I believe that rainwater harvesting in arid regions has limitations that cannot make a sole reliable source of water. The efficiency of the system in meeting different demands depends on the temporal variability of rainfall as well as the annual rainfall amount. In arid regions, we have many days and even months without rain, especially during the high demand seasons. Therefore, in order to save the water for dry periods, the size of the tank becomes excessively large and uneconomical (just like constructing a large dam reservoir in a dry region). Even then, the amount of captured rainfall may not be sufficient since the annual rainfall amount is not high enough. Overall, rainwater systems may be looked upon as complementary systems in arid regions, and not the sole source of water. Conjunctive use is recommended.

Rain water harvesting exist since centuries, especially in arid regions e.g. Rajasthan and Southern India and must be there in other parts of world. Most of these techniques are very scientific they only need scientific data collection and documentation.

As pointed out by Bahram, arid regions get very little rainfall and will need large sized tanks which may not be economical.

Water demands vary depending upon the size of population, social status of persons (high income group and free users tend to waste), animal population and crop requirements.

Rainfall is the only source of water in most of the arid regions, however, with construction of big dams, many arid regions do receive water through canals.

Conjunctive use is possible only if there is sufficient ground water; further, the source of ground water is forest - vegetative cover, mountains, topographic location of a place and adjacent humid regions.

Water harvesting Storage = Demand - Available ground water (quality ? -> purpose of use) - canal water (?)

Cost will depend upon the available material used for construction of storage

Harvested rain water in many arid regions of India is mainly use for drinking purposes, whereas in semi arid regions it is used for multi purpose activities.

Rain water in tanks, if not stored properly causes development of water borne bacteria - E coli which could be easily destroyed by solarization, boiling, UV filter, etc.

Effective water management as is done in Australia, New Zealand, Netherlands, etc by continuous monitoring demand supply on watershed basis is the solution.

Recycle and reuse of water in most of these countries is also the solution to manage the limited water resource.

For assessment = whether the storage is sufficient to meet the demands ? Cost of storage ? Changes in quality of stored water ? Social acceptability and affordability? Water table of the region ? Underground / Above ground storage ?