The management of scarce water resources using GNSS, InSAR and in-situ micro gravity measurements as monitoring tools

Abstract

South Africa is a water scarce country hence the careful monitoring and management of available water resources is critical for the wellbeing of the citizens of the country. A high percentage of the Earth’s water supply is stored underground which can be extracted either through pumping or using artesian pressure. This paper describes the application of GNSS, InSAR and In-Situ Micro Gravity measurements for the monitoring of an artesian wellfield in the Oudtshoorn District in the Western Cape province of South Africa. GNSS receivers were run continuously for a period of 133 days between March and August 2014 to detect possible surface subsidence during pumping and artesian free flow extraction of water in the wellfield. Two InSAR scenes were processed, one during the peak period of water extraction from the wellfield and the other approximately 4 months after all boreholes were closed and pumps switched off. A micro-gravity campaign was conducted over two days in the wellfield with the gravity meter co-located at one borehole which was opened on the second day of the campaign. The results from the GNSS monitoring showed a subsidence of approximately 15 to 20 mm at the peak of the free flow and pump while those from the InSAR and microgravity measurements were largely inconclusive.