The spatial and temporal distribution of growing season water deficits in Ontario are discussed in this publication. The May to September precipitation for all of Ontario and an example of precipitation distribution during an extended dry period in southern Ontario are included. A forage crop grown on soils with 100 mm of available soil water capacity was assumed and four methods were used to estimate the water deficits. Two of the methods used monthly average precipitation and temperature only to determine average water deficits and calculations were made using climate data for a large number of climatological stations. The other two methods required daily sunshine or solar radiation records which limited the number of locations. The latter two methods used models to simulate the daily water use of a forage crop for each year from 1961-1990 to provide the magnitude of water deficit for each season. One model used water budgeting procedures only and the other model simulated water movement in the soil in response to hydraulic gradients and water uptake by plant roots. Regression procedures were used to relate the magnitude of water deficits calculated by the daily water budget model at several locations to the seasonal deficits based on one of the methods that use monthly average precipitation and temperature. This was done to provide water deficits for a sufficient number of locations to map the deficits based on daily budgeting procedures.

The average precipitation during May to September ranged from 350 mm along the north shore of eastern Lake Ontario to more than 450 mm east of Georgian Bay, east of Lake Huron and northwest of Lake Superior. Even though the average growing season precipitation appeared to be sufficient for maximum crop production, it was demonstrated that extended dry periods do occur and cause significant short term water deficits. The magnitude of the growing season water deficits depended on the calculation method and available water conditions in the soil. For example, at Harrow, the average growing season water deficit for the 1961-90 period was estimated to be 32 mm based on the method used in the 1968 Climate of Ontario publications whereas the average deficit was 163 mm based on the model that simulated water movement in the soil in response to hydraulic gradients and water uptake by plant roots based on daily climate data. Therefore, water deficits were mapped based on the daily water budgeting procedure that provided the most conservative water deficits, e.g. 133 mm at Harrow assuming 100 mm ASW capacity and free drainage. The average water deficits based on this method, and the regression relationships with one of the monthly methods, vary from less than 60 mm in most of northern Ontario and areas east of Lake Huron and Georgian Bay to over 120 mm in four regions in southern Ontario. The larger deficits in some areas are caused by physiographic features, such as the Niagara Escarpment, and the effect of the Great Lakes on precipitation.

Water deficits for individual growing seasons can exceed 300 mm in certain areas when dry periods are prolonged, e.g. at Harrow, there is a 5 % probability of 297 mm or greater based on one of the daily simulation methods. The growing season water deficits for soils with different available water capacities an water table situations are discussed to demonstrate the importance of considering these features when interpreting the average water deficits as mapped in this publication. Comparison of the magnitude of water deficits based on the four methods showed that the older methods, using monthly climate normals, grossly underestimated water deficits in all regions of Ontario.