Variation in soil water in Shapotou Area under straight-tube root irrigation

In order to improve water use efficiency of afforestation drive in Shapotou Area, straight-tube root irrigation test was carried out on biennial sand-fixation Haloxy lonammodendron in Shapotou Nature Reserve Area in Zhongwei, Ningxia. The test aimed to study the variation rules of soil moisture in the 0-100 cm soil profile under straight-tube root irrigation. The aqueduct length of straight-tube root irrigation was 40 cm and water seepage micro-porous were distributed at 30-40 cm end section of the aqueduct. Soil water content was measured by the TDR soil moisture recorder at a time interval of 1 h. Based on the recorded data, the variations in soil water under straight-tube root irrigation was analyzed. The results showed that:1) for the period of root irrigation, the Logistic curve well described the variations in soil water content with irrigation time. After the stop of irrigation, however, the variation in soil water content degeneration was best described by power function model. 2) The maximum soil water infiltration rates of different layers were in the ranked sequence of 60 cm > 40 cm > 80 cm > 100 cm > 20 cm. The shortest time needed to reach the maximum infiltration rate was in the 40 cm soil layer (with an average time of 1.22 h) and the longest time was in the 100 cm soil layer (with an average time of 4.57 h). The average maximum infiltration rate of 1 m soil profile was 1.65%·h^-1 and the average time needed to reach the maximum infiltration rate of 1 m soil profile was 2.16 h. 3) Based on the fitted curves, it was suggested that the optimum straight-tube root irrigation cycle of H. lonammodendron forest in Shapotou was approximately 4 d and each single irrigation time was 6-10 h. 4) After stopping irrigating for 2 h, the rate of recession of soil moisture increased with increasing soil depth. Also after stopping irrigation for 48 h, the rate of degeneration of soil water content in all soil layers was almost zero. During the growing period of H. lonammodendron, the rate of degeneration of soil water in the 1 m soil layer reached the maximum of 2.20%·h^-1 during grain-filling period and reached the minimum of 1.31%·h^-1 during aestivation period. 5) The straight-tube root irrigation had the minimal effect on soil water content in the 20 cm soil layer, but the biggest influence on the 60 cm soil layer. During irrigation, the constant value line of soil water content centered on the 60 cm deep line which radially distributed into the surface and deep soil layers. After irrigation, average soil water contents in the 20 cm and 60 cm soil layers were significantly different (P < 0.05) from those of the other layers. The research showed that the law of soil water infiltration in straight-tube root irrigation followed the Logistic curve, the regression rule followed the power function curve, and the straight-tube root irrigation had the minimum impact on soil water in the 20 cm layer and had the maximum impact on soil water in the 60 cm layer. The irrigation cycle of straight-tube root irrigation for H. lonammodendron in Shapotou Area was approximately 4 days and the recommended single irrigation time was 6-10 h.