Isotope Mass Balance Method to Partition Evaporation from Soil Total Water Loss in Winter Wheat and Spring Maize Cropping Systems in North China Plain

Evaporation (E) is one of main paths of soil water loss in most dryland farming systems. Since it is considered to be of no benefit to crop production, E should be minimized as much as possible. However, the partitioning of E from total water loss (TWL) via transpiration (T) and seepage (S), under field condition is not an easy task. The isotope mass balance (IMB) method is one of the few ways to do so. This study was designed to separate E from TWL in winter wheat and spring maize cropping systems in the North China Plain using the IMB method. In field experiments conducted over two years, plot surfaces were covered with plastic film, with straw mulch, left bare or left as normally in the field, and stable isotope values and amounts of soil water, rainfall, and irrigation water were measured periodically. Results showed that the proportion of E to TWL was 18 percent and 31 percent in winter wheat and spring maize, respectively in the conventional plot, 21 and 12 percent, respectively in plastic film-covered (hereafter referred to as filming) plot, and 47 and 38 percent, respectively in the bare plot. In conclusion, it was found that the IMB method is a simple way to partition average E from TWL over a growing season, and the method is sensitive enough to separate E between straw mulching, filming, conventional practice, and bare soil. The most sensitive factor in using IMB is the determination of the depth of E front within the soil. In the current experiment, isotope measurements of soil water at depths of 0–5 cm were critical to the success of IMB. (author)