5.1.1 General
There are six main objectives with the hydrological estimations:
• Estimations of water losses for energy production, due to irrigation demands;
• Estimations of water losses for energy production due to increased evaporation, when the new reservoirs are in operation;
• Evaluation of influence on irrigation in the Vientiane plains from the filling of the new reservoirs;
• Evaluation of influence on energy production from the filling of the new reservoirs;
• Evaluation of influence on irrigation in the Vientiane plains from operation of the new hydropower projects;
• Estimation of energy production under the assessment’s three scenarios.
The water losses due to irrigation demands in the upper catchments are small, almost negligible, since most existing and future planned irrigation is located in the Vientiane Plains, downstream from all hydropower plants. However, in order to properly describe the real expected future situation according to the expressed needs of Department of Electricity, the impact of the future irrigation development in the upper parts of the basin has been considered in the energy estimations. Therefore, extracted flows for irrigation demands in 2007, 2013 and 2020 are estimated below.
5.1.2 Water losses for energy production due to irrigation
Appendix A provides information on existing and planned areas for wet- and dry-season irrigation for the various areas in the NNRB, but it has not been possible to find specific information about actual planned irrigated areas for each sub-basin. However, since 45 % of the GoL-planned irrigation areas in the whole NNRB are already irrigated in 2007, we have assumed that the same share per district will be true also for the future planned areas. Thus, Table 5.1.1 shows scaled-up areas withdrawing water from each hydropower (intermediate) catchment area. The data is separated into wet and dry seasons with the wet season extending from May through November and the dry season from December through April.
Table 5.1.1: Irrigated areas in the catchments of the Hydropower plants
Planed irrigated areas (ha) Actual irrigated areas 2007 (ha) Hydropower
Plant impact
area May-Nov Dec-Apr May-Nov Dec-Apr
Nam Ngum 5 217 20 98 9
Nam Ngum 3 3 178 919 1 430 414
Nam Ngum 2 1 845 663 830 298
Nam Ngum 1 2 136 173 961 78
Nam Lik 2 3 000 1 537 1 350 692
Nam Lik 1 12 800 4 068 5 760 1 831
Total 23 176 7 380 10 429 3 321
Estimated cumulative extracted flows at each plant at present situation in 2007 are shown in Table 5.1.2. In the estimations it has been assumed that the total extraction during May-Nov is 5 000 m3/ha and during Dec-Apr it is 15 000 m3/ha.
Table 5.1.2: Extracted flows for irrigation demands in 2007
Estimated extracted flows 2007 (m3/s) Hydropower
Plant impact
area May-Nov Dec-Apr Year Year acc.
Nam Ngum 5 0.03 0.01 0.02 0.02
Nam Ngum 3 0.39 0.47 0.42 0.44
Nam Ngum 2 0.23 0.34 0.27 0.71
Nam Ngum 1 0.26 0.09 0.19 0.90
Nam Lik 2 0.37 0.79 0.54 0.54
Nam Lik 1 1.56 2.08 1.78 2.32
Total 2.83 3.78 3.22 3.22
It has been assumed that the irrigated areas will, in practice, increase by 10% from 2007 to 2013. Between 2013 and 2020, a further 10% increase is expected.
The irrigation demands in 2007, 2013 and 2020 are assumed to be independent of the hydropower development in the NNRB.
Table 5.1.3 shows the reductions in available runoff for hydropower production due to increased irrigation demands estimated for the years 2013 and 2020.
Table 5.1.3: Extracted flows for irrigation in 2013 and 2020.
Extracted flows for irrigation 2013 (m3/s) Extracted flows for irrigation 2020 (m3/s) Hydropower
Plant impact
area May-Nov Dec-Apr Year Year acc. May-Nov Dec-Apr Year Year acc.
