Transmission loss estimation for ephemeral sand rivers in Southern Africa

Ephemeral sand rivers represent an important water resource in Southern Africa. These rivers only flow for a few days in a year. However, much of this water infiltrates the underlying river bed sediments where it is protected from evaporation and utilized by farmers throughout the dry season. Despite their importance, little is known about how much recoverable water is annually stored within the sand. A particular difficulty concerns obtaining reliable estimates of transmission losses (the amount of water that infiltrates the river bed). The objective of this article was to develop an improved methodology for quantifying transmission loss from ephemeral sand rivers by calibrating a lumped rainfall-runoff model to observed river flow data. Fifteen years of daily river flow data were obtained from four sand rivers in Botswana, namely, Shahshe, Ntshe, Tati and Metsimotlhabe. These data were supplemented with meteorological data from AgMERRA (Ruane et al., 2015) and precipitation data from CHIRPS (Funk et al., 2015). Our simplified rainfall runoff model had four unknown parameters including a river bed infiltration factor, a surface storage capacity, a river bed storage capacity and an average river channel width. Posteriori parameter distributions were derived using a GLUE (Beven and Binley, 1992) methodology. Our study confirms that upper and lower bounds for transmission loss can be obtained by calibrating a lumped rainfall runoff model to a single set of river flow gauging data. Transmission loss was found to represent between 55% and 85% of the total surface runoff at these locations.

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