Water balance for agriculture production in the dry seasons of the Mekong river delta in Viet Nam

Life ScienceS | Agriculture Vietnam Journal of Science, Technology and Engineering56 September 2020 • Volume 62 Number 3 Background The Vietnamese Mekong river delta (MD) is the largest wetland region in the southernmost part of the Mekong river basin and connects more than 700 km of coastal line to the East Sea and the Gulf of Thailand (Fig. 1). The Delta is four million ha in size (12% of total natural land of Vietnam) and hosts more than 18 million inhabitants (about 22% of the entire country’s population in 2009). For more than 300 years, the local people have lived close to rivers and streams to facilitate the domestic use of water, such as for agricultural cultivation, fishery, and river transportation. Thus, any change in the water source affects their activities and the ecosystem of the area. The MD is recognized as the largest agriculture and aquaculture production region of Vietnam. The delta supplies more than 53% of the nation’s staple food, rice (Fig. 2), 65% of the total fish production, and 75% of the tropical fruit trees. Further, rice production is considered to be the main economic sector, which occupies more than 60% of the labour force in the MD. The total rice production of Vietnam for the 2014-2015 market year reached 45.18 million tons of paddy rice or approximately 28.24 million tons of milled rice. Since the end of the 1980s, Vietnam has been known as one of the top five milled rice exporters to the world market and more than 90% of Vietnam’s rice export comes from the MD. From a series of data obtained from the General Statistical Office [1], the total rice production in the MD exceeded 25.25 million tons in 2014-2015. in this period, about 7.1 million tons of rice was sold to the world food market, which became the record for the largest amount of rice exported from the nation. The cropping calendar for rice and other upland plant cultivation in the MD, in terms of the Water balance for agriculture production in the dry seasons of the Mekong river delta in Vietnam Le Anh Tuan* Research Institute for Climate Change - Can Tho University Received 8 April 2020; accepted 24 July 2020 *Email: latuan@ctu.edu.vn Abstract: Water is the most important component of agriculture production, especially during the dry seasons when all water sources are scarce yet water needs are very high. The Mekong river delta in Vietnam is the largest agricultural and aquacultural region in the country where about half of the Delta’s land area is used for rice and upland crop cultivation. One key strategy to address the regional water utility problem is to estimate the net water requirement during the dry seasons. The needs for irrigation water discharge for rice and other upland crops during the dry seasons of the Delta were quantified using the Penman-Monteith equation for estimating reference crop evapotranspiration along with the CROPWAT model for calculating crop irrigation water requirements. In general, the total water taken from the Mekong river flow for irrigation requirements should be approximately 2,300-2,600 m3/s for normal yields in the current agriculture areas and under local cultivation conditions. Water diversion and upstream hydropower projects are challenging tasks, especially in the context of climate change, to satisfy the water needs for agricultural irrigation in the near future. All water stakeholders among the neighbouring countries that rely on the Mekong river must adjust their regional water-use planning as one of the mitigation solutions for the seasonal drought crisis. Keywords: agriculture, CROPWAT model, irrigation water, Mekong Delta, water balance. Classification number: 3.1 Doi: 10.31276/VJSTE.62(3).56-61 Life ScienceS | Agriculture Vietnam Journal of Science, Technology and Engineering 57September 2020 • Volume 62 Number 3 growing season duration and the number of crops against the available water conditions in rainfed and irrigated areas, is shown in Fig. 3. From the calendar, a very large water requirement occurs at the end of the dry season, i.e. March and April. However, in March and April, a majority of the seasonal rice has been harvested and those fields are vacated. in a few irrigated areas and