Ridge-furrow rainfall harvesting system combined with gravel mulch reduced soil evaporation and stabilized soil temperature. ► Ridge-furrow rainfall harvesting system combined with gravel mulch improved rain water use efficiency and watermelon yield. ► Ridge-furrow rainfall harvesting system with a plastic mulched ridge had an obvious higher effect on yield. ► The ratio of ridge and furrow had a significant effect on improving watermelon yield. The ratio 1:1 was the most beneficial.