James A. Staricka, Soil Scientist, WREC

The accompanying graph shows the soil water content under summer fallow, spring wheat, and safflower at approximately two week intervals during the 2000 growing season. For each cropping treatment, measurements were taken from three plots and averaged. All the cropping treatments being compared have been alternating with spring wheat since 1995. On the graph, each bar is divided into four sections which represent the water content in each of the 1-ft depth increments down to 4 feet. Alternating sections have been shaded to improve the clarity of the graph. The amounts of rain received between measurement dates are listed below the graph.

At the start of the growing season, soil water content varied somewhat in the three treatments . The soil water content was 8.0 in./4-ft in the plots to be fallowed, 7.4 in./4-ft in the plots to be planted to wheat, and 6.6 in./4-ft in the plots to be planted to safflower. This variation occurred even though all the treatment were planted to wheat in 1999 and appeared to be a carryover effect from the 1998 growing season, when the plots had the same cropping treatment as this year.

Also at the start of the growing season, the 0 to 1 ft depth zone was the wettest, the 3 to 4 ft zone was intermediate in wetness, and the 1 to 2 ft and the 2 to 3 ft zones were the driest. This was the result of the 0 to 1 ft depth being partially replenished during the 1999-2000 winter, and the 3 to 4 ft depth being only partially depleted during the 1999 growing season.

The soil in all treatments gained water by the second date (June 7), with the gain occurring solely in the top foot of the soil. During June, the soil in the wheat and safflower plots began to lose water, with the change again occurring solely in the top foot. On June 28, the soil was drier in the wheat plots than in the safflower plots. This occurs nearly every year from mid-June to mid-July when water use by wheat exceeds that of safflower.

Equipment failure delayed the next soil water measurement until July 25. By this time, the soil in all treatments was drier, with the wheat and safflower plots drying to within 1 inch of the wilting point. (For this soil, the wilting point is reached when only about 4 inches of water remains in the 4-ft soil profile). The soil in all treatments dried slightly more by August 8.

By August 23, the soil water content in the fallow and wheat plots had increased slightly, but had remained steady in the safflower plots during this period. This indicates a greater water use by safflower during this time. On September 6, the soil was wetter in all plots, a result of decreased water use and rain occurring during the interval. But by the end of September, all the plots had lost some water.

During the entire growing season, changes in soil water content occurred only in the upper 2 feet of soil in the fallow and spring wheat plots. There was some depletion of water as deep as the 3-4 ft depth in the safflower plots.

From September 27 until December 6, 3.76 inches of precipitation occurred. Of this, 2.23 inches was rain that occurred before snow cover was established. This late season precipitation should have helped replenished the soil water reserves for the 2001 growing season.