I supply water in an urban from a tank at 1250m elevation, and a pipe of 300mm from tank. The route for a pipeline has an up and down profile, and the pipe is tapped along the way, but amazingly when the pipe reaches to a certain point near the end of it, which is about 1235m elevation, the flow of water is so little, sometimes we find no water at all while the tank is full to overflow. What could be the problem?
Your problem is easily identifiable. Along your route you will find one or more points higher than 1250m. Take an elevation measuring device. be exact. find the highest point on that route. Build your 1250m water tank at the new highest level. Add 5 to 10 meters for good measure. If funds to do this are not available then find the points higher than 1250m and splice in a solar powered sump pump. Keeps the pressure strong at the weakest points on your route.