Applicability of a User-Oriented Evaporation Pan for Low frequency Irrigation of Cotton

Authors: Thomson, Steven; Williford, Julius; Fisher, Daniel

Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/11/2007
Publication Date: 1/11/2007
Citation: Thomson, S.J., Williford, J.R., Fisher, D.K. 2007. Applicability of a User-Oriented Evaporation Pan for Low frequency Irrigation of Cotton. In Proceedings of the Beltwide Cotton Production Research Conference, National Cotton Council, Memphis, TN. CDROM.

Interpretive Summary: On-farm irrigation scheduling aids have the potential of aiding the farm manager in water conservation by more timely application only when the plant truly requires it. Irrigation scheduling aids are not widely used, however, 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 scheduling aid, originally developed at the University of Georgia, 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 has been successfully tested 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: The evaporation pan is one field device used to support irrigation scheduling. This device integrates environmental effects and measures evaporation to give an indication of crop water use. However, conventional evaporation pans are sometimes difficult to set up, interpret, and access for field readings. The UGA EASY Pan (Evaporation-based Accumulator for Sprinkler-enhanced Yield), developed at the University of Georgia, allows remote observation of evaporation by a float connected to a pointer that can be seen from the road. The pan has thus far been used successfully for high frequency irrigation of crops grown in soils with relatively low water holding capacities. As a stringent test of this method, the pan was evaluated in Mississippi for low frequency irrigation of cotton in clay soils. To establish a baseline trigger level for irrigation, granular matrix soil water sensors (GMS) were placed at three depths and four stations in a 3-ha field planted in a Sharkey series clay soil. Stoneville BXN 47 was planted for the 2002 and 2003 seasons and Stoneville 4892 BR was planted for 2004. Pan usage and temporal adjustment criteria were developed during the 2003 season and further evaluated using data from 2002 and 2004. Irrigation was initiated based on field manager recommendations without assistance from scheduling aids so that pan readings could be associated with the field manager’s irrigation decisions. Recommendations were made for temporal adjustment of the pan taking into account increased evapotranspiration based on sensor data analyzed post-season. To account for increased temporal water demand, a recommendation was made for a second adjustment approximately 97 days after planting (DAP) or 28 days after white bloom. This additional adjustment was also verified to be suitable to signal irrigation 112 DAP (43 days after white bloom) for one season, coinciding with a trigger signal from sensor readings. Based on subsequent analysis of sensor readings as a baseline reference, one irrigation could have been delayed and another eliminated for 2003 (a 25% water savings) and the first irrigation could have been eliminated for 2004 (a 33% water savings) if pan recommendations had been followed to schedule irrigation.