Effects of uniformity and irrigation amount of drip system on distributions of soil water and nitrogen in arid area

Uniformity ratio is an important parameter in the design and operation of microirrigation systems, and a greater ratio might be of benefit to obtain a more uniform distribution of water and nutrients in the soil. However, the initial installation costs of systems with greater uniformity values are relatively high. Recent experimental researches in semi-humid and semi-arid regions indicated that the system uniformity had no significant influence on the distributions of soil water content and nutrients as well as crop growth and yield. However, the results are not necessarily applied to the arid regions where there is considerably less precipitation than in semi-humid and semi-arid regions. The effects of drip system uniformity and irrigation amount on the distributions of water and nitrate in soil were evaluated in arid environments of Xinjiang Uygur Autonomous Region, China, during the growing seasons of cotton in 2010 and 2011 to amend the current design and evaluation standards of drip system uniformity. Three Christiansen uniformity coefficients (Cu) of 0.65, 0.78, and 0.94 and three irrigation levels of 50%, 75%, and 100% of full irrigation were used. The lower Cu values of 0.65 and 0.78 were obtained by assembling the segments of drip tubes with six different nominal discharges (1.1, 1.4, 1.75, 2.1, 2.7, and 3.0 L/h at 0.1 MPa) randomly along the entire lateral, but a mean emitter discharge of 2.1 L/h at 0.1 MPa, which was similar to the mean emitter discharge of the high uniformity treatment (Cu=0.94), was maintained for the assembled laterals. The distributions of the soil water content were measured from seven equally spaced access tubes located along a dripline using a TRIME-FM. Gravimetric samples of soil were collected regularly to determine the temporal and spatial distributions of nitrate in the experimental plots. A high seasonal mean uniformity coefficient of soil water content (0.80-0.97) within 0-60 cm depth was observed during both seasons. The effects of drip system uniformity and irrigation amount and their interaction on the seasonal mean uniformity coefficient of soil water content within 0-60 cm depth were insignificant at a significance level of 0.05. A highly temporal and spatial varied distribution of nitrate in soil was observed. The uniformity coefficient of soil nitrate, which ranged from -0.27 to 0.92, was substantially lower than that of soil water content. Insignificant influences of drip system uniformity and irrigation amount and their interaction on the uniformity coefficient of soil nitrate were observed. As the considerably less precipitation in arid regions cannot compensate for the negative effects of nonuniformly applied water on the distributions of water in soil, the influence of drip system uniformity on the seasonal mean uniformity coefficient of soil water content within 0-60 cm depth was greater than that of uniformity coefficient of initial soil water content and irrigation amount. The results obtained in this study are useful for modifying the standards of drip irrigation uniformity in arid regions.