I just bought the water pump. I have a power bank 5V 2A 10Ah. I just connected it to my Arduino Uno R3. And added a motor shield to it. If I connect the pump to the motor shield and run it on 5V will it run or I need to power the motor shield with external power ?
Thanks in advance ! 
Probably. Try it!
You need to be sure that the motor current is NOT flowing THROUGH the Arduino board.
...R
Dasaradh:
added a motor shield to it.
Which motor shield?
A lot of h-bridge chips require a higher motor voltage than logic voltage.
It would also be good to know who much current the h-bridge can supply.
If your motor runs at 3W with 5V (not a very safe assumption) then it should draw 600mA of current. This is more current than some h-bridge chips can handle.
The motor's specs list 8V as the lower working range. Still 5V might work.
Its rated at 8V to 15V, so no, it won't be much good at 5V. It will be slower, it'll probably
collect bubbles and stall if running too slow too.
not all water pumps are positive displacement
many are centrifgual and only spin the water to create a pressure.
that pressure gets forced around the blades, but most is pushed out the exhaust port.
at slower speeds, the force will not be very strong,.
pumps are measured in gallons per hour or gallons per minute as one value
the second is the head pressure. how high it can push water up.
if you put it in a birdbath, it might push up an inch and deliver 100 gph.
but add a tube and it may not be able to lift water up 1 foot.