The old and new – this clearly shows the falaj and the fact that it is open. There are very strict social and cultural rules about aflāj systems as they are communal supplies of water in an arid climate. No one pollutes it; the whole village will have contributed to the cost of construction and up-keep, in most areas the amount of water is divided up between each family on a ‘timed’ basis. A form of sun-dial clock (post in the ground with graduated lines) was and in some cases still is used for this purpose.
In very arid areas, the falaj can travel for many kilometres underground with entrance holes so that it can be maintained; finding water, construction and maintenance is skilled and costly work.
A quote from that well known on-line encyclopædia –
In Oman from the Iron Age Period (found in Salut, Bat and other sites) a system of underground aqueducts called Falaj were constructed, a series of well-like vertical shafts, connected by gently sloping horizontal tunnels. There are three types of Falaj: Daudi (داوودية) with underground aqueducts, Ghaili (الغيلية) requiring a dam to collect the water, and Aini (العينية) whose source is a water spring. These enabled large scale agriculture to flourish in a dry land environment. According to UNESCO, some 3,000 aflaj (plural) or falaj (singular), are still in use in Oman today. Nizwa, the former capital city of Oman, was built around a falaj which is in use to this day. These systems date to before the Iron Age in Oman.
David A Lockwood 
Very interesting post. I was reading recently about similar ancient Greek systems where the “pipes” had engineered into them various baffles that caused the right kind of turbulence to keep sediments in suspension so that they did not get clogged up. I wonder if the aflāj systems had similar construction.
ehpem,
Thank you for the comment, I think it is just ‘blood, sweat & tears’ here.
David.