saltwater intrusion-free areas such as the An Giang, Dong Thap, Can Tho, and Vinh Long provinces, farmers may plough the lands to prepare for their new rice crop and a lot of water is pumped to the rice fields in March and April. Fig. 3. Agricultural cropping calendar in the Vietnamese MD. Since 2014, locating fresh water sources has become a huge challenge for agricultural irrigation, especially during the dry season. The available water flow from the Mekong river to the delta has seriously dropped resulting in saline water intrusion of hundreds of thousands of hectares of rice fields. According to the statistics from the Ministry of Agriculture and Rural Development [4], the serious effects of drought and saline intrusion in 2015-2016 caused damage to more than 339,000 ha of Winter-Spring rice paddies, which resulted in a nearly 22% loss of total rice area across the region. Due to limits placed on irrigation water, the Winter-Spring rice crop area dropped by 8.72% in 2016 when compared with the rice area in 2014 (Fig. 4). Fig. 4. Change in the Winter-Spring rice areas in the MD due to drought and saline intrusion. Data source [1]. Rice is a major food source for not only the Vietnamese but also for many people around the world. it is well-known that rice cultivation in the lowlands like the MD requires copious amounts of water. At the flowering stage of rice growth, the need for water is high and the rice yield is very sensitive to water deficit, which results in increased spikelet sterility and thus fewer grains. Determining the amount of field water needed to produce one kilogram of rice is a critical issue for water managers. Many experiences have shown that there are large variations in the water need, i.e., 4 limits placed on irrigation wate , the Winter-Spring ric crop area dropped by 8.72% in 2016 when compared with the rice area in 2014 (Fig. 4). Fig. 4. Change in the Winter-Spring rice areas in the MD due to drought and saline intrusion. Data source [1]. Rice is a major food source for not only the Vietnamese but also for many people around the world. It is well-known that rice cultivation in the lowlands like the MD requires copious amounts of water. At the flowering stage of rice growth, the need for water is high and the rice yield is very sensitive to water deficit, which results in increased spikelet sterility and thus fewer grains. Determining the amount of field water needed to produce one kilogram of rice is a critical issue for water managers. Many experiences have shown that there are large variations in the water need, i.e., to produce one kilogram of rice, about 3,000-5,000 litres of water is required on average, which greatly depends on farming management and weather. In agro-meteorology, evapotranspiration is a word combining evaporation and transpiration and is one of the most significant components of water balance in crop fields. On average, it takes 1.432 litres of evapotranspired water to produce 1 kg of rough rice [5]. In reality, the water need may go up to 2,500 litres, which includes the outflows of evapotranspiration, seepage, and percolation [6]. Water demand for agricultural cultivation is defined as the amount of water required for crops that compensates for the loss of water due to evapotranspiration and the physical conditions of the soil and land that contribute to water storage as well as the growth stages of the crop. Table 1 presents a rough comparison of water balance for irrigated areas in the countries of the Lower Mekong Basin (LMB) versus land area, population, and the Mekong River flow discharge available in the dry season. An estimation of irrigation volume requirements for rice and other upland crops is considered as a regional water security strategy. 1,488.3 1,564.6 1,562.7 1,531 1,426.3 1400 1420 1440 1460 1480 1500 1520 1540 1560 1580 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 R ic e ar ea (x 1 ,0 00 h a) Fig. 1. The Mekong river delta in Vietnam and the delta’s network of rivers and canals. Source [2]. 