Skip to main content
ARS Home » Research » Publications at this Location » Publication #109313
Title: CROP COEFFICIENTS FOR IRRIGATION OF COTTON

Author
item Detar, William
item Maas, Stephan - Steve
item Fitzgerald, Glenn

Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/7/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Water is becoming scarce and expensive, and it is economically and environmentally important to apply the proper amount when irrigating. Crop coefficients (Kc) are used to calculate the daily water needs of plants. A new procedure is presented which makes it easy to get these crop coefficients. It uses soil moisture measurements made in an irrigated field, along with either (1) evaporation measurements from an evaporation pan, or (2) standard Penman potential evapotranspiration (ET) measurements. Four seasons of data were taken using subsurface drip irrigation of cotton on a sandy soil. The crop coefficients were related to growing-degree-days above 60 degrees F. Peak values for the crop coefficients were found to be reasonably close to those found by other researchers. For use with the pan evaporation, the peak was found to be Kc = 0.88; with the Penman ET, the peak was Kc = 1.14. A soil stress factor was also found, which reduces the crop coefficient linearly, when the extractable soil moisture drops below 53%.

Technical Abstract: This report is about a new procedure for getting crop coefficients, which are used with reference evapotranspirations to get actual plant water use in irrigated fields. Water was applied daily to cotton on a 2-acre field of sandy soil, using a subsurface drip irrigation system. Scheduling was controlled by manually-adjusted time clocks. Weekly measurements of soil moisture were made with a neutron probe to a depth of 5 ft. Dripperlines were located under every plant row. Weekly changes in the average soil moisture were plotted versus the relative water application (water applied divided by the reference ET). From a previous experiment it was found that crop coefficients leveled off in the range of 1000 to 1500 heat units. Moisture change data for this range was combined for two years, 1998 and 1999, and regressed against the relative water application. The regression equation had r^2 = 76%, with a slope of 0.777. In the plot of weekly water change vs relative water application for the entire season, a line was drawn through each data point with a slope of 0.777 and projected toward the horizontal line representing zero change in soil moisture. The x-value for the intersection was the crop coefficient for that week. This slope- projection procedure is very simple, and furnished coefficients very similar to those obtained in the same field in 1996 and 1997 by a much more complicated method called a covariance procedure.