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Can crop production intensification through irrigation be sustainable? An ex-ante impact study of the south-central coastal zone of Bangladesh [1]

['Zahirul Haque Khan', 'Institute Of Water Modelling', 'Dhaka', 'Md Saiful Islam', 'Shume Akhter', 'Md Raqubul Hasib', 'Asish Sutradhar', 'Jagadish Timsina', 'Institute Of Study', 'Development Worldwide']

Date: 2024-06 

Abstract In Bangladesh’s south-central coastal zone, there is considerable potential to intensify crop production by growing dry winter season ‘Boro’ rice, maize, wheat, pulses and oilseeds using irrigation from southward flowing and predominantly freshwater rivers. However, the impacts of surface water withdrawal for sustained irrigation and its safe operating space remain unclear. We used field measurements and simulation modeling to investigate the effects of irrigation water withdrawal for Boro rice–the most water-consumptive crop–on river water flow and salinity under different climate change and river flow scenarios. Under the baseline conditions, about 250,000 ha could potentially be irrigated with river water that has salinity levels below 2 dS/m. The impact on river water salinity would be minimal, and only between 0.71 to 1.12% of the cropland would shift from the 0–2 dS/m class to higher salinity levels. Similarly, for the moderate climate change scenario (RCP 4.5) that forecasts a sea level rise of 22 cm in 2050, there would be a minor change in water flow and salinity. Only under the extreme climate change scenario (RCP 8.5), resulting in a sea level rise of 43 cm by 2050 and low flow conditions that are exceeded in 90% of the cases, the 2 dS/m isohaline would move landward by 64 to 105 km in March and April for the Tentulia and Buriswar Rivers. This would expose an additional 36.6% of potentially irrigable cropland to salinity levels of 2 to 4 dS/m. However, Boro rice will already be well established by that time and can tolerate greater levels of salinity. We conclude that there is considerable scope to expand irrigated crop production without negatively exposing the cropland and rivers to detrimental salinization levels while preserving the ecosystem services of the rivers.

Citation: Khan ZH, Islam MS, Akhter S, Hasib MR, Sutradhar A, Timsina J, et al. (2024) Can crop production intensification through irrigation be sustainable? An ex-ante impact study of the south-central coastal zone of Bangladesh. PLOS Water 3(2): e0000153. https://doi.org/10.1371/journal.pwat.0000153 Editor: Sher Muhammad, ICIMOD: International Centre for Integrated Mountain Development, NEPAL Received: June 27, 2023; Accepted: December 5, 2023; Published: February 13, 2024 Copyright: © 2024 Khan et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: The authors affirm that the article and its supplemental material have the data justify the study's conclusions. On reasonable request, the raw data will be provided. Funding: This research was supported by the Bill and Melinda Gates Foundation (BMGF) (Supports to ZHK, MSI, SA, MR, AS, US) under the STARS Grant Agreement number 1094229-2014, as well as Cereal Systems Initiative for South Asia (CSISA; http://csisa.org/) (Supports to JT, TK) The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