3 Fig. 2. Contributions to rice production in Vietnam. Data source [3]. Fig. 3. Agricultural cropping calendar in the Vietnamese MD. Since 2014, locating fresh water sources has become a huge challenge for agricultural irrigation, especially during the dry season. The available water flow from the Mekong River to the Delta has seriously dropped resulting in saline water intrusion of hundreds of thousands of hectares of rice fields. According to the statistics from the Ministry of Agriculture and Rural Development [4], the serious effects of drought and saline intrusion in 2015-2016 caused damage to more than 339,000 ha of Winter-Spring rice paddies, which resulted in a nearly 22% loss of total rice area across the region. Due to Mekong river delta 53% Mekong river delta 18% Northern coastal region 16% Central region 8% Highland region 2% South Eastern region 3% Fig. 2. Contributions to rice production in Vietnam. Data source [3]. Life ScienceS | Agriculture Vietnam Journal of Science, Technology and Engineering58 September 2020 • Volume 62 Number 3 to produce one kilogram of rice, about 3,000-5,000 litres of water is required on average, which greatly depends on farming management and weather. in agro-meteorology, evapotranspiration is a word combining evaporation and transpiration and is one of the most significant components of water balance in crop fields. on average, it takes 1.432 litres of evapotranspired water to produce 1 kg of rough rice [5]. in reality, the water need may go up to 2,500 litres, which includes the outflows of evapotranspiration, seepage, and percolation [6]. Water demand for agricultural cultivation is defined as the amount of water required for crops that compensates for the loss of water due to evapotranspiration and the physical conditions of the soil and land that contribute to water storage as well as the growth stages of the crop. Table 1 presents a rough comparison of water balance for irrigated areas in the countries of the Lower Mekong basin (LMB) versus land area, population, and the Mekong river flow discharge available in the dry season. An estimation of irrigation volume requirements for rice and other upland crops is considered as a regional water security strategy. Table 1. Land, population and rice production in the Lower Mekong basin, 2014. Variable Thailand Laos Cambodia Vietnam Total Area in LMB (km2 x 103) Area in LMB (%) Population in LMB (2014) (x 106) Population in LMB (2014) (%) Population density (persons/km2) Agriculture area in LMB (ha x 103) Paddy area in LMB (ha x 103) Paddy are as % of agric. area irrigated paddy area (ha x 103) irrigated as % of paddy area Paddy prod. (2014) (t x 106) % growth of prod. (2000-2014) Average yield (2014) (t x 106) Prod. as % of country’s total 184.0 28.7 24.2 36.7 132 10,300 1,647 45.1 1,425 30.7 14.7 2.5 2.6 45 202.0 31.5 6.1 9.3 28 1,900 631 33.2 172 27.3 3.9 4.5 4.3 98 161.0 25.0 12.5 19.0 78 3,100 1,647 53.1 505 30.7 8.7 6.1 3.1 94 95.0 14.8 23.0 35.0 279 4,610 2,606 56.5 1,921 73.7 25.2 3.0 5.9 56 642.0 100.0 65.8 100.0 103 19,910 6,531 47.9 4,023 42.2 52.5 3.4 3.8 57 Data source [7]. Methodology CRoPWAT [8] is a decision support system software developed by the Land and Water Development Division of the UN’s Food and Agriculture organization for the calculation of crop water and irrigation requirements based on soil, climate, and crop data. The equations for the efficient quantity of crop water are based the guidelines of [9] and the expected crop yield is based on the water use in [10]. The maximum crop evapotranspiration (ETcrop) is equal to the reference-crop evapotranspiration (ETo) multiplied by the crop coefficient (Kc): ETcrop = Kc x ETo (Eq. 1) The crop coefficient values, Kc, were provided for a large number of crops and the procedures to determine the ETcrop over the growing season were followed. Crop water requirements are defined by the depth of water needed to mitigate water loss through maximum crop evapotranspiration (ETcrop) in order to achieve full production potentials under the given crop growing environment. The Penman-Monteith equation is the FAo’s standard method for modelling evapotranspiration and is formulated as [8]: 5 Table 1: Land, population and rice production in the Lower Mekong Basin, 2014. Variable Thailand Laos Cambodia Vietnam Total Area in LMB (km2 x 103) Area in LMB (%) Population in LMB (2014) (x 106) Population in LMB (2014) (%) Population density (persons/km2) Agriculture area in LMB (ha x 103) Paddy area in LMB (ha x 103) Paddy are as % of agric. area irrigated paddy area (ha x 103) irrigated as % of paddy area Paddy prod. (2014) (t x 106) % growth of prod. (2000-2014) Average yield (2014) (t x 106) Prod. as % of country’s total 184.0 28.7 24.2 36.7 132 10,300 1647 45.1 1425 30.7 14.7 2.5 2.6 45 202.0 31.5 6.1 9.3 28 1900 631 33.2 172 27.3 3.9 4.5 4.3 98 161.0 25.0 12.5 19.0 78 3100 1647 53.1 505 30.7 8.7 6.1 3.1 94 95.0 14.8 23.0 35.0 279 4610 2606 56.5 1921 73.7 25.2 3.0 5.9 56 642.0 100.0 65.8 100.0 103 19,910 9531 47.9 4023 42.2 52.5 3.4 3.8 57 Data Source. [7]. Methodology CRoPWAT [8] is a decision support system software developed by the Land and Water Development Division of the UN’s Food and Agriculture Organization for the calculation of crop water and irrigation requirements based on soil, climate, and crop data. The equations for the efficient quantity of crop water are based the guidelines of [9] and the expected crop yield is based on the water use in [10]. The maximum crop evapotranspiration (ETcrop) is equal to the reference-crop evapotranspiration (ETo) multiplied by the crop coefficient (Kc): ETcrop = Kc x ETo. (Eq. 1) The crop coefficient values, Kc, were provided for a large number of crops and the procedures to determine the ETcrop over the growing season were followed. Crop water requirements are defin d by the depth of water needed to mitigate water loss through maximum crop evapotranspiration (ETcrop) in order to achieve full production potentials under the given crop growing environment. The Penman-Monteith equation is the FAo’s standard method for modelling evapotranspiration and is formulated as [8]: ( ) ( ) ( ) , (Eq. 2) where ETo (mm.day-1) is the reference evapotranspiration, Rn (MJ.m-2day-1) is the net radiation recorded at the crop surface, G (MJ.m-2day-1) is the monitored soil heat flux density, T (°C) is mean daily air temperature measured at 2 m height, u2 (m.s-1) is the wind speed measured at 2 m height, es and ea (kPa) are the actual vapor pressure and saturation (Eq. 2) where ETo (mm.day-1) is the reference evapotranspiration, Rn (MJ.m-2 ay-1) is the net radiation recorded at the crop urface, G (MJ.m-2da -1) is the monitored soil heat flux density, T (°C) is mean daily air temperature measured at 2 m height, u2 (m.s-1) is the wind speed measured at 2 m height, es and ea (kPa) are the actual vapor pressure and saturation vapor pressure such that (es - ea) (kPa) is the saturation vapor pressure deficit, ∆ (kPa °C-1) is the slope of the vapor pressure curve, and γ (kPa °C-1) is the psychometric constant. The water balance equation for a rice field is estimated by Eq. 3: 6 vapor pressure such that (es - ea) (kPa) is the saturation vapor pressure deficit, (kPa °C-1) is the slope of the vapor pressure curve, and  (kPa °C-1) is the psychometric constant. The water balance equation for a rice field is estimated by Eq. 3:     iiisdiiiCi CKePmhh 0 (Eq. 3) where all units are mm, hci is the water depth of the field at the end of calculated period, hoi is the water depth in the field in the start of calculated period, mi is the irrigated water during the ith calculated period, Psdi is the possible precipitation used during the ith calculated period, (ei + Ki) are the water losses during the ith calculated period, and Ci is the drainage water during the ith calculated period. it is necessary to first have an irrigation scheme based on Eq. 3 and then the irrigation water volume to each unit of irrigation land (m3/ha) is estimated. For estimation of the water requirement for an irrigation scheme, the coefficient of irrigation (q), defined as the water discharge needed for providing a plant cultivation area unit, is given in Eq. 4: ij ij iij t m q   4,86  , (Eq. 4) where qij (l.s-1ha-1) is the coefficient of irrigation of the ith plant in the jth irrigation time, i is the ratio of the area between the ith plant and the entire irrigation area, mij (m3.ha-1) is the irrigation discharge of the ith plant at the jth irrigation time, and tij (days) is the time for mij irrigation. Evapotranspiration in the dry season is higher than in the rainy season and the available water discharge from the Mekong River to the Delta is lower in the dry season, therefore this paper focuses on the estimation of the water requirement for the Winter- Spring rice crop. in this study, the meteorological data