A Crop-Coefficient Model for Scheduling Irrigation in Cotton

This research project was conducted by Martina Weiss, a M.S. student at the Technische Universität München in Germany during her internship at the Univeristy of Georgia between July and December, 2012.

The goal of the project was to develop crop coefficient-based irrigation scheduling protocols for cotton grown in the southeastern United States.  The project was conducted at the University of Georgia’s Stripling Irrigation Research Park (SIRP) located near Camilla in Mitchell County.
The model uses a simplified water balance approach to track plant-available soil water during the growing season.  The water balance is calculated like a checkbook, inputs like precipitation and irrigation are credited to plant-available soil water and withdrawals like evapotranspiration are debited from plant-available soil water.  The following parameters are used to calculate plant-available soil water:

Soil water holding capacity
Estimated rooting depth
Measured precipitation and irrigation
Crop evapotranspiration (ETc) estimated from reference ET with a crop coefficient
Minimum allowable depletion of plant-available soil water
Irrigation system characteristics (where applicable)

The crop coefficient (Kc) is widely used to estimate crop water use and to schedule irrigation.  Crop evapotranspiration (ETc) can be estimated from reference ETo and the crop coefficient Kc using the following equation:

Performance of the GDD crop coefficient model

Performance of the GDD crop coefficient model showing plant available soil water, irrigation events, precipitation events and soil water tension at 8, 16, and 24 inches.

Kc and consequently ETc varies by crop phenological stage.  During development of the model, we decide to take two different approaches to triggering Kc changes.  The first required the user to observe the crop and determine when a phenological stage was reached and enter this into the model thus triggering a Kc change.  The second used growing degree days (GDD), also referred to as heat units, to trigger changes in Kc.  GDD are a heuristic tool in phenology used to measure heat accumulation and can be used to predict plant phenology (growth stages) such as the date of flowering or the date on which a crop reaches maturity.

We completed the crop coefficient models in mid-July and used them from July 20 to September 28.  During this period, we irrigated 4 times – twice in late July and twice in September.  For a period of about 3 weeks in late August through mid-September, the model indicated that plant available soil water had been exhausted.  However, the soil moisture sensors we used indicated that soil moisture was still available.  These observations led us to believe that the crop coefficient values we used during this period were perhaps too large.  The crop coefficient values used by the model need to be revaluated prior to the 2013 growing season.

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