We could observe from the [Pump curve] that Total Head decreases with increase in flow rate, but for [System Curve], it is opposite, i.e, Total Head increases with flow rate. Why is that so? I need a detailed explanation or a link where I could understand this curve.
It's quite simple:
When you're pumping a fluid, as the flow rate increases, head losses also increase. That's why the pump head decreases. As there are more internal losses, the pump provides the fluid with less energy. Pump curve: A-B*Q^2 (A and B being positive coefficients).
In the system, the increase of the flow rate increases losses as well, so the resistive head goes up. System curve: C+D*Q^2 (C and D being positive coefficients).
The operation point is the point where the pump head equates the resistive head of the system and that is true for a determinate flow rate.