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Soil Water Bypass and Solute Transport under Irrigated Pasture

OK Tedi Mining Ltd., Environment Dep., P.O. Box 1, Tabubil, Western Province, Papua New Guinea

*Corresponding author.

ABSTRACT

Downward water flow through soil transports solutes and suspended matter. The velocity of flow can be critical in the process of groundwater pollution, and can determine how effectively this water leaches the root zone of crops. This study quantifies fast soil water fluxes (bypass), and provides some insight into the effectiveness of this bypass in leaching of salt. Three water salinity treatments were imposed on replicated 75-m² plots growing irrigated pasture. A tritium-labeled irrigation was applied to the plots to quantify bypass flow. Penetration of tritium-labeled water decreased with increased salinity treatment (P < 0.01) for up to 50 d after irrigation application, despite a greater leaching fraction under saline conditions. Bypass decreased with increased salinity because there was higher antecedent water content (P < 0.02) due to lower crop water use, resulting in less soil cracking under saline conditions. Bypass flow was also calculated from a model that used the profile of Cl^– concentration below the root zone. However, calculations from the model failed to agree with the experimental results, including the direct measurement of bypass made from recovery of the applied radiotracer. The model failed because it assumed that bypass was of low salinity, whereas the experiment indicated that bypass contributed to leaching. Therefore it was concluded that bypass is best defined in hydraulic terms, rather than in terms of salt leaching. It was also concluded that the Cl^– concentration in bypass flow reflected the concentration in the soil matrix. Therefore total water loss below the root zone was adequately quantified by a leaching model of the Cl^– mass balance, incorporating the volume-averaged soil water Cl^– concentration.

Work performed at Inst. for Sustainable Irrigated Agriculture, Ferguson Road, Tatura, Victoria, Australia 3616.

Received for publication April 19, 1993.

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