Water Observations from Space: accurate maps of surface water through
time for the continent of Africa
Abstract
Earth observation of waterbodies through time is a powerful tool in
understanding both the location of waterbodies and their temporal
dynamics. Water Observations from Space (WOfS), developed and
well-tested in Australia, is a service providing historical surface
water observations derived from Landsat satellite imagery from 1987 to
present day. WOfS provides better understanding of where water is
usually present; where it is seldom observed; and where inundation of
the surface has been occasionally observed by satellite. We applied the
WOfS algorithm to Africa and validated its accuracy through image
interpretation of satellite and aerial imagery using an online tool
created by the NASA Servir program, Collect Earth Online. The Digital
Earth Africa Product Development Task Team, composed of four regional
geospatial organisations RCMRD, AfriGIST, AGRHYMET and OSS, conducted
the validation campaign and provided both the regional expertise and
experience required for a continental-scale validation effort. In order
to understand the accuracy and bias of the WOfS algorithm in Africa at
both the continental-scale and regional zones, we generated 2900 sample
points covering the continent including the main islands and distributed
them into 7 Agro-ecological zones. We assessed whether the point was
flooded, dry, or cloud covered, for 12 months in 2018, resulting in
34,800 assessed observations. As water information is available through
WOfS in near real-time, it can be used for environmental monitoring,
flood mapping, monitoring planned water releases, and management of
water resources in highly regulated systems. WOfS is expected to be used
by ministries and state departments of agriculture and water management
in countries, international organizations, academia and the private
sector.