The best and precise way of to measure is using a flow meter, but this would be a costly proposition
Alternatively,
Measure using a timer and a drum, this will give you a fair idea.
Or
Please look at the attached sketch. Make a L shaped scale with 4" on one side and measure X in inches. Give the values of X and D so that I can calculate the flow volume and velocity.
If it is overhead tank it has to be RCC structure. If it costs more than 10 lakhs then solar pumping system would be a cheaper option.
Or you can have a ground level reservoir with brick construction in a elevated place. This will reduce construction cost by 60% compared to RCC overhead tank. Outside of the tank, walls can be supported by soil like a tank a bund.
Also, why specifically overhead tank? Because if this is done using brick and mortar, cost will be enormous. And PVC tanks dont come in large volume holding and isnt economical as well.
If you are not thinking in terms of gravity based irrigation system, consider having a farm sump constructed using LDPE pond liners. This is considerably economical.
And, if the rationale behind the storage is because of non-availability of continuous power during day time, then yes you can consider solar water pumping system. So essentially you should be comparing the cost of having a storage tank vs cost of solar water pumping system.