Zhu Ouyang

A New Method to Define the VI-Ts Diagram Using Subpixel Vegetation and Soil Information: A Case Study over a Semiarid Agricultural Region in the North China Plain.

The VI-Ts diagram determined by the scatter points of the vegetation index (VI) and surface temperature (Ts) has been widely applied in land surface studies. In the VI-Ts diagram, dry point is defined as a pixel with maximum Ts and minimum VI, while wet point is defined as a pixel with minimum Ts and maximum VI. If both dry and wet points can be obtained simultaneously, a triangular VI-Ts diagram can be readily defined. However, traditional methods cannot define an ideal VI-Ts diagram if there are no full ranges of land surface moisture and VI, such as during rainy season or in a period with a narrow VI range. In this study, a new method was proposed to define the VI-Ts diagram based on the subpixel vegetation and soil information, which was independent of the full ranges of land surface moisture and VI. In this method, a simple approach was firstly proposed to decompose Ts of a given pixel into two components, the surface temperatures of soil (Tsoil) and vegetation (Tveg), by means of Ts and VI information of neighboring pixels. The minimum Tveg and maximum Tsoil were then used to determine the wet and dry points respectively within a given sampling window. This method was tested over a 30 km × 30 km semiarid agricultural area in the North China Plain through 2003 using Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) and MODerate-resolution Imaging Spectroradiometer (MODIS) data. The wet and dry points obtained from our proposed method and from a traditional method were compared with those obtained from ground data within the sampling window with the 30 km × 30 km size. Results show that Tsoil and Tveg can be obtained with acceptable accuracies, and that our proposed method can define reasonable VI-Ts diagrams over a semiarid agricultural region throughout the whole year, even for both cases of rainy season and narrow range of VI.

Variation in Yield Gap Induced by Nitrogen, Phosphorus and Potassium Fertilizer in North China Plain

A field experiment was conducted under a wheat-maize rotation system from 1990 to 2006 in North China Plain (NCP) to determine the effects of N, P and K on yield and yield gap. There were five treatments: NPK, PK, NK, NP and a control. Average wheat and maize yields were the highest in the NPK treatment, followed by those in the NP plots among all treatments. For wheat and maize yield, a significant increasing trend over time was found in the NPK-treated plots and a decreasing trend in the NK-treated plots. In the absence of N or P, wheat and maize yields were significantly lower than those in the NPK treatment. For both crops, the increasing rate of the yield gap was the highest in the P omission plots, i.e., 189.1 kg ha−1 yr−1 for wheat and 560.6 kg ha−1 yr−1 for maize. The cumulative omission of P fertilizer induced a deficit in the soil available N and extractable P concentrations for maize. The P fertilizer was more pivotal in long-term wheat and maize growth and soil fertility conservation in NCP, although the N fertilizer input was important for both crops growth. The crop response to K fertilizers was much lower than that to N or P fertilizers, but for maize, the cumulative omission of K fertilizer decreased the yield by 26% and increased the yield gap at a rate of 322.7 kg ha−1 yr−1. The soil indigenous K supply was not sufficiently high to meet maize K requirement over a long period. The proper application of K fertilizers is necessary for maize production in the region. Thus, the appropriate application of N and P fertilizers for the growth of both crops, while regularly combining K fertilizers for maize growth, is absolutely necessary for sustainable crop production in the NCP.

Cultivated land use change in China, 1999–2007: Policy development perspectives

Cultivated land protection (CLP) entered the new era of macro administration in 1999 in China. This paper presents a holistic analysis of cultivated land use change concerning the three goals of the CLP, i.e., grain security, ecological security and harmonizing regional development. Farmers’ willingness to grow grain has been the key factor in safeguarding grain security. Grain-for-green policy has contributed to improving ecological state especially in the western provinces. Effects of the land macro-control of the CLP on harmonizing regional development were significant. Moreover, cultivated land use change in 1999–2007 points out the way of the evolving policy in the future. From the viewpoint of normative concept of multifunctionality, we discuss development of the three land functions, i.e., production function, environmental function and carrier function. Finally, we propose to emphasize multifunctional land management based on regional differences to promote transition of the CLP.

