Analysing the origin of rain- and subsurface water in seasonal wetlands of north-central Namibia

Abstract

We investigated the origins of rain-and subsurface waters of north-central Namibia's seasonal wetlands, which are critical to the region's water and food security. The region includes the southern part of the Cuvelai system seasonal wetlands (CSSWs) of the Cuvelai Basin, a transboundary river basin covering southern Angola and northern Namibia. We analysed stable water isotopes (SWIs) of hydrogen (HDO) and oxygen (H 2 18 O) in rainwater, surface water and shallow groundwater. Rainwater samples were collected during every rainfall event of the rainy season from October 2013 to April 2014. The isotopic ratios of HDO (dD) and oxygen H 2 18 O (d 18 O) were analysed in each rainwater sample and then used to derive the annual mean value of (dD, d 18 O) in precipitation weighted by each rainfall volume. Using delta diagrams (plotting dD vs. d 18 O), we showed that the annual mean value was a good indicator for determining the origins of subsurface waters in the CSSWs. To confirm the origins of rainwater and to explain the variations in isotopic ratios, we conducted atmospheric water budget analysis using Tropical Rainfall Measuring Mission (TRMM) multi-satellite precipitation analysis (TMPA) data and ERA-Interim atmospheric reanalysis data. The results showed that around three-fourths of rainwater was derived from recycled water at local-regional scales. Satellite-observed outgoing longwave radiation (OLR) and complementary satellite data from MODerate-resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer (AMSR) series implied that the isotopic ratios in rainwater were affected by evaporation of raindrops falling from convective clouds. Consequently, integrated SWI analysis of rain-, surface and subsurface waters, together with the atmospheric water budget analysis, revealed that shallow groundwater of small wetlands in this region was very likely to be recharged from surface waters originating from local rainfall, which was temporarily pooled in small wetlands. This was also supported by tritium ( 3 H) counting of the current rain-and subsurface waters in the region. We highly recommend that shallow groundwater not be pumped intensively to conserve surface and subsurface waters, both of which are important water resources in the region.

Impact of rice cultivation on evapotranspiration in small seasonal wetlands of north-central Namibia

“…Furthermore, considering the yearly variability around iishana, more reliable water availability is expected for the small seasonal wetlands (oondombe) which are formed sporadically on slightly elevated areas between the shallow water channels, with the water traditionally being utilized for grazing and fishing (Niipele et al, 2015). Since the surface water of such small wetlands is important for both direct exploitation of the surface water and for underground water storage (Hiyama et al, 2017), the effect of agricultural use on water availability is critical to regional water cycles.…”