Submitted to: Journal of Cotton Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 30, 2006
Publication Date: September 25, 2006
Citation: Thomson, S.J., D.K. Fisher, 2006. Calibration and use of the uga easy evaporation pan for low frequency sprinkler irrigation of cotton in clay soils. Journal of Cotton Science 10(3): 210-223. Available: http://www.cotton.org./journal/2006-10/3/upload/jcs10-210.pdf Interpretive Summary: Irrigation in the United States is still largely practiced without the aid of instruments or methods that have shown clear potential for saving water and pumping costs. This is because many of these tools are perceived to be too costly, too difficult to maintain, or require great interpretive skill to operate properly. A new, easy to use irrigation scheduling method has been presented that uses a simple washtub filled with water, incorporating a float attached to a pointer that can be seen from the road. A back plate against which the pointer travels has two lines; a black line to indicate field capacity or sufficient water; a red line to signal irrigation is needed. This device was originally designed at an experiment station in southern Georgia and tested for sprinkler irrigation of crops grown in sandy soils. For the study presented here, the pan was adapted for sprinkler irrigation of cotton in the Midsouth US, grown in a heavy clay soil. The pan was used along with soil water sensors to infer crop water status; the latter which provided the irrigation trigger level by which to evaluate the pan. Procedures for proper use and setup of the pan are outlined, and experimental results indicated that two simple pan adjustments made during the season were needed to indicate proper time to irrigate. Analysis of data from the 2003 season also showed that altering the timing of irrigation based on pan and sensor recommendations would have eliminated the need for one irrigation, saving a potential 25 - 33% in pumping costs and water applied during the season.
Technical Abstract: A new irrigation scheduling tool that detects evaporation and allows remote observation has been adapted for irrigation of cotton in Midsouth US clay soils. Originally developed at the University of Georgia, the UGA EASY evaporation pan consists of a washtub with a float connected to an adjustable rod that is hinged to allow it to swivel. The rod is connected to a pointer, which indicates crop water status against a back plate that can be seen from the road. The back plate has a black line that indicates field capacity (drained upper limit of water) and a red line that signals to the observer when to irrigate. To change sensitivity, the rod can be adjusted back and forth according to soil/crop combination and period of the season. The pan was used along with Watermark granular matrix soil water sensors, placed at three depths and four stations of a 3-ha field planted with Stoneville 4892BR cotton in Sharkey Clay soil. Watermark sensor readings established a baseline trigger level for irrigation. The pan was calibrated early in the season and was monitored along with sensor readings. No further adjustments were made to the pan during the season. Irrigation was activated based on sensor readings, but the farm manager was given some leeway based on his sequence for irrigating all fields. Recommendations for temporal adjustment of the pan to adjust for increased evapotranspiration were made based on sensor data, analyzed post-season. To account for increased crop water demand, a recommendation was made to adjust the pan 97 days after planting (DAPL) for the 2003 season. This additional adjustment was also suitable to signal irrigation 112 DAPL, coinciding with sensor readings. It was noticed, based on subsequent analysis of sensor readings, that one irrigation could have been delayed and another eliminated. The pan's settings were then applied to sensor and pan data from the previous 2002 season. In contrast with data from the 2003 season, a single adjustment made early season would have been adequate for accurate signaling of irrigation 97 DAPL. Further temporal evaluations could not be made, however, because the crop was not irrigated when sensors called for irrigation. The difference between the two years was attributed to the growth delays seen for the 2002 crop. Procedures are outlined for proper use of the pan in Midsouth clay soils grown in cotton. One pan adjustment at 85% of the time between planting and appearance of first open bolls provided accurate results for the 2003 season.