TA-8267 THA: STRENGTHENING INTEGRATED WATER AND FLOOD MANAGEMENT IMPLEMENTATION

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1 TA-8267 THA: STRENGTHENING INTEGRATED WATER AND FLOOD MANAGEMENT IMPLEMENTATION Mae Ta Sub-basin IWRM Modelling Results Report March 2015 Prepared for ADB

2 Prepared by Prepared for Copyright Suggested citation More information Project Team ICEM ADB YEAR ICEM ICEM International Centre for Environmental Management 6A Lane 49, Tô Ngoc Vân Tay Ho, HA NOI, Socialist Republic of Viet Nam Meelit Shah, Andrew Mittleman, Jeremy Carew-Reid, Jorma Koponen, Keith Avery Ward, Lindsay Furness, Mai Vinh, Simon Tilleard, Charit Tingsabadh, Chanokporn Prompinchompoo, Sinee Chaungcham, Tawatchai Rattanasorn, Vajirasak Vanijja, Vilas Nitivattananon, Beau Damen

3 TABLE OF CONTENTS KEY FINDINGS... 1 INTRODUCTION... 3 TA 8267 THA: Strengthening Integrated Water and Flood Management Integration... 3 Mae Ta sub-basin modelling... 3 MODEL SETUP... 3 IWRM modelling tool description... 3 Mae Tae baseline IWRM model setup... 4 Spatial input data... 4 Hydro-meteorological and flooding input data... 6 Baseline cropping information... 7 Mae-Ta baseline model results... 7 Baseline hydrology... 7 Baseline crop yields... 8 MODEL SCENARIOS... 9 Water management scenarios... 9 List of scenarios... 9 Scenario detail: Reservoir construction with supplementary irrigation for one crop a year Scenario detail: Reservoir construction with supplementary irrigation for double or triple cropping Scenario detail: Climate change scenario Water management scenario results Reservoir development with supplementary irrigation for one crop a year Reservoir construction with supplementary irrigation for double or triple cropping Irrigation efficiency improvement Land use change scenario Climate change scenario ANNEX A: Rice production for baseline compared to single crop supplmentary irrigaton scenario 29 ANNEX B: Crop production for double/triple cropping supplmentary irrigation scenario compared to baseline i

4 KEY FINDINGS Key findings from water management scenario modelling for the Mae Ta basin are outlined below. Baseline The baseline simulated basin outflow ranges from 150 ML to 300 ML depending on the hydrological year. For historical conditions ( ) the optimal planting date for rainfed rice is mid-july. The baseline rainfed rice average yield for a planting date of mid-july is between tonnes per ha. Scenario 1: Three medium scale reservoirs are constructed to provide supplementary irrigation for rice single-cropping In this scenario, apart from the additional water provided by supplementary irrigation, all other cropping details are kept the same as the baseline. Irrigated areas would only be on existing cropped areas and there would be no expansion of current cropped area. The construction of three medium scale reservoirs operated for supplementary irrigation of singlecropping rice would increase the annual total rice production in the zones covered by the command areas by an average 2,000 tonnes, a 30% increase in production compared to the rainfed rice baseline. Analysis of the water volume of Mae Laeng reservoir under this scenario shows that the reservoir is empty or close to empty every dry season. This indicates that the reservoir and command area are well sized for single-cropping but would not be able to support double-cropping. Scenario 2: Three medium scale reservoirs are constructed to provide supplementary irrigation for rice and maize double or triple-cropping For this scenario, planting calendars were adjusted to take advantage of the available reservoir water volumes and allow for second or triple cropping. In addition, the planned irrigation command area for Mae Laeng was found to be too large for the planned reservoir volume. Therefore the command area for Mae Laeng was reduced by approximately half compared to the planned area. Irrigated areas would only be on existing cropped areas and there would not be any expansion of current cropped area. The construction of three medium scale reservoirs in the basin operated for rice/maize double and triple-cropping would reduce flood peaks by up to 10m 3 /s and reduce the annual outflow of the basin by 50 Mm 3 /year. The reservoirs, especially Mae Tae, would dramatically reduce sediment outflow from the basin by up to 55,000 tonnes a year. This may causing starving of sediment in the river channel downstream which could lead to increased channel erosion. The Mae Laeng reservoir has the largest crop command area and is located upstream with a relatively small inflow. Crop yields for the reservoir could reach up to 6 tonnes per ha of maize in the dry season and 3 tonnes per hectare of rice in the wet season. On average the annual production of rice in the command area (including irrigation and rainfed rice) would not change, with some years showing minor increases in production and some years showing minor decreases. This is because the rice crop is planted earlier than optimal, to allow time for additional crops. The reservoir would allow for an average 2,290 tonnes per year production of maize. Despite the significant reduction in command area compared to that planned, the reservoir is still not well sized for the command area, on average it would empty at least once every two years. The sedimentation rate of the Mae Laeng reservoir is low, totaling less than 1% of the reservoir volume over 10 years. The Mae Tae reservoir is located on a large tributary and has a relatively small command area. It therefore has an abundance of water compared to the irrigation demand and could support rice triple-cropping. Crop yields for this reservoir could reach up to 3.5 tonnes per ha of rice in the early dry season, almost 4 tonnes per hectare of rice during the late dry season and more than 4 tonnes 1

