The North China Plain (NCP) is amongst the global "hotspots" in terms of groundwater over-exploitation, and irrigation for winter wheat is the primary cause. Shrinkage of area sown to winter wheat proves to be a practical strategy to reverse groundwater over-exploitation and to promote groundwater storage. Land retirement policy was introduced by the government to encourage the extension of spring maize single cropping system in groundwater over-exploited areas. In addition, since the 1990s, large areas in the NCP in which winter wheat and summer maize are being replaced by the single cropping system of spring maize, due to the active behaviors of farmers. Not all land parcels grow winter wheat at the moment. Therefore, it is crucial to reveal the targeted land parcels for winter wheat abandonment and to assess reasonable and proper standards for ecological compensation, prior to the implementation of land retirement policy. In this paper, a case study was carried out in Cangxian county of Hebei province. Multi-level logit models were constructed using household survey data, in order to detect the determinants across plot, household and village levels on farmers' cropping system decisions, and the opportunity costs for winter wheat abandonment were calculated using cost-benefit analysis. The aim of this study was to recognize land parcels with winter wheat and to give scientific support on standards for ecological compensation. Results showed that: (1) land quality and irrigation condition at the parcel level are two essential elements influencing farmers' cropping system decisions. Nearly 70% in total area of poor land and more than 90% in total area of unirrigated land has suffered winter wheat abandonment. The targeted land parcels for land retirement policy should be the irrigated ones with land qualities being good, relative good or relative bad. (2) There were no significant differences between the net-profits of spring maize and summer maize under similar farming conditions, and the opportunity cost for land retirement should be equal to the net-profit of winter wheat. (3) When only considering the influence of winter wheat on groundwater resources, the primary purpose of the land retirement policy is to reverse the groundwater table and to induce groundwater recovery and restoration at the preliminary stage, and a high level of 350 yuan/mu was recommended as the subsidies for ecological compensation; at a later stage, the primary purpose of the policy transited to the balance of exploitation and supplement of water resources, and a lower level of 280 yuan/mu was recommended.

The study aims to investigate the spatio-temporal changes in crop patterns in China since 1980. In doing so, the analysis methods of time-series trend and spatial cluster were used to cover the major eleven crops at county scale. The results indicate that (1) There are 16 kinds of crop combinations ranking in the China's top 10 during the past 30 years. Yet since 2002, the simplified cropping structure has been gradually replaced by the multiple cropping structure, which suggests an increase in the diversity index of crop patterns. In 1980, about 82.7% of China's counties have a similar crop pattern which is composed of rice, wheat, corn and their combinations, however, this pattern largely changed after 2002 due to the increase in the planting area of fruit and vegetables. (2) In the same period, rice planting area of 47% of the counties, wheat planting area of 61% of the counties of and corn area of 29.6% of the counties experience a significant decrease, while other crops show an increasing trend. As a result, rice-dominated cereal crops in China are slightly adjusted to the coexistence of rice, wheat and maize crops. In particular, maize area proportion shows a significant change, which forms a so-called "corn decreased belt” spanning from northeast to southwest of China. Urbanization had an important impact on crop patterns as fruit and vegetable planting areas rapidly grow so as to meet the increasing demands in urbanized areas. (3) Crop patterns also show an obvious spatial cluster effect in China's 1300 counties. The proportion of high cluster accounts for 2.86%, 5.64%, 6.11%, 4.53%, 1.62%, 7.77%, 8.24%, 12%, 10%, 1.41% and 9.35% of China's counties for rice, wheat, maize, soybean, fibers, cotton, vegetables, potatoes, fruits, sugars and oils, respectively. These crops are distributed in Northeast China, Xinjiang, Northern Shaanxi Plateau, Yunnan-Guizhou Plateau and the metropolis areas. This finding of this study can support the decision making in agricultural restructuring and adaptation to climate change.