Climate change and drought phenomena impacts have become a growing concern for water resources engineers and policy makers, mainly in arid and semi-arid areas. This study aims to contribute to the development of a decision support tool to prepare water resources managers and planners for climate change adaptation. The Hydrologiska Byråns Vattenbalansavdelning (The Water Balance Department of the Hydrological Bureau) hydrologic model was used to define the boundary conditions for the reservoir capacity yield model comprising daily reservoir inflow from a representative example watershed with the size of 14,924 km 2 into a reservoir with the capacity of 6.80 Gm 3 . The reservoir capacity yield model was used to simulate variability in climate change-induced differences in reservoir capacity needs and performance (operational probability of failure, resilience, and vulnerability). Owing to the future precipitation reduction and potential evapotranspiration increase during the worst case scenario (−40% precipitation and +30% potential evapotranspiration), substantial reductions in streamflow of between −56% and −58% are anticipated for the dry and wet seasons, respectively. Furthermore, model simulations recommend that as a result of future climatic conditions, the reservoir operational probability of failure would generally increase due to declined reservoir inflow. The study developed preparedness plans to combat the consequences of climate change and drought.

Overall water resources evaluations including groundwater contributions to river flow are critical for assessing climate change and drought impacts on basin hydrological responses. By utilising precipitation, daily stream flow and simulated river discharge alteration data, this study introduces a simple but comprehensive methodology to better understand the potential impact of river regulation coupled with climate variability and drought on groundwater involvement. The Lower Zab River basin in northern Iraq has been selected as a representative case study to demonstrate the new methodology. Three of the most commonly used base flow separation methods were assessed: Eckhardt algorithm, flow duration curve and Chapman filtering algorithm. In addition, the indicators of hydrologic alteration method and the reconnaissance drought index have been applied. The results demonstrated that some of the underground water responds to precipitation events. A noticeable increase in groundwater contribution has been observed during the hydrological years between 1998-2001 and 2006-2008 as a result of a sharp decline in the average precipitation. However, the opposite has been observed during the hydrological year 1987, which impacted negatively on the basin water resources availability. The reservoir release through the dry periods causes the observed variations in base flow index (BFI) values between the preand post-river damming time periods. Considering the BFI long-term seasonal variation, index values started to rise in April and reached their maximum by the end of June. A steady decline has been recorded between August and September.

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Investigating the spatiotemporal distribution of climate data and their impact on the allocation of the regional aridity and meteorological drought, particularly in semi-arid and arid climate, it is critical to evaluate the climate variability effect and propose sufficient adaptation strategies. The coefficient of variation, precipitation concentration index and anomaly index were used to evaluate the climate variability, while the Mann-Kendall and Sen's slope were applied for trend analysis, together with homogeneity tests. The aridity was evaluated using the alpha form of the reconnaissance drought index (Mohammed & Scholz, Water Resour Manag 31(1):531-538, 2017c), whereas drought episodes were predicted by applying three of the commonly used meteorological drought indices, which are the standardised reconnaissance drought index, standardized precipitation index and standardized precipitation evapotranspiration index. The Upper Zab River Basin (UZRB), which is located in the northern part of Iraq and covers a high range of climate variability, has been considered as an illustrative basin for arid and semiarid climatic conditions. There were general increasing trends in average temperature and potential evapotranspiration and decreasing trends in precipitation from the upstream to the downstream of the UZRB. The long-term analysis of climate data indicates that the number of dry years has temporally risen and the basin has experienced succeeding years of drought, particularly after 1994/1995. There was a potential link between drought, aridity and climate variability. Pettitt's, SNHT, Buishand's and von Neumann's homogeneity test results demonstrated that there is an evident alteration in the mean of the drought and aridity between the pre-and post-alteration point (1994).

Numerous drought indices with various intricacy have been utilised in several climatic regions. Presently, the reconnaissance drought index (RDI), which is considered as a powerful index of meteorological drought, is acquisitioning approval primarily in semi-arid and arid climatologic areas. Because RDI is based on precipitation (P) and evapotranspiration (ET), it assesses the ET estimation effects on the characterisation of drought severity computed by RDI. The current study sheds light on the impact of the ET methods, and the elevation and climate conditions on the RDI annual results, (particularly, the alpha form of the index (RDI α12 )), using three of the most widespread experimental ET estimates with low data requirements. These techniques are known as Thornthwaite, Hargreaves, and Blaney-Criddle, and are utilised in addition to the Food and Agriculture Organization Penman-Monteith reference technique. Data from 24 stations for the period from 1979 to 2014 cover different elevations and climatic conditions. No significant (P > 0.05) impacts on both the standardised (RDI st ) and normalised (RDI n ) forms of the RDI were detected by applying the considered ET methods at various elevations for various climatic conditions. However, the RDI α12 is directly influenced with a significant (P < 0.05) deviation that has been observed by various ET methods at different elevations and climate conditions. Accordingly, consideration should be paid to the ET estimation methodologies, in particular at high elevations. The use of various approaches may lead to flaws in availability of water resources and water quality forecasts.

Climate change impact, drought phenomena and anthropogenic stress are of increasing apprehension for water resource managers and strategists, particularly in arid regions. The current study proposes a generic methodology to evaluate the potential impact of such changes at a basin scale. The Lower Zab River Basin located in the north of Iraq has been selected for illustration purposes. The method has been developed through evaluating changes during normal hydrological years to separate the effects of climate change and estimate the hydrologic abnormalities utilising Indicators of Hydrologic Alteration. The meteorological parameters were perturbed by applying adequate delta perturbation climatic scenarios. Thereafter, a calibrated rainfall-runoff model was used for streamflow simulations. Findings proved that climate change has a more extensive impact on the hydrological characteristics of the streamflow than anthropogenic intervention (i.e. the construction of a large dam in the catchment). The isolated baseflow is more sensitive to the precipitation variations than to the variations of the potential evapotranspiration. The current hydrological anomalies are expected to continue. This comprehensive basin study demonstrates how climate change impact, anthropogenic intervention as well as hydro-climatic drought and hydrological anomalies can be evaluated with a new methodology.

There is scientific evidence of accelerated sea level rise and saline intrusion. Some impacts, such as stratification and estuarine circulation, are subtle; others are dramatic including shifts in salt-sensitive habitats and limited water availability of suitable quality for industrial and municipal uses. These results have become a remarkable reality resulting in a set of integrated surface water organisation issues. Tremendous population increases overwhelming many coastal areas have expanded the problem. These challenges have been studied from many perspectives using various objectives and methodologies, and then arriving at different findings. However, all research assured that significant rises in sea level have influenced estuaries and tidally affected rivers, and these observations are expected to become rapidly worse in the future. This study introduces, categorises, critically investigates, and synthesises the most related studies regarding accelerated sea level rise and challenges of the development associated with the resources of surface water in estuaries and tidally-affected rivers. This critical review reveals that there is a need for research that focuses on the development of sustainable surface water resources.

A baseflow separation methodology combining the outcomes of the flow-duration curve and the digital filtering algorithms to cope with the restrictions of traditional procedures has been assessed. Using this methodology as well as the monitored and simulated hydro-climatological data, the baseflow annual variations due to climate change and human-induced activities were determined. The outcomes show that the long-term baseflow index at the upstream sub-basin is nearly half of that at the downstream from October to April, whereas they are close to each other for the remaining months. Some of the groundwater reacts to precipitation and an evident rise in the groundwater contribution was detected for the hydrological years 1998-2001 and 2006-2008. The contrary was recorded for1987. The water released from the reservoir in the dry periods led to distinctions in the detected baseflow index between the pre-damming and post-damming periods of the river.