Rain is still for free

The excessive water consumption, along with the excessive waste of water in agricultural activities are a fact, and a well-known and long-kept debate. One of the most practical and simple measures that can be taken, at least at first sight, came from the rainwater harvesting. The vertical and horizontal rainwater tanks, built from different materials, UV stable polyethylene for example, are able to store more than 30000 litres of free rainwater and may represent a very efficient alternative for small -medium scale agriculture; allowing a more distributed control of the water management. I am not an expert on this topic, but from just a logic point of view I am wondering why are not their use wider extended. Which are their limitations? Should the governments promote their utilization by subsidizing them?