Effects Of Drip System Uniformity On Distributions Of Water, Nitrogen And Salt In Soil And Cotton Growth Under Arid Conditions

Uniformity is an important parameter in the design and operation of microirrigation systems. A higher level of uniformity leads to 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. Several design and evaluation standards for drip system uniformity have been developed in different countries. However, the redistribution of water and nutrients nonuniformly applied through microirrigation system in the soil and their effects on crop growth and yield have not been adequately and quantitatively considered in establishing the current standards of microirrigation uniformity. 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. Moreover, in arid regions, irrigation is of vital importance for agricultural production. The salinization is increasingly concerned because the irrigation amount was reduced greatly when irrigation method was shifted from border to drip and the region is experiencing a dry climate, small amounts of precipitation and shallow groundwater table. Further research on the dynamics of salts under a wide range of drip system uniformities would be beneficial for reducing the risk of salinity.The effects of drip system uniformity and irrigation amount on the distributions of water and nitrate and salt in soil and cotton growth were evaluated in arid environments of Xinjiang Uygur Autonomous Region, China, during the growing seasons of cotton in2010and2011to amend the current design and evaluation standards of drip system uniformity. Three Christiansen uniformity coefficients (Cu) of0.65,0.78, and0.94and three irrigation levels of50%,75%, and100%of full irrigation were used. The lower Cu values of0.65and0.78were obtained by assembling the segments of drip tubes with six different nominal discharges randomly along the entire lateral. Moreover, a model simulating the transport of water and nitrate in soil was established and solved numerically by using the HYDRUS-2D package. The model was calibrated and validated by the field experiments conducted in Urumqi, Xinjiang Uygur Autonomous Region, China, during the cotton growing seasons of2010and2011and was applied to simulate the distributions of water and nitrogen in soil as affected by drip system uniformity under arid conditions. The main conclusions of this study are as follows: (1) 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 within0-60cm depth in this region was greater than in semi-humid regions. However, a high seasonal mean uniformity coefficient of soil water content (0.80-0.97) within0-60cm depth was observed during both seasons when drip system uniformity varied from0.65to0.94.(2) The soil water content and bulk electrical conductivity (ECb) was monitored continuously during the growing seasons of cotton. The results indicated that drip system uniformity had a substantial influence on the distribution of the soil water content and ECb at60cm depth. A great fluctuation in Christiansen uniformity coefficient (Cu) of soil water content and ECb at60cm depth was observed for the low uniformity treatment during the irrigation season, while a relatively stable variation pattern was observed for the high uniformity treatment. The ECb Cu was substantially lower than the water content Cu and its value was greatly related to the water content Cu and the initial ECb Cu.(3) A highly temporal and spatial varied distribution of nitrate in soil was observed. The uniformity coefficient of soil nitrate, which ranged from-0.27to0.92, 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. A greater irrigation amount produced a significantly lower electrical conductivity of saturated-soil extract (ECe) at the end of the irrigation season, while the influence of the system uniformity on the ECe was insignificant at a significance level of0.1.(4) A lower system uniformity significantly reduced the Cu of plant height, leaf area index, nitrogen uptake, and lint yield. The influence of system uniformity on lint yield was related to the level of irrigation and the favorability of weather conditions for obtaining the potential yield. When weather conditions (e.g., temperature) were favorable for crop growth, the low system uniformity treatment produced a significantly lower lint yield than the medium and high uniformity treatments.(5) The emitter discharge rates that were progressively decreasing from the inlet to the distal end along the dripline were discretized as a series of sequential segments each having an equal discharge rate. Assuming no lateral exchange of water in soil between adjacent segments, we used the model to evaluate the effect of drip system uniformity and soil spatial variability on the distributions of water and nitrate in soil under arid conditions. The results indicated that the simulated uniformity coefficient of soil water content and nitrate were greater than the observed data. The soil spatial variability in the experimental field increased the nonuniform distributions of the soil water and nitrate.(6) In arid regions, the determination of the target drip system uniformity should balance the installation and operation costs, crop production, product quality, and potential salinity risk of soil. A Cu value of around0.80could be used as the target uniformity of drip irrigation systems.