Effects of gravel–sand mulch, plastic mulch and ridge and furrow rainfall harvesting system combinations on water use efficiency, soil temperature and watermelon yield in a semi-arid Loess Plateau of northwestern China

► 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. In the northwestern Loess Plateau of China, low precipitation results in poor crop yields, with a great fluctuation from year to year. The adoption of gravel–sand mulching has shown improvements in the growth of crops such as watermelon. The ridge and furrow rainwater harvest system (RFRHS) has been shown as an easy and efficient way to collect rainwater. A field experiment was conducted from 2007 to 2009 at Gaolan, Lanzhou, Gansu, China, to measure the effects of RFRHS, plastic mulch and gravel–sand mulch combinations on soil temperature, evapotranspiration (ET), water use efficiency (WUE) and watermelon yield. There were eight treatments: (1) flat gravel–sand mulched field, (2) RFRHS with a sand mulched furrow, entire plastic mulch and the ratio 1:1 of ridge and furrow, (3) RFRHS with a sand mulched furrow, entire plastic mulch and the ratio 4:3 of ridge and furrow, (4) RFRHS with a sand mulched furrow, entire plastic mulch and the ratio 5:3 of ridge and furrow, (5) RFRHS with a sand and plastic mulched furrow, bare ridge and the ratio 4:3 of ridge and furrow, (6) RFRHS with an entire plastic mulch and the ratio 4:3 of ridge and furrow, (7) conventional ridge planting with a plastic mulched ridge, and (8) flat gravel–sand mulched field plus 23 mm supplementary irrigation. Soil temperature for RFRHS with a gravel–sand plus plastic mulched furrow was slightly lower than that of flat gravel–sand mulch. The RFRHS caused a significant increase in watermelon yield and WUE. The increase in watermelon yield and WUE was greatly influenced by the ratio of ridge and furrow when RFRHS was combined with gravel–sand mulch. Watermelon yield was highest for the 1:1 ratio, and WUE was highest for the 5:3 and 1:1 ratios of ridge:furrow, and these were significantly greater than that of flat gravel–sand mulch, without or with irrigation. The use of ridge with plastic film mulch increased the beneficial effect of RFRHS on yield. The watermelon yield and WUE for non-plastic-mulched ridge were e