Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/4/2002
Publication Date: 1/8/2002
Citation: THOMSON, S.J., FISHER, D.K., SASSENRATH COLE, G.F., FREELAND, T.B., PRINGLE, L. USE OF GRANULAR-MATRIX SENSORS, MODELS, AND EVAPORATION MEASURING DEVICES FOR MONITORING COTTON WATER USE AND SOIL WATER STATUS IN THE MISSISSIPPI DELTA. NATIONAL COTTON COUNCIL BELTWIDE COTTON CONFERENCE. 2002. Paper No. 991137. Interpretive Summary: Irrigation of cotton in the Mississippi Delta is usually practiced by "feel," without the aid of soil water measuring devices, evaporation instruments, or models that estimate actual crop water use. Using one or more of these tools could prevent over-irrigation with corresponding increases in pumping costs or under-irrigation with a corresponding reduction in yield and cotton quality. Farm managers are reluctant to use irrigation aids because of labor required to read sensors or instruments, the need to properly interpret readings, and lack of confidence in models. Although many methods for irrigation scheduling have been reported on over the years, widespread adoption has not occurred in the Mississippi Delta. An approach was undertaken to evaluate potential methods for irrigation scheduling, keeping in mind practical considerations for their use on the farm. To accomplish this, a preliminary study compared several methods for determining crop water use. An evaporative device called an atmometer showed promise as a convenient tool for practical irrigation scheduling, and data compared well with models of cotton water use. A washtub evaporation pan could be used to approximate cotton water use if appropriate factors are applied for the cotton's growth stage and simple methods are adopted to read the device. For soil water sensors to be adopted, wireless transmission of signals and guidelines to help the farmer make irrigation decisions based on their readings may be required. Models of cotton water use may be the most feasible of all potential scheduling methods in the Mississippi Delta if proper adjustments are applied as a function of growth stage.
Technical Abstract: A study was conducted to obtain preliminary data for irrigation management of cotton in the Mississippi Delta. Data from soil moisture sensors, an atmometer, washtub evaporation pan, Class A evaporation pan, and several models were compared for their ability to determine crop water use and derive soil water status for cotton. Soil water potential from Watermark sensors was converted to soil water content by a published curve of the soil represented by each sensor. The resulting value was compared with soil water status derived in a 30-inch soil zone using the FA056 crop water use model. The atmometer showed potential as a tool for estimating ET over periods longer than one day, but it lacked sensitivity for manual, daily readings. The washtub evaporation pan gave acceptable readings and could be used to estimate ET with proper calibration. Correlation between data from the atmometer and washtub was acceptable only ywhen data for periods greater than one day were included. Data from the washtub correlated well with data from the Class A pan (r=0.75), having a slope close to one. The washtub will need to be convenient and easy-to- read for the farmer if it is to be adopted for irrigation scheduling. Model-based representations of ET indicated wide variability using data from the Stoneville weather station. However, if a suitable model is evaluated and proper temporal adjustments are applied, model based ET estimation might be the most feasible of all methods since detailed weather data are readily available in the Delta. For widespread adoption of sensor-based irrigation scheduling methods, wireless transmission of signals and decision support tools may be required.