Head deviation property and its relationship with soil moisture uniformity of subsurface drip irrigation laterals

Abstract: Head deviation ratio is a key technical indicator for length optimization in drip irrigation laterals. The deviation ratio in sub-surface drip irrigation is different from that in surface drip irrigation because the laterals are embedded and compressed in soil, and influenced by the submerged discharging. This study aimed to investigate head deviation property of subsurface drip irrigation laterals and its relationship with soil moisture uniformity. This experiment including 3 large pipe networks of subsurface drip irrigation in cotton field was carried out in Xinjiang. The drip irrigation pipe was buried in 28-33 cm below the soil surface. The length was 100 m. The spacing of pipes was 0.5 m in 2002, and 1 m after tye year of 2004. A total of 3 pairs of laterals were chosen out of upstream, medium-stream and downstream of the branch pipe. For each pipe network, more than 6 laterals were selected for the experiment. Among the laterals, some were ring network and some were tree network. The precision pressure gauge was used to measure the water pressure in the head and tail of laterals, and water meter to measure water volume at the beginning of laterals. During the experiment, the designed working pressure was 10 m. The soil moisture at depths of 30 cm along the laterals was determined by a soil moisture measuring instrument. The head deviation ratio and soil moisture uniformity was calculated based on soil moisture. The relationship between the laterals' head deviation ratio and soil moisture uniformity coefficient in 30 cm depths along the laterals was analyzed. The results showed that the head deviation ratio of the laterals were from 0.58% to 12.80%, meeting the requirement of Chinese microirrigation standard (smaller than or equal to 20%). Overall, the head deviation ratio in each lateral were different in a same pipe network with the value fluctuated in a certain range, whereas the changing trend was stable between tree-like laterals and ring-like laterals in a same branch pipe. The analysis on outliers of measured head deviation ratio showed that the deviation ratio may be affected by thickness of lateral wall. The lateral with thick wall had more stable hydraulic performance than that of the thin wall. During the entrance pressure of 7.32-11.75 m, the entrance pressure had no significant influence on head deviation ratio (P>0.05). The head deviation ratio was extremely negatively correlated with soil moisture uniformity coefficient (P<0.001). According to theoretical analysis, a model was established with soil moisture uniformity coefficient as response variable and head deviation ratio as independent variable. By comparing the calculated values and model-established values of soil moisture uniformity coefficient, we found the model was reliable in predicting oil moisture uniformity coefficient with the absolute error from -6.0% to 5.3% and 85% of laterals had the absolute error smaller than 5%. This study can provide valuable information for the length optimization of laterals, evaluation of working conditions, and management of operation in subsurface drip irrigation system.