Every serious pump datasheet contains the same chart: a curve that slopes gently downhill. It intimidates many buyers, but it tells you more in five seconds than five pages of description.
The two axes
The horizontal axis is flow (Q): how much water the pump moves. The vertical axis is head (H): the pressure it works against, expressed in metres. Read the curve like a promise: "at this head, I will give you this much flow."
Why the line slopes down
A pump trades pressure for flow. Pumping across flat ground it delivers its maximum flow. As the lift increases, flow falls, until at the curve's end (the shut-off head) it moves nothing at all. Any pump can reach any point on its curve; the question is whether your point is on it.
Find your duty point
Take your required head (lift plus pipe losses) and required flow. Find that pair on the chart. It must sit below the curve, with a little room to spare. If it sits above the line, that pump cannot do the job.
The sweet spot: BEP
Curves often mark a Best Efficiency Point, where the pump converts the most shaft power into water movement. Operating near BEP means lower energy bills, less vibration, and longer bearing and seal life. Working for years at the curve's extreme ends, fully throttled or nearly dry, is how pumps die young.
A worked example
You need 30 metres of head and 4 m³/h. Pump A's curve passes through (30 m, 6 m³/h) near its centre, which is ideal. Pump B barely reaches (30 m, 4.2 m³/h) at its extreme edge, so it will do the job while straining at low efficiency. Same price? Take the machine working comfortably in its middle ground.