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PNW CONSERVATION TILLAGE HANDBOOK SERIES
Chapter 3 - Residue Management, No. 6, October-November 1985


Crop Residue Reduces Freeze-Thaw Evaporation Loss

Roger Veseth

Stored soil water is one of the most important crop yield determining factors for dryland crop production in the Pacific Northwest. Since about 'A to % of the annual precipitation comes during the winter months, overwinter soil water storage is greatly affected by fall crop and residue management practices. Frozen soils can also have a major impact on overwinter soil water storage by reducing or stopping water infiltration into the soil. Precipitation or rapid snow melt on frozen soils can result in high water loss by runoff and create severe soil erosion as the surface soil begins to thaw. An additional but less obvious way that overwinter soil water storage can be reduced is through the diurnal (day-night) freeze-thaw cycles during the winter months. This can be particularly important in late-winter when diurnal freeze-thaw cycles are common and daytime temperatures are well above the freezing point, allowing for increased evaporation rates.

Process of Water Loss

This process of evaporative soil water loss through diurnal freeze-thaw cycles, and the effect of crop residue management, is being researched by Joe Pikul, USDAARS soil scientist, and other STEEP researchers at the Columbia Plateau Conservation Research Center near Pendleton, OR.

Pikul explains that when the soil surface freezes, water migrates from deeper soil layers to the freezing front. Because frozen soil holds a large amount of water as ice, thawing during the day often results in a saturated condition at the surface. The glistening effect created by the sun shining on this thawing, saturated surface, something commonly seen on bare, conventionally tilled fields in late winter, is what Pikul terms a "silver thaw. " Evaporative water loss under these conditions is especially high because evaporation rates are similar to those of a free-water surface.

Research on Residue Effects

Pikul and other researchers studied the diurnal freeze thaw cycles and water loss under different residue treatments near Pendleton in March 1982. Two residue treatments were compared in a wheat stubble field on a Walla Walla silt loam soil. One treatment was the original 5 tons per acre of wheat residue as standing stubble with the harvested straw and chaff evenly spread across the field. The other treatment was a bare soil surface where all the residue was removed by burning.

During the 7-day monitoring period between March 19 and 26, nights were clear and cold, and days were warm. No frozen soil was detected during this period under the standing stubble treatment. In contrast, the bare surface treatment soils froze each night to a depth of at least 0.5 inch and thawed again each day. The difference, Pikul points out, is partly that heat radiating out from the soil at night is being reflected back to the soil by the standing stubble, but is lost from the bare soil. By reducing this heat loss at night, soils can remain warm enough so that freezing does not occur when temperatures are at or only slightly below freezing, Stubble also reduces air movement near the soil surface, which reduces heat loss at night as well as evaporation during the day.

During one 24 hour freeze-thaw cycle, the bare soil treatment lost 0.1 inch of soil water from the top 4 inches of soil. Under standing stubble, where the soil had not frozen, only 0.01 inch of soil water evaporated. Pikul emphasizes that the higher water loss from the bare surface treatment should not be considered a consequence only of freeze-thaw induced water movement. The standing stubble creates a unique microclimate that reduces evaporation rates through reduced fluctuations in diurnal soil and air temperatures (particularly cooler daytime temperatures), reduced air movement over the soil surface (windbreak effect) and other factors.

By the end of the monitoring week, the bare surface treatment lost 0.6 inch of soil water compared to 0.4 for the standing stubble treatment. Since each inch of soil water increases the yield potential of spring wheat (for example) about 5 bushels per acre, the 0.2 inch greater loss in soil water corresponds to a potential yield loss of 1 bushel from the following spring wheat crop during this one week period alone.

Repeated periods of diurnal freeze-thaw cycles and soil water evaporation through the winter have the potential of significantly reducing soil water storage and yield of the following crop. Residue management practices, which leave a higher level of residue on the surface overwinter, can effectively reduce diurnal freeze-thaw cycles and evaporative losses of stored soil water. This is one more step toward improving soil water storage efficiency.

     
 

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