Soil water retention

Direct disposal of spent bleaching earth on farmland is another option that has been examined and found to work well. Studies (138) have shown that 60-90% of the oil is decomposed during the course of a normal six-month growing season by soil bacteria this approach works best in warm climates on sandy soils where some fertilizer has been added. Plants grown in soils treated with spent bleaching earth were normal and may even have benefited from improvements in soil water retention caused by the spent clay addition. 

Inspite of the fact that wilting point is a good indicator of lower limit of available water, there is enough evidence to indicate that wilting point is not a true intrinsic soil property As such there does not exist an rmique soil water retentivity value at which the water uptake by plants suddenly ceases, rather plant usually wilts at a point controlled by rate factor (both supply and demand). However, the 15 bar percentage has been found to be closely correlated with the permanent wilting point (Richard and Weaver 1943). 

Assouline S., D. Tessier, and A. Bruand. 1998. A conceptual model of the soil water retention curve. Water Resour. Res. 34 223-231. 

Kosugi, K. 1994. Three-parameter lognormal distribution model for soil water retention. Water Resour. Res. 30 891-901. 

Effects of the fractal nature of soil on water and solute transport can be further complicated by the differences among fractal dimensions in soils at different scales. Avnir et al. (1986) found at least two ranges of radii with different surface fractal dimensions in studied soils. Dependence of fractal dimension on pore radii was demonstrated (Wu et al., 1993 Perfect et al., 1993). Pachepsky et al.( 1995a) found three or four distinct scaling intervals with different fractal dimensions in the range of pore radii from 4 nm to 5 pm. Fractal scaling of soil water retention is usually well pronounced at capillary potentials lower than -30 kPa. 

Comegna, V., P. Damiani, and A. Sommella. 1998. Use of a fractal model for determining soil water retention curves. Geoderma 85 307-314. 

Kravchenko, A., and R. Zhang. 1998. Estimating the soil water retention from particle-size distributions A fractal approach. Soil Sci. 163 171-179. 

Pachepsky, Y.A., R.A. Scherbakov, and L.P. Korsunskaia. 1995b. Scaling of soil water retention using a fractal model. Soil Sci. 159 99-104. 

Bird, N.R.A., and A.R, Dexter. 1997. Simulation of soil water retention using random fractal networks. Eur. J. Soil Sci. 48 633-641. 

Capillary pores are those characterized by capillary forces, so that in them, water moves in the direction of elevation forces and not of gravitational forces. The capillary pore volume determines the soil water retention capacity. The motion of air in capillary pores is strongly restricted. 

Alexandra W., Ning, L., 2012. A transient water release and imbibitions method for rapidly measuring wetting and drying soil water retention and hydraulic conductivity functions. Geotechnical Testing Journal 35(1) 1-15. 

Physical properties SOM significantly contributes to the stabilization of soil structure, because it binds to soil mineral particles thus, it is capable of binding these particles together into water-resistant aggregates. It has an important contribution to soil water retention because of its ability to absorb up to 20 times its mass in water the low solubility of SOM adds to its stability in upper horizons, without leaching, and the dark color of SOM contributes to soil thermal properties. 

LRP PTF was derived from linear regression based on measured soil basic properties and hydraulic properties in sandy soil of Liaoning, the performance of prediction were compared to the existing VER and ANN PTFs, LRP had been ranked as the most suitable PTFs for estimating soil water retention. The inferior performance of VER confirmed that PTFs should be applied from one region to other region with validation and coefficient modification even similar soil texture. 

Merdun H., Cinar O., Meral R., Apan M. (2006). Comparison of artificial neural network and regression pedotransfer functions for prediction of soil water retention and saturated hydraulic conductivity. Soil and Tillage Research, 90(1-2) 108-116. 

Schaap M.G., Leij F.J. Van Genucheten, M. Th. (1998). Neural network analysis for hierarchical prediction of soil water retention and saturated hydraulic conductivity. Soil Science Society of Americal Journal, 62 847-855. 

El-Rehim, H. A. Hegazy, E. S. El-Mohdy, FI. L. Radiation synthesis of hydrogels to enhanee sandy soils water retention and inerease plant performance. J. Appl. Polym. Sei. 93 1360-1371 (2004). 

Effects of Surfactants on Soil Water Retention and Flow... 

Soil water retention (pressure-saturation) relationships are frequently described using the van Gmuchten (VG) equation (2J),... 

Figure 6. Measured andfitted soil water retention curves for F-70 Ottawa sand at 750 mg/L of Triton X-IOO.

The texture of the medium relates in a general way to the pore-size distribution, as large particles give rise to large pores between them, and therefore is a major irrfluence on the soil water retention ctrrve. Additionally, the structure of the medium, especially the pervasive-ness of aggregation, shrinkage cracks, worm-holes, etc. substantially influences water retention. 

Hydraulic conductivity, which is one of the important soil parameters for seepage analysis, is estimated from the pore-size distributions measured by a mercury intrusion porosimeter. Two different relations are indispensable for analysis of partial saturation soil because hydraulic conductivity depends on the capillary potential, which is usually characterized by soil-water retention. 

The network model is developed for estimating the hydraulic conductivity and soil-water retention taking into account the spatial differences of the pore size. The model is checked with published laboratory tests. It is clear that the model demonstrates the hysteresis between the wetting and drying, and shows good similarities of the relative hydraulic conductivity with the laboratory values. 

There are various kinds of the conceptual models for estimating hydraulic conductivity, taking into account the pore size of soil. The models are clustered into the three types from the mathematical treatment of the pore structure of soil as shown in Table 1. All these are models requiring laboratory values on water retention. Therefore, they cannot predict hydraulic conductivity without knowledge of soil-water retention. The hydraulic conductivity of the partial saturation depends on water saturation, which also relates to the capillary potential. It is nessesary to establish two relations for the modelling of pore-water movement through partially saturated soil. 

TABLE 2, Models of soil-water retention characteristics... 

Three different approaches have been proposed for calculating the water retention curve as shown in Table 2. The third approach is the fitting method, in which the laboratory test values on the soil-water retention are indispensable. The first and second are unique, but there are still problems to be overcome, since they cannot indicate the hysteresis between the wetting and the drying. The soil-water retention usually shows a strong hysteresis because of the spatial differences of the pore size distribution, so these models are not capable of predicting the hysteresis. 

Fig. 4, Measured and computed curves of soil-water retention of Toyoura sand (0 controlled air pressure method, instantaneous profile method)...

The soil properties, affecting the pore-water movement in the partially saturated domain, are estimated from the pore-size measurement through the network model. The model can estimate the soil-water retention as well as hydraulic conductivity. No mathematical model exists that can compute both properties. In addition, no model exists indicating the hysteresis between wetting and drying. The network model also predicts the hysteresis in the water retention. 

The network model still has some problems to overcome. The numerical assumption of the probability of pore-size occurrence has to be improved. It is difficult to compute the soil-water retention near the full saturation because... 

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