FIGURE 4.11 Comparison of the Lag-1

FIGURE 4.11 Comparison of the Lag-1 autocorrelation for 10 stations on the Lower Mekong River, compared with 624
global stations (data set compiled by Murray Peel).

The large rivers
of tropical Africa, such as the Congo, have
relatively modest flood regimes in terms of unitdischarge, which is attributable to a combination
of low relief and less extreme tropical rainfall
climates.
By comparison with many other rivers, the
size of the flood pulse in the Mekong is extremely
predictable. Campbell (Chapter 16) noted that
the Cv of the annual flow of the Mekong at
Chiang Saen is only 0.2, while the worldwide
average for rivers with catchments larger than
105 km2 is 0.33 (McMahon et al., 1992). Further
downstream, at Pakse, the site closest to the
Cambodian floodplain, the Cv has dropped to
0.16, less than half the global average.
Even though the flood size is extremely consistent
from year to year, this does not mean
that the size of the flood in one year is a good
guide to the size of the flood in the next year.
The Lag-1 autocorrelation explores the relationship
between floods in successive years. In
catchments with high positive correlation, of
0.6 for example, this would mean that the magnitude
of the flood in one year was quite a
strong predictor of the size of the flood in the
next year. In general, the floods on the Mekong
are only weakly, positively correlated with the
floods of the year before (explaining between
Hình 4.11 So sánh các Lag-1 tự tương đối với 10 trạm trên sông Mêkông, so với 624
trạm toàn cầu (bộ dữ liệu được biên soạn bởi Murray Peel).

10% and 20% of the variation) (Fig. 4.11). This
means that the size of floods from year to year
is essentially independent of each other, which
is typical of rivers of this size.
4. HISTORICAL CHANGES IN
HYDROLOGY
Decadal variation in the hydrology of the
Mekong River provides context for exploring
the possible effects of both human impacts on
hydrology, and climate change. The Mekong
experiences quasi-periodic discharge fluctuations
at an interdecadal scale. This fluctuation
is visible in the annual Mekong dry-season
flows at Vientiane (Fig. 4.12), particularly the
substantial decrease in dry-season flows
between the 1940s and 1950s.
Human impacts on hydrology can be classified
as direct and indirect. The major direct
impact on water volumes in the Mekong is diversion
for irrigation. Indirect effects are caused by
dams and changes in land use, in particular the
conversion of forest to agriculture. These impacts

can alter the gross volume of water in the river,
as well as the timing and duration of flows. There
has been much speculation about the effect of
human impacts on flow regimes, but little investigation
of the evidence, as described below.
4.1. Irrigation
Removal of water for irrigation is the largest
direct hydrological impact on the Mekong
River. Simulations of a 20-year flow period for
the Mekong river basin indicates irrigation
water requirements of 13.4 km3 year, which corresponds
to a 2.1% and 2.3% decrease in the
mean annual streamflow at the outlet (Haddeland
et al., 2006). Half of the diverted water is
estimated to be lost via evapotranspiration,
and half returned to the river (Jackson et al.,
2001). While this is a substantial volume of
water, when compared with irrigation water
demand from other large rivers, this is a modest
diversion. For example, 37% of the total volume
of the Colorado River in North America is
diverted for agriculture (Haddeland et al., 2006).

Although the volume of water diverted for
irrigation is modest, it is important to note that
this diversion occurs in the dry season, when
the relative effect is greatest. For example, in
the delta at Phnom Penh in February, March,
and April, it is estimated that 60%, 45%, and
40% (respectively) of the flow is abstracted for
irrigation (MRC, 2003). It is worth noting that
the majority of dams planned for the Mekong
Basin are designed for hydropower generation
rather than for water extraction. The effect of
these dams will be to increase dry-season flows
(Podger et al., 2004) which could compensate
for increases in dry-season irrigation extractions.
4.2. Effects of Deforestation
Forest degradation in the Mekong Basin has,
according to Giril et al. (2001), been occurring at
an unprecedented rate and scale, particularly
from the 1960s onwards (Table 4.5). On the
Korat Plateau in Thailand, which includes the
Mun and Chi tributary systems, forest cover
was reduced from 42% in 1961 to 13% in 1993

(MRC, 2005). Furthermore, logging pressure on
the forests of Lao PDR, Cambodia, and Burma
was intensified after 1989, when Thailand introduced
a logging ban within natural forests, and
consequently sought increased imports from its
neighbors.
Two potential hydrological impacts of deforestation
might be distinguished:
1. Total water yield may be increased as annual
evapotranspiration decreases, and
2. Seasonal distribution of flows may be modified
as flood runoff increases and dry-season

Kết quả (Việt) 1:

