Determining cotton water use in a semiarid climate with the GOSSYM cotton simulation model

Simulation models are useful tools that provide information about potential changes to production systems before committing time and resources. GOSSYM, a physiologically based cotton (Gossypium hirsutum L.) simulation model, was developed as an experimental tool but has been modified and adapted as a crop management tool. For GOSSYM to be used successfully in crop management, especially as an irrigation scheduling predictor, its ability to accurately calculate and partition total water use between transpiration (T) and soil water evaporation (E) must be evaluated. A field study with cotton was conducted at Lubbock, TX, in 1994 in an Olton soil (fine, mixed, thermic Aridic Paleustoll) to compare GOSSYM's calculations of E, T,and evapotranspiration (ET = E + T) with values measured by neutron attenuation, microlysimetry, and stem how gauges. During a 12-d period when E and T were measured separately, GOSSYM underestimated cumulative E by 18%, while cumulative T was in close agreement with measured values. Underestimation of E was due to overestimation of leaf area index (LAI), thus reducing simulated incident irradiance at the soil surface. A comparison of calculated and measured values of daily T over 60 d showed that calculated daily T's were 40% greater than measured values during the initial 10 d, 50% less during the last 10 d, and in close agreement in between. Differences between calculated and measured daily T-values were attributed to GOSSYM underestimating potential ET and overestimating LAI. However, ET over 102 d was calculated within 10% at the end of the measuring period. These data suggest that GOSSYM can be used to assess water use in cotton, and as a tool for scheduling irrigations in a semiarid region, provided that current algorithms used to calculate potential ET are modified to include air humidity.