minimum storage level at acceptable levels; 2) Kirua swamp was identified as a major water losing swamp specially when water release from the reservoir exceeds 30 m3/s. This undermines the improved release situation from NYM reservoir, which would otherwise boost the hydropower production at Hale and Pangani Fall. Thus, the gain in water release from the reservoir should either be accompanied by improved channel system (such as construction of river training works) or irrigation may be encouraged in areas between the reservoir and the swamp to convert the additional water for beneficial use and 3) if the new reservoir operation procedure is strictly followed, in normal hydrological years an additional area of 10,000 ha can be brought under irrigation upstream of NYM reservoir. Finally, the study results should be viewed in the context of the study period and needs further improvement as new data becomes available.
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    GIS-based distributed water balance model for lake Victoria basin.
    (University of Dar es Salaam, 1998) Moges, Semu Ayalew
    A GIS-based distributed hydrological model that operates at a regional scale is developed for Lake Victoria basin. It operates on monthly time scale and at a spatial resolution of 10 by 10 minutes grid cells that represent the basin. The model enables, one to estimate the complex hydrologic regimes of the basin, simulate discharge from the various catchments that drain into Lake Victoria, and eventually to estimate the inflow and net basin supply of Lake Victoria. The model consists of two coupled components; a Grid-based Water Balance Model (GWBM) and a Flow Accumulation & Routing Model (FARM). The water balance model, GWBM, is developed based on the Thornthwaite and Mather monthly moisture accounting technique, and it has been used to create GIS based high resolution data sets of the major hydrological regimes of soil moisture, actual evapotranspiration and the runoff at each grid cell in the region. The GWBM uses soil-water retention properties, like the field capacity, the wilting point and the available water content that are derived from the soils & vegetation cover of the region using GIS. The flow routing model. FARM, is a multigrid, dynamic model that computes discharge through each cell within the region. It combines runoff produced by GWBM with information on network topology of the Lake Victoria, flood plain inundation & transfer rates. It is a linear routing technique applied through each grid with the assumption that the excess runoff generated by GWBM creates a subsurface storage pool over each grid cell. The coupled model has been applied to Lake Victoria basin and calibrated using measured discharges of 15 catchments for the period 1970-75. The performance of the model has been found to be reasonably good as judged by low annual & monthly percentage volumetric errors. Furthermore, long-term hydroclimatological data sets over the region have been used to represent the hydrologic regimes of the region in terms of variation of soil moisture, actual evapotranspiration and runoff. The net basin supply (NBS) has been finally computed.