The Nilometer in Cairo, on the southern tip of Roda Island, is a unique historical site
which is often overlooked by visitors. It is one of the oldest existent structures which dates
back to the period just after the Arab conquest of Egypt. Nilometers, known as al-Miqyas in Arabic,
were used to measure the levels of the Nile River and are a remarkable reminder of Egypt's illustrious
past. These water measurement structures continued to be useful up until the modern era when the
Nile's natural flows were disrupted by large water storage reservoirs. Since the Nile River has been
of critical economic importance to both ancient and modern civilizations alike, officials have gauged
its water levels for more than 5,000 years, writing them down for more than 13 centuries. The Nilometer
is an excellent example of an historic water measurement device.
Although a Nilometer has existed in the Cairo area since the Pharaonic Period, the Umayyads (an
early Arab dynasty) constructed a Nilometer on Roda Island in about 715 AD. This structure was restored
in 815, but was destroyed by a flood in 850. The Nilometer existent on Roda Island today was designed
by Abu'l 'Abbas Ahmad ibn Muhammad ibn Kathir al-Farqhani, a native of Farghana, West Turkestan, who is
known in the West as the astronomer Alfraganus. The structure was restored in the 870's and again in
1092. It remains mostly original, except for the wooden conical roof (domed on the inside) which is a
modern restoration (see Photograph 1). The earlier dome was destroyed by
an explosion during the French occupation in 1825. It was rebuilt using, as a reference, an
18th century drawing by a Danish traveler
(see Photograph 2).
Photograph 1. Exterior of the Nilometer on Roda Island.
Photograph 2. Interior of the Nilometer (Photographed by Bill Hocker).
The Cairo Nilometer is a more sophisticated instrument than earlier pharaonic and roman examples
such as the one on Elephantine Island at Aswan (see Photograph 3). The
Cairo model consists of a large pit (or stilling well) that extends below the level of the Nile. The
stilling well is connected to the Nile by three tunnels (each at different elevations) on the structure's
eastern side (see Photograph 4). These tunnels are now blocked off from the
Nile River, so that the Nilometer is no longer functional. There are 45 steps leading down to the bottom
of the stilling well. The height of each step is 24 cm. The steps allowed for a quick reading of the
Nile River's water level.
Photograph 3. The Nilometer on Elephantine Island, Aswan, consists of a flight of stairs and staff gauges.
Photograph 4. The Nilometer on Roda Island is connected to Nile River by three tunnels, one of which is shown here.
In the center of the large well is a marble octagonal column was a Corinthian capital. The top of
the column is held in place by a wooden beam spanning the Nilometer. To measure the water level, the column
is graded and divided into 19 cubits (a cubit equals approximately half a meter), and thus can measure water
levels up to about 9.5 meters. The floods measured by this Nilometer were important to both the rulers of
Egypt, the Caliph, and the general population. During the summer months, the Cairo Nilometer was used to
regulate the distribution of water as well as to compute the levy of taxes paid as tribute by Egypt to the
Arab Caliph. An ideal flood filled the Nilometer to the 16 cubit mark; less could mean drought or famine
(see Figure 1) and more could mean a catastrophic flood.
Figure 1. Real-world interpretation of the readings from the Nilometer.
Because of its importance in determining the area's prosperity, the Cairo Nilometer was important
trigger for the medieval celebration of Fath al-Khalij, the festival of the Opening of the Canal. The Khalij
Canal originated opposite Roda Island and was blocked with an earth dam. It would be opened when the water
level in the Nile River reached 16 cubits. At this level, the summer flood from the river was used to fill
the canal. During the celebrations, decorated boats would crowd the river. Those who witnessed it referred
to it as Cairo's most spectacular festival. Near the Nilometer was a mosque for prayers. The grand celebration
was not a guaranteed annual event. Years when the Nile flood water failed to reach 16 cubits, the celebrations
were canceled, and prayers and fasting were held instead.
The Nilometer at Roda Island provides an important long-term record of the water levels in the Nile River.
In 1936, Jarvis speculated: "In spite of all the changing, uncertain, and erroneous factors that must be considered
in connection with records of stages of the Nile River, it is believed that they disclose some important information;
and there is a fair prospect that they may yield more data with further study . . ." This statement turned out to
be very prophetic. Data collected from the Nile River have spurred the development of a whole field of mathematics
(fractional Brownian motion and fractional Gaussian noise) along with a field of statistics concerned with the
behavior of long memory time series. Gathered by Toussoun (1925), there exists a remarkable hydrological time
series of minimum and maximum water levels for the Nile River. Starting in 622 AD and ending in 1922 AD, the
first missing observation in the annual minima occurs in 1285. This leaves several continuous records to analyze,
the longest one (662 years) is shown in Figure 2.
Figure 2. Time-series record of the Nile River minimum water levels from 662-1284 AD
Click on the image for a larger version.
In an interesting historical note, Whitcher et al (2002) studied the statistical properties of the long-term
Nile River levels from 622 to 1284 AD. Their test confirm an inhomogeneity of the variance and identify the change
point at around 720 AD. This date coincides with the construction of the "new" Nilometer on Roda Island.
One way to anticipate future climate changes is to understand the past. But good instrumental records of
such things as temperature and precipitation extend back only 150 years. This time series is too short to gain a
complete understanding of how climate systems vary. With an eye toward extending the history of climate further
back in time, recent studies have employed various types of data as "proxies" for climate indices. These in turn
can be related to other large-scale phenomena such as El Nino frequency.
Eltahir and Wang (1999) studied the historic water levels from the Nile River (at Cairo) extending back to
622 AD. The researchers found a strong relationship between the Nile's water level and the existence of El Nino-so
strong, in fact, that the Nile record can be used as a proxy for El Nino occurrence. Using this proxy, they
reconstructed the history of El Nino for the past 1,300 years. They found that the frequency of El Ninos in the
1990s has been greatly exceeded in the past. In fact, the period from 700 to 1000 AD was a very active time for
El Ninos, making our most recent decade or two of high El Nino frequency look comparatively mild.
I (Roger Hansen) visited the Nilometer in December 2003 (see Figure 3 for a
location map). I was the first to visit on that particular day and the facilities had not yet been opened. The
cost of admission was 6 pounds E and the caretaker was very pleasant and accommodating.
Figure 3. Map showing general location of the Nilometer
Inside, the Nilometer is much more stunning than I had anticipated
(see Photograph 2 above). The caretaker opened a gate so I could descend to the
bottom of the stilling well. The Cairo Nilometer setup anticipates a modern water measurement structure with
its stilling well, connection(s) to the river, enclosure, and staff gauge. All that is missing is some sort
of mechanical continuous recording device (like a strip chart). A visit to the Nilometer on Roda Island is
From downtown Cairo, the easiest way to get to the Nilometer is by taxi.
Abaza, I. (undated), "The Nilometer on Rawda (Roda) Island in Cairo"
Eltahir, E.A.B. and G. Wang, 1999. "Nilometers, El Nino, and Climate Variability,"
Geophysical Research Letters, 26, 489-492.
Jarvis, C., 1936. "Flood-Stage Records of the River Nile," Transactions of the American
Society of Civil Engineers 101, 1012-1071.
Toussoun, O., 1925. "Memoire sur l'Histoire du Nil." In Memoires a l'Institut d'Egypte,
Volume 18, pp. 366-404.
Whitcher, B., S. Byers, P. Guttorp, and D. Percival, 2002. "Testing for Homogeneity of Variance in
Time Series: Long Memory, Wavelets and the Nile River" Water Resources Research, 38 (5).