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Summary: Groundwater table fluctuations (WTF), chloride mass balance (CMB), apparent groundwater chlorofluorocarbon (CFC-12) ages and tritium (3H) mean residence times were used to assess recharge rates on Jeju Island (Korea), where groundwater is the main source of potable water. Given the limitations of various techniques and the respective data, the methods yield median values of 687mm/yr (WTF), 429mm/yr (CMB), 423mm/yr (CFC-12) and 394mm/yr (3H), which are lower than the multi-annual (1993–2002) average value calculated using the soil water budget (SWB) method (911mm/yr). These underestimates are mainly due to most groundwater samples used for the analyses being located in the more arid lowland areas of the island. All methods yield highest recharge rates in the southern and eastern districts of Jeju implying a strong control of rainfall on the spatial recharge distribution. The spatial variability of recharge at the catchment scale is caused by spatially and temporally variable rainfall and evapotranspiration as well as the wide range in effective porosity and specific yield values of the aquifer lithologies. The WTF method yields reliable results in the coastal regions where low hydraulic gradients prevail. The CMB technique underestimates the recharge values of the SWB in all geographic districts probably as a result of anthropogenic Cl additions to groundwater and unaccounted for atmospheric Cl dry deposition. Median recharge estimates from the CFC-12 and 3H data show the lowest spatial correlation with those from the SWB mainly as a result of vertical anisotropies and uncertainties in the determination of effective aquifer thicknesses particularly in the perched, high level areas. The here applied methods are useful for local calibration and validation of SWB estimates in lower parabasal regions where thin unsaturated zones prevail; however, due to their inability to predict recharge in the more remote upslope areas from which, as of yet, no groundwater data could be obtained, they cannot be applied to predict average recharge values representative for the whole island. [Copyright &y& Elsevier]

NETPATH-WIN is an interactive user version of NETPATH, an inverse geochemical modeling code used to find mass-balance reaction models that are consistent with the observed chemical and isotopic composition of waters from aquatic systems. NETPATH-WIN was constructed to migrate NETPATH applications into the Microsoft WINDOWS® environment. The new version facilitates model utilization by eliminating difficulties in data preparation and results analysis of the DOS version of NETPATH, while preserving all of the capabilities of the original version. Through example applications, the note describes some of the features of NETPATH-WIN as applied to adjustment of radiocarbon data for geochemical reactions in groundwater systems. [ABSTRACT FROM AUTHOR]

BSTRACT: Techniques employed to simulate infiltration and subsurface ground-water flow were examined for a number of available watershed models. The large number of processes that these models simulate prohibits detailed analysis of subsurface flow, due to excessive computer and data requirements. Such models emphasize surface flow and include only that portion of water lost to the subsurface and the portion returned to the stream as baseflow. Problems were examined in adopting conjunctive use models, which allow the coordinated exploitation and management of both surface and ground-water resources. The application of conjunctive use models in water resources management is expected to increase dramatically over the next decade. [ABSTRACT FROM AUTHOR]

Despite recent advances in numerical methodology, the solution of the complete equations describing saturated-unsaturated flow in an unconfined aquifer remains a difficult problem. Therefore in practical engineering calculations the formulation must be simplified. It is shown here that Boulton's equation (involving a time-dependent drainable porosity) and also the linearized Boussinesq equation can provide results that are in agreement with certain aspects of the more realistic Richards solution, provided that an estimation of their parameters is possible. However, the performance of Boulton's formulation is only marginally superior to that based on the Boussinesq equation. [ABSTRACT FROM AUTHOR]

A laboratory experiment is considered that was carried out by Higgins (1980) on a sand tank to study the problem of water table geometry during flow to an adjacent stream. The purpose of the analysis is to determine whether the effects of partial saturation and of compressibility are likely to be important in this type of problem. Through numerical simulations and other considerations, it is concluded that compressibility alone, however large, cannot reproduce these experimental results. The results can only be simulated if proper account is taken of the partly saturated flow above the water table. Compressibility effects of aquifer material and of the water may be safely neglected. [ABSTRACT FROM AUTHOR]