Con số 4,11 so sánh của Lag-1 autocorrelation cho 10 trạm trên sông Mekong thấp hơn, so với 624toàn cầu trạm (dữ liệu bộ biên soạn bởi Murray vỏ).Các con sông lớnnhiệt đới châu Phi, chẳng hạn như Congo, cóchế độ tương đối khiêm tốn lũ lụt trong điều khoản của unitdischarge, mà là nhờ vào một sự kết hợpcứu trợ thấp và ít mưa nhiệt đới cựckhí hậu.Nguồn nhiều con sông khác, cácKích thước xung trận lụt sông Mê Kông là cực kỳdự đoán được. Campbell (chương 16) lưu ý rằngCv của dòng chảy hàng năm của sông Mê Kông tạiChiang Saen là chỉ 0,2, trong khi các trên toàn cầulà sông với catchments lớn hơn105 km2 là 0,33 (McMahon et al., 1992). Xem thêmhạ lưu, tại Pakse, nơi gần nhất để cácCánh đồng ngập lụt Campuchia, Cv đã giảm xuống0,16, ít hơn một nửa mức trung bình toàn cầu.Mặc dù kích thước lũ lụt là vô cùng phù hợpnăm, điều này có nghĩa làKích thước của lũ lụt trong một năm là một tốthướng dẫn để kích thước của lũ lụt trong năm tới.Tụt hậu-1 autocorrelation khám phá mối quan hệgiữa lũ lụt trong năm kế tiếp. Ởcatchments với cao tương quan tích cực, của0,6 ví dụ, điều này có nghĩa là có tầm quan trọngcủa lũ lụt trong một năm là khá mộtCác dự báo mạnh mẽ của kích thước của lũ lụt ở cácnăm tới. Nói chung, lũ lụt trên sông Mê Kôngcó chỉ yếu, tích cực tương quan với cáclũ lụt của năm trước (giải thích giữaHình 4.11 So sánh các Lag-1 tự tương đối với 10 trạm trên sông Mêkông, so với 624 trạm toàn cầu (bộ dữ liệu được biên soạn bởi Murray Peel).10% and 20% of the variation) (Fig. 4.11). Thismeans that the size of floods from year to yearis essentially independent of each other, whichis typical of rivers of this size.4. HISTORICAL CHANGES INHYDROLOGYDecadal variation in the hydrology of theMekong River provides context for exploringthe possible effects of both human impacts onhydrology, and climate change. The Mekongexperiences quasi-periodic discharge fluctuationsat an interdecadal scale. This fluctuationis visible in the annual Mekong dry-seasonflows at Vientiane (Fig. 4.12), particularly thesubstantial decrease in dry-season flowsbetween the 1940s and 1950s.Human impacts on hydrology can be classifiedas direct and indirect. The major directimpact on water volumes in the Mekong is diversionfor irrigation. Indirect effects are caused bydams and changes in land use, in particular theconversion of forest to agriculture. These impactscan alter the gross volume of water in the river,as well as the timing and duration of flows. Therehas been much speculation about the effect ofhuman impacts on flow regimes, but little investigationof the evidence, as described below.4.1. IrrigationRemoval of water for irrigation is the largestdirect hydrological impact on the MekongRiver. Simulations of a 20-year flow period forthe Mekong river basin indicates irrigationwater requirements of 13.4 km3 year, which correspondsto a 2.1% and 2.3% decrease in themean annual streamflow at the outlet (Haddelandet al., 2006). Half of the diverted water isestimated to be lost via evapotranspiration,and half returned to the river (Jackson et al.,2001). While this is a substantial volume ofwater, when compared with irrigation waterdemand from other large rivers, this is a modestdiversion. For example, 37% of the total volumeof the Colorado River in North America isdiverted for agriculture (Haddeland et al., 2006).Although the volume of water diverted forirrigation is modest, it is important to note thatthis diversion occurs in the dry season, whenthe relative effect is greatest. For example, inthe delta at Phnom Penh in February, March,and April, it is estimated that 60%, 45%, and40% (respectively) of the flow is abstracted forirrigation (MRC, 2003). It is worth noting thatthe majority of dams planned for the MekongBasin are designed for hydropower generationrather than for water extraction. The effect ofthese dams will be to increase dry-season flows(Podger et al., 2004) which could compensatefor increases in dry-season irrigation extractions.4.2. Effects of DeforestationForest degradation in the Mekong Basin has,according to Giril et al. (2001), been occurring atan unprecedented rate and scale, particularlyfrom the 1960s onwards (Table 4.5). On theKorat Plateau in Thailand, which includes theMun and Chi tributary systems, forest coverwas reduced from 42% in 1961 to 13% in 1993(MRC, 2005). Furthermore, logging pressure onthe forests of Lao PDR, Cambodia, and Burmawas intensified after 1989, when Thailand introduceda logging ban within natural forests, andconsequently sought increased imports from itsneighbors.Two potential hydrological impacts of deforestationmight be distinguished:1. Total water yield may be increased as annualevapotranspiration decreases, and2. Seasonal distribution of flows may be modifiedas flood runoff increases and dry-season