The hydrological cycle is driven by a complex, interrelated ensemble of dynamic natural processes (climate forcings). The Earth’s tilt and rotation around the Sun are among the primary drivers of seasonal variations in precipitation and water availability. Atmospheric and oceanic circulation patterns and their interactions are equally important drivers of weather, climate and the hydrological cycle.

Humans have a significant impact on some components of the cycle, in which some interventions are deliberate, such as modifying runoff through storage and inter-basin transfers. The former impacts floods and droughts to ensure water is available when needed and damage is averted or minimized when there is an excess; the latter brings water to where it is needed. Other interventions such as changing land surfaces for urban settlements or agriculture can severely alter the hydrological cycle through changes in infiltration, runoff and evapotranspiration rates.

The state of water resources is one of constant change, resulting from the natural variability of the earth’s climate system and the anthropogenic alteration of that system and the land surface through which the hydrological cycle is modulated. Specific changes to water resources and the hydrological cycle include:

- Changes in mean surface flows due to natural climate variability at interannual and multidecadal time scales and climate change
- Increased flood potential due to climate change
- Increased losses due to temperature increase
- Changes in the seasonality (or timing) of flows, especially in snow melt basins
- Changes in flows from glaciers due to their retreat
- Decreasing snow and permafrost
- Groundwater depletion – losing the buffer against rainfall variability
- Changes in soil moisture

The state of water resources is also influenced by withdrawals to meet socio-economic demands. These are in turn influenced by population growth, economic development and dietary changes, as well as by control measures exerted to protect settlements in flood plains and drought-prone regions. These sources of change and the interactions between them create a new level of uncertainty associated with the use and availability of water resources – in addition to existing uncertainties related to the earth’s climate system and hydrological cycle. As a result, it is no longer possible to assume that the future hydrological record will follow the course of the historical record.

Hydrological cycle in Namibia

Namibia is the most arid African country south of the Sahara with low and varied precipitation, from a maximum of ± 650mm in the north east to less than 50mm per year along the coast. It is estimated that only 2% of the rainfall ends up as surface run-off and a mere 1% becomes available to recharge groundwater. The balance of 97% is lost through evaporation (83%) and evapotranspiration (14%).

Namibia’s rainfall is skewed, with the northeast getting more than the west and south-western parts of the country. Namibia’s international boundaries, both northern and southern are marked by the Kunene River in the northwest, the Okavango River in the central north and the Zambezi and Kwando Rivers in the northeast. The Orange River marks Namibia’s southern border. It is only in these rivers that perennial surface water resources are found. These rivers are all shared with neighboring riparian states with an obligation for them to be managed and used in terms of the relevant rules of International Water Law.

Water cycle
Water cycle
Quantities for the water cycle applicable to Namibian conditions
Quantities for the water cycle applicable to Namibian conditions