Managing to get the most from available water

Water is the most important limiting input for crop production. In more than a couple of the past years, producers across the province have been forced to watch while their crops suffer from dry conditions in the heat of summer.

Other than irrigation (which is rare for field crops in Ontario), there’s really nothing we can do to manage how much water we get and when we get it. For that reason, it makes more sense to think about how we might lose it – and how we can keep it.

Striking the Water Balance

On the opposite side of the water balance equation from inputs (precipitation and irrigation) are losses – runoff, deep drainage, and evapotranspiration – and the soil storage component. The right lever to pull in this system will depend on the specifics of each operation and its soil landscape, so here are the broad concepts and some general recommendations for how to influence them to manage your risk for drought stress.

Deep drainage

As water moves through the soil, it eventually goes below the rooting zone and is essentially lost for crop production. This is an important natural process for recharging groundwater and the way that soils provide a water purification service.

Tile drainage essentially short-circuits this process, shunting groundwater into surface waterways. Water table management like this is critical for timely planting in most of the province and improves the drought tolerance of crops as it allows roots to develop deeper before dry conditions occur. However, we’re often left pining after that spring water once it gets dry in the summer. Controlled drainage systems allow for more refined water table management and allow us to keep water in the field when it’s needed. Once thought to be suitable only for flat fields, this technique is now also being applied on sloping terrain with drains installed on contour.

Evapotranspiration

Water evaporates from soil more quickly in hot, dry weather. It also evaporates more quickly from bare soil than from soil that is covered, either by residue or by plant canopy (Figure 1). Bare soil competes with the crop for water just like weeds do. Overwintering cover crops can provide the best of both – transpiring excess water in the spring and providing mulch to cover the soil until the crop canopies. “Planting green” – into living cereal rye terminated soon after planting, for example – is an effective strategy for soybeans.

Figure 1. Relative evaporation rate from bare soil and residue-covered soil. (van Donk et al., 2010)

Runoff is a terrible way to lose water, because it usually also means losing soil. The erosive power of runoff is related to its speed, so anything that can slow the water down will help. Residue cover is the first step – 100% residue cover results in negligible erosion, but 50% cover still reduces erosion by 80%. This level of residue is usually achieved with no-till. A recent meta-analysis of runoff and tillage systems showed that no-till reduced runoff by over 20% on average compared with other tillage systems (Figure 2). The residue cover in a no-till system reduces crusting, and the lack of disturbance results in higher pore continuity, meaning more infiltration and percolation.

Figure 2. Change in runoff from no-till compared to reduced tillage and plowing. From Sun et al., 2015.

The devil is in the details, though. No-till fields may experience more runoff than tilled soils if they are left bare from residue harvest (i.e. straw removal or low-residue crops (e.g. soybeans) if their surfaces are smoother than tilled fields. This is where cover crops come in. Even in tilled systems, the pores created by cover crop roots can increase infiltration of a heavy rainfall event by 17% (Yu et al., 2016). Figure 3 shows a soil infiltration demonstration that provides a good visual of how coarse and fine root channels help move water through the soil. Cover crops with coarse roots (e.g. daikon radish) and those with very dense root systems (e.g. cereal rye) provide the most benefit.

Figure 3. Water infiltrates quickly and preferentially through continuous macropores created by daikon radish and oat cover crop roots.

Store it in the Soil

Making the best use of the water we get in a season means getting it in the ground, but also keeping it there and making sure it’s accessible to crop roots. A well-structured soil with a range of pore sizes will hold the most plant-available water. In a compacted soil, root access to water may be limited by soil resistance even before water levels fall below the permanent wilting point (Figure 4.)

Figure 4. The least-limiting water range (LLWR) is between field capacity and permanent wilting point in low-density soil, but soil resistance (SR, i.e. compaction) or aeration (AFP) may be more limiting in compacted soil. (from Keller et al., 2015)

If what’s noted above makes you think that you don’t need to worry about water because you are already no-tilling, think again. Tillage is a tool for dealing with compaction. If you take it out of the toolbox, extra care is required to avoid squeezing your soil of its water holding capacity.

Weather Summary

Weekly Weather Summary August 10-16
Location	Date	Temperature		Rainfall	Totals
	August 10 -16	Max	Min	(mm)	Rain	GDD₀	GDD₅	CHU
		°C	°C	(mm)	01-Apr	01-Apr	01-Apr	01-May
Harrow	2020	31	13	11	307	2266	1609	2447
Harrow	2019	28	13	2	370	2243	1579	2344
Ridgetown	2020	30	12	16	314	2176	1522	2316
Ridgetown	2019	28	11	9	514	2123	1461	2209
London	2020	30	12	8	338	2112	1466	2230
London	2019	31	11	25	452	2030	1390	2130
Brantford	2020	32	12	0	211	2143	1495	2220
Brantford	2019	30	10	N/A	274	2104	1449	2191
Welland	2020	31	13	0	269	2162	1516	2321
Welland	2019	28	10	5	362	2165	1495	2286
Elora	2020	30	10	1	281	1975	1340	2078
Elora	2019	29	8	2	359	1673	1083	1713
Mount Forest	2020	30	11	9	396	1952	1327	2086
Mount Forest	2019	28	9	N/A	129	1903	1260	2001
Uxbridge	2020	31	11	8	264	2005	1380	2140
Uxbridge	2019	28	11	N/A	227	1898	1249	1971
Peterborough	2020	31	9	14	209	2024	1376	2098
Peterborough	2019	29	6	2	334	1892	1243	1901
Trenton	2020	29	14	12	276	2115	1461	2275
Trenton	2019	29	11	9	354	2092	1420	2188
Kemptville	2020	31	12	36	261	2171	1506	2272
Kemptville	2019	29	8	N/A	203	1999	1345	2037
Earlton	2020	32	10	6	316	1851	1241	1972
Earlton	2019	27	6	1	349	1589	1013	1653
Sudbury	2020	29	13	23	384	1872	1256	2005
Sudbury	2019	27	9	0	351	1610	1040	1689
Thunder Bay	2020	29	9	1	181	1746	1129	1814
Thunder Bay	2019	27	6	4	267	1556	968	1575
Fort Francis	2020	28	7	79	289	1862	1227	1947
Fort Francis	2019	27	6	0	292	1680	1061	1701
This table developed by OMAFRA using data from Agriculture and Agri-Food Canada and Environment Canada. Max and Min Temps show the extremes that occurred for the 7-day period.