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Evaporation rate estimation is important for water resource studies. Previous studies have shown that the radiation-based models, mass transfer models, temperature-based models and artificial neural network (ANN) models generally perform well for areas with a temperate climate. This study evaluates the applicability of these models in estimating hourly and daily evaporation rates for an area with an equatorial climate. Unlike in temperate regions, solar radiation was found to correlate best with pan evaporation on both the hourly and daily time-scales. Relative humidity becomes a significant factor on a daily time-scale. Among the simplified models, only the radiation-based models were found to be applicable for modelling the hourly and daily evaporations. ANN models are generally more accurate than the simplified models if an appropriate network architecture is selected and a sufficient number of data points are used for training the network. ANN modelling becomes more relevant when both the energy-and aerodynamics-driven mechanisms dominate, as the radiation and the mass transfer models are incapable of producing reliable evaporation estimates under this circumstance.

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Groundwater recharge studies are essential for investigating the feasibility of using the reclaimed lands in Singapore for subsurface storage and recovery of water. Through time-series and spectral analyses, net recharge percentages and stressfiltering characteristics at the reclaimed land were found to depend strongly on the stress transfer velocity, which was a combined function of rainfall pattern and vadose zone thickness. Based on stress transfer velocity, a theory was established to provide logical explanations for the rainfall-recharge relationship, the observed stress-filtering characteristics and the recharge percentage characteristics at the unconfined sandy aquifer. Although the reclaimed land site has a lithollogically homogeneous soil profile, a non-uniform recharge pattern was observed to be influenced pronouncedly by the uneven density distribution of bush grasses. Under a bare soil condition, significantly lower recharge percentages were observed for areas under the influence of offshore tides. The unconfined sandy aquifer appears to dampen out the wave propagation of offshore tides rapidly within a short distance from the tidal source, though it has a fairly straight shoreline and is created from highly compacted sand fills.

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A thorough understanding of rainfall recharge processes and their controlling factors is essential for management of groundwater systems. This study investigates the effects of various meteorological and hydrogeological factors on the gross recharge percentages, the rainfall-recharge relationships and the recharge threshold values for unconfined sandy aquifers under an equatorial climate. Among the meteorological factors investigated, rainfall intensity was found to have the most significant impact on the gross recharge rate. The effects of potential evaporation rate, relative humidity and air temperature on the gross recharge percentage were significant when the vadose zone thickness is larger than 2Ð5 m. The recharge threshold values were found to depend strongly on the vadose zone thickness. The rainfall-recharge relationships could generally be well defined by a normal-log relationship. The rainfall-recharge relationships derived here are applicable to yield estimates of gross recharge percentages for unconfined sandy aquifers under an equatorial climate, using rainfall intensity and vadose zone thickness as input variables. In this study, a theory was developed and validated to provide physical explanations for the observations, based on the residence time of the percolated rainwater within the vadose zone. Among the soil hydraulic parameters tested, porosity and saturated hydraulic conductivity were found to have the most pronounced effects on the gross recharge percentage. Utilizing the sensitivity results and the theory derived, an approach was developed for extending the application of the derived rainfall-recharge relationships to other sand textures. The approach was found to be capable of producing rough and fast estimations of gross recharge percentage for other sand textures.

Abstract:A new approach is demonstrated that permits a reliable estimate of specific yield using published values of the van Genuchten water retention parameters and effective grain sizes and the measured effective grain sizes of soil samples. The specific yield distribution of the soil texture was computed using the published values of the van Genuchten parameters. The specific yield values and the published values of effective grain sizes were then used to construct a specific yield-effective grain size curve, which estimates the 'point' specific yield of the soil samples. Applying the central limit theorem, the point specific yields could be transformed into an 'areal' specific yield for a study area. Compared with other commonly used approaches, the present procedure requires relatively low computational efforts and readily obtainable data. It is cost effective and does not depend on soil texture classification. More importantly, it incorporates the depth to water table and the variations in grain sizes inherent in natural soil conditions in the estimation. The approach developed was applied for estimating the specific yield of an unconfined sandy aquifer created by land reclamation in the equatorial region. The values obtained were compared with field measurements and the typical ranges of specific yield from the literature. Instead of a single estimate of the specific yield, the method yields a confidence interval with a high confidence level of 95% and with a narrower range than the typical ranges from the literature. In addition, the estimated values are close to the field measurements; hence, the procedure provides a cost-effective alternative to field measurement. The applicability of the present approach could be extended to sites with heterogeneity in the horizontal direction. Nevertheless, the applicability of the present approach for layered soil profiles requires further evaluations.