DETERMINING COTTON WATER-USE IN A SEMIARID CLIMATE WITH THE GOSSYM COTTON SIMULATION-MODEL

Simulation models are useful tools that provide information about pote ntial changes to production systems before committing time and resourc es. GOSSYM, a physiologically based cotton (Gossypium hirsutum L.) sim ulation model, was developed as an experimental tool but has been modi fied and adapted as a crop management tool. For GOSSYM to be used succ essfully in crop management, especially as an irrigation scheduling pr edictor, 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 comp are 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 inci dent irradiance at the soil surface. A comparison of calculated and me asured values of daily T over 60 d showed that calculated daily T's we re 40% greater than measured values during the initial 10 d, 50% less during the last 10 d, and in close agreement in between. Differences b etween calculated and measured daily T-values were attributed to GOSSY M underestimating potential ET and overestimating LAI. However, ET ove r 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 cott on, and as a tool for scheduling irrigations in a semiarid region, pro vided that current algorithms used to calculate potential ET are modif ied to include air humidity.