I am volunteering at an orphanage in Guatemala. There is no well, and the city water is a relatively expensive. I would like to install a rain catchment system on our roof for use in the outhouse bathroom and garden. I've diagrammed it and attached the image. My question is this: Will the water tank completely fill up, or will the water level in the water tank stay at the water level of the red pipe? My education says that the rain will fill up the very bottom of the tank, then the flat part of the red pipe, and then continue to fill the tank and up the red pipe in such a way that the respective water levels are equal. However, common sense seems to be saying that the tank will not fill up past the level of the flat red pipe section because the water has to "push" its way up the tank. Will this system work? Is my common sense concern reflective of reality?