Numerical and analytical assessment of stormwater infiltration via wells and infiltration trenches

Arid and semi-arid regions, also referred to as drylands, are areas characterised by low water availability. A solution commonly used in drylands to cope with water scarcity is the collection of stormwater, which is stored during wet seasons or rain events and subsequently used during drier periods. Storage is often provided by infiltration into shallow aquifers. Common problems with the storage of stormwater in surface water bodies or during infiltration phases are high evaporation rates, salinisation, mosquito-breeding potential, among others. These problems are majorly a consequence of too low infiltration rates, which is caused by clogging layers in the topsoil and the presence of a thick vadose zone over the underlying aquifer. This study aims to increase the groundwater recharge rates in stormwater harvesting systems by bridging the vadose zone and the low permeable upper layers of the soil profile. We explored the efficiency of vertical structures that allow high infiltration rates by means of analytical and numerical models. Two structures are studied in detail, namely shafts and infiltration trenches. We assessed the validity of the numerical scenarios and the infiltration structures modelled using analytical models. Due to the variety of hydrogeological conditions and stormwater harvesting systems that can be found in drylands, we focused on the Kingdom of Saudi Arabia and the dams built in the channel of wadis (i.e. ephemeral streams) as a study case to construct the numerical scenarios. The results of the modelling scenarios suggest that the shafts and infiltration trenches are effective in bridging the upper layers of a typical soil profile in a wadi dam. By using such structures, recharge was significantly enhanced in comparison with the scenarios at which stormwater infiltrates directly through the vadose zone in a wadi dam. We also carried out a sensitivity analysis of key hydraulic and infiltration-structure parameters. Therefore, the results of this study have applicability beyond the Kingdom of Saudi Arabia as similar hydrogeological conditions related to wadis can be found in other parts of the world such as northern Africa, southeastern Asia and the southwestern USA. Furthermore, dams are presently used in other dryland countries as the main structure for groundwater recharge; the methods investigated here to increase infiltration rates, therefore, provide meaningful options to enhance aquifer water storage.