Decision-making on water-nitrogen irrigation for Isatis indigotica in oasis based on the coupling of fuzzy evaluation model and particle swarm optimization algorithm.

To quantitatively assess the responses of the growth, yield, quality and resource utilization of Isatis indigotica to water-nitrogen regulation and determine the optimal water-nitrogen management plan for Isatis indigotica, an oasis medicinal plant, field-grown Isatis indigotica under drip irrigation with plastic film mulching was taken as the research object. Three water levels (low W1: 60%-70%θf , medium W2: 70%-80%θf, high W3: 80% -90%θf) and three nitrogen application levels (low N1: 150 kg·ha-1, medium N2: 200 kg·ha-1, high N3: 250 kg·ha-1) were set, with no water control (W0) and no nitrogen application (N0) as the control. Based on two-year experimental data, a multi-level weighted fuzzy evaluation model was used to comprehensively evaluate the growth, yield, quality and resource utilization indicators. A regression model between water-nitrogen factors and evaluation indices was established, and the Particle Swarm Optimization (PSO) algorithm was coupled for optimization and solution. The evaluation indices of yield and growth indicators under the medium water and medium nitrogen (W2N2) treatment were the highest (with average values of 0.1173 and 0.1125); the evaluation indices of resource utilization indicators under the low water or medium water and low nitrogen treatments were relatively high (with average values of 0.1220 and 0.1224); and the evaluation indices of quality indicators under the low water treatment were relatively high (with an average value of 0.1025). The secondary fuzzy comprehensive evaluation showed that W2N2 was the optimal treatment. The interaction between water and nitrogen was significant, and the effects of both on comprehensive growth showed a trend of increasing first and then decreasing. The optimal water-nitrogen combination obtained by PSO was: water level of 75.10%-85.10%θf and nitrogen application rate of 224.53 kg·ha-1. This study can provide a basis for the scientific management of water-fertilizer in the local field cultivation of Isatis indigotica.