GROWING SEASON WATER DEFICITS IN ONTARIO

Transcription

1 GROWING SEASON WATER DEFICITS IN ONTARIO D.M. Brown, Land Resource Science, O.A.C., University of Guelph, Guelph, Ontario N1G 2W1 A. Bootsma and R. de Jong, Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa K1A 0C6 The water needs of crops are provided from stored soil water which is replenished by precipitation. Water available for extraction by plant roots is usually plentiful when perennial crops begin to grow in the spring throughout most of Ontario. The amount of available soil water depends on the type of soil, its organic matter content, and the depth to which roots grow. Loam and silt loam soils have a greater capacity to hold water available for plant roots than coarse-textured sandy and fine-textured clay soils. The roots of some crops penetrate the soil to a greater depth than others, e.g. alfalfa is deep-rooted whereas grasses, potatoes and most vegetable crops are shallow-rooted. Therefore, the magnitude of water shortage or deficit depends on the crop, the soil type on which it is grown and of course the local weather conditions. The term water deficit may be defined as any shortage of available water that results in a reduction of the amount of water that is evaporated from the soil and transpired by the crop. When the soil water content is adequate evaporation and transpiration occur at their potential rate. This is referred as potential evapotranspiration (PET) and is dependent on atmospheric conditions such as radiation, temperature, wind speed and humidity. Optimum crop growth occurs when water is not limiting. When soil water is inadequate, leaf stomates close, resulting in lower transpiration and reduced crop growth. This is referred to as actual evapotranspiration (AET). Water deficits can be calculated by determining the difference between PET and AET on a daily, monthly or seasonal basis. Water deficits vary widely from year to year and from region to region. These variations can cause wide fluctuations in crop yields and may influence the quality of the harvested product. Crop insurance premiums and indemnities are also dependent on the frequency and intensity of occurrence of water deficits. Information on May to September rainfall and the variability of water deficits for the forage crop growing season in Ontario are presented here. This will provide producers and extension specialists with information on the frequency and severity of water deficits in Ontario and the potential need for irrigation. May to September Precipitation This five month total of precipitation is often used to provide to provide the geographic distribution of growing season precipitation. The average precipitation for the May through September period varied from less than 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 (Fig. 1). Precipitation statistics for this period at 25 climate

2 stations are summarized in Table 1. Average monthly precipitation varied from 51 mm in July at Belleville to 109 mm in September at Muskoka and 112 mm at Fort Frances in June. The average standard deviation (SD) for the five- month total precipitation over the 25 sites was 89 mm and varied from a SD of 68 mm at Brucefield and Peterborough to a SD of 31 mm at Harrow. The monthly average SD for the 25 stations varies from 33 mm in May to 43 mm in September. SD is a measure of how much precipitation varies from year to year. Higher SD s indicate greater variation. Dry periods. Extended dry periods lasting from one to two months of the growing season have occurred, when only a trace to a few mm of rain fell. These dry weather patterns may cover fairly large regions or may be limited to small areas. For example, during the 41-day period from June 16 to July 26, 1966, two regions in southern Ontario received less than 15 mm. The long-term average rainfall for this period in these regions is over 100 mm. The driest regions that year extended southeast from Lake Huron - Georgian Bay to Lake Ontario and another smaller area from southern Lake Huron to Lake Erie. Less severe dry periods occurred in southern Ontario in 1963 and 1978 during a period of comparable length to the one in A more severe dry spell accompanied by much above normal July temperatures occurred in Several dry periods in some southern Ontario areas have also occurred in the 1990's. Occurrences of such dry periods lead to considerable variation in water deficits for crop production from one year to the next and result in the periodic need for irrigation. Major dry periods occur about once in every five years in southern Ontario and less frequently in northern Ontario. Growing Season Water Deficits The sporadic occurrence of dry periods, the spatial distribution of rainfall during these dry periods, and variation in evaporative demand (i.e. PET) emphasize the need for information on both the spatial and temporal distribution of water deficits. Examples of both are provided below. Spatial variation in water deficits. The average water deficits for soils with 100 mm of available soil water (ASW) capacity based on the climatic records for are mapped in Fig. 2. ASW capacity is the amount of water within the rooting depth that the soil can hold between field capacity and the permanent wilting point. Water deficits were calculated using the Forage Aridity Index (FAI) model applied to daily climate data for the forage crop growing season. (This season begins sometime after March 1st each year, when the average air temperature of three consecutive days stays above 5ºC and the season ends when the average air temperature of three consecutive days drops below 5ºC in late fall.) The total water required (in addition to rainfall) each season to maintain the available soil water above one-half of its ASW capacity (i.e. above 50 mm for a soil with 100 mm ASW capacity) was calculated for each of the 30 seasons and then averaged to obtain the water deficits shown on the map (Fig. 2). Large areas east of Lake Huron and east of Georgian Bay and all of northern Ontario, except for small areas near Kenora (the Lake-of-the-Woods area) and north of Georgian Bay, had average water deficits of less than 60 mm. The average deficit increased to over 120 mm in four regions: south Essex county and south of Sarnia; between the Niagara escarpment and west Toronto and around the western end of Lake Ontario and as 2

