Ken'ichirou Kosugi

Lognormal Distribution Model for Unsaturated Soil Hydraulic Properties

The soil water retention model developed by Kosugi was modified to be compatible with Mualems model in order to derive an analytical expression for the relative hydraulic conductivity Kr. The modified water retention model is to be derived by applying a lognormal distribution law to the soil pore radius distribution function. Parameters of this retention model have physical significance on the water content (θ)– capillary pressure (ψ) curve and are related directly to the statistics of the pore radius distribution. The accuracy of the resulting combined water-retention-hydraulic-conductivity model is verified for observed data sets for six soils. Results showed that the proposed model produces acceptable matches with observed water retention curves and adequate predictions of hydraulic conductivities in five out of six cases. The θ − ψ and Kr− ψ (or Kr − θ) curves generated by this model are generally similar to those generated by van Genuchtens model.

Three-parameter lognormal distribution model for soil water retention

Many models for soil water retention have been proposed. However, most of these models are curve-fitting equations and do not emphasize the physical significance of their empirical parameters. A new retention model that exhibits increased flexibility was developed by applying three-parameter lognormal distribution laws to the pore radius distribution function ƒ(r) and to the water capacity function, which was taken to be the pore capillary pressure distribution function ƒ(ψ). This model contains three parameters that are closely related to the statistics of ƒ(ψ): the bubbling pressure ψc, the mode ψ0 of ƒ(ψ) and the standard deviation σ of transformed ƒ(ψ). By comparison of this model with three existing models (the van Genuchten model, the Brooks-Corey model, and the modified Tani model), it was shown that ψc, ψ0, and σ are all essential for a general retention model.

A new model to analyze water retention characteristics of forest soils based on soil pore radius distribution

The soil water retention model proposed by Kosugi was modified to have a relatively simple functional form. The resulting water retention model (the lognormal distribution model) contains two parameters which have physical significance on the water retention curve and are related directly to the statistics of the soil pore radius distribution functiong(r). The observed retention data sets of 282 undisturbed forest soils taken from Mashimo were analyzed using the lognormal distribution model, and it was shown that the model performs fairly well for every data set. The estimated parameters of the model indicated that the water retention characteristics of the undisturbed forest soils are related to the soil structure more closely than to the soil texture. Many crumb-structure forest soils have greater median ofg(r) than the crumb and granular-structure soils. Disturbed loamy soils have smaller median ofg(r) than many of the undisturbed forest soils and disturbed sandy soils have smallerg(r) width than the forest soils. The brown forest soils taken from A-horizon generally have the greater median ofg(r) than the brown forest soils taken from B-horizon. Some of the brown forest soils have the greater width ofg(r) than the black soils. The median ofg(r) of brown forest soil taken from A-horizon generally becomes greater as the soil moisture condition becomes wet.

Spatial variability of soil hydraulic properties in a forested hillslope

Spatial variability of soil hydraulic properties was measured in a forested hillslope and analyzed by applying combined water-retention-hydraulic-conductivity models (the LN and VG models) and a power function model for soil hydraulic conductivity (the Leibenzon model). Results showed that the pore-tortuosity parameterl in the LN and VG models should be treated as a fitted parameter for accurate descriptions of the unsaturated conductivity. The simultaneous optimization of both retention and conductivity curves is preferable to obtain appropriate descriptions of hydraulic properties when both retention and conductivity data are available for the parameter estimation. The Leibenzon model produced slightly poorer estimates than the LN and VG models. The exponent parameter in the Leibenzon model exhibited a spatial variation with the standard deviation of 2.38. The spatial variability of hydraulic properties was analyzed based on the spatial variation in parameters of the LN model obtained by the simultaneous optimization procedure. The parameter ψm, which has a positive correlation with median pore-radius, was generally small at the crest and upper slope locations and large at mid-slope to footslope locations. Except for the crest, surface soils had larger ψm values than the subsurface soils, suggesting a well-developed crumb structure in surface horizons of forest soils. For most soils, σ was greater than I, indicating a relatively large width of pore-size distribution. Saturated hydraulic conductivity (Ks) was generally small at crest and upper slope locations, and large at mid-slope to footslope locations. The largerKs values were attributable to largerψm values.

Baseflow concentrations of nitrogen and phosphorus in forested headwaters in Japan

A comprehensive investigation on all dissolved nitrogen and phosphorus components at both local and regional scales in the headwaters from forested watersheds is valuable to improve our understanding of the factors controlling water quality. Here, we investigated the baseflow concentrations of dissolved nitrogen and phosphorus components, N:P ratio, and their associations with region and vegetation type in forested headwaters in fives regions of Japan. We found that inorganic nitrogen and phosphorus were the dominant components in the 26 temperate forested streams, rather than organic forms. There were significant positive correlations between the concentrations of N and P components. Furthermore, the regional patterns of the concentrations of nitrate, dissolved inorganic P (DIP), and dissolved total N (DTN) and P (DTP) were similar. Our results suggest that the regional patterns of the concentrations of N and P components should be related to the regional atmospheric deposition of both N and P nutrients. We also found that the nitrate and DTN concentrations were higher in man-made evergreen conifer (EC) than those in the natural deciduous broadleaf (DB). In contrast, the DIP and DTP concentrations in EC were lower than those in DB. The uniformly higher N:P ratio in EC- than in DB-forested streams for each region suggest that EC-forested streams could be more affected by P-limited than DB-forested streams when N inputs from atmospheric sources increased.

