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Improving water productivity through deficit irrigation has become a major goal for sustainable agriculture amidst global decline in water availability. The study evaluated the yield, crop water use and water productivities of field-grown drip-irrigated tomato in response to regulated deficit irrigation, and subsequent simulation under different deficit and irrigation method scenarios, using AquaCrop model, in Afaka, Nigeria. The field experiment, laid in randomized complete block design, comprised three deficit irrigation levels (80, 60 and 40% of reference evapotranspiration, ETo) imposed at the vegetative, flowering and maturity growth stages, with 100% ETo at the three crop growth stages as the control. The highest fresh fruit yield (19.0 t/ha) was obtained irrigating with 100% ETo value at all growth stages but the highest water productivity of fresh fruit (4.94 kg/m3 ) was obtained irrigating with 60% ETo at maturity stage, then full irrigation at vegetative and flowering stages. On fruit dry yield basis, the highest simulated crop water productivity (0.46 kg/m3 ) for the deficit scenarios was obtained irrigating with 80% ETo at all the three growth stages, having the highest fruit dry yield (1.67 t/ha) and the lowest seasonal water applied (447 mm). Under the scenarios of irrigation methods (drip, basin and furrow), the fruit dry yield was similar in each treatment, but water productivity was highest (0.53 kg/m3 ) under drip irrigation system. Irrigating with 80% ETo at all the entire crop growth cycle of UC 82B tomato is recommended for the highest crop water productivity.

The complexity of hydrological models has been a setback in their evaluation particularly for long-term simulations. Deficit and constant loss (DCL) method has been introduced in Hydrologic Engineering Center's Hydraulic Modeling System (HEC-HMS) model for continuous based simulations. However, studies on climate change impacts using the method are still very few. This study used the method to evaluate potential impacts of climate change on streamflow at Bernam Basin, Malaysia for 2010-2039, 2040-2069 and 2070-2099 to the baseline period (1976-2005) under two RCP scenarios (RCPs 4.5 and 8.5). The model efficiency during evaluation is found satisfactory. Compared with the baseline period, the predicted streamflow decreased in all future periods during main and off-seasons. However, the changes have become more pronounced during the off-season with a significant decrease of 9.14% under the worst-case scenario (RCP8.5). Therefore, the Basin would likely experience tremendous pressure in the late century due to low streamflow, particularly in off-season months.

Irrigated agriculture has been identified as an important practice to achieving food security and socio-economic development in the face of rapid population growth and climatic uncertainties. In northern Nigeria, irrigation has long been identified as the key to achieving the much-desired increase in food production to meet the ever-increasing population. However, the existing irrigation schemes encountered several challenges coming from different dimensions including economic, social, environmental, institutional and technological. To attain sustainable crop production, this paper attempts to uncover the underline challenges confronting irrigation schemes in northern Nigeria that cut across sustainability pillars. The findings revealed that irrigation schemes contributed immensely toward achieving food security and improving the wellbeing of rural dwellers. However, the huge investment in large- and medium-scale irrigation schemes have resulted in massive economic losses. This could be attributed to their under-utilization, poor management and abandonment although few ones are performing remarkably well. The study recommends the need to adopt new water allocation and application methods that can improve water use efficiency, users-managers join approach (participatory), effective and competent institutions which include improved monitoring, evaluation and surveillance systems, frequent policy review to suit the situation, law enforcement, and timely sensitization and awareness campaigns.

Water allocation is a key component of good water management in an irrigation system. Water imbalance between upstream and downstream has been a major issue at Tanjung-Karang Rice Irrigation Scheme Malaysia. This study applied the HEC-RAS model to estimate water for supply and analysed its allocation to the demand in the scheme. Water demand was computed based on water requirement, effective rainfall and seepage/percolation. Results of R2, NSE, PBIAS and RSR during the model calibration and validation were 0.66, 0.64, 0.94 and 0.60; and 0.65, 0.59, 1.77 and 0.64, respectively. The irrigation scheme was under-supplied from January-March and over-supplied from April-June during the off-season. While in the main-season was under-supplied from July-September and over-supplied from October-December. Similarly, the excess water during the off and main seasons for the period was 40.10 and 52.40 Mm3 respectively. Whiles the deficit water during the same seasons was 52.46 and 53.14 Mm3, respectively. This suggests providing an adequate storage facility, which could store excess water during low water demand and use it in the period of water shortage. The developed model could therefore assist in estimating the over/under-supply with respect to the demand thereby storing the excess for use during the period of high demand.&nbsp

