Assessing the Impact of Irrigation Return Flow on River Salinity for Colorado’s Arkansas River Valley

A river salinity model has been developed on the 21 major canal areas in the lower Arkansas River Basin in Colorado to address the impact of irrigation return flow on the river. The quantity of the return flow is predicted by constructing response functions for tailwater, canal leakage, and in-field deep percolation so that the spatial and temporal distribution of the return flow can be simulated. A groundwater table surface is generated using water table elevation data from 974 wells in the study area to establish flow paths and travel time for groundwater. The quality of the return flow is predicted by simulating the evapoconcentration process in the root zone soil in which hydro-chemical reactions occur and affect the salinity of in-field deep percolation water. The effects of shallow water table and high soil salinity on crops are simulated to account for the impacts that these two factors have on crop consumptive use. Model calibration and validation over a 192-month period from January 1986 to December 2001 show strong agreement between the observed and simulated values of river flow volume and river salinity. The simulation results show that irrigation return flows, including tailwater and groundwater return flows, significantly increase river quantity, but that groundwater return flow is also a major component of river salinity. There is significant seasonal fluctuation in river salinity and soil water salinity. The increase of soil water salinity from the soil surface to the bottom of the root zone is significant and will cause salt to be loaded to the groundwater. The simulation from 1991–2001 indicates that 20.9% of the 19,944 million ${\mathrm{m}}^3$ of irrigation water applied to Colorado’s lower Arkansas Valley becomes canal leakage, 22.5% is in-field deep percolation, and 22.9% becomes tailwater. This study assesses the effect of reducing agricultural irrigation on river salinity. Consider the scenario of three major canals stopping irrigation and transferring a portion of the curtailed water to off-basin cities with the remaining portion released to the river. The results indicate that, in order not to increase river salinity for downstream reaches, the portion of water transferred to cities should not exceed 50%.