5 per hectare of rice in the wet season. Rice production in the command area would increase by an average 3,500 tonnes per year compared to the baseline. The Mae Ta reservoir has a high sedimentation rate, culminating to over 300ML during the 10 years of the simulation. This is equal to 4% of the total reservoir capacity. The Huay Mae Suag reservoir has a small inflow compared to the design volume and command area. Despite the reduction in command area compared to that planned, the reservoir is not well sized for the command area and on average would empty once every 1.5 years. Despite the insufficient reservoir volume, crop yields for the reservoir could reach up to 7 tonnes per ha of maize in the dry season and 4 tonnes per hectare of rice in the wet season. On average the annual production of rice in the command area would increase by 500 tonnes but may also decrease in some years. Maize production would average 1,130 tonnes per year. The sedimentation rate of the Huay Mae Suag is low, totaling less than 1% of the reservoir volume over 10 years. Scenario 3: Irrigation losses are eliminated by improving irrigation efficiency With major improvements in irrigation efficiency, significantly more water would be stored in Mae Laeng and Huay Mae Suag reservoirs. This would lead to crop yield improvements of 0.2 to 2 tonnes per ha for the Mae Laeng and Huay Mae Suag command areas compared to Scenario 2. Scenario 4: 16,460 ha of the upper Mae Tae basin forest area is degraded Degradation of the upper Mae-Ta watershed would lead to an increase in the basin outflow throughout the year including up to a 5 m 3 /s increase during flood peaks. Sediment outflow from the basin would substantially increase by up to two times in the wet season. This may cause sediment aggradation downstream which can smother fish habitat and cause increased flooding. Sedimentation of the Mae Tae reservoir over the 10 year simulation period would double to over 700ML (or 8% of the total reservoir capacity). Scenario 5: Climate change increases rainfall during the wet season, decreases rainfall during the dry season and increases temperatures Under the climate change scenario there will be a slight decrease in flows throughout the year. This is due to increased evapotranspiration because of higher temperatures. There will be only a minor impact on erosion and sediment outflow from the basin. Earlier maturation of crops will lead to decreased yields of up to 1 tonne per ha during the wet (rice) and dry (maize) seasons compared to Scenario 2. The increased evaporation and transpiration, decreased maturing time and increased (wet season) and decreased (dry season) rainfall will slightly decrease irrigation use in the basin. There will be only minor changes in the volume of water stored in the reservoirs. Flooding Flooding maps available from GISTDA were checked to assess flood levels in Mae-Ta basin. It was found that significant flooding does not occur in the basin. According to GISTDA flood maps, between 2005 to 2012 the only year that flooding occurred was in 2005, and this was only very limited flooding. Consequently no flood simulation has been conducted for the basin. 2

