Evaporation and evapotranspiration in a watermelon field mulched with gravel of different sizes in northwest China

Trials were conducted to study the effects of decreasing evapotranspiration and evaporation under mulching with coarse sand and gravel of different sizes in a watermelon field in the Loess Plateau of northwest China in 2004. The trials involved four treatments: mulching with coarse sand and gravel of various diameters (2–5, 5–20, and 20–60 mm) and no mulching (control). In addition, the soil surface was mulched with gravel of five size classes in 10 microlysimeters and the same initial water content to determine the relationship between evaporation and gravel size. Ten microlysimeters were filled with six soil samples with different initial water content, and the soil surface was mulched with uniformly sized gravel to study the effect of soil moisture on evaporation. The results showed that there was a significant difference in evapotranspiration and evaporation between treatments: evaporation increased linearly with gravel size. Mulching with 2–5 mm diameter sand and gravel resulted in significantly less evapotranspiration than mulching with 5–20 and 20–60 mm diameter sand and gravel. By comparison, evapotranspiration with 2–5 mm diameter sand and gravel was not significantly different from that with the control. The relationship between evaporation and soil moisture under gravel mulch could be represented mathematically by a cubic curve. Evaporation was decreased little when the soil moisture was reduced from 27% to 8% with gravel mulch. Evaporation and the ratio of evaporation to evapotranspiration (E/ET) increased with the diameter of sand and gravel. The E/ET ratio was 40.7% in the growing period for the control, and it was only 17.8–25.0% for treatments mulched with sand and gravel. Soil evaporation with nonmulching was reduced by 78.0–93.7 mm when plastic film was mulched on the gravel surface and by 16.9–26.3 mm with gravel mulching only. The yield was not obviously different between treatments with gravel of various sizes, but water use efficiency was significantly higher for mulching with 2–5 mm diameter gravel than for mulching with 20–60 mm diameter gravel. In addition, the soluble carbohydrate content decreased for small-diameter gravel.