Methodology - Response of Flood Events to Land Use and Climate Change - page 86

Then, for each observation the left sided probabilities were calculated by:

R
N þ 1

P L ¼ ð 3 : 37 Þ

where:
P L is left sided probability (probability of having less value in the series); R is
the rank; and N is number of observations.
The return period for each observation was determined using the following
equation:

T ¼ 1
P R

1
1 P L

¼ ð 3 : 38 Þ

The plotting position for each observation was determined as follows:

y ¼ ln ð ln P L Þ ð 3 : 39 Þ

In the total 31 years of data, the maximum discharge per year was extracted
from the originally observed data, and all the above process have been done in
excel and the ''CumFreq'' program. The statistical methods discussed are applied
to extend the available data, and hence predict the likely frequency of occurrence
of a flood event.

3.12 Trend Detection in Hydrometeorological Variables

Hydrologists have been interested in trend testing for hydrometeorological vari-
ables such as precipitation, temperature, and stream flow. The trend analysis of
hydrological and meteorological series is important and even more relevant when
considering the regional effects of global climate change. Trend analysis provides
an idea about climatic variability in the watershed. Trend analysis is done with
Kendall's method to check the significance of trends. The Mann-Kendall test as a
nonparametric method is often used in trend analysis and suggested by many
investigators as a very strong method to detect trends. For example, Hirsch [ 103 ]
applied this test in a water quality trend test application. In another example,
Helsel and Hirsch [ 104 ] provided a comprehensive review of statistical approaches
used for trend analysis of water recourses time series using Mann-Kendall. This
test was originally derived by Mann [ 105 ] and developed by Kendall [ 106 ] sub-
sequently. This part of the research is an attempt to develop an integrated meth-
odology to analyze statistical significant trends in the time series of precipitation,
discharge, and temperature data to monitor the changes over time.