Developing a Model for the Sustainable Use of Alluvial Aquifers for Small-scale irrigation: A Case study of Shashani River

Abstract

Water resources are under increasing pressure globally. This is due to several factors, including changing weather patterns, population growth, and increased agricultural activity. Sustainable use of water resources is critically needed to conserve the resource and utilize the resource efficiently. Sustainable water resource management can be achieved through scientific study and demand management. In arid and semi-arid regions of Sub-Saharan Africa, some communities rely on water stored in alluvial aquifers within river channels (sand rivers) for dry season water supply. The main objective of this study was to develop a model for the sustainable use of the alluvial aquifer on Shashani River for small-scale irrigation. To achieve this, the study focused on three specific objectives: to quantify the potential water supply of the alluvial aquifer on Shashani River, to model its sustainable yield for small-scale irrigation, and to determine a suitable sand abstraction system to support its sustainable use. The volume of the alluvial aquifer was estimated using probing (mechanically driving a steel rod into sediment to measure depth and bedrock profile along three cross-sections) combined with satellite imagery (tracing river boundaries to estimate the river section). The sustainable yield was modelled based on literature and validated using field investigations. To validate the model, the sustainable yield was estimated for the Alluvial Aquifer on Shashani River. Satellite imagery and Geographical Information Systems were used for catchment sizing, rainfall-runoff modelling for runoff estimation, and field investigations and data collection for estimating alluvial aquifer abstraction. Desktop studies also estimated flow into and out of the river basin. To determine suitable abstraction technology, current methods were studied to inform the design of a sustainable system for irrigation. SolidWorks 2023 was used to design and simulate the performance of the abstraction technology. The volume of the Shashani Alluvial Aquifer was found to be 23.9 x 106 m3. The sustainable yield rate was estimated to be 505 432 700 m3/yr for a typical wet year, 329 404 000 m3/yr, for a typical normal year and 135 730 800 m3/yr for a typical dry year. This suggests that current abstraction can be safely increased by over 100% without exceeding sustainable yield or causing adverse environmental impacts. Therefore, users of these alluvial aquifers can safely utilize them sustainably for agricultural purposes. A suitable sand abstraction system for sustainable abstraction was modelled based on user demands in the catchment. SolidWorks 2023 software was used to model pump components and simulate their motion during operation. Simulations suggest that if the pump operates at a minimum of 50% efficiency, the flow rate will be 1.2 l/s. This would enable a farmer to pump approximately four cubic meters of water to a head of ±7m in an hour. If this water is discharged to and stored in a 4m3 reservoir, it will be sufficient to meet the crop water requirements of one hectare of leafy vegetables for three weeks. The Shashani River alluvial aquifer can be sustainably utilized for small-scale irrigation, with the proposed sand abstraction system providing sufficient water to meet crop demands without exceeding environmental limits