for a 10-year series of water requirement calculations are collected from the Provincial Weather Stations of An Giang, Can Tho, and Soc Trang which serve as three rice production regions that represent the flood plain areas, middle areas, and coastal areas, respectively. Hydrological data for water balance analysis during the dry seasons are collected from the Mekong River Commission [10, 11]. The monthly discharge from the hydrological stations of the Lower Mekong mainstream from Chiang Sean (in Thailand) to Luang Prabang, Vientiane, Nakhon Phanom, Mukdahan and Pakse (all in Laos) and Kratie (in Cambodia) is presented in Table 2. The mean monthly discharge flows at Tan Chau and Chau Doc (in Vietnam) are available over the period 1979-1996 as presented in Table 3. The soil texture groups of the surveyed fields are from the Provincial Department of Agriculture and Rural Development. other secondary data, such as reports, papers, and irrigation water utility events from the upstream countries of the Mekong Basin are reviewed for discussion [12-19]. (Eq. 3) where all units are mm, hci is the water depth of the field at the end of calculated period, hoi is the water depth in the field in the start of calculated period, Σmi is the irrigated water during the ith calculated period, ΣPsdi is the possible precipitation used during th ith calculated p riod, Σ(ei + Ki) are the water losses during the ith calculated per od, and ΣCi is the drainage water during the th calculated period. It is necessary to first have an irrigation scheme based on Eq. 3 and then the irrigation water volume to each unit of irrigation land (m3/ha) is stimated. For estimation of the water requirement for an irrigation scheme, the coefficient of irrigation (q), defined as the water discharge needed for providing a plant cultivation area unit, is given in Eq. 4: 6 vapor pressure such that (es - ea) (kPa) is the saturation vapor pressure deficit, (kPa °C-1) is the slope of the vapor pressure curve, and  (kPa °C-1) is the psychometric constant. The water b lance equation for rice field is estimated by Eq. 3:     iiisdiiiCi CKePmhh 0 (Eq. 3) where all units are mm, hci is the water depth of the field at the end of calculated period, hoi is the water depth in the field in the start of calculated period, mi is the irrigated water during the ith calculated period, Psdi is the possible precipitation used during the ith calculated period, (ei + Ki) are the water losses during the ith calculated period, and Ci is the drainage water during the ith calculated period. it is necessary to first have an irrigation scheme based on Eq. 3 and then the irrigation water volume to each unit of irrigation land (m3/ha) is estimated. For estimation of the water requirement for an irrigation scheme, the coefficient of irrigation (q), defined as the water discharge needed for providing a plant cultivation area unit, is given in Eq. 4: ij ij iij t m q   4,86  , (Eq. 4) where qij (l.s-1ha-1) is the coefficient of irrigation of the ith plant in the jth irrigation time, i is the ratio of the area between the ith plant and the entire irrigation area, mij (m3.ha-1) is the irrigation discharge of the ith plant at the jth irrigation time, and tij (days) is the time for mij irrigation. Evapotranspiration in the dry season is higher than in the rainy season and the available water discharge from the Mekong River to the Delta is lower in the dry season, therefore this paper focuses on the estimation of the water requirement for the Winter- Spring rice crop. in this study, the meteorological data for a 10-year series of water requirement calculations are collected from the Provincial Weather Stations of An Giang, Can Tho, and Soc Trang which serve as three rice production regions that represent the flood plain areas, middle areas, and coastal areas, respectively. Hydrological data for water balance analysis during the dry seasons are collected from the Mekong River Commission [10, 11]. The monthly discharge from the hydrological stations of the Lower Mekong mainstream from Chiang Sean (in Thailand) to Luang Prabang, Vientiane, Nakhon Phanom, Mukdahan and Pakse (all in Laos) and Kratie (in Cambodia) is presented in Table 2. The mean monthly discharge flows at Tan Chau and Chau Doc (in Vietnam) are available