Simulating the effect of subsurface tile drainage on watershed salinity using SWAT

Subsurface tile drains in areas with intensive agriculture can be a major contributor of salinity loadings to streams and rivers, as salts in soil and groundwater are transported into the tile drains. Modeling tools can be used to assess baseline conditions, quantify salt mass export, and assess management scenarios to decrease salt removal. In this study, a newly developed version of the SWAT model for salinity transport, SWAT-Salt, was modified to include salt ion transport in subsurface tile drains and used to explore the effects of region-wide implementation of subsurface drainage on salinity transport and export from an irrigated semi-arid watershed. The model includes the transport of 8 major salt ions (SO4, Ca, Na, Cl, Mg, K, CO3, HCO3), and with the inclusion of tile drain transport simulates the fate and reactive transport of these ions in soil water, groundwater, tile drain water, and stream water. The SWAT-Salt model is applied to a 732 km2 salinity-impaired irrigation region within the Arkansas River Valley in southeastern Colorado. The model is first tested against salt ion data in effluent from a 15 km2 tile drainage district, and then applied to the entire region to assess the impact of region-wide implementation of subsurface tile drainage on in-stream salt ion concentrations, in-stream salt ion loading, and total salt export from the watershed. The model can be a useful tool in simulating salinity transport in tile drained watershed and investigating the effect of salinity management practices at a variety of spatial and temporal scales.