6 INTRODUCTION AD B TA-8267 THA Strengthening Integrated Water and Flood Management Implementation I C EM TA 8267 THA: Strengthening Integrated Water and Flood Management Integration The objective of the TA 8267 THA: Strengthening Integrated Water and Flood Management Integration project is to reduce loss and damage from flood in Thailand through improved decisionmaking processes for integrated water resources, flood and drought management. The TA is using the Yom River basin and Mae Ta sub-basin as case studies for improved participation processes and education on integrated water resource management (IWRM) principles. The objective of the modelling component of the project is to support water management planning in the Yom River Basin and Mae Ta sub-basin. This is achieved by demonstrating and building capacity in the use of integrated watershed modelling to assess the hydrological impacts of water resource management options. Mae Ta sub-basin modelling The objective of the Mae Ta sub-basin modelling, and this report, is to provide a basis for sector analysis of water resources management scenarios. The modelling is designed to demonstrate how climate change and hydrological modelling can provide information on water management scenarios and inform water management planning. The results described in this report will be used by the sector teams to analyse impacts on their sector under each water management scenario. This modelling report describes the setup of the Mae Ta sub-basin model, outlines the set of water management scenarios identified by the TA team in consultation with government agencies and provides a summary of the key modelling results for the Mae Ta sub-basin. MODEL SETUP IWRM modelling tool description The IWRM model is a physically based hydrological model that enables users to test the hydrological impact of different water resource management development in the context of a whole basin. The IWRM model consists of four main components including i) rainfall runoff; ii) river hydrology and hydraulics; iii) reservoir operations; and iv) agro-economic parameters. Each component is flexible so that management scenarios can be incorporated into the model and assessed. A detailed description of the selection of the IWRM model for use in the TA is provided in the TA Inception Report. A detailed description of the model computation methods and model equations can be found in the IWRM model manual 1. 1 Koponen, J., Lauri, H., Veijalainen, N., and Sarkkula, J. (2010) HBV and IWRM Watershed Modelling User Guide, MRC Information and Knowledge management Programme, DMS Detailed Modelling Support for the MRC Project, Available at: 3

7 Mae Tae baseline IWRM model setup Spatial input data The Mae Tae model has been extracted from the broader Yom Basin model developed as part of TA Model resolution is 400 m which results in 3360 active grid cells. This is a very modest grid cell count. However, as the modelling study is for basin level planning the 400 m resolution has been deemed sufficient. The model grid consists of topographic (DEM), land use, soil class and soil depth layers. River data layers are generated based on the DEM. The model layers are presented in Figure 1 - Figure 3. Figure 1. Mae Tae model topographic grid and comparison of model river channels (blue lines) with mapped channels (red lines). For clarity not all model river channels are shown. 4

8 Figure 2. Mae Tae baseline land use (left) and planned reservoirs and irrigation areas (right figure). Figure 3. Mae Tae baseline soil classes (left) and test soil depths (right figure). In the final simulations effective soil depths are calibrated and the soil depth layer is not utilised. 5

9 Hydro-meteorological and flooding input data There are no temperature or precipitation stations located in the Mae-Ta sub-basin. Therefore temperature and precipitation were interpolated from nearby stations. Analysis of the precipitation data shows that the quality of the rainfall data deteriorates in later years (Figure 4). Average wet season rainfall is 5.7 mm/d and dry season is mm/d depending on the location. Interpolated average wet season daily maximum temperature is presented in Figure 5. No hydrological measurements were available for the Mae Ta sub-basin, therefore the calibrated parameter values for the broader Yom Basin model were used. Figure 4. Cumulative rainfall for the Mae Tae stations. Figure 5. Average wet season daily maximum temperatures. Flooding maps available from GISTDA were checked to assess flood levels in Mae-Ta basin. It was found that significant flooding does not occur in the basin. According to GISTDA flood maps, between 2005 to 2012 the only year that flooding occurred was in 2005, and this was only very limited flooding. Consequently no flood simulation has been conducted for the basin. 6

