thumb|Measuring shaft of the nilometer on [[Roda Island, Cairo built in 861 AD on the orders of the Abbasid Caliph al-Mutawakkil]]
A nilometer is a structure for measuring the Nile River's clarity and water level during the annual flood season in Egypt.
Nilometers originated in pharaonic times, were also built in Roman times, and were highly prevalent in Islamic Egypt in Rashidun, Ummayad, Abbasid, Tulunid, Mamluk, Alawiyya and Republican periods, until the Aswan Dam rendered them obsolete in the 1960s.
Description
thumb|Nilometer shown 1800 at the southern tip of Roda Island
Between July and November, the reaches of the Nile running through Egypt would burst their banks and cover the adjacent floodplain. When the waters receded, around September or October, they left behind a rich alluvial deposit of exceptionally fertile black silt over the croplands. The akhet, or Season of the Inundation, was one of the three seasons into which the ancient Egyptians divided their year.
The annual flood was of great importance to Egyptian civilization. A moderate inundation was a vital part of the agricultural cycle; however, a lighter inundation than normal would cause famine, and too much flood water would be equally disastrous, washing away much of the infrastructure built on the flood plain. Records from AD 622999 indicate that, on average, 28% of the years saw an inundation that fell short of expectations. The Palermo Stone reports systems of measurements utilizing units such as cubits, palms, and fingers.
The ability to predict the volume of the coming inundation was part of the mystique of the ancient Egyptian priesthood. The same skill also played a political and administrative role, since the quality of the year's flood was used to determine the levels of tax to be paid. This is where the nilometer came into play, with priests monitoring the day-to-day level of the river and announcing the awaited arrival of the summer flood.
Religious attributes related to the Nile intertwined with the ideology or belief in Ma'at (a system of natural balance). Nilometers were accessible to only members of the city's priests and nobles. The restriction on who may access these structures ensured both accountability in proper readings and political control for the religious communities and ruling classes. overseen by the astronomer al-Farghani. Another nilometer had been ordered in 715 by Usāma b. Zayd b. ʿAdī, who was in charge of collecting the land tax (kharaj) in Egypt for the Umayyad caliph Sulaymān ibn ʿAbd al-Malik.
thumb|upright|Nilometer on [[Elephantine|Elephantine Island ]]
The second nilometer design comprises a flight of stairs leading down into the water, with depth markings along the walls. This location was also particularly important, since for much of Egyptian history, Elephantine marked Egypt's southern border and was therefore the first place where the onset of the annual flood was detected.
The most elaborate design involved a canal or culvert that led from the riverbank – often running for a considerable distance – and then fed a well, tank, or cistern. Yaqut al-Hamawi, writing in the 12th–13th centuries, writes that in the beginning of year 247 AH (March 861), during Yazid ibn Abd Allah al-Turki's governorship of Egypt, the Abbasid caliph al-Mutawakkil had the 'new' nilometer constructed, and ordered that the privilege of measuring the river 'be taken away from the Christians.' The governor then appointed a man from Basra named Abu'r-Raddad who had emigrated to Egypt and taught hadith. He died in 266 AH (879/80), with Yaqut recounting "the supervision of the Nilometer has remained in the hands of his descendants until the present day" (which would have been around 1225 for the author). K. A. C. Creswell states that the main inscription of the nilometer was probably tampered with by Ibn Tulun, saying "there can be little doubt that it was he who removed the name of the Abbasid Khalif." Ibn Tulun would however refrain from substituting his own name, with Creswell saying he may have not felt sufficiently secure to do so. The emir, Ahmad ibn Tulun during 872-873 would spend 1,000 dinars on works carried out on the nilometer.
Amr ibn al-As after the conquest of Egypt reported to the Caliph Umar that when the Nile rose to 14 cubits there was a sufficient harvest, 16 an abundant harvest, 17 being the most advantageous height of all, and that at 18, one-fourth of Egypt would be inundated and usually be followed by plague. The Baghdad doctor, Abd al-Latif al-Baghdadi who traveled Egypt between 1192-1201 would also relate similar measurements. In the exceedingly low water levels of 1201 and 1202, the nilometer remained dry. In 1201, the year commenced with a rise in 2 cubits before rising an average cubits. In 1202, the year began with 1½ cubits before rising to cubits. During the Abbasid Period, the nilometer was used to measure the river level and hence determine the rates of tax in Egypt.
In 1937, efforts to drain and excavate the nilometer were begun by Kamil Bey Ghalib, the Under-Secretary of State for Public Works, employing a new method used by Rothpletz and Lienhard. In this method, the mud could be removed completely and the structure examined.