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. 13.
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. 14)
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 and 16)
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. 17).