IGCP 565 Project Workshops

IGCP 565 Workshop 4: Integration of geodetic observations and products in models of the hydrological cycle Support for water management through hydrological models and data assimilation November 21-22, 2011 Johannesburg, South Africa

Space-Geodetic (Earth) Observations in Support of Groundwater Resource Assessment and Drought Management

Andiswa Mlisa (1), Chris Hartnady (1)
(1) Umvoto Africa (Pty) Ltd

Since the inception of sustained interest in the large-scale fractured-rock aquifers in the Table Mountain Group (TMG), e.g., the Deep Artesian Groundwater for Oudtshoorn Municipal Supply (DAGEOS) Project in 1999-2000 (Umvoto Africa, 2005), a quiet scientific revolution has occurred and is gathering momentum with regard to the application of modern Earth Observation (EO) and space-geodetic methods. New technologies for groundwater monitoring and resource assessment are now emerging, with implications for the exploration, development and management of the hidden resources within deep artesian basins that are characteristic of the TMG aquifer systems.

The sustainable management of deep groundwater resources depends on knowledge of physical subsurface properties, fluxes in and out of the groundwater reservoir, and estimates of the total storage capacity. Today, observation techniques to provide this knowledge are limited and often require large human and economic resources for operation. With its proven ability to continuously monitor 3-D positions at the sub-5 mm level, GNSS is a promising avenue for developing complementary observational techniques that would provide valuable information in support of the resource management.

The proposal outlines a design strategy for the monitoring of changes in continental water storage (surface and subsurface) and the remote-sensing of the hydromechanical structure and properties of the deep confined fractured-rock aquifer systems of the Western Cape province by land- and space-based systems, with particular reference to the application of EO technologies. Its focus is an experimental system using a combination of land-based microgravity and Global Positioning System (GPS) observations, complemented by satellite gravity and satellite radar methods for (i) monitoring deep-aquifer storage changes and (ii) determining fundamental hydromechanical properties of the aquifer such as its bulk compressibility.

This requires an extensive array of permanent, continuously-recording GPS stations. The Chief Directorate National Geo-spatial Information (CDNGI) within the South African Department of Rural Development and Land Reform operates a nationwide "TrigNet" array of GPS stations, often co-located with South African Weather Service stations. Although the primary objective for the installation of the network was to provide an accurate and precise positioning service for surveying, engineering and GIS applications, TrigNet data is being used for a variety of applications in geophysics, meteorology and atmospheric physics. TrigNet provides a convenient platform for developing a space- and ground-based system for monitoring natural and anthropogenic fluctuations in aquifer storage through detection of associated small surface deformations.

The current development by the Oudtshoorn Municipality of the Blossoms Wellfield, between George (south) and Oudtshoorn (north) in the Western Cape province, and its proximity to the TrigNet stations GEOR and BWES, provides an opportunity to link its groundwater-monitoring systems to a local surface deformation monitoring experiment, in the context of a long-term (~1 year), large-scale abstraction test. It is anticipated that this experiment will result in an innovative methodology for regional-space-based monitoring and continuous assessment of deep groundwater resources, adding to the conventional methods of water resource monitoring described in the provisions of Chapter 14 of the South African National Water Act.