3 far north as south Simcoe county, southwest of Barrie; Prince Edward county; and in Renfrew county south of Petawawa, northeast of the Precambrian Shield. The western parts of Metro Toronto and south Peel region had an average deficit exceeding 150 mm. The area that is shown by the 120 mm finger southwest of Barrie is in the lee of the Niagara Escarpment. The Escarpment enhances convection (vertical lifting or motion of the air due to atmospheric instability) to the west and diminishes it to the east resulting in less precipitation in the Nottawasaga river valley than in areas to the west and to the northeast. Temporal variation in water deficits. The average, and the 25 and 10% chances of seasonal water deficits are summarized for 34 locations in Table 2. At locations where average seasonal water deficits are in the 120 to 130 mm range (e.g. Vineland, Albion, Smithfield and Picton) there is a 1 in 10 chance of the seasonal deficit exceeding about 235 mm and a 1 in 4 chance of exceeding about 180 mm. At locations in the region east of Lake Huron where the average water deficit (shown in Fig. 2) is less than 60 mm, (e.g. Stratford, Owen Sound and Mount Forest), there is a 1 in 10 chance of the deficit exceeding about 160 mm. In northern Ontario and the region east of Georgian Bay, the deficits typically exceed 130 mm at the 10% probability level (e.g. Muskoka, North Bay and Kapuskasing) and exceed 180 mm in northwestern Ontario (e.g. Kenora) one year in ten. The highest deficits were found at Toronto, where values exceed 280 mm at 10% probability (1 year in 10) for soils with 100 mm ASW capacity. Effects of ASW capacity on water deficits. The average water deficits shown in Figure 2 and Table 2 assume the ASW is 100 mm, which is typical for soils such as a Caledon sandy loam. These deficits decrease as the ASW capacity increases (see Table 3). For example, water deficits for soils with 200 mm ASW capacity, typical of soils such as a Guelph loam, would be about 50 percent less than the water deficits for soils with 100 mm ASW capacity at the 25% probability level in southwestern Ontario. Similarly, soils with 250 mm ASW capacity, such as a Clyde silt loam, would be about 75 percent less, at the 25% probability level in southwestern Ontario. The percentage decrease in water deficits as the ASW capacity increases depends on the probability level and the region in Ontario. All regions have years in which the water deficits are zero for all soils. Effect of soil texture on ASW capacity. An illustration of the range in ASW capacity for soils of different textures is provided in Fig. 3. The information was developed by Dr. R. A. McBride and associates at the University of Guelph assuming a bulk density (δ b ), organic matter content (OM), soil water tension and drainage to a deep water table as indicated. Changing these assumptions will affect the upper and lower limits of volumetric soil water content for specific textures, and thus the ASW capacity. The ASW capacity for a soil profile can be estimated from the model that was used to derive the diagram in Fig. 3, provided the texture and the above characteristics are known for each layer of the soil profile in question. The ASW capacity is estimated by determining the differences between the lower limit of ASW (i.e. the lower dashed curve in Fig. 3) and the point in the textural class polygon in the diagram that applies to the soil profile horizon. For example, at the mid-point in the loam soil polygon, with 20% clay content, the lower limit of ASW is at 14% moisture and the upper level of ASW is at 36%. Therefore, the ASW capacity for this loam would be 22% of the depth of the soil horizon in the profile in question, (i.e x 100 mm depth is 22 mm of water in a 100 mm soil horizon). Therefore, a soil profile with this same texture, 3

4 and plant roots able to extract water to a depth of 50 cm, would have 110 mm of ASW capacity. Dry period frequency/irrigation needs. An indication of the frequency of dry periods when irrigation would be beneficial can be determined from statistics on growing season water deficits. For example, if it is assumed that irrigation would be required for years when the water deficit exceeded 100 mm, then the number of years requiring irrigation varies from about 50 to 70 percent (15 to 21 years out of 30) for well drained soils with 100 mm ASW, and from 5 to 15 percent (2 to 5 years out of 30) for well drained soils with 250 mm ASW in most of southern Ontario. The frequency of years in which water deficits exceed certain thresholds can be estimated from the average seasonal deficit in Figure 2 for soils with 100 mm ASW capacity by using the information in Table 4. Effects of a water table. Water deficits presented here assume a freely draining soil profile without the influence of a high water table. Water deficits can be significantly reduced when additional water is available to plants due to the presence of a high water table. A different soil water balance model was used to evaluate the effect of high water tables on deficits for specific soil profiles with different soil textures (i.e. a sandy loam, a clay loam, and a clay soil). Those results indicated that, for similar water table situations in all three soil types, deficits were reduced much more in the sandy loam profile, due to greater upward movement of water in comparison to the clay loam and clay soils. Further Reading and Contacts Re: (1) Dry periods and water deficit calculations- Brown, D. M., Bootsma, A. and De Jong, R Analyses of growing season water deficits for Ontario. University of Guelph, O.A.C. Land Resource Science Tech. Memo. 98-1, 23 pp. (2) Effects of soil textures on available soil water (ASW) capacity- R.A. McBride, School of Environmental Sciences, Univ. of Guelph McBride, R.A and MacIntosh, E.E Soil survey interpretation from water retention data. I. Development and validation of a water retention model. Soil Sci. Soc. Am. J. 48: (3) Water table effects- De Jong, R. and Bootsma, A Estimates of water deficits and surpluses during the growing season in Ontario using the SWATRE model. Can. J. of Soil Sci. 77: (4) The FAI model - De Jong, R., Bootsma, A., Dumanski, J. and Samuel, K Variability of soil water deficiencies for perennial forages in the Canadian prairie region. Agric. Water Manage. 20:

5 Table 1. Average precipitation (mm) for each month from May to September, total seasonal and its standard deviation (SD) for 25 locations in Ontario for the period. Location May June July Aug. Sept. Total (SD) Harrow (131) Ridgetown (103) London (105) Woodstock (103) Brantford (85) Welland (69) Vineland Station (71) Waterloo Wellington A (101) Brucefield (68) Wroxeter (83) Orangeville (87) Albion Field Centre (90) Peterborough (68) Belleville (76) Cornwall (83) Ottawa (89) Renfrew (77) Haliburton (91) Muskoka (81) Sudbury (96) Earlton (70) Kapaskasing (92) Thunder Bay (106) Kenora (114) Fort Francis (90) Average SD (89) 5

6 Table 2. Water deficits (mm) for soils with 100 mm of available water-holding capacity (ASW) for 34 locations in Ontario for the 1961 to 1990 period. Location Average xx % Probability of deficits exceeding given value for each location 25% 10% Harrow/Kingsville Ridgetown London Woodstock Delhi Welland Vineland Station Stratford Guelph Toronto Southhampton Owen Sound Mount Forest Midhurst Albion Field Centre Lindsay Orono Smithfield Picton Kemptville Cornwall Ottawa Arnprior Bancroft Petawawa Muskoka North Bay Sudbury Gore Bay Sault Ste. Marie Timmins Kapaskasing Thunder Bay Kenora

7 Table 3. The average level of water deficits (mm) that are exceeded at four probability levels based on application of FAI model for three regions in Ontario for soils with four different ASW capacities (mm) in the period. Region in Ontario ASW Probability level (mm) 50% 25% 10% 5% Southwestern < Eastern < <1 < Northern < < < Table 4. Approximate frequency (%) with which selected water deficit thresholds are exceeded for soils with 100 mm ASW capacity. Probability of deficit exceeded (%) Average seasonal Selected Water Deficit Thresholds water deficit (mm) 1 50mm 100mm 150mm 200mm Water deficits as shown in Figure 2. 7

8 8

9 9

10 45 40 silt silty clay loam clay loam silty clay clay Volumetric Soil Water Content (%) silt loam sandy loam/ loamy sand/ sand loam sandy clay sandy clay loam upper limit of available water content (ASW) based on texture lower limit of available soil water (ASW) Assumptions ρ b = 1.3 Mg m -3 (g cm -3 ) OM = 1.7 % Field Capacity Tension = 10 kpa (100 mb) Soil is free draining Clay Content (%) Figure 3. Available water content (% by volume) for soils of different textures in relation to clay content (% by weight) to illustrate how ASW capacity can be calculated. (Source: R.A. McBride, Dept. Land Resource Science, University of Guelph). 10

11 ABSTRACT A daily soil water balance model was used to estimate the spatial and temporal occurrences of water deficits for Ontario for the perennial crop growing season, based on climate records for the 1961 to 1990 period. In addition information on the May through September precipitation is provided. The average seasonal water deficits varied from less than 60 mm in most of northern Ontario and east of Georgian Bay and Lake Huron to more than 120 mm in four small regions of southern Ontario and exceeded 150 mm in the metro Toronto area. In order to provide a risk assessment of seasonal water deficits for irrigation purposes, the magnitude of deficits at specific probability levels were calculated assuming a sandy loam soil with 100 mm of available soil water. Regions with average water deficits greater than 120 mm were found to exceed a deficit of 225 mm at the 10% probability level. In regions where the average water deficits were less than 60 mm, the probability analyses showed that water deficits were about three times as large at the 10% probability level as the average deficits for individual sites, e.g. the average deficit at Stratford was 55 and at the 10% probability it was 161 mm. In the south Essex county area, where the average deficit exceeded 120 mm, one model calculated water deficits that exceeded 290 mm at the 10% probability level and was near 300 mm at the 5% level. The 30-year average total May through Sept. precipitation varied from <350 mm to >450 mm. The variability in precipitation from year to year can be larger than the spatial variability of averages across Ontario. 11