Planting cotton in wheat residue (WR) is a common practice in the Southern High Plains of TX. Advantages are that the WR provides a barrier to protect cotton against blowing sand, increased infiltration of rain, reduced soil evaporation, and greater infection of root-mycorrihizae; disadvantages are soil warming in the Spring is delayed, weed control is more difficult, and competition for water increases. Information about the effects of the WR on the water balance of a cotton crop is not known. The objective of this study was to test the hypothesis that the total water use of a cotton crop grown in a WR and in a conventional way, for the same environmental conditions, is the same. The corollary of our hypothesis is that the WR modifies the partitioning of total water use by decreasing soil evaporation (E) and increasing crop transpiration (T). To test our hypothesis we conducted field experiments in Lubbock, TX. Cotton, Paymaster HS-26, was planted 20 May 1991 in 1.0 in rows in a WR and in a conventional 100-m x 250-m plots. In each plot, over the growing season, we measured profiles of soil temperature with thermocouples and water content by neutron attenuation. Soil evaporation was measured with microlysimeters for a period of 7 days after an irrigation. In addition, leaf area, crop height, plant mapping was measured weekly over the growing season. A weather station was used to record hourly values of air and dewpoint temperature, wind speed, and short-wave irradiance. Simulated daily values of E and T were calculated with the ENWATBAL model, a mechanistic model that integrates theories of E, T, infiltration, and root-water uptake. The model was modified to simulate the effects of a WR on the water and energy balance of a cotton crop by using an additional resistance to the sensible and latent heat fluxes developed from an earlier study. The model was executed for 100 days and simulated values of daily E and cumulative E+T were compared to measured values. The cumulative E+T was 327 mm in the conventional cotton and 321 mm in the WR. Cumulative E was 50% of cumulative E+T in the conventional cotton and 31 % of cumulative E+T in the WR. Our results showed that the model correctly calculated E and E+T for both the conventional the WR supporting our hypothesis and corollary.