Climate change is expected to cause ever greater reductions to the yield of durum wheat due to declining rainfall in the durum growing areas of the world. Improving water use efficiency of this crop is crucial. This PhD study investigated the impact of deficit irrigation on the growth, yield and water use efficiency of eight durum wheat varieties, based on greenhouse and field experiments. The greenhouse experiment consisted of three levels of water replacement, 50, 75 and 100% of full point, under two CO₂ concentrations (350 and 750 ppm). The field experiment imposed four irrigation water replacements, 0, 50, 75 and 100% of fully irrigated crop water use, in 2017, and repeated in 2018. The field experiment indicated that irrigation replacement at 50% increased aboveground biomass, grain yield, water use and water use efficiency by 18, 33, 14 and 20% respectively, when compared with the 0% irrigation water replacement in 2017. Reducing irrigation from 100 to 0% generally reduced biomass linearly from 15 to 12 tonnes per hectare and this was true for Caparoi, Jandaroi, DBA Aurora, DBA Lillaroi and EGA Bellaroi in 2018. The interaction between irrigation replacement and varieties showed that Hyperno, DBA Lillaroi and DBA Aurora showed increases of 31, 18 and 16% respectively for water use efficiency under the 50% water replacement over the dryland treatment, while Jandaroi appeared unaffected. The remaining varieties declined in water use efficiency ranging from Caparoi (-3%) and DBA Bindaroi (-13%). The greenhouse experiment demonstrated that the levels of irrigation and CO₂ significantly affected water use efficiency of biomass production (WUE_b). The interaction between irrigation and CO₂ levels shows that the greater WUE_b achieved at 750 ppm CO₂ increased more as irrigations were reduced than for the ambient CO₂ level (5.1 to 6.0 verses 3.6 to 4.1 g kg^-1 ). In all cases varieties produced their highest WUE_b at 50% irrigation and lowest at 100%, however for the 75% treatment WUE_b sometimes equalled that for the 50% and for other varieties the 100% irrigation.

Soil salinization is a pernicious problem in the coastal areas of Bangladesh that presents substantial challenges to agricultural productivity and difficulties in addressing the sustainable development goals (SDGs). While the biophysical nature of salinity is well understood, dealing with the uncertainties that emerge from the differential nature of its impact on agricultural livelihoods remains unexplored. In order to address the uncertainties in adaptation decisions, this study applied the adaptation tipping points (ATPs) approach to determine the yield loss threshold in a coastal saline environment, and to show the potential signal for tipping points of dry season rice cultivation in the coastal areas. To apply the ATP approach in the agricultural system, this study developed three sequential steps, and showed its potential for an ATP-driven research-extension policy agenda. Using a semi-structured survey, data were collected from 280 farmers (rice farmers; n=109, shrimp farmers; n=107, salt farmers; n=64) who were randomly selected from two coastal sub-districts (Assasuni from Satkhira district (south-west) and Banskhali from Chittagong district (south-east). In a follow-up survey, in-field salinity measurement data were collected from the rice field of the selected farmers (n=36), and how farmers' perceived these salinity level in their rice field were recorded. Then, the literature based salinity classification of these measured salinity data and farmers' perceptions about the same salinity measured data were compared. To complement the semi-structured survey data, key informant interviews and focus group discussions were also conducted.

In the first step (Chapter 2), this study explored the rice farmers' understanding of the salinity problem and their locally practised adaptation strategies to inform the policymakers as to when farmers are concerned about the salinity problem. Rice farmers perceived the reproductive stages of rice plants as being the most vulnerable to salinity. The findings also demonstrated that farmers perceived a field as being affected by high salinity levels when the levels were lower than the scientific literature recommends. In response to the salinity problem, most of the rice farmers undertake early transplanting and apply irrigation to overcome its harmful effects in Boro season rice cultivation. The study demonstrated that farmers' adaptation strategies (e.g. early transplanting) potentially avoid high salinity during the reproductive stages of rice development in Boro season (dry season), indicating that farmers understand the salinity problem and are concerned about the harmful effects of salinity on rice production. Farmers' actions to cope with salinity are pre-emptive of when salinity would have severe effects on rice plant growth and yield.

In the second step (Chapter 3), this study examined differential framing of the salinity problem (i.e. perceptions of diverse types of farmers) in order to understand farmers' perceptions of salinity and the underlying causes, and adaptation preferences. The findings indicated that farmers overall have perceived an increasing salinity trend over the last 20 years. However, salinity perceptions and underlying causes varied among the different types of farmers (i.e. rice, shrimp and salt farmers). The majority of rice farmers (87%) reported increased salinity, while over half of the salt (52%) and shrimp farmers (52%) perceived that salinity has decreased over the past 20 years. The results also indicated that most of the rice farmers (62%) perceived that anthropogenic factors are the main cause of increased salinity, while the majority of the shrimp and salt farmers focused more on natural factors as being responsible for salinity. The results also indicated that while rice farmers perceived severe yield loss due to high salinity, shrimp and salt farmers perceived that they were not disadvantaged and reported higher production from their farming enterprises. According to the study, the rice farmers preferred salinity-tolerant rice varieties that have greater tolerance at the reproductive stages, while shrimp and salt farmers considered that engineering solutions such as strengthening embankments and canal excavation as the preferred adaptation options to cope with salinity. Thus, this study demonstrated that farmers' perceptions of salinity are directly related to the ways in which their livelihoods are being impacted by salinity.

In the third step (Chapter 4), this study applied the adaptation tipping points (ATPs) approach to investigate threshold yield loss from the perspectives of all types farmers affected by salinity, which could signal plausible discontinuation of growing rice in the Boro season (i.e. Dry season). Adaptation tipping points are defined as the points where the current strategies may fail to achieve their objectives. This study considered promoting saline-tolerant rice varieties in the dry season as a management strategy, with a policy objective of coping capacity under a high saline environment. The findings revealed that despite government actions to promote dry season rice cultivation, local farmers have not expanded rice growing in the Boro season, and salinity-induced lower rice yields were found to be the prime reason behind this lack of expansion. This study has shown the existence of a farmer-derived threshold yield loss in the coastal dry season rice farming system that is based on their experience and observations. However, yield loss thresholds varied among the different types of farmers, with rice farmers tolerating a greater rice threshold yield loss (23%) compared to shrimp and salt farmers (at 16% and 14% of threshold yield loss, respectively). Thus, differential perceptions of the salinity affected yield loss thresholds suggest government actions to support and encourage integrated land management for rice, shrimp and salt farming rather than research and extension efforts that are focused on dry season rice expansion alone. These actions could strengthen sustainable livelihood options to ensure food security, and contribute to the achievement of the sustainable development goals for instance, no poverty (SDG-1), zero hunger (SDG-2) and good health and well-being (SDG-3).