SOIL-WATER BYPASS AND SOLUTE TRANSPORT UNDER IRRIGATED PASTURE

Downward water flow through soil transports solutes and suspended matt er. The velocity of flow can be critical in the process of groundwater pollution, and can determine how effectively this water leaches the r oot zone of crops. This study quantifies fast soil water fluxes (bypas s), and provides some insight into the effectiveness of this bypass in leaching of salt. Three water salinity treatments were imposed on rep licated 75-m(2) plots growing irrigated pasture. A tritium-labeled irr igation was applied to the plots to quantify bypass Bow. Penetration o f tritium-labeled water decreased with increased salinity treatment (P < 0.01) for up to 50 d after irrigation application, despite a greate r leaching fraction under saline conditions. Bypass decreased with inc reased salinity because there was higher antecedent water content (P < 0.02) due to lower crop water use, resulting in less soil cracking un der saline conditions. Bypass flow was also calculated from a model th at used the profile of Cl- concentration below the root zone. However, calculations from the model failed to agree with the experimental res ults, including the direct measurement of bypass made from recovery of the applied radiotracer. The model failed because it assumed that byp ass was of low salinity, whereas the experiment indicated that bypass contributed to leaching. Therefore it was concluded that bypass is bes t defined in hydraulic terms, rather than in terms of salt leaching. I t was also concluded that the Cl- concentration in bypass flow reflect ed the concentration in the soil matrix. Therefore total water loss be low the root zone was adequately quantified by a leaching model of the Cl- mass balance, incorporating the volume-averaged soil water Cl- co ncentration. -