12. Example Zambezi

Note

In development

Zambezi basin

The hydrological model CWATM is intended to be scalable and can be applied over finer spatial scales (e.g., basin). CWATM has been calibrated for the Zambezi, using six sub catchments and measured discharge

Assessment of water stress

The CWATM calibrated model is used to assess water scarcity till 2050 in the Zambezi basin. Water resources at each grid cell are depended of climate, of water managements (e.g. reservoirs) and of water use for irrigation, livestock, domestic or industry. For each cell (at 5 arcmin ~ 9x9 km2) and for aggregated regions water resources can be related to water demand from different sectors. Results from the distributed hydrological model CWatM are aggregated to 21 sub-basins.

Water Stress Index for Zambezi

The WSI is defined in Falkenmark et al., 1989, Falkenmark, 1997 and Wada et al., 2011 as comparing blue water availability with net total water demand. A ration defines water scarcity in terms of the ration of total withdrawals across the sectors domestic, industrial and agriculture to water resources.

{WSI= Water demand / Water availability}

where:
WSI: Water Stress Index
Water demand: Net total water demand as sum of livestock, irrigation, industrial and domestic water demand
Water avail: Freshwater availability from river, lakes and renewable groundwater

A region is considered “severely water stressed” if WSI exceeds 40% (Alcamo et al., 2003). The yearly WSI shows no water stress for the whole basin in 2010 but this will change till 2050 for the BAU scenario (business-as-usual composed from SSP2 and RCP6.0 scenarios) mainly due to increasing agricultural and domestic water demand (increase by a factor of 5) as annual mean river discharge is only increasing by 6%. August is chosen for monthly comparison as this is the month with the highest rate of water withdrawal (WW) and a mean monthly discharge (MMD) with is only slightly higher than in November:

plot from API (1)

Figure 1: Annual water stress 201- and 2050