Evaluation method of soil water distribution uniformity under conditions of field subsurface drip irrigation

Uniformity is one of the important indicators for the evaluation of irrigation quality, and soil water distribution uniformity is the ultimate expression of irrigation uniformity. Selection of suitable sampling numbers of soil water is the base for economical and accurate evaluation of soil water distribution uniformity after irrigation. Field measurement of soil moisture distribution was measured in the cotton field under subsurface drip irrigation in Xinjiang Uygur Autonomous Region, 2 days after irrigation when the soil moisture had reached a relative stability. Relationship between sampling numbers and soil moisture distribution characteristics, and soil water distribution uniformity were analyzed. Results showed that the coefficient Cu of soil water distribution was close to its mean value with increasing of samples. From the quantitative analysis of samples, the mean of Cu was not obvious change as sample number increase from 15 to 60. Under the condition of experiment, the desirable number of samples should be more than 24 for field soil moisture monitoring and evaluation. There was a significant relevance between Cu and the distribution uniformity (Du). But a certain deviation occured between the coefficients derived from the measured data and that recommended by the United States Department of Agriculture. And this should be taken into account when such exchange formula was used. Using uniform 25-point sampling method, soil water distribution of subsurface drip irrigation in cotton fields was evaluated. Results showed that soil moisture uniformity in the vicinity of drip line (20-40 cm) was lower than that in the upper and below layer of drip line before irrigation, and would increase after irrigation, compared with the upper and lower layers. Soil moisture variation near the drip line is the largest in the soil under subsurface drip irrigation.