Estimates of water deficits and surpluses during the growing season in Ontario using the SWATRE model

Abstract

The spatial and temporal variations in soil water play a key role when considering issues such as irrigation and drainage requirements, crop growth and yield potential and contamination of groundwater by agricultural chemicals. In this study a previously validated deterministic model was used to estimate water (evapotranspiration) deficits and surpluses during the growing season of a perennial forage crop at 14 locations in Ontario. Three different hydraulic soil profile characteristics, representing a clay, clay loam and sandy loam soil, and three different lower boundary conditions (free drainage, fixed and fluctuating water table) were considered. Seasonal deficits ranged from less than 15 mm in abnormally wet years (90% probability of exceedence) in the north, to more than 300 mm in exceptionally dry years (5% probability of exceedence) in the extreme southwest region of the province. With a water table fluctuating between 25 cm depth during the middle of April and 140 cm in early October, 28 to 49% of the evapotranspiration deficit was caused by excess water in all three soils. The flux of water at 100 cm depth, accumulated during the growing season, was used as a measure of the net amount of surplus water moving through the root zone or being extracted from the water table. This flux ranged from significant upward values in dry years in a sandy loam with a fixed water table at 120 cm depth, to downward fluxes of more than 300 mm in abnormally wet years when a fluctuating water table was present. Our simulations show important spatial and temporal interactions among soil and climatic factors and suggest that each one, including the lower boundary condition, should be properly characterized in soil water investigations. Key words: Soil water, modelling, spatial and temporal variability, forage crop