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Water distribution pattern in soils under surface and micro-irrigation systems Khan, Maqsood Ahmad


The work reported here describes the water distribution patterns in sandy loam and silt loam soils under surface and micro-irrigation at different temperatures simulating the arid region conditions such as those experienced in the Balochistan Province of Pakistan. Specific consideration was given to the temperature variations before and after irrigation, the elucidation of spatial distribution of applied water in the root zone, and'the evaluation of affects of soil temperature on water movement within the rooting zone. Results of a series of four studies at three different soil temperatures (30, 35, and 40 °C) are presented. Experiments were conducted in 1 m³ wooden boxes with sandy loam and silt loam soils. The operating temperatures were maintained by using electric heat lamps. Soil moisture content was measured by Time Domain Reflectometry (TDR) technique at four different rooting depths simulating the root zone of onion crop. Both the soils showed distinguishable changes in subsurface temperature prior to and after the application of irrigation water at different operating temperatures. Subsurface temperature variations were more pronounced at higher temperature (40 °C) as compared to lower temperatures under both irrigation systems. However, there was not any significant change in soil temperature beyond 15 cm depth in either of the soils. Temperature changes were observed to deeper soil depths in silt loam soil under trickle irrigation as compared to surface irrigation because trickle irrigation favors better water retention in soils as compared to surface irrigation. Lateral spatial distribution of irrigation water along the soil depth was significantly affected by soil temperature in both soils. At lower temperatures (30 and 35 °C) percent volumetric moisture content was higher in top layer (D₁) as compared to higher temperature (40 °C) . The pattern reversed in the lower layers (D₂, D₃, and D₄) where higher operating temperature favored the storage of irrigation water under both irrigation systems. Nevertheless, the lateral distribution of irrigation water was different in sandy loam and silt loam soils. Sandy loam soil had a rapid decrease in volumetric moisture content laterally away from the application source due to higher conductivity whereas silt loam soil with lower conductivity and higher capillary forces exhibited more gradual lateral decline in the volumetric moisture content away from the point source. One plausible reason might be that the gravity forces have a limited effect on the water movement in fine-textural soils such as silt loam where capillary forces dominate the flow. Contours of soil moisture content identified the differences in wetted patterns at different soil temperatures. The maximum percent volumetric moisture content after irrigation was found just below the irrigation source in majority of the experiments for both soils. However, irrigation source did not behave as an idealized point source even though it was true in some experiments. The volumetric moisture content were different at different places within the same radii from the point of application probably due to the effect of temperature on soil water viscosity, surface tension and hydraulic conductivity. The spread of wetting front increased with the increase in temperature for both soils and under both irrigation methods. The wetting fronts moved vertically and laterally more deeper and wider at 40 °C as compared to lower temperatures (30 and 35 °C). The possible reasons could be greater conductivity, permeability, and diffusivity at higher temperature.

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