6. Outlook for the near future (2025)
Here at the World Water Forum we will deal with the possibility and the
necessity of rainwater harvesting in the future:
China was one of the first countries carrying out studies using the GIS
- Geographic Information Systems- method to identify potential rainwater
harvesting areas. The factors influencing the rainwater harvesting potential
considered in that study are: the amount of precipitation, the annual
days of rain, the rainfall fluctuation rate and the topography. GIS were
used to overlay the different sheets of factors. The result was a final
ranking of four different areas of rainwater harvesting demand. The final
result is shown in Fig.
In Brazil's semiarid region in the Northeast we worked out a similar map
of ranks of rainwater harvesting demand based on two factors:
The first factor is annual rainfall: the Brazilian semiarid region is
located near the Equator, has an annual rainfall between 250 and 1000
mm and has a very high evaporation rate (open surface evaporation near
or more than 3000 mm a year). Furthermore, the rain falls very irregular
/ uneven. (Fig.
The second factor of ranking considers the hydro-geology of the semiarid
region. The crystalline subsoil contains no or very low contents of often
saline subsoil water. This crystalline region within, but also outside
the semiarid part of Northeastern Brazil, has a high demand for rainwater
harvesting. In the limestone regions the groundwater is already overexploited.
There is normally a medium demand for rainwater harvesting. In the regions
with alluvium there is groundwater exploitable through shallow wells.
In the sandstone area there are very big amounts of subsoil water. In
the last two regions there is a low demand for rainwater harvesting. (Fig.15
The surface water potentials were not considered in this study. In the
semiarid region the only all year water source is the big Sao Francisco
River. Its water is exploited for irrigation of 50000 ha land, especially
for export cash crops. Within the semiarid region, this area along with
some other small spots is considered an exception consisting of potentially
4 % of the total area.
To define the areas with high rainwater harvesting demand is an effective
political instrument. It can be used in working on rural development plans
for the local people which are economically viable, socially just and
ecologically sustainable (Fig.