Agronomy Library > Soil Conservation

Grain Yield Calculations based on Stored Soil Water and Growing Season Precipitation
Author: Dr. Paul Brown, modified by Don Wentz
Date Created: November 30, 1990
Last Reviewed: February 16, 2007

Data for north central Montana was used to calculate yield equations for high and low yielding cultivars on winter wheat, barley, oats, and safflower.

                                    High                                                    Low

Winter Wheat      Y = 5.8 (ET – 3.9*)                           Y =  4.0 (ET – 3.8*)

Spring Wheat      Y = 5.1 (ET – 3.8*)                            Y =  3.4 (ET – 4.1*)

Barley                  Y = 7.6 (ET – 3.1*)                           Y =  5.9 (ET – 4.5*)

Oats                     Y = 10.3 (ET – 4.8*)                         Y =  8.9 (ET – 6.0*)

Safflower             Y = 164 (ET – 7.3*)                          Y =  103 (ET – 7.8*)

* Initial Yield Point

Example :
You have a medium textured loam soil and can push the Dr. Brown Moisture Probe into the soil an average of 60 cm (2 feet).
Based on the PAWC chart, you have about 100 mm (4 inches) of stored soil water. (Check the chart on Plant Available Water Capacity for Soil Texture Classes)
The area you live in has an average growing season precipitation of 200 mm (8 inches) of rain.
You therefore have a total of 300 mm (12 inches) of available moisture for crop growth. ( 100 mm of stored soil water and 200 mm of rainfall)
Using the spring wheat formula with the low range: Y = 3.4 ( ET – 4.1) = 3.4 (12 – 4.1) = 3.4 (7.9) = 26.86 bushels per acre.
Using the spring wheat formula with the high range: Y = 5.1 (ET - 4.1) =  5.1 (12 - 4.1) = 5.1 (7.9) = 40.29 bushels per acre.
These tables provide valuable information to assist farmers in making cropping and fertilizer decisions and are especially valuable for the Flexible cropping system. This data is based on yield estimates for Montana where there are climatic, variety and precipitation differences. These charts are provided for guides only and may not reflect true yields for your area. 

1. Brown, Paul L., and G.R.Carlson, Grain Yields Related to Stored Soil Water and Growing Season Rainfall.  Montana State University, Agricultural Special Report 35, November 1990