Impact of Shallow Water Table Depths on Surface Irrigation Requirements and Crop Performance

Abstract

The impact of shallow water table depths on surface irrigation requirements of major crops was studies through a series of experiments conducted in 18 lysimeters situated in Lahore during 1975 to 2004. These lysimeters are of the size 3.05m x 3.05m at the surface and 6.1m deep filled with representative soil profile of the Punjab. Each lysimeter was equipped with tensiometers, soil water extractors and automatic arrangements for the quantitative measurement of up flux of water from water table into root zone or deep percolation losses occurring due to surface irrigation or heavy rainfall. Water table depths in these lysimeters were maintained from 0.9m to 4.5m with an interval of 0.9m in phase I; where as in phase-II water table depths were maintained from 0.3 m to 3.0 m with an interval of 0.3 m. The crops experimented were wheat, maize, sugarcane, sunflower, berseem and sorghum. In each experiment each treatment was replicated twice and repeated for three years. The results indicate that movement of water from the water table into root zone is negligible if the water table is located at 3 m depth or more. Groundwater contribution to crop water requirements increases as water table depth decreases. It was observed that crops can meat all their water requirements from water table if situated at a depth shallower than 1.2 m. This condition is generally detrimental to crop performance due to harmful semi-saturation conditions and accumulation of salts in the top layers due to upward movement of salts with the upflux of water. Periodic flushing of such salts is necessary to minimize toxic effect of salts for sustained crop production. Summarizing the results it was concluded that: 1) In the areas where water table is located at a depth shallower than 3 m, a considerable amount of surface irrigation water can be saved by making use of contribution of shallow water table. 2) Soluble salts also move upward with the water and accumulate in the top layers causing toxic condition for crop growth. 3) Periodic flushing of such salts is necessary for sustained crop production. 4) The optimum water table depth for most of the crop is about 2 m to 2.5 m. 5) In water logged areas, depth of tile drainage system should be selected such that at least 2 m well drained soil in available.