Determination of crop water use and coefficient in drip-irrigated cotton fields in arid regions

A 3-year field experiment was conducted in Aksu region in Northwest China to determine crop water use, suitable irrigation mode and actual crop coefficient (K-c) in drip-irrigated cotton fields. In the experiment, 12 non-weighing large lysimeters were used to measure cotton water use under various irrigation modes. Then a micro-lysimeter was placed in each large lysimeter to monitor soil evaporation (E). Adjustments were done for a number of factors (including irrigation, meteorology, plastic mulching, water stress and salinity stress) in order to compare FAO-adjusted K-c(Kc-FAO) with the local actual K-c (Kc-loc). The results showed that: 1) plastic mulching with 0.6 cover ratio and 37.5 mm drip irrigation quota gave a crop water requirement of 536.4 mm; which was suitable for the study area. Also ic.105 were respectively 0.28, 0.97 and 0.31 for early, middle and late growth stages. 2) Kc-FAO generally reflected the trend in Kc-loc for the entire growing season. The suggested 0.1 adjusted coefficient under plastic mulching for the initial growth stage was relatively low, and was most rational for the study area when increased to 0.25. 3) E/ET ratio was low (19.1-24.5%) under mulching treatment and relatively high (27.7-32.7%) under non-mulching treatment. 4) While change in E/ETo ratio was linear or logarithmic, that in T/ETo ratio was a power function of the canopy cover. Also the two rates responded to the interaction effects of canopy cover and soil water content. The results could lay the basis for developing scientific, data-driven water-saving strategies in the study area and elsewhere.