10 Baseline cropping information The baseline setup for single-crop rainfed rice paddy is as follows: Planted area of 10,170 ha Wet season planting day in mid-june (optimized to utilize rainfall) No losses other than seepage and evaporation Soil fertility stress 30 %. The rainfed rice planting day was optimized in respect to average yield. The optimal date was found to be 195, which equates to mid-july. Mae-Ta baseline model results Baseline hydrology The Mae Ta Basin simulated outflow is presented in Figure 6. Anecdotal estimates for annual flow are around 129 ML but no recorded estimates could be found. Model simulations give larger values between 150 ML and 300 ML depending on the hydrological year (see Figure 13). This discrepancy can be further investigated if reliable estimates of the basin outflow can be found. Figure 6. Mae Tae Basin simulated daily discharge. 7

11 Baseline crop yields AD B TA-8267 THA Strengthening Integrated Water and Flood Management Implementation I C EM The average rainfed rice yield for a planting date of mid-july is between tonnes per ha, with a basin average of 2.55 tonnes per ha (Figure 7). The largest yields are close to the main rivers. Figure 7. Average rainfed rice yield (left) for planting day 195 (mid-july) 8

12 MODEL SCENARIOS AD B TA-8267 THA Strengthening Integrated Water and Flood Management Implementation I C EM Water management scenarios List of scenarios Four management scenarios have tested as outlined below: 1. Reservoir construction with supplementary irrigation for one crop a year: In this scenario three reservoirs are constructed for irrigation water supply. Supplementary irrigation is provided for only one crop a year (i.e. supports single-cropping only). Further details for this scenario are provided below. 2. Reservoir construction with supplementary irrigation for double or triple cropping: In this scenario three reservoirs are constructed for irrigation water supply. Supplementary irrigation is provided for two or three crops a year (i.e. supports double or triple-cropping). Further details for this scenario are provided below. 3. Irrigation water saving: In this scenario the three reservoirs are constructed for double or triple-cropping and 30% irrigation losses are eliminated. Seepage and evaporation from the irrigated paddies is still included in the scenario. 4. Land degradation: In this scenario 16,460 ha of the upper Mae Tae basin forest area is degraded to shrub and grassland (Figure 8). 5. Climate change: Climate change increases rainfall during the wet season, decreases rainfall during the dry season and increases temperatures. More details for the climate change scenario are provided below. Figure 8. Degraded area (grey) in the land use scenario. 9

13 Scenario detail: Reservoir construction with supplementary irrigation for one crop a year In this scenario three reservoirs are constructed and supplementary irrigation provided for singlecropping. The three reservoirs include: i) Mae Laeng with 11.5 million m 3 volume and 1,250 ha effective command area; ii) Mae Tae with 7.36 million m 3 volume and 400 ha effective command area; and Huay Mae Suag with 6.16 million m 3 volume and 605 ha effective command area. An 18% environmental flow has been forced for each reservoir. The environmental flow is defined as the forced release of a minimum percentage of incoming flow to ensure that river flows are maintained. Rainfed rice crop was included on 7,900 ha outside of the irrigation command areas. Apart from the additional water provided by supplementary irrigation, all other cropping details are kept the same as the baseline including: Single-cropping; Wet season planting day in mid-june; No losses other than seepage and evaporation; and Soil fertility stress of 30 %. Scenario detail: Reservoir construction with supplementary irrigation for double or triple cropping In this scenario the three reservoirs are constructed and supplementary irrigation provided for double or triple-cropping. The same three reservoirs are used as described in the section above. An 18% environmental flow has been forced for each reservoir. It has been assumed that 30% of the irrigation water is lost (i.e. 30% of the water is not returned to the river). It has been assumed that Mae Laeng soil is less fertile so 40% soil stress is applied to the Mae Laeng command area, and 30% soil stress applied in the other areas. The reservoirs are operated based on crop irrigation demand and providing all irrigation water required if available (excluding the environmental flow allowance). For this scenario planting calendars were adjusted to take advantage of the available reservoir water volumes but not fully optimized in terms of water utilization, costs or benefits. Two crops were used for Mae Laeng and Huay Mae Suag: 1. Wet season rice, planting date May 30 th 2. Early dry season maize, planting date September 27 th. Mae Tae has more water available and therefore triple rice cropping were used: 1. Wet season rice, planting date May 30 th 2. Early dry season rice, planting date September 27 th 3. Late dry season rice, planting date February 4 th Note that changing the wet season planting date (i.e. not mid-june as used in the baseline) means that the planting is not undertaken at the optimal time for wet season rainfall. Therefore the wet season rice yields for this scenario may decrease compared to the wet season rice yields of the baseline. Although the wet season yields may decrease compared to the baseline, the total annual yields will be greater due to the introduction of a second and third crop. The planned irrigation command areas were found to be too large for the volume of water captured in the reservoirs, particularly the Mae Laeng. Therefore the command areas were modified based on 10

