Ontario Grain Farmer February 2021

28 Water is a precious resource. The success of farm businesses and the health of our families depend on having a clean and abundant supply. Historically, agricultural technology has allowed us to manipulate the quantity and quality of water supplies to increase productivity. Today, this continues, with new technologies and a better understanding of natural processes. This article is the fifth in a series focusing on modern management of water for grain farms. Go to www.ontariograinfarmer.ca for more information and previous articles. LAST MONTH, IN this water series, we explored drainage and the need and challenges associated with managing too much water in the off-season when the crops are not growing and using it. This month we follow with a critical look at whether storing some of that excess drainage water for later use as an irrigation supply is a good option. In Ontario, rainfall is usually sufficient during the growing season, but some years and in pockets of the province there have been significant droughts causing poor yields, leading some to consider irrigation. If those drought years become more frequent and more severe, could irrigation be a key part of maintaining grain crop yields in Ontario’s future? WATER LOSS To answer these questions, the first thing we may want to assess is just how much water actually leaves via tile drainage systems in the non-growing season? Edge-of-field monitoring investigations sponsored by the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) have shown that, depending on the year, between 140 mm — 210 mm (5.5” – 8.3”) rainfall equivalent of water can be expected to leave a field through its systematic tile drainage system. Of course, the actual amount in a year depends on the field’s soil characteristics, the intensity of the drainage system, and rainfall amounts. Healthy soils with good structure and organic matter (OM) levels will tend to store more water within the soil root zone. Heavier soils, or those with poor structure or compaction which limit infiltration, will result inmore overland runoff. Nevertheless, this amount of water drained in the non- growing season, if it could be stored for later use, is likely more than adequate to get through a typical Ontario drought. To capture runoff/tile drain water for later use, however, requires a storage pond. The size of that pond depends on howmuch water might be needed in the summer months. Irrigation specialists in southern Ontario estimate that, keeping soil moisture at an optimal level for grain crops, typically requires two to three irrigation events applying 25 mm of water each time (75mm/summer on average). To irrigate a 40 hectare (100 acre) field, the water required would fill a 125 m x 125 m reservoir that is 4 m (12 ft) deep has a footprint of 3.9 acres (assuming sloped walls and 0.9 m (3 ft) freeboard). To construct this pond would therefore require four per cent of a field’s area to be used for water storage instead of production. This drainage water storage effort would also require purchasing and operating irrigation equipment, such as a travelling gun or subsurface drip installations, to apply the water in a timely manner. Some have proposed that, instead of purchasing standard irrigation equipment, existing tile drains could be used to distribute the irrigation water underground across the field. This idea works best if fields are relatively flat and a naturally impermeable soil layer exists just beneath the tile lines to prevent downward water movement. These conditions, however, are not often present in most fields, making standard irrigation equipment the more likely approach. Water management IS IRRIGATION OF GRAIN CROPS FEASIBLE? Rebecca Shortt, Qin Xu, Kevin McKague, and Ian McDonald Drain water capture for subsequent irrigation becomes more viable where: the existing site conditions favour drain water collection; the field is drought prone; and the crop has a high value. CLIMATE CHANGE Could climate change affect the viability of all this? Climatemodeling by Natural Resources Canada predicts that both temperature and precipitation will increase in the time period of 2020 – 2070. However, the variability may also increase, and this variability is likely to be greater for precipitation, leading to increased incidents of summer dry periods. Dr. Qin Xu, a researcher at the University of Guelph, developed a model to look at corn and soybean yields under these models of future climate (Base, C1, C2 and C3) (Figure 1) and compared those modeled yields with and without irrigation. Her modeling shows corn and soybean yields increase with the projected increasing temperatures and precipitation but that yield variability from year-to-year could also increase because of the possible increased variation in rainfall. Irrigation increases both corn and soybean yield in the models. The projected difference in yield between irrigated and non-irrigated in 2070 ranges from two per cent to 11 per cent for corn, and two per cent to eight per cent for soybeans. These are the average increases, considering that there will be both wet and dry years. In dry years, the benefits will be much greater. The ability to irrigate means that farmers can make use of the increased temperatures without the risk of yield losses that occur from potentially more variable rainfall. Xu’s model suggests irrigation has a larger positive impact on corn yields than soybean yields. This finding makes sense when we compare to a state such as Michigan with Research