Trading off hydropower potential for irrigated agriculture – an example from the Sesan river

Timo A. Räsänen, Olivier Joffre, Paradis Someth and Matti Kummu

Aalto University, FinlandICEM, VietnamITC, Cambodia

timo.rasanen@aalto.fiwww.wdrg.fi

Background• Sesan River • Hydropower development

• 6 existing and 5 planned large hydropower projects

• Agriculture and crops• Cultivation of cash crops is increasing: Rubber,

Coffee, Cassava, Cashew, Soya• Irrigated rice: 28,500 ha in Kontum and Gia Lai

and 4,350 ha in Ratanakiri• In Cambodia rice is largely rainfed

• Food security• Cambodia and Vietnam are hot spots for food

insecurity (UN ESCAP, 2009; FAO, 2012)

• Food demand is estimated to double by 2050 in the region (FAO 2010)

• Climate variability has increased -> Challenges for food production

• Multipurpose reservoirs could potentially contribute to increased agricultural production

Aims, focus & approach

Aim: To assess the impacts of A) irrigation water abstraction from reservoir on hydropower generation and B) hydrological impacts of irrigation and hydropower

Focus: On 7 existing and 4 planned hydropower reservoirs

Approach: A) Detailed assessment with case study reservoir and B) assessment of catchment scale irrigation development

Assessment methodology• Hydrological modelling (VMod)

• Hydropower modelling (CSUDP)

• Assessment of land use suitability for irrigated rice (LUSET)

• Assessment of crop water requirement (FAO CROPWAT 8.0)

• Assessment of irrigation potential of each reservoir• Modelling of the trade off’s (CSUDP)

Assessment of land use suitability for irrigated rice

• Land suitability was estimated for 5 km distance from the Sesan River

• Results:• Three land suitablity

classes• Altogether 86,000 ha

indentified as potentially suitable

• Assessment is on coarse scale and includes areas already under cultivation

Moderately suitableMarginally suitable

Highly suitable

Assessment of crop water requirement

• Crop water requirement was defined using for• Dry season rice, 105 days• Wet season rice, 125 days

• Average dry season irrigation volumes• Cambodia: 13,200 m3/ha• Vietnam: 12,000 m3/ha

• Average wet season irrigation volumes• Cambodia: 2,700 m3/ha• Vietnam: 2,200 m3/ha

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 520

500

1000

1500

2000

2500

Cambodia

Vietnam

[week]

[m3/

ha]

Weekly irrigation schedule

Irrigation potential and scenarios

• Irrigation potential of each reservoir was based on• Land suitability• Crop water requirement• Specific dry season water allocations: 3%, 5% and 7% of dry season water budget

• Seven reservoirs were selected for further assessment• Three irrigation scenarios for Sesan 3A

• B2. 3,894 ha dry and wet season rice• B3. 6,490 ha dry and wet season rice• B4. 9,086 ha dry and wet season rice

• One catchment scale irrigation scenario for seven reservoirs• 28,348 ha dry and wet season rice (rate of irrigation corresponds to B3)

Reservoirs with irrigation and assessment scales

Catchment scale with seven reservoirs

Case study reservoir for more detailed assessment

Case study reservoir with high irrigation potential

• Sesan 3A with high downstream irrigation potential

• An example where irrigation from reservoir could have been implemented

• Dowstream area has already large irrigation projects

• Sesan 3A reservoir storage is small, requires co-operation with upstream dams Sesan 3 and Yali

Case study: Yali-Sesan 3-Sesan 3A cascade

• Simulated annual average hydropwer generation of Sesan 3A

• 425-428 GWh

• Reduction in annual hydropower generation:

• 3,894 ha: -0.6…-0.7% • 6,490 ha: -1…-1.2%• 9,086 ha: -1.4…-1.7%

• Reduction in dry season hydropower generation

• -1.8…-5.6%

Catchment scale irrigation scenario for seven reservoirs: Hydropower• Catchment scale reduction in annual hydropower generation

• 28,348 ha: -1.6 % (209 GWh)

• Most significant impacts were experienced in Lower Sesan 3• -3.4% (56 Gwh) reduction in annual hydropower generation

Baseline annual average hydropower

generation

Irrigated area Change in annual average hydropower

generation

Baseline dry season average hydropower

generation

Change in dry season average hydropower

generation

[GWh] [ha] [%] [GWh] [%]

Upper Kontum 1,057 600 -1.9 398 -4.1

Plei Krong 497 2,817 -1.4 149 -3.7Yali 3,850 0 -0.6 1,333 -1.6Sesan 3 1,228 0 -0.6 405 -1.6Sesan 3A 454 6,490 -1.7 148 -5Sesan 4 1,478 3,474 -2 449 -6.1Sesan 4 A - 5,091 - - -Sesan 1 641 0 -3.2 271 -7.5Lower Sesan 3 1,634 7,843 -3.4 636 -8.6Lower Sesan 2 2,218 2,033 -1.3 638 -4.3TOTAL 13,056 28,348 -1.6 4,427 -4.2

Catchment scale irrigation scenario for seven reservoirs: Hydrology

• The hydropower operations increased dry season flows and reduced wet season flows

• Lower Sesan 3: dry season +167%, wet season -11%

• Irrigation of 28, 348 ha reduced the annual river flow by 0.39 km3 which corresponds to 1.9% of the Sesan’s annual average flow

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0

200

400

600

800

1000

1200

1400

1600

1800

[m3/

s]

Lower Sesan 3

Catchment scale irrigation scenario for seven reservoirs: Land cover• Hydropower reservoirs inundate land

• Existing reservoirs 198 km2• Planned reservoirs 837 km2• Lower Sesan 3 will inundate 45 000 ha of land

defined in this study as potentially suitable for irrigated rice

• Expansion of agriculture has already caused deforestation in Sesan catchment

• e.g. in Ratanakiri 40% reduction in forest cover between 1997-2005

• Irrigation development from Sesan 4, Lower Sesan 3 and Lower Sesan 2 reservoirs have high likelihood of causing further deforestation

Conclusions and final remarks• Main finding: Irrigation from reservoirs contributed

to minor losses in hydropower generation• Sesan 3A: 7.3 GWh (1.2%) were traded to 9,100 ha of

irrigated rice • Catchment scale: 208.9 Gwh (1.6%) were traded to 28,400

ha of irrigated rice

• Impacts of hydropower on river flow were more significant compared to irrigation and impacts were cumulative downstream

• Co-ordination between hydropower projects may be required for successful irrigation

• This study focused only on technical and hydrological aspects

• Social, livelihood and political issues have a major role in sustainable development

• Findings of this study prompt further investigations on multi-purpose reservoirs