14 modelled water availability. The planned and modified irrigation command areas are provided in Table 1. Rainfed rice crop was included on 7,900 ha outside of the irrigation command areas. In addition 752 ha of rainfed rice was included within the planned Mae Laeng irrigation area, to make up for the reduction in command area size. Table 1. Planned versus modified irrigation command areas Reservoir Planned irrigation command area (rai) Modified command area (rai) Mae Laeng 1,360 wet season, 320 ha dry season 544 ha wet season, 408 ha dry season Mae Tae 400 ha both wet and dry seasons 400 ha both wet and dry seasons Huay Mae Suag 608 ha for both wet and dry seasons 608 ha for wet season, 182 ha dry season Scenario detail: Climate change scenario Climate change impacts have been assessed by incorporating projected changes in precipitation and temperature for The projected changes for 2050 assume the worst case scenario outlined by IPCC regional scatter diagrams for South-East Asia 2. The regional scatter diagrams are based on four emissions scenarios (A1FI, A2, B1 and B2) and seven GCMs (CCSRNIES, CSIRO, Mk2, CGCM2, ECHAM4/OPYC3, HadCM3 and NCAR DOE PCM). The worst case scenario has been taken as the largest projected increase in rainfall during the wet season, largest projected decrease in rainfall during the dry season and the year-round largest projected increase in temperature. The details of the climate change scenario are outlined in Table 2. Table 2. Climate change scenario Parameter IPCC range in projected changes 2 Change to be incorporated into climate change scenario Precipitation March to May change of -10% to +10% June to August change of -10% to +5% September to November change of -10% to +4% December to February change of -10% to +13% March to May decrease of 10% June to August increase of 5% September to November increase of 5% December to February decrease of 10% Temperature Year round increase of 1 to 3 ⁰C Year round increase of 3 ⁰C 2 Available at 11

15 Water management scenario r esults Reservoir development with supplementary irrigation for one crop a year Comparison of Figure 9 (baseline) and Figure 10 (single crop supplementary irrigation) shows that providing supplementary irrigation to rice single-cropping of the three command areas would increase the annual total rice production in these areas by an average 2,000 tonnes per year, a 30% increase in production for the command areas. Tabulation of the results, showing the change in crop production by command area, is provided in Annex A. Figure 9. Annual total rice production for baseline scenario (rainfed only) in Mae Laeng, Mae Tae and Huay Mae Suag planned command areas (cumulative). Note that the results are cumulative. Thus Mae Laeng line is sum of outside areas and Mae Laeng command area; Mae Tae is the sum of Mae Ta command area, Mae Laeng command area and outside areas. Figure 10. Annual total rice production for single-cropping supplementary irrigation scenario. Note that the results are cumulative. Thus Mae Laeng line is sum of outside areas and Mae Laeng command area; Mae Tae is the sum of Mae Ta command area, Mae Laeng command area and outside areas. Outside planned command areas is rainfed rice, the remainder is irrigated rice. Analysis of the water volume of Mae Laeng reservoir shows that the reservoir is empty or close to empty every dry season (Figure 11). This indicates that the reservoir and command area are well sized for single-cropping but would not be able to support double-cropping within the command 12

16 area. If double cropping is to be implemented the command area size would need to be reduced to ensure there is sufficient water for multiple crops each year. Figure 11. Mae Laeng reservoir water volume for the reservoir development with supplementary irrigation for one crop a year scenario Reservoir construction with supplementary irrigation for double or triple cropping The construction of three medium scale irrigation reservoirs reduces basin outflow throughout the year. The reduction is more pronounced during the wet season and flood peaks (Figure 12). The reduction in flood peaks can reach up to 10 m 3 /s, as illustrated for 1994 in Figure 12. The reservoirs also reduce the total annual outflow of the basin by around 50Mm 3 (Figure 13) Figure 12. Basin outflow with and without irrigation for baseline compared to Scenario 2, three reservoirs operated for double or triple cropping 13