Nam Ngum 5 0.03 0.01 0.02 0.02 0.03 0.01 0.02 0.02
Nam Ngum 3 0.43 0.52 0.46 0.49 0.47 0.57 0.51 0.53
Nam Ngum 2 0.25 0.37 0.30 0.79 0.27 0.41 0.33 0.86
Nam Ngum 1 0.29 0.10 0.21 0.99 0.32 0.11 0.23 1.09
Nam Lik 2 0.40 0.87 0.60 0.60 0.44 0.95 0.66 0.66
Nam Lik 1 1.72 2.29 1.96 2.55 1.89 2.52 2.15 2.81
Total 3.11 4.16 3.55 3.42 4.57 3.90
In Table 5.1.4, extracted flows for irrigation in each development scenario (see Chapter 4), have been put together.
Table 5.1.4: The extracted flows (m3/s) for irrigation for the three development scenarios.
NN5 NN3 NN2 NN1 NB2 NB1 NL2 NL1
2007 - - - 0.90 - - - -
Scen. 1a - - 0.79 0.99 - - - -
Scen. 1b - - 0.86 1.09 - - - -
Scen. 2a - 0.49 0.79 0.99 - - - -
Scen. 2b - 0.53 0.86 1.09 - - - -
Scen. 3a 0.02 0.49 0.79 0.99 0.00 0.00 0.60 2.55
Scen. 3b 0.02 0.53 0.86 1.09 0.00 0.00 0.66 2.81
5.1.3 Water losses for energy production due to evaporation
Due to evaporation losses, the inflows to the existing NN1 will be reduced when new reservoirs are constructed in the up-stream parts of the catchment.
Table 5.1.5 show the net reservoir losses (mm/year) according to EdF (2007). The evaporation rates have been calculated on the basis of measurements carried out by Department of Meteorology and Hydrology for Phonehong, altitude 179 m.a.s.l.
Table 5.1.5: Evaporation, net reservoir losses (mm/year) at impounded areas
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total
58 93 110 70 0 0 0 0 0 0 0 0 331
Since the reservoirs are drawn down during the period January to April from approximately Full Supply Level (FSL) down to Minimum Operation Level (MOL), the reservoir area at the average level in each reservoir is used in the estimations of the inflow reductions due to evaporation.
The data in the table 5.1.6 show the evaporation losses in m3/s from each reservoir.
Table 5.1.6: Flow reduction in m3/s due to evaporation from reservoirs
Area km2
Jan (m3/s)
Feb (m3/s)
Mar (m3/s)
Apr (m3/s)
May (m3/s)
Jun (m3/s)
Jul (m3/s)
Aug (m3/s)
Sep (m3/s)
Oct (m3/s)
Nov (m3/s)
Dec (m3/s)
Year (m3/s) NN1 331.8 7.19 12.76 13.63 8.96 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.54 NN2 90 1.95 3.46 3.70 2.43 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.96 NN3 18.4 0.40 0.71 0.76 0.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.20 NN5 9 0.19 0.35 0.37 0.24 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.10 NB1 4 0.09 0.15 0.16 0.11 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 NB2 9 0.19 0.35 0.37 0.24 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.09 NL1 17 0.37 0.65 0.70 0.46 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.18 NL2 10 0.22 0.38 0.41 0.27 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.10
5.1.4 Available flows for hydropower production
Annual average available flows at each dam site, taking irrigation demands and evaporation losses into consideration, have been calculated for the present situation in 2007 and the three scenarios. The result is shown in Table 5.1.7. The flows include diversions to the NN1 reservoir from Nam Xong and Nam Leuk.
