Modelling the effect of irrigation on the hydrological output from a small prairie watershed

Abstract

In a typical irrigation practice the soil water content is maintained at or near field capacity, which can affect hydrological processes such as deep percolation and runoff in different ways. The aim of this study was to evaluate these effects for a partially irrigated, small watershed in the Canadian prairie using the Soil Water Assessment Tool (SWAT). The watershed was defined by upstream (inlet) and downstream (outlet) monitoring stations located along a short reach of a river. SWAT was calibrated and validated using the net flow between the upstream (inlet) and downstream (outlet) locations because the watershed was defined by this reach. Runoff contribution to the incremental streamflow was minimal, as indicated by the reduced values of the calibrated initial curve number (CNII) (CNII range 49–59). Irrigation activity increased runoff depth but the differences between irrigated and non-irrigated areas were not statistically (α ≤ 0.05) significant. The low runoff contribution was also corroborated by the streamflow record that demonstrated the low potential for runoff generation in the watershed. The only apparent runoff occurred after a major rainfall event of a cumulative depth of more than 200 mm. The modelling also showed that the shallow aquifer discharge was the main streamflow constituent. Precipitation during May to July was responsible for 70–90% of the seasonal deep percolation. By the end of the season the deep percolation from the irrigated areas exceeded that of the non-irrigated areas by up to 70%. Thus, the irrigation activity in the watershed did not change the water partitioning among the existing hydrological pathways but had temporal effects on the magnitudes of runoff and, more importantly, deep percolation and the subsequent groundwater discharge in the main reach.