Effects of depth and irrigation amount of subsurface infiltration irrigation pipes on water use efficiency and yield of jujube

Irrigation water in Yellow River is decreasing due to global warming, the uneven distribution of rainfall, serious surface evaporation and underground leaks. Water-use efficiency has posed a great challenge in the arid and semi-arid areas, particularly on western China, such as Tongxin County, Ningxia. To meet challenge, it is necessary to explore an efficient water-saving method using the depth of underground infiltration irrigation and pan evaporation coefficient. In this study, an experiment was performed on two factors and 12 treatments in three depths of underground infiltration irrigation: D15 (15 cm), D30 (30 cm), D45 (45 cm), and four irrigation volume: W0.6 (Kcp = 0.6), W0.8 (Kcp = 0.8), W1.0 (Kcp = 1.0), W1.2 (Kcp = 1.2), where the crop-pan coefficient (Kcp) was used to evaluate irrigation volume, and the 7-year Tongxin round jujube was selected as the subject. The field experiments were conducted from January 2017 to December 2018 in the Water-saving and Efficient Agricultural Science and Technology Park (36°50'N, 105°60'E) in the dry area of Wangtuan Town, Tongxin County, Ningxia, China. The spatial and temporal changes were investigated by soil moisture, annual growth of root system, agronomic traits, yield and water-use efficiency. Then, a spatial analysis was used to search the areas, where jujube yield and water-use efficiency were better matched, and to obtain the optimal underground level, where the irrigation range was determined by the depth of infiltration irrigation and crop-pan coefficient. The test results showed that the water contents in the soil from 0-100 cm in the vertical direction, and from 0-80 cm in the horizontal was higher than others with the increase of irrigation volume, whereas, the peak of soil water content moved downward with the increase in the depth of underground infiltration irrigation. The buried depth of 15 cm or 30 cm can increase the moisture content in the soil depth of 0-40 cm, while the buried depth of 45 cm can increase that in the soil depth of 40-60 cm. The irrigation volume and the depth of subsurface infiltration irrigation have a great impact on the root system distribution of 20-80 cm soil layer. There were significant differences in the root system increments under various subsurface infiltration irrigation depths. Specifically, the D30W0.6 and D30W0.8 treatments increased the root dry density from 20-60 cm soil layer, and the D45W1.0 and D45W1.2 treatments increased that from the 60-80 cm soil layer, compared with other treatments. Two-year experiments showed that the D30W1.0 treatment was optimum to increase the hanging length of jujube, the number of flowers per hang, the number of fruits per hang, fruit set rate and yield. The highest water-use efficiency of D30W0.8 was 4.68kg/m3 in 2017 and 5.32 kg/m3 in 2018, increased by 17.88% (2017), 16.41 % (2018), but the yield decreased by 9.32% (2017) and 5.94% (2018), compared with D30W1.0 treatment, indicating D30W0.8 treatment was significantly different from other treatments (P<0.05). Therefore, the yield and water use efficiency (WUE) can be used to optimize the irrigation strategies for jujube trees by using multivariate regression and spatial analysis. In this case, the optimal irrigation amount and the depth of subsurface infiltration irrigation can be selected as follows: 370-410 mm and 28-33 cm, respectively. The finding can provide a sound basis and technical support for high efficient management of jujube trees during subsurface irrigation in arid areas of Ningxia, China. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.