17 Figure 13. Mae Tae Basin simulated annual outflow volume for baseline compared to Scenario 2, three reservoirs operated for double or triple cropping Reservoir water volumes reflect water availability from inflows and water outflow to meet irrigation demand and environmental flows. The reservoir volumes for the three reservoirs are provided in Figure 14 - Figure 16. Mae Laeng has the largest crop command area and is located upstream. It therefore has the least available water and longest dry-out periods (six times in the 10 year simulation period). The Mae Tae reservoir is located on the Nam Mae Tae main tributary and has a relatively small command area. It therefore has an abundance of water compared to the irrigation demand. Huay Mae Suag also often dries out, including 15 times in the 10 year simulation period. Figure 14. Mae Laeng reservoir water volumes for Scenario 2, three reservoirs operated for double or triple cropping 14

18 Figure 15. Mae Tae reservoir water volumes for Scenario 2, three reservoirs operated for double or triple cropping Figure 16. Huay Mae Suag reservoir water volumes for Scenario 2, three reservoirs operated for double or triple cropping 15

19 The spatially averaged crop yields for the command area of each reservoir are presented in Figure 17 - Figure 19. Due to the supplementary irrigation provided by the reservoirs, the yields are consistent between years except for the very dry years. Figure 20 shows the average rice yield for agricultural areas outside of the reservoir command areas. These areas do not receive the supplementary irrigation, therefore only have one crop per year, and therefore have much lower and inconsistent yields. Figure 17. Mae Laeng average crop yield per hectare for Scenario 2, three reservoirs operated for double or triple cropping. Lower (in general) wet season yields are rice and early dry season yields are maize. Figure 18. Mae Tae triple rice crop yields per hectare for Scenario 2, three reservoirs operated for double or triple cropping 16

20 Figure 19. Huay Mae Suag average crop yield per hectare for Scenario 2, three reservoirs operated for double or triple cropping. Lower (in general) wet season yields are rice and early dry season yields are maize. Figure 20. Rainfed rice crop yields per hectare (average over the cropping area) for Scenario 2, three reservoirs operated for double or triple cropping Use of the three reservoirs to provide supplementary irrigation for double and triple-cropping would increase total rice production for the basin and also allow a crop of maize every year. Total annual production of rice for the basin could reach up to 40,000 tonnes ( Figure 21), including a 4,000 tonnes increase in annual rice production in the three command areas. Most of this increased rice production would come from the Mae Ta command area which would see an increase of 3,500 tonnes of rice per year. The results for Mae Laeng and Huay Mae Suag are mixed, some years will see minor increases in rice production whilst some will see minor decreases in rice production. The mixed results for the Mae Laeng and Huay Mae Suag reservoirs is because the rice crop is planted earlier than optimal, to allow time for a second crop. The reservoirs would also enable supplementary irrigation for dry season maize production of up to 3,500 tonnes a year (Figure 22) and averaging 3,425 tonnes per year. A tabulation of the results by command area is provided in Annex B. 17

21 Figure 21. Paddy rice production for Scenario 2, three reservoir construction and operation for double and triple cropping. Note that the results are cumulative. Thus Mae Laeng line is sum of otuside areas and Mae Laeng command area; Mae Tae is the sum of Mae Ta command area, Mae Laeng command area and outside areas. Mae Laeng command area production includes irrigated and rainfed rice production within the command area. Figure 22. Maize annual production Reservoir sedimentation is presented in Figure 23. The Mae Ta reservoir shows a significantly faster rate of sedimentation. Over the 10 years of simulation the reservoir will receive 300 ML of sedimentation, or 4% of total reservoir volume. Basin annual sediment outflow with and without the reservoirs is provided in Figure 24. The reservoirs, especially Mae Tae, reduce sediment outflow by up to 55,000 tonnes a year. 18

