Field study on the benefits of the variable rate irrigation strategies for a center pivot system

Potential production benefits can be expected to easily quantify for the Variable Rate Irrigation (VRI) over the conventional Uniform Rate Irrigation (URI) systems in a given field. An effective VRI management can greatly contribute to the investment in the systems. In this study, a field test was conducted from October 2017 to September 2018 during the growing seasons of winter wheat and summer maize in Zhuozhou (39°27'N and 115°51' E), Hebei Province, China. The mean annual temperature and precipitation were 11.6°C, and 563.3 mm, respectively, particularly with more than 70% probability between July and September. The VRI system was set as a center pivot with three-span 140 m long with an overhang. Variable-rate water was applied along the lateral and travel direction. A control system was also used to control the duty cycle of a solenoid valve ahead of each sprinkler, and the travel speed of the center pivot. One 1.64-hm2 quadrant irrigated by the VRI system was delineated into four management zones (zones 1 to 4) with the available soil water holding capacity (AWC). Among them, two conventional sprinkler URI strategies (U1 and U2 treatments), and two VRI strategies (V1 and V2 treatments) were arranged in the four sub-zones for each management zone. The mean soil water contents were daily measured to determine the irrigation date in the treatments of U1, V1 and V2 in zone 1 with the lowest AWC values. Furthermore, the average soil water content was approximately represented at 0-0.4 m depths. Particularly, the mean soil water contents were measured for the four management zones in the U2 treatment. The irrigation was triggered, when the measured soil water content dropped close to 66% and 70% of the field capacity for the winter wheat and summer maize, respectively. The same application was adopted for the two URI treatments. Specifically, the 20 and 30 mm depths were set before and after the booting stage for the winter wheat, respectively, whereas, the 20 mm depths were for the summer maize. The application depth was equal to the consumption of soil water calculated from the upper limit value and the measured soil water content for the V1 treatments. In the V2 treatments of winter wheat, 84%, 99%, and 68% of the rate in zone 1 were recommended for zones 2, 3, and 4, respectively, where the deliver depth for zone 1 was the same as the URI treatments. The V2 treatments of summer maize were also combined with the V1 treatments and the rain forecast. The applied water decreased by 0%, 20%, and 40% for the light (less than 10 mm), moderate (between 10 and 25 mm), and heavy rain (more than 25 mm), respectively, according to the forecast report from the National Meteorological Center of China Meteorological Administration. There were some positive effects of VRI strategies on water-saving, yield, and WUE, spatial distribution of plant height, and Leaf Area Index (LAI). Results showed that the VRI strategy was an optimal way of irrigation management, according to the upper limit value and the meteorological forecast rainfall level. Compared with the URI, the irrigation amount of the optimal VRI strategy was reduced by 31%, and 40%, while the WUE values increased by 6%, and 27% for the winter wheat and summer maize, respectively. There was no significant difference between the VRI and URI strategies in the yield for the winter wheat and summer maize. Besides, no significant effect was found on the spatial distribution of plant height, LAI, yield, and WUE. Therefore, the irrigation prescription maps can be expected to serve as the VRI management, according to the soil water content and meteorological forecast about rainfall, particularly more suitable for both the semi-arid and semi-humid climates. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.