Groundwater contamination by nonaqueous‐phase liquids (NAPLs) has become a serious environmental issue. Therefore, it is necessary to estimate the NAPL content (θNAPL) in saturated soil to detect and monitor the NAPL contamination in groundwater. The objective of this study was to investigate three estimation methods for the θNAPL in saturated sandy soil, namely, the noncalibration method (NCM), the permittivity regression method (PRM), and the best‐fit α method (BFαM). These methods utilize the permittivity (K) measured by amplitude domain reflectometry (ADR) and two dielectric mixing models: a theoretical Maxell–de Loor (MD) model and an empirical α model. The NCM applies the measured K to the models directly. The PRM applies the measured K to the models indirectly using the regression equation between the measured and estimated K The RMSEs of the NCM for the α model were 0.038 m3 m−3 for light NAPL (LNAPL) and 0.015 m3 m−3 for dense NAPL (DNAPL) when the constant α value was set at 0.5. The RMSEs of the MD model, however, were 0.090 m3 m−3 for LNAPL and 0.070 m3 m−3 for DNAPL. Using the PRM, the RMSEs of the MD model became much better than those of the NCM. The RMSEs for θNAPL estimated by the BFαM were 0.007 m3 m−3 for LNAPL and 0.018 m3 m−3 for DNAPL. These results demonstrate that θNAPL was estimated easily and with sufficient accuracy with the α model and the PRM.
Tillage near the soil surface may greatly influence drainage discharge and pressure head values in the subsurface zone. In this study, a controlled comparative experiment was conducted in the field under natural weather conditions, using a tilled and an untilled column, to evaluate the effects of tillage on subsurface drainage discharge and pressure head values. There were no significant differences between the two columns in subsurface drainage before tillage treatment before the study, as checked by a preliminary experiment. After tilling one column of the two columns, cumulative subsurface drainage discharge was larger and occurred earlier for the tilled column than for the untilled column. The measured drainage discharge and pressure head values were evaluated using the water movement model, HYDRUS version 6.0, which numerically solves the Richards equation. The HYDRUS model reproduced measured values well for subsurface drainage discharge and pressure head values after the tillage, as determined by root mean squared error (RMSE), mean bias error (MBE), and R2 Therefore, it is concluded that the model, which includes the effects of tillage on the hydraulic properties, can explain the reasons for differences in observed drainage in the two columns. Moreover, the effects of tillage depth on subsurface drainage discharge were simulated. The results implied that the effect of tillage on subsurface drainage were induced by the conditions of pressure head just before the first rainfall, and this effect was equal to, or greater than, the effects of changes in hydraulic properties due to tillage.
Abstract:The characteristics of evapotranspiration estimated by the complementary relationship actual evapotranspiration (CRAE), the advection-aridity (AA), and the modified advection-aridity (MAA) models were investigated in six pairs of rural and urban areas of Japan in order to evaluate the applicability of the three models the urban area. The main results are as follows: 1) The MAA model could apply to estimating the actual evapotranspiration in the urban area.2) The actual evapotranspirations estimated by the three models were much less in the urban area than in the rural.3) The difference among the estimated values of evapotranspiration in the urban areas was significant, depending on each model, while the difference among the values in the rural areas was relatively small. 4) All three models underestimated the actual evapotranspiration in the urban areas from humid surfaces where water and green spaces exist. 5) Each model could take the effect of urbanization into account.
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