1. Introduction Bangladesh, a deltaic country in South Asia, has a land area of 14.5 million ha and a human population of over 170 million. While the population is still growing at about 1% per year, agricultural land availability decreased from 9.4 million ha in 1976 to 8.5 million ha in 2011, resulting in 0.05 ha of arable land per person [1]. Food security remains a concern, especially in the coastal zone [2]. New avenues are needed for low-cost and sustainable food production and to create opportunities for farmers to generate more income, as poverty and hunger are closely interlinked [3]. The United Nations’ sustainable development goal (SDG) 2, "Zero hunger," advocates that increased agricultural productivity must be achieved in a sustainable manner. Expansion of the cropland area entails risks, including further encroachment into forests and other natural habitats [4]. In response, sustainable intensification (SI) has been proposed as a set of principles to increase crop and livestock yields and associated economic returns without negative impacts on soil and water resources or the integrity of non-agricultural ecosystems [5, 6]. The diversion of river water for irrigation in a delta region is risky, as it can alter water flow and thus cause saltwater intrusion, which is likely to get exacerbated by sea level rise caused by global warming [7]. This study explores whether it is possible to intensify crop production with irrigation in the delta region of Bangladesh while staying within the safe operating space and thus preserving the ecosystem services of the rivers [8, 9]. Exempting Bangladesh’s coastal zone and mountainous eastern fringes, most land is already cropped with 2–3 crops per year, mainly rainfed rice during the monsoon ‘Aman’ season, and irrigated rice and other crops during the dryer winter months (‘Rabi’ season). In the north, groundwater is the primary source of irrigation water. However, in the low-lying coastal zone in the south, which is about 1 to 3 m above sea level, easily accessible groundwater is generally too saline for irrigation [10]. About 0.24 million ha of cropland is left fallow during the dry winter months in southwestern Bangladesh [11–13]. Most of the winter fallow land (0.074 million ha) is in the Barisal division in the south-central hydrological coastal zone, with a cropland area of approximately 0.54 million ha [14]. In that division, about 0.32 million ha are cultivated under low input conditions with little to no irrigation and fertilizer in the winter. Only 0.15 million ha are cropped using higher rates of water and nutrients [14]. Krupnik et al. [14] also mentioned that, using diversified crop (maize and wheat instead of ‘Boro’ rice) with proper irrigation management at least 20,800 and 103,000 ha of fallow and rainfed agriculture can be converted into enhanced double cropping, respectively. Hence, there is a considerable potential to intensify crop production by growing Rabi season rice (also known as ‘Boro’), maize, wheat, pulses and oilseeds during the winter in this division. However, most of these crops will require irrigation, which can potentially be supplied using surface water when and where it is sufficiently fresh [15–17]. Schulthess et al. [18] reported yields as high as 7 t ha-1 for maize and higher than 2 t ha-1 for wheat for this area. Bhattacharya et al. [19] concluded that by draining surface water just before monsoon rice maturity, it is possible to practice highly productive and profitable triple-cropping systems in low soil salinity portions of the coastal zone by including maize or sunflower in the winter season, a short duration rice in pre-monsoon (Kharif-1) season, and a medium duration rice in the full monsoon (Kharif-2) season. The use of surface water for irrigation responds to Bangladesh Government’s priorities articulated in a policy that encourages substantial investments to increase cropping intensity on currently winter fallow and rainfed croplands and to expand the use of surface water irrigation during the dry season [20]. Such developments could be a logical place to start defining the environmentally sound ‘envelope’ for intensification potential in this region. In the Barisal division, five major rivers and numerous canals cross the landscape. Canals are natural and assist in bringing tidally mediated water inland and drain out excess water during the rainy season, both of which are important ecosystem services [21]. Considering the ecosystem service of freshwater supply, Krupnik et al. [14] reported that nearly 0.06 million ha of land could be irrigated in this division with surface water pumps using river and canal water during the winter. That study limited the area that can be irrigated to a buffer of 0.4 km on both sides of the rivers and major canals due to the limited lift and water conveyance capacity of most surface pumps. In our study region, canals have been partially used for irrigation, transport, and supply of fish species, but many have become silted up, rendering some sluice gates that have been installed to manage fresh and saline water flow inactive [22, 23]. However, with rehabilitation and appropriate water flow infrastructure, rivers and canals could be utilized efficiently by irrigating most of the land [14]. Hence, an ex-ante analysis of the safe operating space is a prerequisite for their restoration, as it would require a significant investment. Conversion to large scale irrigation during the winter months may need a considerable amount of water, which in turn could reduce river flow. This, combined with sea level rise (SLR), may cause an increase in salinity conditions at the downstream end of these tidal rivers. The dense network of canals and rivers, tidal amplitude and flow dynamics, the extent of landward entry of tides, the volume of freshwater flow from upstream catchments, and salinity fluctuations in the river basins strongly govern the availability of fresh water for dry season cropping in the southern delta [24, 25]. Assessment of water flow and salinity of the regions’ rivers and interlinked natural and man-made canal systems are essential starting points to determine the quantity of available freshwater during the Rabi season. The water flow and salinity could be estimated by field sampling and measurements, although such measurements and monitoring at every point of interest would neither be feasible nor cost-effective. Instead, well-calibrated simulation models can be applied to understand and quantify the water flow, as well as soil and water salinity under the current and future climate change. Hence, the objectives of the current study were to (i) improve the river water flow and river water salinity models for their applications in southern coastal Bangladesh and (ii) apply those models under different climate change scenarios to quantify the effects of river water withdrawal on surface water availability and salt intrusion to assess the potential for crop intensification in the coastal region at present and under future climate change conditions. These objectives were achieved by calibrating the models for the rivers and canal systems of the south-central coastal area based on field measurements and secondary data and applying them under various scenarios of climate change and SLR. The overall goal was to determine the safe operating space for expanding irrigated dry-season agriculture using available surface water. We wanted to determine whether critical levels of river flow could be maintained to safeguard the river ecosystem and prevent the intrusion of saline water into the delta under various climate change scenarios. We present it as a case study that links different disciplines and addresses SDG 2 (Zero hunger), SDG 6.4 (sustainable freshwater withdrawals) and SDG 13 (combat climate change and its impacts). Such a study could largely be applied to most deltas globally exposed to SLR and salt intrusion and with a scarcity of quality irrigation water for sustainable crop intensification. The manuscript is divided into the five sections. Section 1 is introduction. Section 2 describes the climate conditions, river data collection plan (water level, flow and salinity), description of river basins and hydrological situation of the study area, description of the mathematical model framework, calibration and validation of the model, and scenario generation with climate change projection and upstream flow condition. Section 3 elaborates on the calibration and validation of the models that simulate water level, river flow and surface water salinity and quantifies the impact of climate change on salinity intrusion due to potential abstraction of river water for irrigation. The discussion (section 4) puts the study results in a larger context and section 5 summarizes the outputs of the study and mentions topics that require further inquiries.

