Comprehensive Growth Evaluation of Subsurface Drip-Irrigated Walnuts Based on the TOPSIS-GRA Coupled Model

To identify optimal water and fertilizer management for walnut trees under subsurface drip irrigation in Southern Xinjiang, a field experiment was conducted on 16-year-old ‘Wen 185’ walnuts in Aksu. Four irrigation levels—75% ETc (W1), 100% ETc (W2), 125% ETc (W3), and 150% ETc (W4)—and three fertilizer levels—N 270, P 240, K 300 kg·hm-2 (F1), N 360, P 320, K 400 kg·hm-2 (F2), and N 450, P 400, K 500 kg·hm-2 (F3)—were applied, resulting in 12 treatments. This study assessed the impact of different water and fertilizer treatments on walnut growth dynamics, yield, fruit quality, water and fertilizer use efficiency, and soil nitrate residue. Principal component analysis (PCA) was employed to develop comprehensive growth and photosynthesis indices (CGI and CPI). Pearson correlation analysis was used to identify parameters significantly correlated with yield and quality. A game theory-based combination weighting method was then applied to integrate six categories of indicators: growth, photosynthesis, yield, quality, resource use efficiency, and environmental impact. A coupled TOPSIS-GRA model was established for comprehensive evaluation, and binary quadratic regression was used to optimize the water and fertilizer application ranges. The results demonstrated that the W2F2 treatment provided the most effective synergistic enhancement in terms of growth, photosynthetic performance, yield, and quality. It also maintained high water use efficiency (WUE) and partial factor productivity of fertilizer (PFP), while effectively mitigating nitrate accumulation in deep soil layers. The CGI and CPI indices, developed through PCA, successfully quantified phenological growth and photosynthetic characteristics. Correlation analysis identified seven core parameters. Among these, IV-CPI showed the strongest correlation with yield, whereas II-CPI contributed notably to increasing single-fruit weight and reducing tannin content. Within the comprehensive evaluation system employing game theory-based combination weighting, yield was assigned the highest weight (0.215), and IV-CPI received the lowest (0.011). The TOPSIS-GRA model ranked W2F2 highest, with an optimal irrigation range of 5869.94–6519.81 m3·hm-2 and fertilizer application rate of 975.54–1107.49 kg·hm-2. The coupled TOPSIS-GRA model effectively balanced the objectives of high yield, superior quality, resource use efficiency, and environmental sustainability. It offers a solid theoretical basis and practical technical guidance for enhancing both the productivity and sustainability of walnut orchards under subsurface drip irrigation in the Southern Xinjiang region.