WATER & SOIL - These are not the droids you are looking for.

Êv2
( ko)
Reduced Voriote y
I'O
t1
50 too
ttl
5to20
, Return Period, yeors
Figure 3.3 Differentiation of the three types of extreme value d¡stribution.
I
200
Table 3.1 The relationship between y and T values for the EVI
distribution.
Equations similar to 3.15 and 3.16, but this time relating
y and the probability of exceedence, may be obtained by
substituting P(X > x), as given in Equation 3.5, into the two
equations. This results in
1.O1
2.OO
2.33
5
10
20
30
50
100
200
500
1000
- 1.53
0.37
o.58
1.50
2.25
2.97
3.38
3.90
4.60
5.29
6.21
6.91
-2.O
- 1.5
- 1.O
-0.5
o
o.5
1.O
1.5
2.O
2.5
3.0
3.5
4.O
4.5
5.0
5.5
6.0
6.5
7.O
1.OO
1.01
1.O7
1.24
1.58
2.20
3.25
5.OO
7.90
12.69
20.59
33.62
55.1 0
90.52
148.9
245.2
403.9
665.6
1 097
and
P(X>x) = I -6-e-I
y = -fn(- fn(l-P(x>x))) .....3.18
3. I .4 Sampling properties
317
The data sample, from which a flood frequency analysis
infers a design flood magnitude, must possess the following
properties if the analysis is to make the proper inferences
about the population's distribution.
Sufftcient Length The data sample should be sufficiently
long, i.e., it should contain a large number of items. Often
l0 annual flood peak items are considered sufficient (Neill
1973; Beard 1977), though even then the sample is of limited
use in design (Linsley et al, 1975) unless supplemented
with additiongl hydrological information, €.g., a correlation
with a longer streamflow record from a nearby station.
t6
Water & soil technical publication no. 20 (1982)