Figure 9 shows examples of areal
rainfall derived from APHRO_EA_V0804. During May and June, monthly rainfall is increasing
for both Nam Lik and Nam Ngum watersheds, while such a trend is not seen for Nam Khan and
Nam Suong watersheds. Part of the reason for the increase can be attributable to the
underestimation of areal rainfall in 1985 and 1986, when the gauging data is only available at
Luang Prabang (Table 1). For this reason, the seasonal Kendall Test of the monthly areal rainfall was applied to the five watersheds
during the period 1987 - 2002. The
slope estimator B for data representing
a trend with the significance level of
5% was shown in Figure 10. All the
watersheds except for Nam Ou
watershed showed increasing trends in
May with the significance level of 5%,
while Nam Ou and Nam Ngum
watersheds showed distinct trends in
July and August, respectively (Figure
10). The increasing trend in May is
consistent with the earlier onset of wet
season alleged from local people.
Trends of monthly point rainfalls were
also analyzed with the same
significance level for the same period
(actual period of analysis depends on
the data availability shown in Table 1).
In this case, significant increasing
trend was found at Vang vieng for May
and June, and two other points also
showed positive trends in May. Trends
of monthly areal rainfall during the wet
season are likely due to a composite
result of point rainfall with different
trend characteristics.
The result of trend test for the
monthly streamflow was shown in
Figure 12, in which streamflow was
converted to the flow depth by
considering the catchment area of each
watershed. In contrast to the areal
rainfall trends (Figure 10), a strong
increasing trend of the flow depth was
found for July, except for the Nam Lik
watershed. In principle, runoff is
generated with some delay due to the
water storage in relatively dry
watersheds (Figure 6), thus the trend
detected for the streamflow in July is
possibly corresponds to the increasing
trends of rainfall in May. However, areal rainfall of Nam Suong and Nam Ou watersheds shows
smaller or even no trend in May, while the increasing trend of streamflow in July is larger for the
Nam Ou river (Muon Ngoy station) and the Nam Suong river (Ban Sibounhom station, see also
Figure 13). These results indicate that drastic land use changes due to the abandoned swidden and increased shrub over the northern part of the Mekong basin can be related not only to the
increasing runoff ratio but also to the increasing trends of streamflow. These hydrological
impacts can be explained by some reasons such as reduced evapo-transpiration and soil
permeability. For Nam Lik watershed, where no trend of streamflow was found for July (Figure
13), it is considered that the impact of increasing areal rainfall is alleviated by the forest that
covers more than 40% of the watershed. It is interesting to note that the normalized slope
estimator shows increasing trends of streamflow even in the dry season (from October to April),
especially for Ban Na Luang in Nam Ngum watershed (Figure 14). As indicated in Figure 9, a
strong increasing trend of areal rainfall in May is followed by a moderate trend in August for
Nam Ngum watershed. This implies that the larger amount of rainfall during the wet season,
especially in May and August, contributed to the increased streamflow in the subsequent periods
including the dry season.