Terrestrial Water Storage from GRACE and Satellite Altimetry in the Okavango Delta (Botswana)
New technology can for the first time enable the accurate retrieval of the global and regional water budgets from space-borne and ground-based gravity surveys. Water is mankind’s most critical natural resource, but it is being heavily used throughout the
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Terrestrial Water Storage from GRACE and Satellite Altimetry in the Okavango Delta (Botswana) O.B. Andersen, P.E. Krogh, P. Bauer-Gottwein, S. Leiriao, R. Smith, and P. Berry
Abstract New technology can for the first time enable the accurate retrieval of the global and regional water budgets from space-borne and ground-based gravity surveys. Water is mankind’s most critical natural resource, but it is being heavily used throughout the globe. The aim of this paper is to outline the HYDROGRAV project dealing with improving large scale hydrological model with time variable gravity observations. Also preliminary HYDROGRAV investigationsa of terrestrial water storage variations in the Okavango delta in Botswana are presented. Data from 4 years of satellite altimetry, GRACE derived TWS and GLDAS hydrological model all show a clear annual variation corresponding to the well known seasonality of the delta. However, they also all show an increasing trend in the amount of water storage in the region over the last 4 years.
69.1 Introduction One of the fundamental connections between hydrology and geodesy stems from the fact that a 2.4 cm thick, infinitely extended layer of pure water located at any depth below a gravimeter generates an incremental gravitational acceleration of 1 μgal (= 1 × 10-8 m/s2 ) (Knudsen and Andersen, 2002). Temporal change in terrestrial water storage (TWS) in the earth
O.B. Andersen () DTU Space, Juliane Maries Vej, 2100 Copenhagen, Denmark e-mail: [email protected]
system is therefore proportional to the temporal change in the measured gravitational acceleration. The potential of time-lapse gravity surveys to monitor the status of water resources systems has been recognized since a long time and can benefit a number of disciplines (Crossley et al., 1999). Ground-based time-lapse gravity surveys were used successfully to determine alluvial aquifer storage and specific yield, which is a key parameter for the sustainable management of groundwater resources (Pool and Eychaner, 1995). Moreover, it has been demonstrated, that superconducting ground-based gravimeters reflect hydrological signals on the order of several microgals (Amalvict et al., 2004; Bower and Courtier, 1998; Neumeyer et al., 2006; Hinderer and Crossly, 2004). The new generation of Scintrex portable gravimeters has enabled the possibility of local time-lapse surveys. The Scintrex instruments deliver local measurements with an accuracy of 4 μgal or 10 cm of water (Scintrex A-10, Scintrex CG-5) (www.scintrex.com). With the launch of the GRACE satellites (Tapley et al., 2004) water storage estimates with an accuracy of 0.4 μgal or 1 cm of water are now achievable from space for spatial scales larger than 1,300 km (Andersen and Hinderer, 2005; Andersen et al., 2005). However these accuracies are constantly being improved with the longer timescale of data and improved GRACE data processing. Recent investigations show that the inherent spatial and temporal resolution of GRACE data is around 300–400 km and 10 days (Rowlands et al., 2005). Su
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