over the period 1979-1996 as presented in Table 3. The soil texture groups of the surveyed fields are from the Provincial Department of Agriculture and Rural Development. other secondary data, such as reports, papers, and irrigation water utility events from the upstream countries of the Mekong Basin are reviewed for discussion [12-19]. (Eq. 4) where qij (l.s-1ha-1) is the coefficient of irrigation of the ith plan in the jth irrigation time, αi is the r tio of the area between the ith plant and the e tire irrigation area, mij ( 3. ha-1) is the irrigation d scharge of the ith plant at the jth irrigation time, and tij (days) is the time for mij irrigation. Evapo rans iration in the dry s ason is higher an in Life ScienceS | Agriculture Vietnam Journal of Science, Technology and Engineering 59September 2020 • Volume 62 Number 3 the rainy season and the available water discharge from the Mekong river to the delta is lower in the dry season, therefore this paper focuses on the estimation of the water requirement for the Winter-Spring rice crop. in this study, the meteorological data for a 10-year series of water requirement calculations are collected from the provincial ưeather stations of An Giang, Can Tho, and Soc Trang which serve as three rice production regions that represent the flood plain areas, middle areas, and coastal areas, respectively. Hydrological data for water balance analysis during the dry seasons are collected from the Mekong river commission [10, 11]. The monthly discharge from the hydrological stations of the Lower Mekong mainstream from Chiang Saen (in Thailand) to Luang Prabang, Vientiane, Nakhon Phanom, Mukdahan and Pakse (all in Laos) and Kratie (in Cambodia) is presented in Table 2. The mean monthly discharge flows at Tan Chau and Chau Doc (in Vietnam) are available over the period 1979-1996 as presented in Table 3. The soil texture groups of the surveyed fields are from the Provincial Department of Agriculture and Rural Development. other secondary data, such as reports, papers, and irrigation water utility events from the upstream countries of the Mekong Basin are reviewed for discussion [12-19]. Table 3. Mean monthly flows at Tan Chau (TC) and Chau Doc (CD) Stations (m3.s-1). St. Jan. Feb. Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov. Dec. TC 6,220 3,720 2,600 2,010 2,640 7,180 11,270 16,390 21,140 20,340 15,260 10,180 CD 1,360 700 420 330 460 1,450 2,390 3,970 5,200 5,480 4,700 2,710 Tot. 7,580 4,420 3,020 2,340 3,100 8,630 13,660 20,360 25,430 25,820 19,960 12,800 Data source [12]. Results and discussion Based on the Penman-Monteith equation (Eq. 2), the reference evapotranspiration ETo calculation results of An Giang, Can Tho, and Soc Trang provinces during the dry seasons are given in Table 4. Table 4. The reference evapotranspiration in ETo (mm/day) for 6 months of the dry seasons (data taken from 2008-2017). Province/city Nov. Dec. Jan. Feb. Mar. Apr. An Giang 3.27 3.04 3.28 4.48 5.43 5.82 Can Tho 3.35 3.16 3.43 4.39 5.21 5.73 Soc Trang 3.32 3.08 3.36 4.35 5.06 5.62 Based on the water balance equations, results from calculated irrigation rate from CRoWAT for the Winter- Spring rice crop is 9,500±400 m3ha-1 and the coefficient of irrigation is in the range of 1.36-1.39 l.s-1ha-1. When compared with the Vietnamese Standards (TCVN 8641- 2011) for the Winter-Spring rice crop in the southern region of Vietnam, the irrigation rate during growing periods should be from 7,500 to 8,000 m3ha-1 [20], excluding the irrigation rate for the land preparation period, which is about 900- 1,000 m3ha-1. The higher calculated irrigation rate can be explained by the higher air temperature in combination with stronger air-wind speeds over the last 10 years resulting in higher evapotranspiration rates, which provides evidence of water insecurity compounded by climate variability. For other agriculture products (upland crops, aquaculture, and animal husbandry), the water needs are experimentally estimated to be 30-35% [19] of the water amount for rice Table 2. Lower Mekong mainstream monthly discharge 1960 to 2004 (m3.s-1). Sites Month Mainstream sites Chiang Saen Luang Prabang Vientiane Nakhon Phanom Mukdahan Pakse Kratie January 1,150 1,690 1,760 2,380 2,370 2,800 3,620 February 930 1,280 1,370 1,860 1,880 2,170 2,730 