Objective The aims were to investigate the effects of brackish water irrigated flat and sloping land on soil water and salt distribution characteristics and spatial variability in gravel-sand mulched field, in order to provide a theoretical basis for the sustainable utilization of brackish water in the dry zone of Northwest China.

Methods Taking Red Circle Village, Xiangshan Township, Zhongwei, Ningxia as the investigation area, the watermelon gravel-sand mulched field planted with brackish water drip irrigation was selected, soil samples were collected from 0 ~ 40 cm depth on the flat and sloping land by using the grid sampling method in the field after the end of the 8-month drip irrigation. The soil water content and conductivity were determined, and the spatial variability of the water content and conductivity of the soil were analyzed on the sand-suppressed flat and sloping land by using the methods of classical and geostatistics.

Results The soil water content in 20 ~ 30 cm layer of flat land had a block-base ratio of 39% greater than 25%. The block-base ratio of the rest of the soil layers was less than 25%, indicating that the soil water content of the pressurized sand had a strong spatial correlation. The water contents in the soil layer of 0 ~ 40 cm in the slope land were 11.2%, 14.33%, 17.95%, and 19.02%, respectively, with an increase with the depth of soil layer. The optimal model of the semi-variance function of soil water content in slope land was the spherical or Gaussian model, while the rests were the exponential model except for the 10 ~ 20 cm soil layer. The water contents of soil on slope land were 11.2%, 14.33%, 17.95% and 19.02% in the 0 ~ 40 cm layer, respectively, increasing with the depth of the soil layer. The theoretical optimal model for the semi-variance function of soil water content on slope land were the spherical model or Gaussian model, whereas on the flat land, except for the spherical model in the 10 ~ 20cm layer, the rest of the models were the exponential models. The soil conductivity was enhance with the increase of soil depth in both slope and flat lands, and it was smaller in 0 ~ 30 cm soil layer in slope land than that in flat land. While it was higher in 30 ~ 40 cm soil layer in slope land than that in flat land, and the coefficient of variation in slope land was also larger than that in flat land. The theoretical models of semi-variance function of the soil conductivity in slope land were gaussian and globular model, while the rest of the flat land were all spherical model except for the exponential model in 20 ~ 30 cm soil layer. From the contour plots of soil water content and conductivity, soil water content was low while conductivity was high at the top of the slope. However, soil water content was high and conductivity was low at the bottom of the slope.

Conclusion  The topography of brackish water-irrigated in gravel-sand mulched field alters the soil water and salt distribution patterns.