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THE DERIVATION OF IDF CURVES FOR PRECIPITATION FROM M5 VALUES
from two different localities and one is a 40 years series while the other
is a 10 years series.
5 The program MT_REIKN
A spreadsheet program has been designed to calculate rainfall inten-
sities by the method described here. It is called MT_REIKN. It is a free-
ware program designed for use by engineers and designers who need
“box rains” (values for rainshowers of constant intensity for a prede-
fined duration and return period) in their design work. 9 duration val-
ues and 6 return period values can be chosen by the user. Fig. 6 shows
the input side and the intensity output table from the program. The text
is in Icelandic as is in the program. It is necessary to insert the M5 value
which is supposed to be picked frotn a map. It is also necessary to insert
the Ci value, but in the program there are given 3 values to choose from,
each for a predefined locality, see table 3.
Inntaksstærbir
Varandi Endurkomu- 1M5
(mín) tími, TS 70
10 (ár)
20 1
30 3 Ci
60 5 0,205
120 10
180 20
360 50
720 1M5 gildi fyrir
1440 Wussuw reikninga
65
1M5 Ci
70 Reiknuð úrkomugildi (l/s/ha) 0,205
Varandi
TS 1 3 5 10 20 50
10 59 79 88 100 112 128
20 45 59 66 75 85 97
30 38 50 56 64 72 82
60 29 38 42 48 54 62
120 21 28 32 36 41 46
180 18 24 27 31 34 39
360 14 18 20 23 26 29
720 9 12 14 16 18 20
1440 6 7 8 9 11 12
Fig. 6 Input (left side) and output (right side) from MT_REIKN.
Text in lcelandic
1969-1999
30 ára reynsla
Einangrunargler
GLERVERKSMIÐJAN
Samvchk
Eyjasandur 2 • 850 Hella
® 487 5888 • Fax 487 5907
Table 3 Cj Values
Locality Region
Reykjavik Capital Area 0.205
Keflavik Reykjanes 0.19
Selfoss Southern Lowlands 0.21
As one can see the C; values are very close to each
other so the results will always be very similar. This does
not mean that it is recommended to use these values
everywhere. On the contrary it is expected that C^ values
are somewhat higher in the highlands than in the low-
lands, but there are not enough observations available to
support this theory yet. These C( are all compiled from
24h data using all available meteorological stations in
the respective areas. But besides that they are all very
close to each other, they are all very close to the Cj val-
ues for the shorter duration series in table 2. This is even
more remarkable, but has simplified both the analysis
and the programming work considerably.
6 Conclusion
The formula (12) may be used as a generalized IDF
relationship for “box rains” in Iceland
I = M5 f(T) g(tr) (12)
The five year 24h annual maximum rains, M5, can be
used as index variable to calculate rains for all return
periods by using the multiplication factor:
f(T) = 1 + q(y- 1.5) (13)
í^^)2+o.oo,
14)
Results from 3 regions in South and Southwest-
Iceland indicate that a Ci value of 0.20 is a close approx-
imation, within 5% from the real value. This does not
have to hold outside these areas however.
7 References
ASCE; Manual on Engineering Practice No. 37 Design and
Construction of Sanitary and Storm Sewers. WPCE
Manual of Practice No. 9.
Novotny, V., K. R. Imhoff, M. Olthof, and PA. Krenkel,
1989; Karl Imhoff’s Handbook of Urban Drainage and
Wastewater Disposal. J. Wiley & Sons, N.Y.
Jónas Elíasson, Vatnafrœðilegar forsendur fráveituhön-
nunar á höfuðborgarsvceðinu. Árbók Verkfræðinga-
félags íslands 1994/1995 ( In Icelandic)1996
Elíasson, Jónas, A statistical model for extreme precipi-
tation, Water Resources Research, Vol. 33, NO 3, pp.
449-455. 1997
Flood Studies Report (FSR), Vol. II; Natural
Environment Resource Council (NERC); ; 1975
Förland, E. and Kristoffersen, D. ; Estimation of Extreme
Precipitation in Norway; Nordic Hydrology, Vol. 20,
No 4/5; 257-276; 1989
g(tr) = 0,02474

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