Investigating the controlling factors on salinity in soil, groundwater, and river water in a semi-arid agricultural watershed using SWAT-Salt.

Soil salinity can have a significant impact on crop yield, particularly in arid and semi-arid irrigated watersheds wherein irrigation and inadequate drainage often combine to increase salt ion concentrations in soil water. To control salinity, the principal step is to identify the key environmental and hydrologic factors that govern the fate and transport of salts in these irrigated areas. To accomplish this objective, global sensitivity analysis is applied to the newly developed SWAT-Salt model, which simulates the reactive transport of 8 major salt ions (SO, Ca, Mg, Na, K, Cl, CO, and HCO) in major hydrologic pathways in a watershed system. The model is applied to a saline 1118 km irrigated stream-aquifer system located within the Lower Arkansas River Valley in southeastern Colorado, USA. Multiple parameters including plant growth factors, stream channel factors, evaporation factors, surface runoff factors, and the initial mass concentrations of salt minerals MgSO, MgCO, CaSO, CaCO, and NaCl in the soils and in the aquifer are investigated for control on salinity in groundwater, soils, and streams. The Morris screening method is used to identify the most sensitive factors, followed by the Sobol' variance-based method to provide a final ranking and to identify interactions between factors. Results show that salt ion concentration in soils and groundwater is controlled principally by hydrologic factors (evaporation, groundwater discharge and upflux, and surface runoff factors) as well as the initial amounts of salt minerals in soils. Salt concentration in the Arkansas River is governed by similar factors, likely due to salt ion mass in the streams controlled by surface runoff and groundwater discharge from the aquifer. Results can be used in decision making regarding the most impactful land and water management strategies for controlling salinity transport and build-up in soils, both for this watershed and other similar semi-arid salinity-impacted watersheds.