Drought is usually supposed to be a rainfall deficiency problem. Most studies and practices to mitigate drought disaster are focusing on water development and irrigation, while neglecting that the agriculture system i...Drought is usually supposed to be a rainfall deficiency problem. Most studies and practices to mitigate drought disaster are focusing on water development and irrigation, while neglecting that the agriculture system is a compounded human-natural system. Drought situation and tendency is also driven by human coping strategies. This paper takes Hebei Province in north China as an example, studing the spirally interact process of drought resisting and hydrological ecological feedback. The result shows that large scale water projects construction facilitated irrigation. With improved irrigation, farmers enhanced multiple crop index and land-use intensity greatly and increased the sowing area of water consuming crop, winter wheat. At the same time, both crop yield and gross output are raising steadily. Water demand and depletion in agricultural system increase year by year. This gradually leads to highly dependent on over exploitation of water resources, especially overdraw of groundwater. The process deteriorated the stability of hydrological-ecological system and made the ecological environment drying up. Drying up environment is breeding greater vulnerability and risk of drought in the long term. For sustainable development, integrated drought risk management should be based on the balance between sustainable water supply and water demand. The key is to improve agricultural system＇s adaptive and resilient capacity to drought.展开更多
This paper deals with the FEEDBACK VERTEX SET problem on undirected graphs, which asks for the existence of a vertex set of bounded size that intersects all cycles. Due it is theoretical and practical importance,the p...This paper deals with the FEEDBACK VERTEX SET problem on undirected graphs, which asks for the existence of a vertex set of bounded size that intersects all cycles. Due it is theoretical and practical importance,the problem has been the subject of intensive study. Motivated by the parameter ecology program we attempt to classify the parameterized and kernelization complexity of FEEDBACK VERTEX SET for a wide range of parameters.We survey known results and present several new complexity classifications. For example, we prove that FEEDBACK VERTEX SET is fixed-parameter tractable parameterized by the vertex-deletion distance to a chordal graph. We also prove that the problem admits a polynomial kernel when parameterized by the vertex-deletion distance to a pseudo forest, a graph in which every connected component has at most one cycle. In contrast, we prove that a slightly smaller parameterization does not allow for a polynomial kernel unless NP coNP=poly and the polynomial-time hierarchy collapses.展开更多
基金Acknowledgments China National Natural Science Foundation （No. 41171402） and Doctoral Fund of Hebei Normal University （No. 103237）.
文摘Drought is usually supposed to be a rainfall deficiency problem. Most studies and practices to mitigate drought disaster are focusing on water development and irrigation, while neglecting that the agriculture system is a compounded human-natural system. Drought situation and tendency is also driven by human coping strategies. This paper takes Hebei Province in north China as an example, studing the spirally interact process of drought resisting and hydrological ecological feedback. The result shows that large scale water projects construction facilitated irrigation. With improved irrigation, farmers enhanced multiple crop index and land-use intensity greatly and increased the sowing area of water consuming crop, winter wheat. At the same time, both crop yield and gross output are raising steadily. Water demand and depletion in agricultural system increase year by year. This gradually leads to highly dependent on over exploitation of water resources, especially overdraw of groundwater. The process deteriorated the stability of hydrological-ecological system and made the ecological environment drying up. Drying up environment is breeding greater vulnerability and risk of drought in the long term. For sustainable development, integrated drought risk management should be based on the balance between sustainable water supply and water demand. The key is to improve agricultural system＇s adaptive and resilient capacity to drought.
基金supported by the European Research Council through Starting Grant 306992 "Parameterized Approximation"
文摘This paper deals with the FEEDBACK VERTEX SET problem on undirected graphs, which asks for the existence of a vertex set of bounded size that intersects all cycles. Due it is theoretical and practical importance,the problem has been the subject of intensive study. Motivated by the parameter ecology program we attempt to classify the parameterized and kernelization complexity of FEEDBACK VERTEX SET for a wide range of parameters.We survey known results and present several new complexity classifications. For example, we prove that FEEDBACK VERTEX SET is fixed-parameter tractable parameterized by the vertex-deletion distance to a chordal graph. We also prove that the problem admits a polynomial kernel when parameterized by the vertex-deletion distance to a pseudo forest, a graph in which every connected component has at most one cycle. In contrast, we prove that a slightly smaller parameterization does not allow for a polynomial kernel unless NP coNP=poly and the polynomial-time hierarchy collapses.