March 830 1,060 1,170 1,560 1,600 1,840 2,290 April 910 1,110 1,190 1,530 1,560 1,800 2,220 May 1,300 1,570 1,720 2,410 2,430 2,920 3,640 June 2,460 3,110 3,410 6,610 7,090 8,810 11,200 July 4,720 6,400 6,920 12,800 13,600 16,600 22,200 August 6,480 9,920 11,000 19,100 20,600 26,200 35,500 September 5,510 8,990 10,800 18,500 19,800 26,300 36,700 october 3,840 5,750 6,800 10,200 10,900 15,400 22,000 November 2,510 3,790 4,230 5,410 5,710 7,780 10,900 December 1,590 2,400 2,560 3,340 3,410 4,190 5,710 Data source [11]. Life ScienceS | Agriculture Vietnam Journal of Science, Technology and Engineering60 September 2020 • Volume 62 Number 3 cultivation or 594-693 m3ha-1. in general, the total water taken from the MD’s river system for nearly 1,360,000 ha agricultural land during the dry season is approximately 2,500-2,600 m3s-1, which is the minimum requirement for normal yields. Considering the mean monthly flow discharges of the Mekong river that passes Pakse (Laos), Kratie (Cambodia), Tan Chau (Vietnam), and Chau Doc (Vietnam) during the dry season, it is easy to find that the expected water requirement for irrigation in the MD is greater than the inflows of the Mekong mainstream. Thus, this figure indicates that this water source during the dry season is the greatest limitation to the extension of cultivation areas not only in the MD but also to other upstream countries. Conclusions and recommendation Lowland rice consumes much more water than any other crops. Although this study is based on a rough water balance equation that only uses three places (i.e. An Giang, Can Tho and Soc Trang) along the Hau river, the calculated results show that it is impossible to satisfy this huge water amount for the irrigation of all the rice areas in the Mekong river delta in Vietnam during the dry seasons. There are many future uncertainties for which scenario- based studies might be required. For example, increasing air temperature, higher solar radiation, stronger wind velocity, less precipitation distribution, extended drought and salinity duration, as well as the effects of present and future agricultural transformation are main factors affecting rice planting areas and the growth of rice. Rice and other crop productivity can be damaged by an increasing scarcity of water resources for irrigation in the future. When the extreme scenario of historical drought and salinity intrusion occurred in 2016, the cultivation areas in the MD were narrowed down by more than 35.5%. if upstream countries continue to develop their mega- irrigation projects with 3-fold larger irrigated areas than what exists currently, the water supply capacity for rice and upland crops in the delta will decrease dramatically. in view of economic and social aspects, rice farmers in the Delta will pay more money to pump water when the water level and flow discharge of the Mekong River decreases. Accordingly, their income will be further reduced as a consequence of climate change, water diversion, as well as abnormal operation of hydropower dam projects. The rice production areas must be reduced as what has already been elaborated in resolution 120 of the Government of Vietnam because water and food security for the downstream nations will be threated. Water for farming should be used as efficiently as possible; the water productivity (crop per drop) should increase and the water profitability (income per litre) should also be increased by switching from rice to high-value crops. The development of water saving methods for farmers in parallel with farming systems improvement and cropping patterns adjustment is strongly recommended. Furthermore, strategic solutions on hydro-diplomacy and the legal and institutional aspects of water resource governance on international, regional, and local scales should be promoted to share both water benefits and risks among all the Mekong countries. ACKNOWLEDGEMENTS This study is partly funded by the Can Tho University improvement Project VN14-P6, and supported by a Japanese oDA loan. The author declares that there is no conflict of interest regarding the publication of this article. REFERENCES [1] General Statistical Office (2016), Statistical Yearbook of Vietnam, Statistic Publishing House. [2] S. 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