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Behavior Study of Cracking Clay Soil. Ii. Simulation of Water Balance, Cracking & Subsidence.

Dr. M. H. Ali
SSO
BINA

H.N. Turral
Research Fellow
The Uni. of Melbourne

Abstract/ Executive Summary:

Behaviour of cracking clay soil, in terms of cracking, subsidence, and water balance & redistribution of infiltrated water were studied under three different irrigation regimes using simulation model ‘FLOCR’ with 7 years climatic data. The simulation results demonstrate how the cracks affect the infiltration and redistribution of water under different circumstances. This information can be used in irrigation management in cracking clay soil to minimize the hazard of rising shallow water-table and optimizing the crop yield.

Keywords: Water balance, Cracking, Subsidence, Simulation

Location:

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Water management

Objectives:

To provide simulated results of water balance, cracking and subsidence of the field soil with a simulation model ‘FLOCR’.

Methodology:

FLOCR is a one-dimensional model that simulates water balance, cracking and subsidence of clay soils (Bronswijk, 1988; Oostindie and Bronswijk,1992). The main feature of ‘FLOCR’ that distinguishes it from other soil-water simulation models is that this model computes a continuously changing system of soil matrix and shrinkage crack. The model considers only the vertical water flow through the soil matrix and through the shrinkage cracks. To enable the analytical solution of the Darcy’s equation, the K(h) relation is described by three different sections. The boundary condition at the bottom of the soil profile is defined by a flux to the open water system via drains (drain intensity) and a flux to the underlying aquifer; and the boundary condition at the top of the soil profile is defined by meteorological data (precipitation, potential evapo-transpiration ). The soil profile may contain upto 5 different horizons with different soil physical properties; and the profile may be divided into approximately 30 compartments. For the complete soil profile, the model computes the total change of volume, crack volume, the surface subsidence, the bypass flow, the groundwater level, the drain discharge, the actual ET and the surface runoff.Climatic data required as driving variable in the model. The irrigation regimes considered here are : irrigating weekly basis, equal to the amount of 1.2 times of potential evapo-transpiration (PET) [IR-1], equal to the amount of PET (IR-2), and 0.8 times of PET (IR-3).

The input parameters in the model may be classified into 3 groups :

Group-1: Directly measured, e.g. moisture retention characteristics, saturated hydraulic conductivity, shrinkage characteristic, and saturation capacity.

Group-2: Indirectly estimated, e.g. unsaturated hydraulic conductivity, potential evapotranspiration

Group-3: Known within a closed limit, e.g. number of soil profile, aquifer depth, drainage intensity.

Source of Information: J. of the Institution of Engineers, Bangladesh, Vol. 27/AE, No.1: 19 – 26, (2001)

Article Link (Online Journal):

Funding Source:

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Total Budget (Tk.) :

Achievement/Findings:

It is evident from the result that the number of unsaturated compartments, crack volume, and surface subsidence are very small in IR-1. With the decrease in irrigation amount, these values increased by significant amount. Also, the depth to water-table follows the same trend. The results also suggest that, with a decrease in infiltration, depletion of soil moisture from the 90 cm profile increase. Actual evapo-transpiration decreases with a decrease in supplied water. The simulation results demonstrate that bypass flow (crack infiltration) [CF] were a large component of irrigation amounts. For the first studied irrigation schedule (IR-1), CF was 63.28 % of the infiltrated water. The second had the slightly smaller, 59.88 %; and the third one had 51.28 % of the infiltrated water.

A large amount of bypass flow indicates that water can be transported rapidly to the subsoil & the ground water-table by bypass flow via the cracks; and there is a risk of rise of shallow water-table. So, to minimize water loss and rise of water-table, and to optimize the yield, water may be applied between 1.2 & 1.0 times of PET, depending on the yield – ET function & tolerance of water stress of a particular crop.

Knowledge Management: Journal