Hydrology and Earth System Sciences Discussions, 2011
Accurate and up to date information on the status and trends of water balance is needed to develo... more Accurate and up to date information on the status and trends of water balance is needed to develop strategies for conservation and the sustainable management of water resources. The purpose of this research is to estimate water balance in a semi-arid environment with limited in-situ data by using a remote sensing approach. We focus on the Lake Manyara catchment, located within the East African Rift of northern Tanzania. We use remote sensing and a semi-distributed hydrological model to study the spatial and temporal variability of water balance parameters within Manyara catchment. Satellite gravimetry GRACE data is used to verify the trend of the water balance result. The results show high spatial and temporal variations and characteristics of a semi-arid climate with high evaporation and low rainfall. We observe that the Lake Manyara water balance and GRACE equivalent water depth show comparable trends a decrease after 2002 followed by a sharp increase in 2006-2007. Despite the small size of Lake Manyara, GRACE data are useful and show great potential for hydrological research on smaller un-gauged lakes and catchments in semi-arid environments. Our modelling confirms the importance of the 2006-2007 Indian Ocean Dipole fluctuation in replenishing the groundwater reservoirs of East Africa. The water balance information can be used for further analysis of lake variations in relation to soil erosion, climate and land cover/land use change as well as different lake management and conservation scenarios. We demonstrate that water balance modelling can be performed accurately using remote sensing data even in complex climatic settings.
Tanzania is embarking on a height modernization programme in support of the AFREF initiatives. Th... more Tanzania is embarking on a height modernization programme in support of the AFREF initiatives. This modernization program is such that in future it will not be necessary for the Surveys and Mapping Division (DSM) to create and maintain the old spirit leveled orthometric height benchmarks. The DSM shall only be required to measure and publish ellipsoidal heights and GPS derived orthometric heights for various survey markers. This paper focuses on the determination of the transformation parameters between the Tanzania National Levelling Datum (TNLD) and the geoid for facilitating GPS heighting in Tanzania. Least squares estimates of the transformation parameters have been determined from orthometric heights in the Tanzania Primary Levelling Network (TPLN) and GPS derived orthometric heights referred to a preliminary model of the African geoid called AGP2003 and a geoid implied by the EGM96 geopotential coefficients. Two transformation models were considered: The Molodensky's transformation formula and a 2D algebraic polynomial. The values of the transformation parameters which were obtained using the AG2003 model produced GPS derived orthometric heights that are closer to the TPLN heights than those produced using the EGM96 geopotential model. It is being proposed here that the new gravimetric geoids that have been developed for Tanzania be used with the GPS observations to obtain new GPS based orthometric heights for Tanzania.
Hydrology and Earth System Sciences Discussions, 2011
Accurate and up to date information on the status and trends of water balance is needed to develo... more Accurate and up to date information on the status and trends of water balance is needed to develop strategies for conservation and the sustainable management of water resources. The purpose of this research is to estimate water balance in a semi-arid environment with limited in-situ data by using a remote sensing approach. We focus on the Lake Manyara catchment, located within the East African Rift of northern Tanzania. We use remote sensing and a semi-distributed hydrological model to study the spatial and temporal variability of water balance parameters within Manyara catchment. Satellite gravimetry GRACE data is used to verify the trend of the water balance result. The results show high spatial and temporal variations and characteristics of a semi-arid climate with high evaporation and low rainfall. We observe that the Lake Manyara water balance and GRACE equivalent water depth show comparable trends a decrease after 2002 followed by a sharp increase in 2006-2007. Despite the small size of Lake Manyara, GRACE data are useful and show great potential for hydrological research on smaller un-gauged lakes and catchments in semi-arid environments. Our modelling confirms the importance of the 2006-2007 Indian Ocean Dipole fluctuation in replenishing the groundwater reservoirs of East Africa. The water balance information can be used for further analysis of lake variations in relation to soil erosion, climate and land cover/land use change as well as different lake management and conservation scenarios. We demonstrate that water balance modelling can be performed accurately using remote sensing data even in complex climatic settings.
Tanzania is embarking on a height modernization programme in support of the AFREF initiatives. Th... more Tanzania is embarking on a height modernization programme in support of the AFREF initiatives. This modernization program is such that in future it will not be necessary for the Surveys and Mapping Division (DSM) to create and maintain the old spirit leveled orthometric height benchmarks. The DSM shall only be required to measure and publish ellipsoidal heights and GPS derived orthometric heights for various survey markers. This paper focuses on the determination of the transformation parameters between the Tanzania National Levelling Datum (TNLD) and the geoid for facilitating GPS heighting in Tanzania. Least squares estimates of the transformation parameters have been determined from orthometric heights in the Tanzania Primary Levelling Network (TPLN) and GPS derived orthometric heights referred to a preliminary model of the African geoid called AGP2003 and a geoid implied by the EGM96 geopotential coefficients. Two transformation models were considered: The Molodensky's transformation formula and a 2D algebraic polynomial. The values of the transformation parameters which were obtained using the AG2003 model produced GPS derived orthometric heights that are closer to the TPLN heights than those produced using the EGM96 geopotential model. It is being proposed here that the new gravimetric geoids that have been developed for Tanzania be used with the GPS observations to obtain new GPS based orthometric heights for Tanzania.
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