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 fourth in a series focusing on modern management of water for grain farms. Go to www.ontariograinfarmer.ca for more information and previous articles.
IT IS WELL known that tile drainage, by optimizing soil moisture conditions in the rootzone during spring, allowing farmers to get on the field earlier, and reducing the risk of soil compaction, delivers agricultural productivity benefits. Therefore, it can certainly be regarded as a best practice when it comes to improving farm returns. Many, however, particularly those outside of agriculture, question its environmental benefits.
Tile drainage can change lands that, before drainage, were too wet to farm and acted as important water retention areas within the landscape. Some question whether this contributes to more sediment and nutrients in surface streams. Others also see tile drainage as removing too much water from a field too quickly resulting in a loss of water for crops later in the year and increased flooding concerns downstream.
This article looks at how much soil water tile drainage removes, conditions that can cause elevated sediments and nutrients in tile water, and whether we can hold back some of this water for later use.
For water to enter a subsurface tile drain, it must first infiltrate the soil surface and move downward through the soil profile, either as “matrix” flow or as “macropore” flow. With matrix flow, the water penetrates outward and downward throughout the soil mass as it is acted upon by a combination of matric (water attraction to the soil itself) and gravitational forces. This steady entry and movement of water into and through the entire soil profile slowly fills all sizes of voids and pores throughout the rootzone, replenishing the water reservoir used by crops.
Pore space (or voids) in a soil matrix come in many different shapes and sizes. However, only the water contained in larger soil pores can be drained by tile drains. When tiles are done draining a field, there is still a significant amount of water remaining in the soil and much of the water that crops utilize remains in the soil. This is water that is held between field capacity — the point where tile drains stop flowing — and permanent wilting point — the point where plants cannot pull out any more water. Figure 1 gives an idealized visualization of these three soil moisture regimes.
When soils are saturated, essentially all pore space within the soil is filled with water. The soil surrounding a tile drain must approach these saturated conditions before soil water will begin to enter the drain pipe. A water table is defined as the point in the soil where all pores are filled with water. For this reason, tile drains generally do not start flowing until the water table rises to the depth of the tile line. Water that moves through the soil as matrix flow to the water table, experiences the natural filtering ability of soils. As a result, water that reaches drainage tiles via matrix flow, tends to have one-tenth the concentration of sediments and nutrients than is present in water that runs off the soil surface.
There is an exception to the general rule that tiles only flow under saturated soil conditions. This exception occurs when water moves to tile drains via “macropore” flow. Rain or meltwater that cannot infiltrate as fast as it is falling on the soil surface will begin to flow small distances overland until it reaches a “macropore”. These are large cracks, worm holes and root channels that are open at the soil surface and may reach far down into the soil matrix.
If these macropores extend and terminate near a tile drain, they can result in surface waters bypassing much of the soil’s water holding and filtering capacity resulting in rapid entry of water into tile drain lines. The narrower the tile drain spacing the more likely a macropore connection will be present between the surface and a tile drain. In the summer months, if tile flow is observed immediately following a rainfall event, it is likely that it originated from macropore flow. Because macropores deliver water directly from the soil surface, the quality of that water is likely to be poorer than water that is filtered through the soil matrix.
In reality, macropore flow and matrix flow happens simultaneously in a field. When a field experiences declines in soil health (more compaction, lower organic matter, erosion etc.) less infiltration (and thus less matrix type flow) occurs and drainage of surface waters is left to rely more on macropore connections, ever narrowing tile drain spacings and even installed surface or blind inlets to help redirect the increasing overland flow underground. While the water may still get removed, there is less water retained in the soil matrix pores for future crop needs and the water that is sent via macropores and surface inlets to the tile drains is of poorer water quality.
DRAINAGE CONTROL GATES
There has been a growing interest in the idea of using drainage control gates in critical times of the year to help reduce tile flow and nutrient loadings. If the volume of water leaving a tiled field can be reduced, then it stands to reason that the potential exists to reduce both water quantity and quality concerns. Drainage control gates are in-line structures that contain boards which can be removed to allow tile systems to flow when drainage is needed or inserted to hold back water when water retention is desired. Retrofitting existing tile drainage systems with control gates can really be easily done only in situations where the field is flat or where the tiles have been installed on a very shallow grade by following the contours.
In Ontario, given our relatively humid climate and normally adequate yet unpredictable rainfall amounts, it can be difficult identifying the ideal times to open and close these control gates to optimize field water levels. Generally, the gates will remain open so free drainage can occur, and soils can be dried out enough for equipment traffic during the planting and harvest periods. The gates are then closed in the peak of summer, when crop growth and water demand is at its maximum. Blocking tile drains in the summer will largely intercept only the macropore flow contributions to tile drainage as the water table below the field at this time is often significantly below the tile depth. The water that gets blocked then is more likely to soak into the soil immediately around and below the tile. Crop roots may penetrate to this depth, but it is unlikely to generate a huge yield gain relative to irrigating on the soil surface where water gets delivered to the crop’s primary root zone.
There have been U.S. (Ohio) and Ontario studies investigating the economic and production benefits of controlled drainage in the absence of any irrigation. Observations have been mixed, with small yield gains experienced some years, and other years showing no yield benefit depending on the timing and nature of the growing season’s rainfall events. (See Table 1 Example – Essex County)
Figure 2 illustrates the amount of tile water that might be available to hold back and keep from leaving a field in the summer months. The figure shows observations of daily rainfall, water table depth and tile flow leaving a field located in Huon County. The critical June through September period generates relatively little tile flow despite the fact that rainfall amounts in these months are comparable to or greater than other months of the year. The graph clearly illustrates that tiles tend to flow consistently only when the groundwater table rises to the tile depth. There are some minor periods of tile flow in the 2016 summer months, likely due to macropores contributing to the tile. Holding back this small amount of water in the June through September period, however, would have done little to contribute to that growing season’s water needs. In the subsequent (2017) crop year, rainfall amounts appeared adequate and actually triggered tile flow in the summer with a rise in water table. Therefore, it is unlikely additional irrigation water was even needed that season to supply crop needs.
Tile drainage provides definite production benefits but can have both negative and positive environmental impacts. Determining the net benefit they deliver by reducing compaction risk and maintaining the health of the soil matrix while in contrast offering a more convenient pathway for the quick exit of macropore water and associated nutrients is proving very difficult.
Controlled drainage which can block off summer macropore flow still needs further study as well to assess its full benefit. Click on these links for more information on “How drainage works”, “How fields dry”, and “The impact of surface inlets”.
For more information on contour and controlled tile drainage and work being done in Ontario to investigate the production and water quality benefits of this practice, visit the Huronview Demonstration Farm site at www.huronview.net/.
In the next article of our series, we will explore the opportunities for using drainage water as irrigation, and how demand/benefits of irrigation may change in the future. •
Other articles in the series:
• Producing the best grain crop possible (September 2020)
• The water cycle and why should we care (October 2020)
• The role of farm drainage (November 2020)
• Improvements through soil health (December 2020)