A general system to measure and calculate daily crop water use

There is a need for an accurate method to calculate and to measure crop wat er use on real-time. We implemented a system that combines knowledge of cro p water use and available technology to control the timely application of w ater, Our objective was to test the system and compare it to the empirical engineering approach that uses a crop coefficient to relate crop water use to a reference evapotranspiration. Technologies involved are the measuremen t of plant water use with stem now gauges, of soil water with time domain r eflectometry, and weather variables. Measurements are coupled with calculat ed values of crop water use obtained with the model ENWATBAL. A single comp uter controls all functions, for example, measurements, model execution, ac tivation of water delivery system. The system was tested for a 2-yr period with cotton (Gossypium hirsutum L.) in Lubbock, TX, using surface drip irri gation, Field experiments were conducted on an Olton clay loam (fine, mixed , superactive, thermic Aridic Paleustolls), Comparison of measured and calc ulated values of crop transpiration and soil water evaporation were in clos e agreement. Simulated results indicated that for a 3-d frequency irrigatio n with small quantities of water the engineering approach lacks the resolut ion to accurately calculate daily requirements of cotton under the semiarid conditions of the Texas High Plains (THP). This is particularly true early in the growing season when predominant evaporative losses are from the soi l and not from the crop. We conclude that the proposed system is general an d can be applied to schedule irrigations based on accurate estimates of wat er loss.