Looking for very low tectonic deformation in GNSS time series impacted by strong hydrological signal in the Okavango Delta, Botswana
Abstract
Located in northern Botswana, the Okavango Delta is a vast wetland, fed from the Angolan highlands and constrained
by a half-graben in the Kalahari depression. Since the 70’s, the Okavango graben is usually considered
as the terminus of the East African Rift System. But a recent geodetic study showed there has been no extension
on the tectonic structure over the past 5 years, and recent geophysical studies began to call this hypothesis into
question. The deformation in the area could instead be related to far-field deformation accommodation due to the
motion of the Kalahari craton relative to the rest of the Nubian plate and to the opening of the Rift Valley.
Getting to the vertical deformation isn’t trivial. The GNSS time series show a strong annual deformation of the
ground surface (3 cm of amplitude). On the vertical component, this periodic signal is so strong that it hides the
tectonic long-term deformation, while this information would give a crucial insight on the geodynamic process at
play.
This periodic signal is related to the seasonal loading of water due to the rainy season. This hypothesis is corroborated
by the modeling of the surface deformation based on the GRACE satellites data, interpreted as the variation
of groundwater amount. In the Okavango Delta, the peak of water level isn’t paced with the local precipitations,
but is driven by a flood pulse coming from the Angolan Highlands. The migration of this massive water body isn’t
visible at first order in GRACE data. Yet, local precipitations are supposed to undergo too much evapotranspiration
to be significant in the hydrological balance. Thus this later water body isn’t supposed to produce a mass anomaly
in GRACE time series. This paradox could highlight a relationship not yet defined between groundwater and local
rainfall.
The wide spatial resolution of GRACE data (about 300 km) doesn’t allow a modeling accurate enough to give access
to the slow tectonic deformation, nor to determine the groundwater behavior within the basin. While GRACE
data show a strong groundwater variation in the area, very few direct data are available on this hydrological reservoir.
We thus decided to implement a new geodetic and piezometric network in the Okavango Delta.
The first results show an unpredicted influence of the local rainfall on the water table elevation, with disturbance
or even stop of decrease of the water table. Signals differ between stations, in response to daily evapotranspiration
as well as monthly behavior of the water table