Objectives Improper water and fertilizer management is common in watermelon production under gravel-mulched field of Northwest China. We investigated the suitable combination of water, N, and K inputs to attain optimum yield and quality of watermelon, as well as economic and environmental benefits.

Methods A medium-late maturing watermelon cultivar ‘Jincheng 5’ was used as the test material in a microplot experiment using drip irrigation under gravel-mulching technology. The experiment adopted the three factors’ quadratic saturating D-optimal design method to make water (W), N and K combination treatment, fertilizer effective equations of yield and quality were set up by adopting the condition of drip fertigation of grafted watermelon.

Results Principal factor effect analysis showed that W, N, and K significantly influenced the watermelon yield and quality parameters, the order of influencing factors on watermelon yield was W>N>K, and the positive effect of water on yield was the most significant, while the negative effect of N was more obvious than that of water, and potassium had lesser effect on watermelon yield. The order of influencing factors on watermelon quality was N>K>W, and nitrogen and potassium markedly improved watermelon quality, while water had significant negative effects. The interactive effect of various factors on watermelon yield were shown as N×W>N×K>W×K. Mean while, W×K was greater than the main effect of nitrogen. Meanwhile regarding watermelon quality, K×W had a greater effect than W×N, and W×N improved crop quality better than N×K. With reference to the model, it was concluded that the water and fertilizer schemes for watermelon yield exceeding 60 t/hm² under the experimental conditions were as follows: irrigation amount 808–1017 m³/hm², N 231–293 kg/hm², K₂O 177–258 kg/hm². And the water and fertilizer schemes for average sugar content of watermelon exceeding 11% were as follows: irrigation amount 555–876 m³/hm², N 226–279 kg/hm², K₂O 217–271 kg/hm².

Conclusions Through comprehensive analysis and comparison, the optimum water and fertilizer scheme for a high yield and good quality of grafted watermelon production under fertigation condition was: irrigation amount 808–876 m³/hm², N 231–279 kg/hm², and K₂O 217–258 kg/hm².