Is It Doable?/LoGO

Hello eveybody!Can you suggest me a solution for
this "watering greenhouse" automation problem?

A)FACTS:

i)PLC used:LOGO!
ii)5 greenhouses
iii)Water supply: 3 wells(pumps),city water network,and a water tank.
iv)A given timetable where it is represented which of the greenhouses are watered at which time period.
v)3 classes of demand: a)high: water network and 2 wells and water tank. b)medium: water network and 2 wells. c)low: water network and IF THE WATER PRESSURE OF THE NETWORK IS LOW A WELL MUST BE ACTIVATED!
vi)At the end of the day each one of the 3 pumps must have been activated for the same total time.


B)OBJECTIVE:

The FBD of the solution

C)MY QUESTION:

How can i ensure that the 3 pumps will work the same total time when-in the low demand state- it is possible that pumps might or might not be activated for an unknown period of time?I belive i have to find a combination of special function blocks in order to control the period of time that a pump is on (e.g. comparing the 3 "on" times at specific time intervals)

I would be gratefull if anyone could point to me such a combination of SFs or even better suggest to me a relative application example.
Thanks in advance!

P.S.1 I believe is clear that i cannot use a weekly timer.
P.S.2 HOW can i convert a digital output of a SF to an analog input of an other one.(I need to compare the TIMES-TO-GO of diffirent hours-counter)

Since the pumps in each well become more active when the city water pressure is low, it will be hard to guess how long each pump should be active to ensure the same activity per day. You cannot predict accurately when the city water pressure will drop.

I'd try to worry less about total time per day being equal, and just do a simple pump rotation that would even out total run time over the course of a couple weeks or a month.

What I would do is look at "misting", this can provide watering and proper humidity for cooling; which may also reduce issues with plant disease. You only need to mist during daylight hours using a short cycle timer in other words you may not need to mist more then a few seconds every 15-30 minutes. It is possible this method may also reduce your water use and allow you to run the pumps as needed or through a rotation as mentioned.

Do a web search for Humidifying Greenhouses or Greenhouse Misting.

Thank you both! I appreciate your "qualitative" suggestions!
The problem i posted is not a real-world problem though.It's more like a puzzler.So the approach should be more "quantitative".I mean i have to find a way to compare the times-to-go.For a more comprehensive standpoint i have uploaded the timetable.
I believe that i should do the following:
1)In the low demand state (5-6 & 8-14) i should rotate the backup pumps every "y" minutes, trying to achieve an equal possibility for each pump to be activated.
2)In the medium demand state (6-8) i should assign specific pumps.
3)After the end of the interval (12-14) i should compare the times-to-go (BUT HOW????) and then try to equalize the "on" times during the refilling of the tank and the high demand state (20-22).
Any opinions?????????????????????????