Overhead Tank Cooling System

None if you have 40 deg Celsius air temperature.

How cold is your underground water?

Is that at 38 deg C?

Put the tank underground.

What is the coldest water you can get in the area?

Maybe use a chiller unit.

Put a double roof on you tank to prevent some heating of the water. A shading system.

People want it hot in summer. You seem to be one of the few thats going the other way around.

Outback Queensland used to have similar problems.

There was a simple evaporative structure that had a length of the water pipe passing through it to provide cool water.

These "safes" or sometimes called "drip safes"(built to keep food safe) had charcoal chunks in a wooden structure that enabled air flow through the charcoal. Evaporation gave cooing effect.

The water quality on the evaporator was not really important, so most used groundwater (not drinking water) for the evaporator. They were run so that there was always water dripping from the charcoal, otherwise they would eventually choke with the accumulated salt and suspended solids from the water.

They were usually build in the shade of a verandah to eliminate direct sunlight exposure.

Insulate the tank and install a small recirculation loop between the underground and above ground water tanks.

Either bury the overhead tanks or delete them and pump the water from the underground tank.

You could lag the tanks with some thermal insulation, couldn't you?

What's the water being used for and how much are you using daily?

I am guessing this is for some sort of process rather than staff drinking out of the tap, but the solution really depends on your application.

Please provide more information. Also, how large is your storage tank and where is your site location (and hence ambient and possible sun strike temperature rise)? What's your target temperature?

Refrigeration system is ideal to cool the water to the desired temperature but it is energy intensive. If the flow rate of water is high and consider to use stores under ground water with pumps and pressurization unit along with proper control system and eliminate over head tank. You need to work out the energy consumption in this case also. Alternatively cover the shed of overhead tank with proper insulation to minimize the heat transmission.

Install a mixing valve joining the two water sources! Adjust flow to the desired water temperature... In my mind that would be your simplest and cheapest solution!

I suspect you will need a chiller (depending on the size of the building you are supplying water to), which is basically a larger refrigeration cycle. If you already utilize evaporative cooling in the area, then evaporative cooling on the supply line from the tanks will work (as a heat exchanger only), as long as the drinking tap water is not in direct contact with the outside. Otherwise use a refrigeration cycle to provide cold source to chill the water as it leaves the tanks into the facility. Depending on how you size the chiller unit, you could have tap water as cold as 15-20 C if you can afford the energy bill.