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Evaluation of Unsaturated Hydraulic Conductivity of Soil Based on Soil Moisture Retention Data

N C Dey
Department of Agricultural Extension, Khamarbai, Dhaka.

M A Bhuiyan
Professor
Department of Water Resources Engineering, BUET, Dhaka, Bangladesh.

Abstract/ Executive Summary:

Hydraulic conductivity is one of the most important hydraulic properties that governs water flow in the soil. Groundwater recharge and contaminant transport mainly depend on the hydraulic properties of soil. The present study was undertaken for the determination of unsaturated hydraulic conductivity (K) of two selected soils by using internal drainage method. The values were also estimated by four popular closed form models namely, Brooks-Corey (1966), Campbel (1974), Bresler et al. (1978) and van Genuchten (1980). Hydraulic conductivity for silty clay loam soil ranged from 24.7 to 0.4 mm/day corresponding to moisture contents of 48 to 35 percent. For silty clay soil it varied from 24.4 to 0.1 mm/day corresponding to moisture contents of 49 to 36 percent. Evaluation of unsaturated hydraulic conductivity was done based on statistical analysis and graphical interpretation. A comparison of estimated values with the measured hydraulic conductivity reveals that van Genuchten model gives the values close to the field values.

Keywords: Soil Moisture ,

Location: Bangladesh University of Engineering and Technology campus and Bangladesh Agricultural Research Institute campus.

Start Date: 00-03-1999

End Date: 00-02-2000

Program Area: Crop-Soil-Water Management

Commodity: Contamination of soil

Objectives:

To determine the unsaturated hydraulic conductivity for two selected soils by field methods and using empirical equations.

Methodology:

Field experiment The experiments were conducted at Bangladesh University of Engineering and Technology (BUET) campus and Bangladesh Agricultural Research Institute (BARI) campus. The experiments were carried out during March 10 to April 4, 1999 and February 2 to 24, 2000 at BUET and BARI campus, respectively. The soil samples were collected for determination of saturated hydraulic conductivity (Ks) and particle size analysis in the laboratory. The soils are silty clay loam (SiCL) in BUET campus and silty clay (SiC) in BARI campus. The saturated hydraulic conductivity (Ks) for both types of soils were determined by constant head method. Saturated moisture content (ø_s) were measured by Time Domain Reflectometry (TDR) when field was fully saturated. Residual moisture content (ø_r) corresponding to soil moisture suction of 15000 cm and air entry matric potential (h_e) were needed for the estimation of unsaturated hydraulic conductivity using empirical equations. Soil water capacity (S_p) was estimated from the slope of the soil-moisture characteristics curve corresponding to the average value of 85 and ør.

At each location, a plot of 3 m x 3 m W leveled and bounded with 0.3 m high wood dikes. Four tensiometers of 30 cm, four of 45 cm and four of 60 cm lengths were installed BUET site and three of 30 cm, three of 45 c and three of 60 cm lengths were installed BARI site.

Unsaturated hydraulic conductivity Was measured by using internal drainage method Each experimental plot was ponded for several days until tensiometer readings were stabilized The excess surface water was drained out and the time was recorded as zero. The plot was covered with thick black plastic sheet to prevent, evaporation. A column of soil between 30-45cm was considered for the calculation of hydraulic gradient and flux of water. The flow rate per unit cross-sectional area of the column was determined from change in moisture content. The average moisture content of the soil column at the beginning and end of each time step was determined using Time Domain Reflectrometry Wave-guides of the Trase system were inserted into the soil up to the depths of tensiometers in order to determine moisture contents for different soil water suctions. Both Trase system and tensiometer readings were taken at six- hour intervals during the first two days, twelve- hour intervals during the third and fourth days, every day for the next seven days and finally at an interval of two days for the rest of the test period

Source of Information: Journal of the Institution of Engineers, Bangladesh Agricultural Engineering Division, Vol. 29/AE, No. I, Dec. 2002

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The unsaturated hydraulic conductivity both SiCL and SiC were found to reduce by an order of magnitude due to decrease of moisture content by about 10 percent. van Genuchten model was found to match well with the measured values whereas the Campbel, Bresler and Brooks-Corey models overestimated the unsaturated hydraulic conductivity. The others empirical equations overestimate the unsaturated hydraulic conductivity. So, van Genuchten (1980) equation can be used as most suitable method for estimation of unsaturated hydraulic conductivity.

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