4. Discussion This study, at the nexus of SDG 2 (Zero Hunger), SDG 6 (Clean Water) and SDG 13 (Climate Action), seeks to determine whether it is possible to abstract surface water for irrigation in the dry winter months to increase agricultural production, while ensuring sustainable management of river water by staying within the safe operating space. The ex-ante analysis considers the baseline water level and salinity conditions of 2015. It assesses the potential impact of climate change in 2030 (RCP 8.5) and 2050 (RCP 4.5 and 8.5) on river water flow and salinity. The output will support planning for adaptation to climate change. We assessed several factors that may negatively impact freshwater availability from the rivers of the southcentral zone of Bangladesh: 1) Withdrawal of river water to meet the increased demand for irrigation, 2) a change of upstream flow, and 3) rise in sea level under plausible climate change scenarios. Previous analyses have shown that under baseline conditions, there is plenty of fresh water for irrigation in much of the Barisal division throughout the dry season [26, 46, 47]. Freshwater availability is abundant due to the connectivity of these rivers to the lower Meghna River. Water abstraction for irrigation would not impact salinity levels in the rivers (Figs 12 and 13). Simulation results showed mean monthly water flow varying from 5,823 to 7,074 m3 s-1 over the dry season (January to April) in the Bishkhali River. In the Buriswar River, the mean monthly flow ranged from 5,143 to 5,971 m3 s-1. Water flows are also abundant during the dry season in Tentulia, Baleswar and Lohalia Rivers. Tentulia and Baleswar Rivers have significant water flows both in the ebb and flood tides from spring to neap tides; the mean monthly water flows at the downstream river stretches of Tentulia River are within 9,456 to 12,173 m3 s-1. The salinity levels in the five rivers exhibit distinct seasonal variation with the change of upstream freshwater flow. Freshwater flow from upstream rivers and tidal effects from the BoB together determine the area’s salinity level and extent. The daily salinity level in the river changes from spring to neap tides and with the season. The higher water levels along the coast during spring tides result in a higher volume of saline water flow to the upstream of the rivers compared to neap tides. For the Buriswar River, the salinity level remains below 0.2 dS/m over the dry season at the middle and upstream stretches, confirming a reliable source of irrigation water and other domestic and industrial uses. The salinity level at the downstream end of this river varies over the year where salinity starts to build from December, peaks in late March or early April, and drops from late May to December. For the Tentulia River, the salinity level remains below 1.8 dS/m in the upstream stretches, while it is within 3 dS/m in the downstream stretches. However, climate change may cause less favorable conditions for the people living in the Buriswar and Tentulia river basins. The simulations revealed that the salinity levels of these two rivers are likely to increase under RCP 8.5 by 2050. This is due to less flow from the upstream and the SLR by 0.43 m. Therefore, 2 dS/m salinity isohaline shifted upward by more than 100 km. Managing irrigation with water that has salinity levels higher than 2 dS/m requires careful and skillful management practices, especially during the establishment of the crops, when they are most sensitive to high salinity levels. Fortunately, the major shift of the isohaline occurs in March only, whereas maize and wheat can be established right after the harvest of the Kharif-2 season aman rice crop in December. Boro rice can be transplanted as early as late January. All in all, the exposure analysis showed that the area of high potential cropland, i.e., exposed to low salinity levels in the range of 0 to 2 dS/m, currently is 276,300 ha. This is about 78% of the total cropland of the five river basins. Thus, there is a high potential for the intensification of irrigated agriculture in the southcentral zone. As irrigation and water management experiments by Krupnik et al. [14], Bhattacharya et al. [19], and Schulthess et al. [18] and modeling scenarios by Timsina et al. [17] have shown, relatively high yield levels can potentially be achieved for Boro rice, wheat, maize, sunflower, soybean and mungbean in southern Bangladesh. Our study did not consider potential changes in upstream boundary flow due to the construction of dams along the Ganges and Brahmaputra rivers and the redirection of water into other basins. Nor did it consider salt intrusion into landward due to cyclones, storm surges, and land subsidence [9]. The salinization of large parts of the south-western zone can be taken as an example to illustrate the consequences of a reduction in upstream boundary flow. The operation of the Farakka Dam in Murshidabad district in the Indian state of West Bengal from 1975 and the diversion of fresh water from the Ganges River towards India during dry season have already decreased the amount of freshwater entering the Ganges delta. The diversion of water reduces the supply of water from rivers and ultimately threatens crop and fish diversity [48]. Tuong et al. [49] also reported that salinity intrusion during the dry season is more sensitive to transboundary flows than SLR. Hence, ensuring transboundary flows during the dry season is highly important for sustainable agriculture and aquaculture in the southern coastal regions. Managing water in the delta region is a complex task, as it needs to balance different users’ interests. These resources are largely shaped by tidal dynamics and transboundary and upstream flows and are affected by natural, socio-economic, and institutional changes. Transboundary river basin management is more complex than for rivers flowing through one country due to the challenges in the design and implementation of joint monitoring programs [50]. Since rivers in Southern Bangladesh originate from the Himalayas and flow through India, a transboundary river basin management involving all countries is paramount. Water management in the coastal delta is generally planned and performed through participatory approaches involving water management organizations, local government institutions and farmers [51]. The internal canals and peripheral rivers and regulators and sluice gates form the integral parts of the water management system, and involve effective drainage and irrigation with the appropriate operation of the control structure and pumps [52]. However, in practice, there is inadequate involvement of local governments and communities in water management and a lack of maintenance of flap and vertical lift gates and regulators, many of them becoming non-functional [53]. Past studies have revealed that lack of appropriate water management at the field level is one of the crucial factors limiting the intensification of agriculture and the increase of water productivity. Tuong et al. [49] emphasized that participatory water management including water governance and equity is essential for sustainable water management in the polders of southern coastal Bangladesh. For sustainable coastal water management that would require strengthening and formalizing the role of local governments in local water management and ensuring their access to permanent maintenance funds, severe hydrological and socio-economic challenges facing the coastal zone would need to be addressed [53]. Improved governance and equity and access to water management would be important as these can play a vital role in the intensification of Rabi crops and further development of aquaculture-agriculture systems [54]. Improved water resource management in coastal regions would need frameworks that recognize the importance of rivers and aquatic resources in providing various ecosystem services. Meynell et al. [55] developed a framework of ecological importance as a tool for river basin planning and water resource management, obtaining baseline information for impact assessment of infrastructure, and protecting ecologically important areas for rivers of mainland southeast Asia. The framework maps out the relative contributions of river reaches to a wide range of ecosystem services and allows prioritization of river ecosystem services to be assessed and mapped according to importance in different river reaches and basins within a region. Likewise, Tickner et al. [56] developed a conceptual framework for a coherent approach to river management research, policy and planning to encourage informed, equitable and sustainable river management. They applied it to the Great Ruaha River basin in Tanzania. The framework integrates concepts from ecosystem science, water resource management, social science and political economy, thereby linking concerns about the river ecosystem with the concerns of decision makers and allowing broader analysis that supports an understanding of how and why different groups within society benefit from the services a river provides. Such frameworks are currently lacking in Bangladesh. Similar frameworks are needed to identify and prioritize the critical ecosystems services provided by the networks of rivers and canals and applying them to policy and to plan for sustainable management of river basins in southern coastal Bangladesh.

