Results - Response of Flood Events to Land Use and Climate Change - page 203

6.12.3 Detection of Trend: Mann-Kendall Test

A nonparametric Mann-Kendall test was used to determine the trends of the
hydroclimate variables of precipitation, discharge, and temperature. This section
shows the results of a nonparametric Mann-Kendall test. Table 6.22 shows the
Mann-Kendall statistics (z-value) and related P value for precipitation, discharge,
and maximum/minimum temperature time series.
From the Table 6.22 , it is clear that there is negative trend at 5 and 1 % signifi-
cance levels for precipitation at the two stations, Barcelonnette and Allos-Nord, and a
positive trend for maximum temperature series at the Barcelonnette station. The
trend analysis determined that the null hypothesis (H 0 ) is rejected in precipitation
(at the Barcelonnette and Allos-Nord stations) and maximum temperature time series
(at the Barcelonnette station), because the absolute value of Z is greater than Z 1-p/2 ,
where Z 1-p/2 is obtained from the standard normal cumulative distribution tables.
This means that there is a significant downward trend in the precipitation time
series and a significant upward trend in maximum temperature at Barcelonnette
station. However, the P value of monthly discharge was greater than the signifi-
cance level a = 0.05 and one cannot reject the null hypothesis (H 0 ). This means
that there is no significant trend in the discharge series (Table 6.16 ).
The obtained results, in this section, confirm the results of the IPCC in Europe
which indicate a decreasing trend in precipitation and an increasing trend in

Table 6.22 Statistics of the Mann-Kendall test for selected variables

Parameter Station P value (5 %) Kendall's tau P value (1 %) Kendall's tau

Precipitation Barcelonnette 0.023 -0.063 0.023 -0.063

H a : There is a trend in the series

Discharge Barcelonnette 0.192 -0.036 0.192 -0.036

H 0 : There is no trend in the series

Max-temperature Barcelonnette 0.024 0.062 0.024 0.062

H a : There is a trend in the series

Min-temperature Barcelonnette 0.739 0.009 0.739 0.009

H 0 : There is no trend in the series

Precipitation Jausiers 0.379 -0.024 0.379 -0.024

H 0 : There is no trend in the series

Precipitation Uvernet 0.414 -0.021 0.414 -0.021

H 0 : There is no trend in the series

Precipitation Condamine 0.105 -0.044 0.105 -0.044

H 0 : There is no trend in the series

Precipitation Allos-Nord 0.054 -0.060 0.054 -0.060

H 0 : There is a trend in the series

Precipitation Sain-Paul 0.512 -0.021 0.154 -0.021

H 0 : There is no trend in the series
H a : There is a trend in the series (as the computed p-value is lower than the significance level
a = 0.05, one should reject the null hypothesis H 0 , and accept the alternative hypothesis H a ). H 0 :
There is no trend in the series (as the computed p-value is greater than the significance level
a = 0.05, one cannot reject the null hypothesis H 0 )