Automated home irrigation system

It's vacation time and among other things, the issue of watering the plants when we are basking on the beach remains a perpetual concern. I stay in an apartment and have a tiny garden in my balcony, which requires no more than 10-15 liters of water a day. I wanted an automated watering system which should satisfy the following three primary concerns.

• Upper bound on water consumption - I can't afford to have the main line water supply run through out the day, even at a very low throttle. The risk of flooding is too great and given that I stay in a society, this is completely unacceptable.
• Automated system - The system should be able to run unattended for 5-10 days, with predictable water discharge.
• Cheap - Total cost of building the system should be very low. Ideally I should be able to assemble the system with household scrap.

Having evaluated quite a few options on the Internet, I came to the harsh realization that none of them can be self sustaining for 5-10 days without an external inflow of water. Gravity drip was the only viable option.

My first attempt was to create a simple gravity drip as below. I made it using thin leveling tubes and straws tipped with syringe needles as emitters.

While in theory this looks great, it suffered from one fundamental problem - throttling. The water just gushes out and runs off fast. I could reduce the outflow aperture by using a thinner outflow pipe, but then it does not scale for a length of 10-15 meters with 20 emitters. I needed to control the outflow and the obvious solution was to put a throttling cock. What better component than an IV drip throttle ;)

With this I could tune and control the outflow - no issues. It ran successfully for a few hours and then showed a strange behavior which took me a full two days to figure out. The issue was that once in a while certain random emitters would dry up even when others kept their drip on. Very strange indeed. After two days of scalp scratching, I figured out that if the cumulative outflow from a subset of emitters equaled the outflow at throttle, it would create a low pressure in the delivery tube. This low pressure tries to suck in water/air from other emitters. Phew!

The solution although obvious was not easy to fabricate. The solution was the put a anti reverse flow valve at the emitters. Now, how do I create an anti reverse flow fluid valve with household scrap?

The above design worked flawlessly. It operates on the principle of partial vacuum. Water enters an air tight container, pre-filled more than half its volume. The water is drained out of this by a needle tipped outflow tube which taps into the lower portion. The theory being that it is very difficult for any low pressure in the tube to lift out all the water and in case of higher outflow, a partial vacuum is created which sucks in more water from the reservoir. The only care I had to take was coat all joining surfaces with araldite to make it water proof.

Voila! all things put together, I had an indefinitely self-sustaining gravity drip irrigation system which would operate at extremely low pressure thresholds.  

If the reservoir capacity falls short of sustaining the supply for more than 5 days, I could attach a sink valve to the reservoir (same principle as that of water closet) and my reservoir stays topped up indefinitely.