In an arid to semiarid country, such as Namibia, water may be the single most important limiting resource in sustainable development. The planning and operation of surface water supply must therefore take into account the natural variability and unpredictability of the rainfall and river flow. Environmental water requirements of unique wetlands at the mouths or Deltas of internationally shared rivers have to be duly considered should any form of development be undertaken. Historically the westward flowing rivers were opportunistically utilized by nomadic people and their cattle. An increase in population has resulted in increased pressure on the limited available resources. In the event of construction of dams on the ephemeral rivers, careful consideration has to be given to the possible effects on the downstream sites in these linear oases. All of the above factors point to the need for improved hydrological monitoring and modelling in arid zones with inherently uncertain hydrological regimes.