A comparison of drip and furrow irrigated cotton on a cracking clay soil

Summary

The potential of drip irrigation to increase irrigation water use efficiency, to decrease waterlogging caused by irrigation, and to conserve soil structure, especially when combined with reduced tillage was studied in an experiment comparing surface (SD) and buried (BD) methods of drip irrigation with furrow irrigation (F) of cotton in a cracking grey clay (vertisol) over four seasons. Drip-irrigated treatments were maintained at a nominal deficit of 45 mm below the fully-irrigated soil water content, while F was irrigated when the deficit reached about 90 mm. Water use efficiency (WUE) was calculated as the ratio between lint yield and total water received by the crop. The WUE of cotton was 16% higher under drip irrigation (2.23 kg ha⁻¹ mm⁻¹) than under F (1.89 kg ha⁻¹ mm⁻¹) when supply channel losses and runoff losses in F were considered. However, similar WUE was obtained for the net water stored in the field using either method of irrigation. The ability of drip irrigation to improve WUE by avoiding deep percolation losses of irrigation water was not an advantage in this soil because furrow irrigation water does not infiltrate beyond the root zone of cotton. Waterlogging occurred in all treatments in the wet season. In the absence of heavy rainfall, waterlogging was not an important factor in this study because drip irrigation avoided waterlogging and good field slope kept the ridges well-drained during furrow irrigation. However, in waterlogging-prone fields or wet seasons, less root activity and low yields are likely to further reduce the WUE. The crop achieved its final root distribution through the soil during early boll filling, with most of the root system in the 0–0.3 m depth of soil in all treatments, but more so under BD. There were more roots and more deep roots under F than under drip. The distribution of roots and root activity followed the distribution of water in the soil. Early-season water extraction profiles were similar for each treatment, but by mid-season there was more water extracted from below the 0.6 m depth in F (16–19% of total) than in BD (8–12%). The clod specific volume of the surface 0–0.1 m of soil was higher in BD (0.718m³ Mg⁻¹) than in F (0.693 m³ Mg⁻¹), and was related to less cultivation and slower wetting in BD than in F, which help to conserve soil aggregates and pores and maintain soil structure. We conclude that the WUE of furrow irrigation in this soil is relatively high and that similar efficiency to drip irrigation can be achieved by improving furrow irrigation management: by reducing transmission losses between pump and field, by reducing runoff losses from the field, by recirculating runoff water, and by reducing waterlogging.