Relationship between Evapotranspiration and Land Surface Temperature under Energy- and Water-Limited Conditions in Dry and Cold Climates

Remotely sensed land surface temperature- (LST-) dependent evapotranspiration (ET) models and vegetation index- (VI-) LST methods may not be suitable for ET estimation in energy-limited cold areas. In this study, the relationship of ET to LST was simulated using the process-based Simultaneous Heat and Water (SHAW) model for energy- and water-limited conditions in Mongolia, to understand the differences in ET processes under these two limiting conditions in dry and cold climates. Simulation results from the SHAW model along with ground observational data showed that ET and LST have a positive relationship when air temperature () is less than or equal to the temperature () above which plants transpire and have a negative relationship when is greater than under the energy-limited condition. However, ET and LST maintain a negative relationship with changes in under the water-limited condition. The differences in the relationship between ET and LST under the energy-limited and water-limited conditions could be attributed to plant transpiration and energy storage in moist/watered soil and plants. This study suggests that different strategies should be used to estimate ET under the energy-limited condition in dry and cold climates.

Evaluation of the VI–Ts method for estimating the land surface moisture index and air temperature using ASTER and MODIS data in the North China Plain

Two conditions are required when a remotely sensed vegetation index–land surface temperature (VI–T s) diagram is used to estimate the land surface moisture index (LSMI) and air temperature (T a). First, a suitable sampling window size is required to define an ideal VI–T s diagram. Second, T a must be homogeneous across the sampling window. In this study, the Shannon diversity index (SDI) and the semivariogram method were used to evaluate the VI–T s diagram for estimating LSMI and T a from Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) and MODerate-resolution Imaging Spectroradiometer (MODIS) datasets. The results show that T a is homogeneous across a sampling window with a width of several tens of kilometres (46.0–83.6 km) based on the semivariogram method and spatial autocorrelation analysis of the T a from 83 meteorological stations in the North China Plain (NCP) in 2003. When the SDIs of VI and T s are respectively larger than 77% and 63% of their maximums within predetermined sampling windows, LSMI estimations by ASTER and T a estimations by ASTER and MODIS are reliable.

Further evaluation of the Sim-ReSET model for ET estimation driven by only satellite inputs

Abstract A simple remote sensing evapotranspiration (ET) model (Sim-ReSET) has been proposed but only tested using field measurements at a site with a semi-arid climate. Its performance for mapping ET using only satellite data remained unknown. In this study, the Sim-ReSET model was further evaluated for ET estimation driven by only MODIS data products. The estimated ET rates were compared with ground-based observational data from a variety of ecosystems and climates across China. The results show that MODIS-based ET estimates are consistent with both the ET measurements from eddy covariance flux towers and those from the Penman-Monteith method combined with micrometeorological data. Evaporation fraction (EF) is indicative of land surface moisture. The derivative EF maps demonstrate that the proposed ET data set obtained from the Sim-ReSET model and MODIS data is capable of capturing the spatio-temporal pattern of land surface moisture for different land covers with different climates. Editor Z.W. Kundzewicz Citation Sun, Z.G., Wang, Q.X., Matsushita, B., Fukushima, T., Ouyang, Z., Watanabe, M., and Gebremichael, M., 2013. Further evaluation of the Sim-ReSET model for ET estimation driven by only satellite inputs. Hydrological Sciences Journal, 58 (5), 994–1012.

Effects of field experimental warming on wheat root distribution under conventional tillage and no-tillage systems

Abstract Despite the obvious importance of roots to agro‐ecosystem functioning, few studies have attempted to examine the effects of warming on root biomass and distribution, especially under different tillage systems. In this study, we performed a field warming experiment using infrared heaters on winter wheat, in long‐term conventional tillage and no‐tillage plots, to determine the responses of root biomass and distribution to warming. Soil monoliths were collected from three soil depths (0–10, 10–20, and 20–30 cm). Results showed that root biomass was noticeably increased under both till and no‐till tillage systems (12.1% and 12.9% in 2011, and 9.9% and 14.5% in 2013, in the two tillage systems, respectively) in the 0–30 cm depth, associated with a similar increase in shoot biomass. However, warming‐induced root biomass increases occurred in the deeper soil layers (i.e., 10–20 and 20–30 cm) in till, while the increase in no‐till was focused in the surface layer (0–10 cm). Differences in the warming‐induced increases in root biomass between till and no‐till were positively correlated with the differences in soil total nitrogen (R 2 = .863, p < .001) and soil bulk density (R 2 = .853, p < .001). Knowledge of the distribution of wheat root in response to warming should help manage nutrient application and cycling of soil C‐N pools under anticipated climate change conditions.