Integrated Hydrological Modeling for Water Resources Management of Heeia Coastal Wetland in Hawaii.

Abstract

The integrated hydrological models are an important tools that can be used to assess the water resources availability and sustainability for food security and ecological health of the coastal regions. In addition, such models are useful in assessing the current and future water budget under different conditions of climate and land use changes. This study addresses the Heeia Wetlands Restoration whereby different scenarios were developed to assess the effects of land cover change (LU), climate change (CL), and sea level rise (SLR) on the water balance components (WBCs), fresh water submarine groundwater discharge (FSGD), seawater intrusion, dissolved silicate (DSi) fluxes, and heat transport within the Heeia Coastal Wetland. The watershed (SWAT) model, the groundwater flow (MODFLOW) model, and the density dependent groundwater flow (SEAWAT) model were utilized in this integrated approach. The SWAT model was used to assess the impact of CL and LU on the WBCs. The LU mainly focused on the conversion of a fallow wetland covered by california grass (invasive plant) to taro field (native plant). The groundwater recharge of the SWAT model output was used as input for both the steady state and transient-MODFLOW model to study the interaction between surface water and groundwater and its effect on the FSGD within the Watershed. The SEAWAT model was used to study the seawater intrusion, DSi fluxes and cold groundwater transport under several CL, LU, and SLR scenarios. The results indicated that the baseflow was the main components of the Heeia streamflow, especially during dry season. The annual recharge, surface runoff, lateral flow and ET comprised about 34%, 6%, 15%, and 45% of the annual rainfall, respectively. The WBCs were more impacted in the late of 2080s compared to the 2050s period. To understand the comprehensive relationships between coastal hydrological processes and ecosystems, the FSGD was estimated under different scenarios of LU, CL, and SLR. The current daily average of the Heeia coastal FSGD was about 0.43 m3/d/m, but expected to decrease by about 10% by the end of 21st century due to the combined effects of various changes. The FSGD comprised 18%, 11%, and 3% of the annual baseflow, recharge, and rainfall, respectively. Moreover, the FSGD fluxes would decline more during the dry season compared to the wet season. The FSGD fluxes were about 1.5 to 3.5 times than the fresh water delivered to the Kaneohe Bay via total Heeia streamflow. The outputs of SEAWAT model indicated that the seawater intrusion was not significantly influenced by SLR, CL, and LU. The average DSi fluxes was about 48 mole per day that increased by 15% during the wet season, but decreased by16% during the dry season. The DSi fluxes were a function of the FSGD. The CL more negatively affected the DSi fluxes compared to the SLR. The respective average heat energy reduction within wetland under california grassland and taro cultivation would be 0.81and 1.12 (Kj/m3) for inflow of cold groundwater, and 4.69 and 3.13 (Kj/m3) for outflow of groundwater. The cold groundwater discharge at the shoreline was significantly mitigated the seawater temperature due to the high thermal gradient between the FSGD and seawater. Despite data scarcity, the integrated hydrological modeling approach has provided a comprehensive assessment of the water resources that can help in the management of the Heeia Coastal Wetland under various land cover and climate conditions.