An isotope method to quantify soil evaporation and evaluate water vapor movement under plastic film mulch

- The soil evaporating front was presented in 5-10 cm soil layer.
- About 4.5% of the soil water (0-20 cm) was evaporated, with mean evaporation rate at 0.80 mm/d through plastic holes.
- About 72.6% of evaporation vapor exchange to condensation water and 70.0% of condensation water evaporated to vapor.
- Evaporation was relative high (E/ET: ∼21.2%), although the proportion of open-hole area was small.

Plastic film mulch is a cropping system feature for water saving that is used extensively in arid and semiarid areas. However, research on water flow beneath and through the perforated film mulches is still limited, and the detailed processes of vaporization and transport of evaporation vapor in the soil are not examined thoroughly. Here we used an isotope-based method to quantitatively describe the processes of the soil water and vapor exchange in a maize field under plastic mulch. Micro-lysimeter-based methods and sap flow combined with soil water balance method (E(F-B)) were conducted to compare and verify the results. It shows that heavy oxygen isotope (18O) depleted and enriched alternately along with the processes of condensation and distillation, and enriched again when the second evaporation occurred in condensation water adhered in plastic film. Soil evaporating front was presented in 5-10 cm soil layer, and the exchange motions of water vapor occurred in 0-5 cm soil layer before it diffused to the outside. During the whole maize growing season, about 4.5% of soil water in the 0-20 cm soil layer was evaporated, and 72.6% of the evaporation vapor was condensed and 70.0% of this condensation water was evaporated again. About 2.3% of soil water evaporated through plastic film holes, with the mean evaporation rate of 0.80 mm/d. A good agreement was found between the evaporation determined by our isotope-based method (E(Iso)) and (E(F-B)) method. The evaporation rate was relatively high with the ratio of E to ET around 21.2%, although the areas of the holes only taking up 0.5-5.0% of the whole area of the plastic cover. Therefore, the isotope-based method was robust in estimating evaporation under plastic film mulch. These quantitative analyses will improve our understanding of the mechanism of soil water movement and vapor exchange under plastic mulch and provide accurate estimation of evaporation in field.