Runoff and Erosion Events in the Inland Northwest

Chapter 3 — Residue and Management, No. 15, Fall 1989

Don Wysocki

Soil erosion is a serious problem on many agricultural lands throughout the world. In the Pacific Northwest erosion problems have been recognized probably since cultivation began. Average annual erosion losses from dryland areas in this region now range from 2 to 20 or more tons/acre. Although other areas of the country sustain soil loss rates similar to these, the erosion process in the Northwest is different from areas in the Midwest and East. Here, excessive erosion occurs most frequently during the winter as the result of rainfall and/or snowmelt on frozen soils. Contrastingly in the Midwest, excessive erosion is most often associated with high intensity spring storms, with raindrop impact being a critical factor.

Producers in the Northwest know all too well the typical erosion scenario: rainfall, snowmelt and runoff from frozen soils. Understanding and quantifying this erosion process is useful in selecting management strategies and erosion control techniques. Also it is important to distinguish the erosion process in the Northwest from other regions so that policy makers can tailor farm programs and conservation legislation to Northwest production conditions.

Study of Erosion

USDA-Agricultural Research Service scientists John Zuzel, Ray Allmaras and Richard Greenwalt from the Columbia Plateau Conservation Research Center at Pendleton, OR, obtained soil erosion measurements at four sites in the dryland areas of northeastern Oregon during the 1979-80 winter (Zuzel et al. 1982). This research sought to evaluate the climatic, physiographic, cultural and soils factors responsible for runoff and soil erosion. Four erosion monitoring sites were established in newly seeded winter wheat after summer fallow in Gilliam, Morrow, Sherman and Wasco counties. Fields were conventionally tilled and seeded between September 1 and October 20. A deep furrow drill was used at the Sherman and Gilliam County sites, a single disk opener in Morrow County and a double disk opener in Wasco County. Tillage and seeding at all sites were done on the contour. Table 1 lists soil and site characteristics at the four locations. Aspect in this table is the compass direction that the slope faces. Soil temperature at 0.5 inches, air temperature, relative humidity and precipitation intensity were recorded continuously. Frost and snow depth and snow water equivalent were measured weekly. Runoff and eroded soil were collected in holding tanks to characterize erosion events.

Table 1. Site and soil characteristic at four erosion monitoring locations in eastern Oregon, winter 1979-80 (modified from Zuzel et al. 1982).

During the period January 12-February 25, 14 runoff events were observed at the four sites. Data from these events are presented in Table 2. Note the clear relation between runoff and erosion. Major runoff events produced large amounts of soil loss. What was the underlying cause for runoff? Weather and soil conditions that existed during these events are shown in Table 3. Twelve of the 14 recorded runoff events occurred when soil was frozen. Frozen soils prevented the infiltration of rain or snowmelt. In some cases runoff equaled total water added by either rain or snowmelt. Although these data were collected from a limited period during one winter, they are indicative of the erosion process in the dryland Northwest.

An event recorded during this study in Gilliam County on Jan. 12, 1980, typifies the erosion process in our region. A snow cover (water equivalent of 1.3 inches) had accumulated on a soil frozen to a 3-inch depth. Air temperatures had averaged 23°F for the previous week. A warm moist air mass entered the area about 11 p.m. on Jan. 11. In 1 hour the temperature rose from 25°F to 39″F. Runoff began at 7 a.m. and continued for 12 hours. Runoff was 1.3 inches and soil loss was 3.2 tons/acre. A hidden factor in this event was condensation of water from the air onto the snowpack (warm moist air contacting a cold snowpack is the same as breathing on a cold windshield). Condensation adds a small amount of water, but more importantly it adds heat. As water condenses it gives up heat. Heat given up by condensation on a snowpack accelerates snowmelt. Zuzel determined that approximately one fourth (about 0.3 inches water) of the melt on January 12 was due to heat released from condensation.

Table 2. Summary of runoff and soil erosion events observed at four Oregon sites from Jan. 12 to Feb. 25, 1980 (modified from Zuzel et al. 1982).

Clearly frozen soils play a key role in the erosion process. To fully assess this role, we must consider the frequency and duration of soil frost. Average and observed soil and weather conditions at Pendleton for November through March are shown in Table 4. The calculated long term average shows that soils will be frozen approximately 50 days/winter at Pendleton (elevation 1,500 ft). Also these data show the cyclical nature of soil frost and snow cover. Soils are not frozen for a single 50-day period, but rather for several periods of shorter duration. Repeated cycles of cooling and warming exposes our soils to the potential of several runoff and erosion events each year. In addition freezing and thawing of soil reduces its resistance to erosion.

Summary

Rain and/or snowmelt on frozen soils is the major cause of erosion in the dryland grain producing region of the Pacific Northwest. This has been recognized for many years by producers and scientists in the region. Recent measurements of these types of erosion events show the distinctive nature of the erosion problem in the region. Typically during November through March, frozen soil can be expected for approximately 50 days at Pendleton. More northern locations or those at higher elevations can expect more days of soil frost. Three or four cycles of frozen soil and snow cover can be expected in most years.

Table 3. Combinations of soil and weather conditions present during runoff and erosion events observed from Jan. 12 to Feb. 25, 1980 (modified from Zuzel at al. 1982).

Tillage and farming strategies that minimize the impact of frozen soil events are the key to conservation farming systems in the Northwest. Preventing and reducing runoff from frozen soils will significantly reduce erosion losses.

Table 4. Observed and calculated average weather and soil conditions for November through March (modified from Zuzal, ARS. Pendleton).

Reference

Zuzel, J. F., R. R. Allmaras, and R. Greenwalt. 1982. Runoff and soil erosion on frozen soils in northeastern Oregon. Journal of Soil and Water Conservation. Vol. 37, No. 6, November-December.