Table 5.1.7: Available annual average flows for hydropower production in m3/s
NN5 NN3 NN2 NN1 NB2 NB1 NL2 NL1
2007 362.5
Scen. 1a 167.65 361.45
Scen. 1b 167.58 361.35
Scen. 2a 91.41 167.45 361.25
Scen. 2b 91.37 167.38 361.15
Scen. 3a 20.08 91.31 167.35 361.01 - 28.86 83.00 192.57 Scen. 3b 20.08 91.27 167.28 360.91 - 28.86 82.94 192.31
In Appendix A, a hydrological-balance schematic for the NNRB shows natural flows without considering irrigation demands, and only the evaporation from the existing NN1 reservoir is deducted. That gives an outflow from NN1 of 363.4 m3/s. In Scenario 3b in Table 5.1.7, the corresponding flow is reduced to 360.91 m3/s. In Scenario 3 above, evaporation losses from NN5, NN3, NN2, NB2 and NB1; (0.96 + 0.20 + 0.10 + 0.05 + 0.09 = 1.40 m3/s) have been taken into consideration. Furthermore, water losses due to irrigation; (0.02 + 0.51 + 0.33 + 0.23 = 1.09 m3/s) have been taken into consideration. 360.91+1.40+1.09=363.40 m3/s.
5.1.5 Impact on irrigation in the Vientiane plains during the filling period of the new reservoirs
During the filling periods of the reservoirs, the flows in the Nam Ngum River down-stream of the confluence with the Nam Lik river will be reduced. The filling of the NN2 reservoir is the most critical for the Nam Ngum branch, since that reservoir is large, and the annual average inflow of 169.4 m3/s will be utilised in its entirety for almost one year.
The flow reduction down-stream of the confluence will be more pronounced during the filling period of the NL1 reservoir, since the incoming annual average flow to NL1 is 195 m3/s.
However, the NL1 reservoir is very small, and the filling period is only a few days. At NL2, the inflow is an average of 84 m3/s, making the filling period approximately one to two months, depending on during which time of the year the filling is started. The impact on irrigation in the Vientiane plains from the filling of the Nam Lik reservoirs can, therefore, be said to be small.
The filling period of NN2 will last almost one year and, at the time of writing, it is not known at what time of the year the filling of the reservoir will start and end. In order to assess the impact on the Vientiane plains of the filling of NN2 reservoir, an average year with regulated outflows from NN1 during the filling period is constructed on the basis of the whole flow series 1987-2006.
The outflow from NN1 during the filling period of NN2 has been modelled by a reservoir simulation for the hydrological period 1987-2006. In the simulation it has been assumed that the inflow to the NN1 reservoir from NN2 is closed during the modelling period, 1987-2006.
Consequently the main tributaries to NN1 Reservoir 1987-2006 will be the Nam Bak river and the diversions from Nam Xong and Nam Leuk, which are assumed to be operated as usual.
For environmental and social reasons, the discharge from NN1 has been adjusted to the reduced inflow from the NN2 dam site. It has been assumed that, on average, 300 m3/s are dispatched from NN1 during 15 hours per day (at present conditions about 465 m3/s are dispatched during 15 hours per day and during a part of the wet season 465 m3/s are dispatched 24 hours). By using this assumption, the reservoir surface elevation is varying between FSL +212.0 m.a.s.l. and down to approximately +204.0 m.a.s.l. during most part of the modelling period 1987-2006. Detailed results from these simulations can be found in Appendix D.
In order to assess the influence on the downstream-located irrigation plants from the filling of the NN2 reservoir, regulated outflows from NN1 and unregulated flows at NL1 dam sites for the modelling period 1987-2006, have been put together in Figure 5.1.1, which shows an average year on the basis of daily 20-year merged flows for the 1987-2006 period.
Figure 5.1.1: Flows from NN1 and Nam Lik 1 dam site during the filling period of the NN2 Reservoir
0 200 400 600 800 1000 1200 1400 1600 1800
ja
Present condtions:
Regulated outflows from Nam Ngum 1 incl. Nam Xong and Nam Leuk diversions + flows at Nam Lik 1 damsite
The merged flows are reduced by approximately 1/3 during the dry season.
Monthly average merged flows at present situation and during the filling period of NN2 are shown in Table 5.1.8.
Table 5.1.8: Total flows from NN1 and Nam Lik with present conditions, and during the filling of the NN2 Reservoir.
