A new procedure for simulating minimum and maximum daily leaf water potential has been introduced in the new version of the CALGOS cotton model, and improved methods of computing the water stress factors have been suggested. The effect of water stress on the rate of photosynthesis (the PTSRED factor) is a function of the minimum (midday) leaf water potential, and its effect on plant growth and development (the WSTRS factor) is a function of the sum of minimum (midday) and maximum (predawn) leaf water potentials. The effect on vegetative growth (WSTRST) is computed from WSTRS. Results of some test runs are given, illustrating the leaf water potential and water stress factors simulated for three drip irrigation treatments.