5. Conclusions The overall goal of this study was to determine the safe operating space for the expansion of irrigated dry season agriculture using available surface water. We wanted to determine whether critical river flow levels could be maintained to safeguard the river ecosystem and prevent the intrusion of saline water into the delta under various climate change scenarios. Our results showed that the abstraction of river water, even for Boro rice, the crop with the highest water demand, would not change the salinity dynamics in the rivers under baseline conditions (2015) nor the moderate climate change scenario (RCP 4.5) in 2050 or the extreme scenario (RCP 8.5) in 2030. Only under the low flow conditions (90% frequency of exceedance) for RCP 8.5 in 2050 the 2 dS/m isohaline would shift landwards by more than 100 km for the Buriswar and Tentulia River basins. An additional 36% of the cropland in the south-central zone would be exposed to river water salinity ranging between 2 and 4 dS/m. For most crops, this may entail some yield depressions. However, water abstraction per se under the baseline scenarios would increase the 2–4 dS/m area by 0.5% only. Thus, the change would be almost entirely due to climate change, independent of water abstraction. Other factors, which we did not simulate, such as a reduced upstream boundary flow caused by the construction of dams and redirection of water into other basins, may cause further salinization in the estuarian zone. This would pose a great threat to the sustainability of crop production, endanger the entire ecosystems and reduce the ecosystem services provided by rivers and canals in the south-central region of Bangladesh. There is a need for additional research to study the impact of salt water intrusion on groundwater quality and bio-diversity of aquatic flora and fauna. In addition, the government needs to prepare and engineer interventions for the preservation of the freshwater zone, and last but not least, analyse the transboundary flow regime for managing salt water intrusion under sea level rise.

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