Jan (m3/s)
Feb (m3/s)
Mar (m3/s)
Apr (m3/s)
May (m3/s)
Jun (m3/s)
Jul (m3/s)
Aug (m3/s)
Sep (m3/s)
Oct (m3/s)
Nov (m3/s)
Dec (m3/s) Present
Average 334.4 328.8 324.5 323.4 373.7 502.4 708.2 1206.0 1219.9 617.6 417.4 346.4 Filling of NN2 230.4 224.8 220.0 211.7 266.8 398.5 581.2 812.1 822.9 394.0 274.9 242.4
5.1.6 Impact on irrigation in the Vientiane plain when the new hydropower plants are in full-scale operation
When all the new reservoirs are in full operation, the wet-season flows to the Vientiane plains will be lower, while the dry-season flows will increase.
In order to describe the combined flows from the future dam sites of Nam Lik 1 and Nam Ngum 1, reservoir simulations have been carried out for the following options:
• Present situation 2007. Only NN1 in operation;
• NN1 and NN2 in operation (scenario 1);
• NN1, NN2 and NN3 in operation (scenario 2);
• NN1, NN2, NN3, NN5, NB1, NB2, NL1 and NL2 in operation (scenario 3).
n-07 feb-07 mar-07 apr-07 maj-07 jun-07 jul-07 aug-07 sep-07 okt-07 nov-07 dec-07
Flow (m3 /s)
Filling Period NN2:
Regulated outflows from Nam Ngum 1 incl. Nam Xong and Nam Leuk diversions + flows at Nam Lik 1 damsite
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
No considerations have been given to water losses from evaporation and irrigation demands in this case, since the influence of these factors are relatively negligible in the Vientiane plains.
In order to make it possible to assess the influences on the downstream-located irrigation from the hydropower-development scenarios, reservoir simulations have been carried out for each one of the scenarios. The result is shown in Figure 5.1.2, which shows the results on the basis of daily 20 years average flows 1987-06.
Figure 5.1.2: Total flows from NN1 and NL1 at present situation 2007, and during Scenario 1, 2 and 3.
0 200 400 600 800 1000 1200 1400 1600 1800
Present 2007 Scenario 1 Scenario 2 Scenario 3
(m3 /s)
Figure 5.1.2 shows that the impact of the new reservoirs at NN2 and NN3 is negligible on the merged flows from NN1 and NL1 (Scenario 1 and 2). However, the construction of the new reservoirs in Nam Lik has a small but noticeable impact – the merged dry-season flows in December – March increase by approximately 12%.
Monthly average total flows for the present situation and scenarios 1, 2 and 3 are shown in Table 5.1.9. In the scenarios, only the “b” alternatives (situation in 2020) have been considered. The difference between the “a” and “b” alternatives is negligible in this context.
01-jan 01-feb 04-mar 04-apr 05-maj 05-jun 06-jul 06-aug 06-sep 07-okt 07-nov 08-dec
Scenario 3
Present + Scenario 1 and 2
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Table 5.1.9: Total average monthly flows from NN1 and NL1 dam sites for the different scenarios.
Jan (m3/s)
Feb (m3/s)
Mar (m3/s)
Apr (m3/s)
May (m3/s)
Jun (m3/s)
Jul (m3/s)
Aug (m3/s)
Sep (m3/s)
Oct (m3/s)
Nov (m3/s)
Dec (m3/s)
Year (m3/s) Present
Average 330.6 325.0 320.7 319.6 367.2 494.0 703.4 1 202.7 1 217.1 614.8 414.3 342.5 555.6 Scen 1 329.6 324.0 319.8 318.6 369.1 493.2 705.4 1 169.0 1 207.9 630.1 424.5 341.5 554.0 Scen 2 329.6 324.0 319.8 318.6 369.1 493.2 705.1 1 164.9 1 202.2 631.4 429.3 343.2 553.8 Scen 3 365.8 360.9 356.7 340.7 363.4 455.8 631.0 1 086.2 1 158.7 664.4 464.1 380.0 553.3