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Chairman: Dr. Rakesh Kumar & Dr. S. Balasubramanyam

Groundwater Flow Dynamics in the Unsaturated Soil Media

Inayatulla M., Shivakumara I.S., Venkatachalappa M and Ranganna G.


In order to reach a phreatic aquifer, water from precipitation, from irrigation, or from an influent river, infiltrates from the ground surface and percolates downward through the unsaturated zone. The same is true for the pollutants carried with water. These pollutants might already been present in water reaching the ground surface, or they may be added to water by the processes of leaching, dissolution, and desorption along their path, from the ground surface to the underlying aquifer. Solid waste in landfills, septic tanks, fertilisers, pesticides and herbicides, applied over extended areas and dissolved in water applied to the ground surface, may serve as examples of sources of pollutants that travel through the unsaturated zone. Hence the understanding and consequently the ability to assess and predict the movement of water in the unsaturated zone is essential when we wish to determine the (total) replenishment of a phreatic aquifer as part of our groundwater flow model. Information on the movement of water is also needed in order to foretell the movement and accumulation of pollutants in the unsaturated zone and the rate and concentration at which the pollutants reach the water table.

 Irrigation and drainage engineers are faced with the problem  of getting water into or out of the soil. In either case, the flow phenomenon involved is flow through partially saturated porous media. When water enters a soil, air must be replaced; and when water is removed, air must enter. The flow, therefore, involves two largely immiscible fluids: air and water. In the design of drainage and irrigation systems, engineers (with rare exception) have made the simplifying assumptions that soil is either completely saturated water or it is completely unsaturated and that resistance to flow of air (associated with the movement of water into and out of soil) is negligible. Such assumptions are in most cases far from realistic. In real cases, there exist functional relationships among the saturation, the pressure difference between air and water, and the permeabilities   of air and water. The present work describes these functional relationships and the properties of porous media, which affect them. Regardless of the scales involved, the soil hydraulic properties, which affect the flow behaviour, are incorporated into two fundamental characteristics:

(i)                   the soil water retention curve describing the relation between volumetric soil water content and soil water pressure; and

(ii)                 the relation between volumetric water content and hydraulic conductivity

 Soil samples from Thirnahalli, Mallasandra and Makali villages were collected. Suction pressure (y m ) of water for corresponding moisture contents (q in cc/cc) were measured in the laboratory. The soil moisture curves were developed. A rigorous evaluation of several texture based regression models proposed in the literature indicated their superiority in predicting the laboratory SMC rather than the field measured one.

SMC equations for the study site are:

 (i)                   Tirnahalli study area, Chickballapur - y = eq(-23)+2.6

(ii)                 Mallasandra study area, Tumkur - y = eq(-50)+8.0

(iii)                Makali village study area, Bangalore - y = eq(-41.54)+5.4

The unsaturated hydraulic conductivity models developed for the three study areas are –

(i)                   Tirnahalli site – K (q) = 4.41 x 10-5 [(q - 0.01)0.5 (e23 – 1)2]

(ii)                 Mallasandra, Tumkur - K (q) = 1.16 x 10-6 [(q - 0.01)0.5 (e50 – 1)2]

(iii)                Makali village - K (q) = 6.9 x 10-8 [(q - 0.01)0.5 (e41.54 – 1)2]

A one dimensional flow model is developed for predicting vertical flow in the unsaturated zone of the homogenous soils. The model is developed based on the pressure head form of Richards equation. This form is chosen to couple the unsaturated flow model with saturated flow models. The cumulative infiltration derived from the solution is of the form of Green-Ampt infiltration equation. The analytical model is capable of incorporating soil properties ranging from weakly non-linear to those of a highly non-linear Green-Ampt like model. The analytical solution describes the development of the moisture content profile during constant infiltration and capable of predicting the time dependence of both soil moisture content and soil moisture potential.

Address: UGC-DSA Center in Fluid Mechanics,
Department of Mathematics, Bangalore University, 
Central College Campus, Bangalore 560 001.
Faculty of Civil Engineering, Jnanabharathi,
Bangalore University, Bangalore 560 056.