Current Status of Time Series Analysis in Hydrological Sciences - Hydrologic Time Series Analysis: Theory and Practice - page 107

index ( I A), evaporation ratio ( C E) and runoff ratio (CQ) were analyzed by
using Mann-Kendall and Sen's slope estimation trend tests to detect monotonic
trend direction and magnitude of change over time. Rainfall variability index
showed that 1968 was the wettest year ( d = +1.75), while 1983 was the driest
( d = -3.03), with the last three decades being drier than any other comparable
period in the hydrological history of the Volta basin. An increase of 0.2 mm/
yr 2 ( P < 0.05) was observed in E p for the 1901-1969 sub-series while an
increase of 1.8 mm/yr 2 ( P < 0.01) was recorded since 1970. Rainfall increased
at the rate of 0.7 mm/yr 2 or 49 mm/yr between 1901 and 1969, whereas a
decrease of 0.2 mm/yr 2 (6 mm/yr) was estimated for 1970-2002 sub-series.
Runoff increased significantly at the rate of 0.8 mm/yr 2 (23 mm/yr) since
1970. Runoff before dam construction was higher (87.5 mm/yr) and more
varied ( CV = 41.5%) than the post-dam period with value of 73.5 mm/yr ( CV
= 23.9%). A 10% relative decrease in P resulted in a 16% decrease in Q
between 1936 and 1998. Since 1970, all the months showed increasing runoff
trends with significant slopes ( P < 0.05) in nine out of the 12 months.
Yu et al. (2006) evaluated long-term trends in seasonal and annual
precipitations for 33 raingauge stations in Taiwan. Cumulative Deviations,
Mann-Whitney-Pettitt and Kruskal-Wallis tests were employed to analyze the
trends. Analytical results indicated that the annual rainfall has increased in
northern Taiwan, declined in central and southern Taiwan, and exhibited no
clear tendency in Eastern Taiwan. Almost all of these rainfall series changed
significantly around 1960, which date divides historical rainfall series into
two sample groups.
Kumar et al. (2010) studied monthly, seasonal and annual rainfall trends
using monthly data series of 135 years (1871-2005) for 30 sub-divisions in
India. Half of the sub-divisions showed an increasing trend in annual rainfall.
However, the trend was statistically significant for only three (Haryana, Punjab
and Coastal Karnataka) sub-regions. Similarly, only one sub-division
(Chattisgarh) indicated a significant decreasing trend out of the 15 sub-divisions
showing decreasing trend in annual rainfall. In India, the monsoon months of
June to September account for more than 80% of the annual rainfall. During
June and July, the number of sub-divisions showing increasing rainfall is
almost equal to those showing decreasing rainfall. In August, the number of
sub-divisions showing an increasing trend exceeds those showing a decreasing
trend, whereas in September, the situation is the opposite. The majority of
sub-divisions showed very little change in rainfall during non-monsoon months.
The five main regions of India showed no significant trend in annual, seasonal
and monthly rainfall in most of the months. For the whole of India, no
significant trend was detected for annual, seasonal, or monthly rainfall. Annual
and monsoon rainfall decreased, while pre-monsoon, post-monsoon and winter
rainfall increased at the national scale. Rainfall in June, July and September
decreased, whereas in August it increased, at the national scale.