Agro-hydrological water management frameworks help to integrate expected planned management and expedite regulation of water allocation for agricultural production. Low production is not only due to the variability of available water during crop growing seasons, but also poor water management decisions. The Tanjung Karang Rice Irrigation Scheme in Malaysia has yet to model agro-hydrological systems for effective water distribution under climate change impacts. A climate-smart agro-hydrological model was developed using Excel-based Visual Basic for Applications (VBA) for adaptive irrigation and wise water resource management towards water security under new climate change realities. Daily climate variables for baseline (1976&ndash, 2005) and future (2010&ndash, 2099) periods were extracted from 10 global climate models (GCMs) under three Representative Concentration Pathway scenarios (RCP4.5, RCP6.0, and RCP8.5). The projected available water for supply to the scheme would noticeably decrease during the dry season. The water demand in the scheme will differ greatly during the months in future dry seasons, and the increase in effective rainfall during the wet season will compensate for the high dry season water demand. No irrigation will therefore be needed in the months of May and June. In order to improve water distribution, simulated flows from the model could be incorporated with appropriate cropping patterns.

Spatial and temporal climatic variability influence on the productivity of agricultural watershed and irrigation systems. In a large irrigation system, the quantification and regulation of the flow at different locations of the channel is quite difficult manually, leading to a poor delivery of supply and demand. Water shortage is a crucial issue due to mismatch between available water and demand at intake point of Tanjung-Karang Irrigation Scheme. This study assessed the potential impacts of climate change on basin outflow for 2010&ndash, 2039, 2040&ndash, 2069, and 2070&ndash, 2099 to the baseline period (1976&ndash, 2005) and used it as input hydrograph to simulate river discharge. A Hydrologic Engineering Corps Hydrologic Modeling System (HEC-HMS) model driven by projections from ten global climate models (GCMs) with three scenarios (Representative Concentration Pathways (RCPs) 4.5, 6.0, and 8.5) used to simulate the outflow and the Hydrologic Engineering Centers River Analysis System (HEC-RAS) model applied for hydraulic modeling. The projected seasonal streamflow showed a decreasing trend for future periods. The average available irrigation supply for historical period is 15.97 m3/s, which would decrease by 12%, 18%, and 21% under RCPs 4.5, 6.0, and 8.5, respectively. Projected irrigation supply showed oversupply and undersupply to the required supply during the growing season. Simulated discharge could therefore be incorporated into cropping practices to boost the sustainable distribution of water under the new realities of climate change in the future.

Climate change is anticipated to bring variability in future streamflows in various watersheds. Soil Water Assessment Tool (SWAT 2012 version) was evaluated using streamflow data (1976–2006), and assessed the potential impacts of climate change driven by projections from ten global climate models (GCMs) under three Representative Concentration Pathways (RCPs) scenarios (RCPs 4.5, 6.0 and 8.5) on future streamflow (2010-2039), (2040-2069) and (2070-2099) at the Bernam River Basin, Malaysia. The model efficiency during calibration and validation, with coefficient of determination (R2), Nash-sutcliffe efficiency (NS), Percent Bias (PBIAS) and Root mean square error-standard deviation ratio (RSR) is 0.67, 0.62, -5.4 and 0.64; and 0.64, 0.61, -4.2 and 0.65, respectively. This shows that SWAT coupled with GIS extension is a good tool for continuous simulation. Future streamflow is projected to decrease in all future periods during main and off-seasons. However, the changes would be more pronounced during the off-season period with a significant decrease of -9.14 % under the worst-case scenario (RCP8.5). Therefore, the Basin may likely to experience tremendous pressure in the late century due to low streamflow, particularly during the off-season months.