Evaluating the performance of watershed models is essential for a reliable assessment of water resources, particularly in Pacific island watersheds, where modeling efforts are challenging due to their unique features. Such watersheds are characterized by low water residence time, highly permeable volcanic rock outcrops, high topographic and rainfall spatial variability, and lack of hydrological data. The Soil and Water Assessment Tool (SWAT) model was used for hydrological modeling of the Nuuanu area watershed (NAW) and Heeia watershed on the Island of Oahu (Hawaii). The NAW, which had well-distributed rainfall gauging stations within the watershed, was used for comparison with the Heeia watershed that lacked recoded rainfall data within the watershed. For the latter watershed, daily rain gauge data from the neighboring watersheds and spatially interpolated 250 m resolution rainfall data were used. The objectives were to critically evaluate the performance of SWAT under rain gauge data scarce conditions for small-scale watersheds that experience high rainfall spatial variability over short distances and to determine if spatially interpolated gridded rainfall data can be used as a remedy in such conditions. The model performance was evaluated by using the Nash–Sutcliffe efficiency (NSE), the percent bias (PBIAS), and the coefficient of determination (R2), including model prediction uncertainty at 95% confidence interval (95PCI). Overall, the daily observed streamflow hydrographs were well-represented by SWAT when well-distributed rain gauge data were used for NAW, yielding NSE and R2 values of > 0.5 and bracketing > 70% of observed streamflows at 95PCI. However, the model showed an overall low performance (NSE and R2 ≤ 0.5) for the Heeia watershed compared to the NAW's results. Although the model showed low performance for Heeia, the gridded rainfall data generally outperformed the rain gauge data that were used from outside of the watershed. Thus, it was concluded that finer resolution gridded rainfall data can be used as a surrogate for watersheds that lack recorded rainfall data in small-scale Pacific island watersheds. [ABSTRACT FROM AUTHOR]

The integration of hydrology and climate is important for understanding the present and future impact of climate on streamflow, which may cause frequent flooding, droughts, and shortage of water supply. In view of this, we assessed the impact of climate change on daily streamflow duration curves as well as extreme peak and low flow values. The objectives were to assess how climate change impacts watershed-wide streamflow and its extreme values and to provide an overview of the impacts of different climate change scenarios (Representative Concentration Pathways (RCP) 4.5 and 8.5) on streamflow and hydrological extremes when compared with the baseline values. We used the Soil and Water Assessment Tool (SWAT) model for daily streamflow and its extreme value modeling of two watersheds located on the Island of Oahu (Hawaii). Following successful calibration and validation of SWAT at three USGS flow gauging stations, we simulated the impact of climate change by the 2050s (2041–2070) and the 2080s (2071–2100). We used climate change perturbation factors and applied the factors to the historical time series data of 1980–2014. SWAT adequately reproduced observed daily streamflow with Nash-Sutcliffe Efficiency (NSE) values of greater than 0.5 and bracketed >80% of observed streamflow data at 95% model prediction uncertainty at all flow gauging stations, indicating the applicability of the model for future daily streamflow prediction. We found that while the considered climate change scenarios generally show considerable negative impacts on daily streamflow and its extreme values, the extreme peak flows are expected to increase by as much as 22% especially under the RCP 8.5 scenario. However, a consistent decrease in extreme low flows by as much as 60% compared to the baseline values is projected. Larger negative changes of low flows are expected in the upstream part of the watersheds where higher groundwater contributions are expected. Consequently, severe problems, such as frequent hydrological droughts (groundwater scarcity), reduction in agricultural crop productivity, and increase in drinking water demand, are significantly expected on Oahu. Furthermore, the extreme values are more sensitive to rainfall change in comparison to temperature and solar radiation changes. Overall, findings generally indicated that climate change impacts will be amplified by the end of this century and may cause earlier occurrence of hydrological droughts when compared to the current hydrological regime, suggesting water resources managers, ecosystem conservationists, and ecologists to implement mitigation measures to climate change in Hawaii and similar Islands. [ABSTRACT FROM AUTHOR]