APPLICATION OF AIRBORNE GEOPHYSICS IN LARGE SCALE INTEGRATED HYDROLOGICAL MODELLING. CASE STUDY: OKAVANGO DELTA, BOTSWANA

摘要

Airborne geophysics is a remote sensing technique that can be used to derive some parameters that are direct inputs into distributed groundwater models, e.g., aquifer thickness. It can also be used indirectly in estimation of some parameters through model calibration, e.g., hydraulic conductivity of flow barriers knowing their locations accurately from geophysics. This paper focuses on the integrated use of remotely sensed airborne magnetic and ground based gravity data sets in order to determine the aquifer geometry (thickness and structural trends). We further look into another geophysics technique; seismotectonics to evaluate the earth's subsidence below the delta caused by the current extensional forces. The results show that the aquifer thickness is structurally controlled and is on average 180m thick. We estimate the earth's crust to be 32km thick below the delta using gravity data. Using this information together with the calculated earthquake moments from seismic event data (1952 to 2005), we estimate an earth subsidence rate of 0.058 mm/yr. We show thorough the use of a coupled surface-groundwater hydrological model that earthquake movements could have played a role in influencing the surface flow regime shift observed in the Okavango Delta. This is done by tilting the most tectonically active north eastern block and the panhandle by a modest figure of 30cm in a southwest direction and simulating the effect of this on the spatial distribution of flooding frequency. An average increase of about 20% in flooding frequency is observed on the south western branch of the delta.