Scaling hydraulic properties of forest soils

For the purpose of characterizing spatial variability of soil hydraulic properties on a forested hillslope, five differing scaling methods were examined using the data set for 30 undisturbed forest soils: scaling of water retention curves, scaling of hydraulic conductivity curves, and simultaneous scaling of both water retention and conductivity curves. For the scaling of conductivity curves and the simultaneous scaling, the soil-pore-tortuosity parameter in a conductivity model was either fixed at a constant or treated as a fitted parameter. Results showed that the set of scaling factors and reference parameters derived by the separate scaling of water retention curves was different from that derived by the separate scaling of conductivity curves. Moreover, the separate scaling methods resulted in large estimation errors in either effective saturation or conductivity. Optimizing the tortuosity parameter was effective in improving scaling results. It was concluded that the simultaneous scaling with a fitted pore-tortuosity parameter was the best among the five methods tested in this study. The obtained scaling factors obeyed the lognormal distribution rather than the normal distribution. The variance of log-transformed scaling factors at the forested hillslope was larger than those at many crop fields studied by previous studies. While a spatial dependence of soil hydraulic properties was suggested for the surface layer of the forested hillslope, hydraulic properties of the subsurface layer exhibited large variations even in a small distance. Copyright

New diagrams to evaluate soil pore radius distribution and saturated hydraulic conductivity of forest soil

Based on the widely used soil pore classification systems, soil pore ratios α, β and γ were derived. α, β, and γ represent ratios of the fine capillary porosity, coarse capillary porosity, and non-capillary porosity to the effective porosity, respectively. The parametersψm and σ of the soil water retention model developed by Kosugi were related to these pore ratios, and a simple method was suggested to estimateψm and σ from measured soil pore ratios. By analyzing the observed retention data sets of forest soils, it was shown that the soil pore ratios are effectively used to evaluate the soil pore radius distribution. A coordinate system with log(−ψm) on the abscissa and σ on the ordinate, which represents the constant α, β, and γ lines, was developed as a new diagram to evaluate the soil pore radius distribution in connection with the soil water retention characteristic. Then, the saturated hydraulic conductivityKs of forest soils was correlated with the parametersψm and σ, and with the ratios α, β, and γ using the coordinate system and the triangle diagram. Results showed thatKs is higher for the soil with a greater median and with a greater width of the pore radius distribution.Ks increases as the non-capillary pore ratio γ becomes greater and the coarse capillary pore ratio β becomes smaller. Functional relationships betweenKs and the water retention parameters, and betweenKs and the soil pore ratios were derived based on Mualems model.

Is MUSLE apt to small steeply reforested watershed

The reforested headwater watersheds in Japan are very important from the points of view of commercial and environmental aspects. At the present time, much and varied research is running to assess and understand the hydrologic behavior of these watersheds. The present study was conducted to evaluate the applicability of the deterministic model MUSLE in the Mie small steeply reforested watershed. The model was tested and calibrated using accurate continuous suspended sediment data collected during eight storm events in 2004. Results of the original model simulations for storm-wise sediment yield did not match the observed data, while the revised version of the model could imitate the observed values well. The results of the study approved the efficient application of the revised MUSLE in estimating storm-wise sediment yield in the study area with a high level of agreement of beyond 88%, an acceptable estimation error of some 14% and non-significant difference in mean values.

Field Determination of Unsaturated Hydraulic Conductivity of Forest Soils

This study introduces a field method to determine unsaturated hydraulic conductivity which is applicable to sloping terrain with a limited water supply. A single steel ring infiltrometer and an artificial rainfall simulator are used in this method to reduce the amount of water required to attain a steady-state flux condition. Six tensiometers and a time domain reflectometry (TDR) are employed as the soil capillary pressure head and the volumetric soil water content measurement devices, respectively. Water contents measured by the TDR are corrected using a simple calibration method suggested by Hook and Livingston (1996). Unsaturated hydraulic conductivities are computed based on the instantaneous profile method using capillary pressure head and water content changes measured during a drainage process. The proposed method was applied to a forest soil profile in Rokko Mountain range. Results showed that the relationships between the unsaturated hydraulic conductivity and capillary pressure head developed by the proposed method coincide well with those measured by the conventional steady-state laboratory experiment. The proposedin-situ method is the effective simple means to determine unsaturated hydraulic conductivities of forest soils, since this method is enough accurate and consuming less amount of water and time.

A method for estimating unsaturated hydraulic properties of vertically heterogeneous soils from transient capillary pressure profiles

Abstract The hydraulic properties of soil are represented by the relationship between the volumetric water content θ and the soil capillary pressure ψ and the relationship between the hydraulic conductivity K and ψ. A method to estimate the hydraulic properties for each horizontal layer of a soil profile was developed by combining the instantaneous profile method to compute K ( ψ ) values and Mualems model to derive the K - ψ relationship from the soil water retention characteristic. With this method, three parameters that are contained both in the θ-ψ and K - ψ models are optimized by an iterative procedure using transient capillary pressure profiles during water redistribution. Two hypothetical soil columns were used to test the parameter estimation procedure, and it was shown that the proposed method can be applied to soil profiles which have homogeneous or heterogeneous hydraulic properties with depth.