22 Figure 23. Reservoir sedimentation volumes for Scenario 2, three reservoirs operated for double or triple cropping Figure 24. Basin sediment annual outflow for baseline versus Scenario 2, three reservoirs operated for double or triple cropping Irrigation efficiency improvement Irrigation efficiency improvements of 30% would have limited impact on the basin outflow (as shown in in 19

23 Figure 25). The efficiency improvements would lead to significantly more water being stored in Mae Laeng and Huay Mae Suag reservoirs compared to Scenario 2 (Figure 26 and Figure 27). This additional water would lead to crop yield improvements of up to 1 tonnes per ha in the dry season for the Mae Laeng and Huay Mae Suag command areas compared to Scenario 2 (Figure 29 and Figure 30). Figure 25. Basin outflow compared in the lossless and 30% loss scenarios. Figure 26. Mae Laeng reservoir volume for no loss and 30% loss scenarios. 20

24 Figure 27. Mae Tae reservoir volume for no loss and 30% loss scenarios Figure 28. Huay Mae Suag reservoir volume for no loss and 30% loss scenarios 21

25 Figure 29. Mae Laeng crop yields for no loss and 30% loss scenarios Figure 30. Huay Mae Suag average crop yields for no loss and 30% loss scenarios. Land use change scenario Degradation of the upper Mae-Ta watershed would lead to an increase in the basin outflow throughout the year and particularly during flood peaks (Figure 31). Sediment outflow from the basin would also substantially increase by up to two times in the wet season (Figure 32). Compared to Scenario 2, sedimentation of the planned reservoirs would occur at a much faster rate (Figure 33 and Figure 34). For example the sedimentation of the Mae Tae reservoir over the 10 year simulation period would double to over 700ML (or 8% of the total reservoir capacity). In reality the degradation of 16,460 ha of the upper Mae Tae basin forest area is an extreme case. The scenario is unlikely to 22

26 occur but is useful to illustrate the potential major impacts of land degradation on sediment loads in the river and sedimentation of reservoirs. Figure 31. Basin outflow for Scenario 2 (labelled baseline) versus the land use change scenario Figure 32. Basin sediment outflow for baseline versus the land use change scenario 23

27 Figure 33. Mae Laeng reservoir sedimentation for Scenario 2 (labelled baseline) versus the land use change scenario Figure 34. Mae Tae reservoir sedimentation for Scenario 2 (labelled baseline) versus the land use change scenario 24

28 Climate change scenario The following results can be interpreted from the climate change scenario: There will be a slight decrease in flows throughout the year (Figure 35). This is due to increased evapotranspiration because of higher temperatures; There will be only a minor impact on erosion and sediment outflow from the basin (Figure 36); Maize transpiration increases more clearly than rice (Figure 37). The plants mature earlier which leads to decreased yields of up to 1 tonne per ha during the wet (rice) and dry (maize) seasons compared to Scenario 2 (Figure 38); The increased evaporation and transpiration, decreased maturing time and increased (wet season) and decreased (dry season) rainfall will slightly decrease irrigation use compared to Scenario 2 (Figure 39); and There will be only minor changes in the volume of water stored in the reservoirs compared to Scenario 2 (Figure 40). The chosen climate change scenario is too simplistic. It does not reflect increases in climate extremes such as floods and droughts which are expected to increase. Increasing climate extremes would cause major hydrological changes, increase occurrence of flood and drought, increase erosion in the watershed and stress crops leading to lower yields. It is recommended that a more detailed climate change analysis is conducted to understand in more detail the potential effects of climate change in the basin. Figure 35. Climate change impact on basin outflow for Scenario 2 (labelled baseline) versus the climate change scenario 25

29 Figure 36. Climate change impact on the basin sediment outflow for Scenario 2 (labelled baseline) versus the climate change scenario Figure 37. Mae Laeng rice (in wet season, shown on the left of the graph) and maize (in dry season, shown on the right of the graph) water use for Scenario 2 (labelled baseline) versus the climate change scenario. Water use represents evapotranspiration. 26

30 Figure 38. Mae Laeng rice (lower values) and maize (higher values) yields for Scenario 2 (labelled baseline) versus the climate change scenario Figure 39. Mae Laeng cumulative irrigation use for Scenario 2 (labelled baseline) versus the climate change scenario 27

