Agro-ecosystems water cycles of the typical irrigated farmland in the North China Plain

This paper reviewed the research progresses on hydrological cycles and water transformation studies at Luancheng Agro-Ecosystem Experimental Station, Chinese Academy of Sciences (simplified as Luancheng Station) during the past 30 years. As a mostly achieved progress, the evapotranspiration (ET) and its partitioning over irrigated winter wheat-summer maize cropland, which is the most common cultivation pattern in North China Plain, are clarified through multi-years observations using weighing lysimeter, micrometeorological measurements, and isotopic analysis. The annual ET of the irrigated wheat-maize field can reach to 870 mm·a-1, which exceeds the annual precipitation by around 350 mm at an average level. This gap between annual ET and precipitation is bridged by groundwater pumpage. As a matter of long-term over exploitation, the groundwater over the North China Plain experienced rapid depletion and caused wide concerns on sustainability. The study at Luancheng Station suggests that the soil evaporation shares about 1/3 of the total water consumption and the evaporation depth can reach deep to 20 cm under land surface through a isotopic method. Moreover, wheat transpiration is mainly dependent on the soil moisture in the depth of 0~40 cm soil layer, which is largely shallower than the generally used "planing wetting depth", i.e. 100 cm. As for the deep percolation or drainage of the soil water and the vertical recharge rate of groundwater, there exists large range among different studies with different method, such as soil water balance modeling, rainfall-infiltration experiment, etc. It is urgently needed to go to the details of water cycles over different agricultural land uses and soil properties, and to address the spatial heterogeneities in evapotranspiration, infiltration, and soil water balancing, over the plain.