Analysisi on cutoff time optimization of border irrigation to improve irrigated water quality

Abstract: Irrigation cutoff time is one of the important factors of irrigation management, and the determination of optimum cutoff time can effectively improve the irrigation performances. But for different irrigation conditions, the cutoff time of irrigation is very different, and it is difficult to directly determine the cutoff time of any given irrigation event. So, the cutoff control index of basin irrigation is used to study the cutoff control modes of basin irrigation. When the irrigation water is closed before the water advance to the end of basin, cutoff control index is equal to the ratio between the water advance distance when the irrigation water is closed and the basin length. On the contrary, R is equal to the ratio between the cutoff time and the time that water advance to the end. The optimization method of optimum cutoff time for basin irrigation was presented, and the minimum infiltration depth larger than zero and storage efficiency larger than 80% were set as the objective function to optimize cutoff time. One-dimensional hydrodynamic model of basin irrigation based on the hybrid numerical method was used to simulate the basin irrigation process. Combining the irrigation model with optimization method of cutoff time, the irrigation processes of 106176 irrigation events with different soil type, basin length, basin slope, land leveling precision and inflow rate were simulated, and for the same land leveling precision, the spatial variability of surface elevation was considered. For land leveling precision of 2, 3, 4, 5 and 6 cm, 4, 12, 15, 20 and 28 groups micro-topography data were respectively generated. The corresponding optimum cutoff time for each irrigation event was obtained, the influence of the irrigation factors on the optimum cutoff time was discussed, and the irrigation cutoff control modes were proposed. The irrigation data of typical fields were used to validate the rationality of the proposed cutoff control modes by comparing the irrigation performances under different cutoff time. Results showed that the optimum cutoff time of irrigation event was up to different irrigation factors. Cutoff control index changed with the basin length, basin slope, inflow rate and land leveling precision. Among these irrigation factors, the influence of slope and inflow rate on cutoff control index was the most obvious. Cutoff control index decreased with the increase of inflow rate. For the basins with the length of lower than 70 m, cutoff control index increased with the increase of slope, and for the basins with other lengths, cutoff control index conversely decreased with the increase of slope. Generally, cutoff control index decreased with the increase of basin length, but the influence of basin length on cutoff control index was obvious for the basins with >100 m length, when basin length was not more than 100 m, the influence weakened. The influence of land leveling precision was related with other irrigation factors, cutoff control index decreased with the increase of land leveling precision for <70 m basin, and increased with the increase of land leveling precision for other basins. By comparing the irrigation performances of typical basins under actual irrigation cutoff time and proposed cutoff control index, the rationality of proposed cutoff control index of basin irrigation was validated. For different basin length, slope, land leveling precision and inflow rate, the values of irrigation cutoff control index were presented. For the basins with the length of less than 70 m, the range of cutoff control index was 0.9-1.15. For most conditions cutoff control index should not be less than 1, but when basin slope was middle and leveling precision was not good, cutoff control index was about 0.9; and if slope was greater than 3‰ and land leveling precision was good, cutoff control index should not be less than 1.1. For the basins with the length of greater than 70 m and basin slope was less than 1‰, the range of cutoff control index was 0.8-1; but when basin slope was greater than 1‰, the range of cutoff control index was 0.75-0.95. The research results can supply reference for the cutoff control of basin irrigations, and the cutoff control index can be chosen according to the practical irrigation events.