The installation of recharge wells and subsurface flow barriers are among several strategies proposed to control seawater intrusion on coastal groundwater systems. In this study, we performed laboratory-scale experiments and numerical simulations to determine the effects of the location and application of recharge wells, and of the location and penetration depth of flow barriers, on controlling seawater intrusion in unconfined coastal aquifers. We also compared the experimental results with existing analytical solutions. Our results showed that more effective saltwater repulsion is achieved when the recharge water is injected at the toe of the saltwater wedge. Point injection yields about the same repulsion compared with line injection from a screened well for the same recharge rate. Results for flow barriers showed that more effective saltwater repulsion is achieved with deeper barrier penetration and with barriers located closer to the coast. When the flow barrier is installed inland from the original toe position however, saltwater intrusion increases with deeper barrier penetration. Saltwater repulsion due to flow barrier installation was found to be linearly related to horizontal barrier location and a polynomial function of the barrier penetration depth.

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In the numerical investigation of saltwater transport in coastal aquifers, we need to correctly evaluate the hydrodynamic dispersion in the flow field. In this study, we focused on the role of dispersivity in the removal process of residual saltwater in a laboratory scale cutoff wall experiment. From a pulse-type fluorescent tracer injection experiment in a saturated porous media of glass beads with a mean diameter of 0.088 cm, the estimated longitudinal and transverse dispersivities were found to be 0.07 cm and 0.0025 cm, respectively. Numerical analysis of the saltwater intrusion and subsequent removal after cutoff wall installation using SEAWAT and the generated dispersivity ratio (αL/αT) of 28 reproduces well the measured salt concentration changes with time. Whereas, if a dispersivity ratio of 10 is used in the numerical simulation, transverse dispersion in the saltwater and freshwater mixing zone becomes large and the residual saltwater is removed faster than the laboratory experiment. Inversely, if 100 was used, the residual saltwater removal time took longer. The transverse dispersion is a key parameter in the mechanical dispersion of saltwater in the mixing zone after cutoff wall installation.

The use of vegetation indices derived from wavelengths known to be sensitive to plant water status is a fast, reliable, and non-destructive method of identifying the spatial and temporal distribution of crop water requirements. A pot experiment was conducted in a screenhouse to assess the potential of vegetation indices in detecting water stress in rainfed rice during the reproductive growth phase. Rainfed lowland (PSB Rc14) and upland (UPL Ri7) rice varieties were subjected to non-water stress [2.0 irrigation water (IW) per cumulative pan evaporation (CPE)], mild water stress (1.5 IW/CPE), and severe water stress (1.0 IW/CPE) treatments during the reproductive growth stage. Leaf reflectance measurements were taken using a portable field spectrometer (Jazz Spectral Sensing Suite, Ocean Optics, Inc.) with a spectral range of 650–1050 nm and 0.36 nm bandwidth. Vegetation indices – namely, water index (WI), normalized water index-1 (NWI-1), normalized water index-2 (NWI-2), normalized water index-3 (NWI-3), and normalized water index-4 (NWI-4) – were then calculated from the leaf spectral reflectance measurements and correlated with leaf relative water content (RWC), crop water stress index, and grain yield. The leaf reflectance in the NIR region (700–1050 nm) of both rice varieties under severe water stress conditions (1.0 IW/CPE) was lower compared to those under mild water stress (1.5 IW/CPE) and non-water stress treatments (2.0 IW/CPE). Significant differences in the vegetation indices were detected at the flowering stage when the onset of water stress was also identified by the crop water stress index (CWSI). NWI-2 had the strongest correlation with grain yield of PSB Rc14 (r = –0.7815), whereas WI and NWI-1 correlated well with grain yield for UPL Ri7 (r = –0.876). These results suggest the potential of using hyperspectral vegetation indices as indicators of water stress during the flowering stage of rice.