31 Figure 40. Mae Laeng irrigation reservoir water volume for Scenario 2 (labelled baseline) versus the climate change scenario 28

32 ANNEX A: RICE PRODUCTION FOR BASELINE COMPARED TO SINGLE CROP SUPPLMENTARY IRRIGATON SCENARIO Parameter Scenario Command Area Baseline (no supplementary irrigation i.e. all rice is rainfed) Outside planned command areas Mae Laeng Mae Ta Huay Mae Suag Rice Production (tonnes/year) Single-cropping supplementary irrigation (i.e. mix of irrigated and rainfed rice) Rainfed rice outside planned command areas Mae Laeng Mae Ta Huay Mae Suag Outside planned command areas Increase Mae Laeng Mae Ta Huay Mae Suag ,001 3, ,171 22,001 4,774 1,514 2,283-1, , ,318 3, ,349 24,318 4,779 1,519 2, ,883 3, ,397 21,883 4,803 1,528 2,330-1, ,092 4,293 1,268 1,822 27,092 4,732 1,495 2, ,009 3,805 1,290 2,221 24,009 4,396 1,508 2, ,511 2, ,511 4,377 1,505 2,272-1,602 1,100 1, ,191 4,151 1,374 1,929 26,191 4,762 1,526 2, ,389 4,182 1,314 1,860 26,389 4,690 1,500 2, ,843 4,095 1,112 1,536 25,843 4,764 1,523 2, ,219 3, ,413 23,219 4,685 1,483 2,264-1, ,563 4,526 1,364 2,110 28,563 4,781 1,519 2, ,104 4,137 1,272 1,806 26,104 4,704 1,493 2, Average 24,427 3,871 1,099 1,600 24,427 4,687 1,509 2,

33 ANNEX B: CROP PRODUCTION FOR DOUBLE/TRIPLE CROPPING SUPPLMENTARY IRRIGATION SCENARIO COMPARED TO BASELINE Parameter Scenario Command Area Baseline (no supplementary irrigation i.e. all rainfed rice) Outside planned command areas Mae Laeng Mae Ta Huay Mae Suag Rice Production (tonnes/year) Double/triple-cropping supplementary irrigation (i.e. mix of rainfed and irrigated) Outside planned command areas Rainfed Mae Laeng Irrigated Mae Ta Huay Mae Suag Outside planned command areas Increase ,001 3, ,171 22,001 2,092 1,689 4,680 2, ,318 3, ,349 24,318 2,312 1,579 4,629 1, ,883 3, ,397 21,883 2,081 1,644 4,748 2, ,092 4,293 1,268 1,822 27,092 2, ,624 1, ,009 3,805 1,290 2,221 24,009 2,283 1,586 4,518 2, ,511 2, ,511 1,665 1,495 4,610 1, ,191 4,151 1,374 1,929 26,191 2,490 1,639 4,724 2, ,389 4,182 1,314 1,860 26,389 2,509 1,686 4,592 2, ,843 4,095 1,112 1,536 25,843 2,457 1,681 4,688 2, ,219 3, ,413 23,219 2,208 1,667 4,567 2, ,563 4,526 1,364 2,110 28,563 2,716 1,692 4,623 2, ,104 4,137 1,272 1,806 26,104 2,482 1,582 4,687 2, Average 24,427 3,871 1,099 1,600 24,427 2, ,641 2, , Mae Laeng Mae Ta Huay Mae Suag 30

34 Parameter Scenario Baseline (no supplementary irrigation) Maize Production (tonnes/year) Double/triple-cropping supplementary irrigation Command Area Mae Laeng Huay Mae Suag Mae Laeng Huay Mae Suag 1994 NA NA 2,480 1, NA NA 2,420 1, NA NA 2,480 1, NA NA 1,850 1, NA NA 2,250 1, NA NA 1,850 1, NA NA 2,480 1, NA NA 2,300 1, NA NA 2,420 1, NA NA 2,250 1, NA NA 2,300 1, NA NA 2,420 1,230 Annual average NA NA 2,292 1,133 31