The water supply in Saudi Arabia is already depleted. Climate change will exacerbate the demand for these resources. This paper examines how climate change affects the water demands of Saudi Arabia’s most important food crops: wheat, clover, vegetables, and dates. To reduce the adverse climate change impacts on these crops’ productivity, as well as their irrigation water requirements (IWR), a number of adaptation techniques were investigated. The study was carried out for the Ar Riyadh region, Saudi Arabia, with a cultivated area of 179,730 ha. In this study, five climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) for two Shared Socio-economic Pathways (SSPs), SSP2-4.5 and SSP5-8.5, were used to forecast and investigate the potential impacts of climate change on agricultural water resources in the Al-Riyadh Region of Saudi Arabia. To simulate IWRs under the present and projected climate change scenarios, CROPWAT8.0 was used. The results showed that the maximum increase ratio in 2100 under SSP2-4.5 and SSP5-8.5, respectively, will be 4.46% and 12.11% higher than in the current case (2020). The results showed that the projected maximum temperatures in 2100 will be increased by 4.46% and 12.11%, respectively, under SSP2-4.5 and SSP5-8.5, compared to the current case (2020), supporting past research on the Arabian Peninsula that revealed that both short- and long-term temperature increases are anticipated to be considerable. Under SSP2-4.5 and SSP5-8.5, the projected ET_o was found to be increased by 2.18% and 6.35% in 2100, respectively. Given that evapotranspiration closely mirrors the temperature behavior in the study region from June to August, our data suggest that crop and irrigation demand may increase in the mid to long term. The findings indicate that Riyadh, Saudi Arabia’s capital and commercial hub, will require more water to irrigate agricultural land because of the expanding ETo trend. Under SSP2-4.5 and SSP5-8.5, the projected growth irrigation water requirement (GIWR) will be increased by 3.1% and 6.7% in 2100, respectively. Under SSP5-8.5, crop areas of wheat, clover, dates, maize, citrus, tomato, potato, and other vegetables in Ar Riyadh will decrease by 6.56%, 7.17%, 5.90%, 6.43%, 5.47%, 6.99%, 5.21%, and 5.5%, respectively, in 2100. Conversely, under SSP2-4.5, the crop areas will decrease by 3.10%, 3.67%, 2.35%, 3.83%, 2.32%, 4.18%, 1.72%, and 2.38% in 2100, respectively. This research could aid in clarifying the adverse climate change impacts on GIWR in Ar Riyad, as well as improving water resource management planning.

Simple analytical and numerical solutions for confined and unconfined groundwater-surface water interaction in one and two dimensions were developed in the STRIVE package (stream river ecosystem) as part of FEMME (flexible environment for mathematically modelling the environment). Analytical and numerical solutions for interaction between one-dimensional confined and unconfined aquifers and rivers were used to study the effects of a 0.5 m sudden rise in the river water level for 24 h. Furthermore, a two-dimensional groundwater model for an unconfined aquifer was developed and coupled with a one-dimensional hydrodynamic model. This model was applied on a 1 km long reach of the Aa River, Belgium. Two different types of river water level conditions were tested. A MODFLOW model was set up for these different types of water level condition in order to compare the results with the models implemented in STRIVE. The results of the analytical solutions for confined and unconfined aquifers were in good agreement with the numerical results. The results of the two-dimensional groundwater model developed in STRIVE also showed that there is a good agreement with the MODFLOW solutions. It is concluded that the facilities of STRIVE can be used to improve the understanding of groundwater-surface water interaction and to couple the groundwater module with other modules developed for STRIVE. With these new models STRIVE proves to be a powerful example as a development and testing environment for integrated water modeling.

In this study, a decentralized new sewage water treatment system is suggested and designed in Ar Riyadh, Saudi Arabia, to safeguard the environment and reuse treated water for irrigation purposes. The system consists of a primary treatment (septic tank), a subsurface horizontal flow constructed wetland (HSSF-CW), and a storage ground tank. The research methodology employed in this study is (i) to define the wastewater characteristics, where air temperature in winter is 18.6 °C, the wastewater flow per person (q) is 150 L/d, demonstrating an inlet design discharge of 300 m³/d, the influent pollutant concentrations for biological oxygen demand (BOD), total suspended solids (TSS), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and fecal coliforms (FC) are 350, 1000, 700, 50, 12 mg/L, and 10⁶ CFU/100 mL, respectively; (ii) to design the septic tank based on a retention time of two days and a surfacing load rate of 1.5 m/d; (iii) the P-k-C* model was used to determine the HSSF-CW surface area based on reed beds of Phragmites australis (common reed) and papyrus plants, where the removal rate was constant at 20 °C for BOD, TP, and FC in the effluent concentrations not exceeding 20 mg/L, 3.0 mg/L, and 2000 CFU/100 mL in order to satisfy Saudi Arabia’s wastewater reuse requirements; and (iv) to design the clean water tank for a hydraulic retention time of 10 h. The results demonstrate that the removing pollutants design area is 1872 m² divided into nine cells, each of width 8 m and length 26 m, with a hydraulic loading rate (LR) of 0.16 m/d and a hydraulic resident time (RT) of 1.1 d. The effluent pollutant concentrations for the BOD, FC, TN, and TP were 245 mg/L, 10³ CFU/100 mL, 35, and 8.5 mg/L, respectively. The wastewater treatment system total removal efficiencies for BOD, TN, TP, and FC were estimated to be 91.8, 70, 57, and 98.5%, respectively. Design curves were developed to ease the design steps. The HSSF-CW is a green wastewater treatment technology that offers greatly decreased investment costs, and service particularly for small-scale applications up to 6000 persons.

When considering three-dimensional groundwater flow, the hydrogeological community widely uses the MODFLOW model based on the head-oriented approach (HOA). However, a great deal of hydrogeological situations requires an extremely accurate assessment of the Darcy velocity in three dimensions. Despite the good numerical approximation of the piezometric head offered by MODFLOW, it does not guarantee sufficient accuracy in estimating all three components of the flow velocity. As an alternative, this article presents the MODFLOW-based velocity-oriented approach (VOA), which can approximate the three components of the groundwater velocity with high accuracy. The article gives comprehensive theoretical background and a comparison of the HOA and VOA approaches. Importantly, the authors show how the VOA equations with the VOA boundary conditions can be implemented and solved with MODFLOW. Recently, the VOA has become a basis for new version of MODFLOW. The two approaches are effectively combined and compared using Visual MODFLOW for a simple case of river–aquifer interaction.