Spatial and Temporal Changes in Vapor Pressure Deficit and Their Impacts on Crop Yields in China during 1980–2008

Vapor pressure deficit（VPD） is a widely used measure of atmospheric water demand. It is closely related to crop evapotranspiration and consequently has major impacts on crop growth and yields. Most previous studies have focused on the impacts of temperature, precipitation, and solar radiation on crop yields, but the impact of VPD is poorly understood. Here, we investigated the spatial and temporal changes in VPD and their impacts on yields of major crops in China from 1980 to 2008. The results showed that VPD during the growing period of rice, maize, and soybean increased by more than 0.10kPa（10 yr）^–1 in northeastern and southeastern China, although it increased the least during the wheat growing period. Increases in VPD had different impacts on yields for different crops and in different regions. Crop yields generally decreased due to increased VPD, except for wheat in southeastern China. Maize yield was sensitive to VPD in more counties than other crops. Soybean was the most sensitive and rice was the least sensitive to VPD among the major crops. In the past three decades, due to the rising trend in VPD, wheat, maize, and soybean yields declined by more than 10.0% in parts of northeastern China and the North China Plain, while rice yields were little affected. For China as a whole, the trend in VPD during 1980–2008 increased rice yields by 1.32%,but reduced wheat, maize, and soybean yields by 6.02%, 3.19%, and 7.07%, respectively. Maize and soybean in the arid and semi-arid regions in northern China were more sensitive to the increase in VPD. These findings highlight that climate change can affect crop growth and yield through increasing VPD, and water-saving technologies and agronomic management need to be strongly encouraged to adapt to ongoing climate change.

Contributions of climate, varieties, and agronomic management to rice yield change in the past three decades in China

The long-term field experiment data at four representative agro-meteorological stations, together with a crop simulation model, were used to disentangle the contributions of climate change, variety renewal, and fertilization management to rice yield change in the past three decades. We found that during 1981-2009 varieties renewal increased rice yield by 16%-52%, management improvement increased yield by 0-16%, and the contribu- tions of climate change to rice yield varied from - 16% to 10%. Varieties renewal and management improvement offset the negative impacts of climate change on rice production. Among the major climate variables, decreases in solar radiation reduced rice yield on average by 0.1% per year. The impact of temperature change had an explicit spatial pattern. It increased yield by 0.04%-0.4% per year for single rice at Xinbin and Ganyu station and for late rice at Tongcheng station, by contrast reduced yield by 0.2%- 0.4% per year for single rice at Mianyang station and early rice at Tongcheng station. During 1981-2009, rice varieties renewal was characterized by increases in thermal requirements, grain number per spike and harvest index. The new varieties were less sensitive to climate change than old ones. The development of high thermal require- ments, high yield potential and heat tolerant rice varieties, together with improvement of agronomic management, should be encouraged to meet the challenges of climate change and increasing food demand in future.

Rainfall-Related Weather Indices for Three Main Crops in China

Rainfall-relatedhazards—deficitrainand excessive rain—inevitably stress crop production,and weather index insurance is one possible financial tool to mitigate such agro-metrological losses.In this study,we investigated where two rainfall-related weather indices—anomaly-based index(AI)and humidity-based index(HI)—could be best used for three main crops(rice,wheat,and maize)in China’s main agricultural zones.A county is defined as an“insurable county”if the correlation between a weather index and yield loss was significant.Among maize-cropping counties,both weather indices identified more insurable counties for deficit rain than for excessive rain(AI:172 vs 63;HI:182 vs 68);moreover,AI identified lower basis risk for deficit rain in most agricultural zones while HI for excessive rain.For rice,the number of AIinsurable counties was higher than the number of HI-insurable counties for deficit rain(274 vs 164),but lower for excessive rain(199 vs 272);basis risks calculated by two weather indices showed obvious difference only in Zone I.Finally,more wheat-insurable counties(AI:196 vs 71;HI:73 vs 59)and smaller basis risk indicate that both weather indices performed better for excessive rain in wheatplanting counties.In addition,most insurable counties showed independent yield loss,but did not necessarily result in effective risk pooling.This study is a primary evaluation of rainfall-related weather indices for the three main crops in China,which will be significantly helpful to the agricultural insurance market and governments’policy making.

Performance of Temperature-Related Weather Index for Agricultural Insurance of Three Main Crops in China

In this study, two categories of weather index—absolute index and relative index—for chilling injury and heat damage of three main crops in China were assessed to identify insurable counties. First, correlations between selected weather indices and yield losses were examined for each county. If a correlation was significant, the county was categorized as ‘‘insurable'' for the corresponding hazard or index. Second, the spatial distribution of insurable counties was characterized and finally, their correlation coefficients were analyzed at various spatial scales.The results show that the spatial patterns of insurable areas varied by categories of weather indices, crops, and hazards.Moreover, the weather indices based on relative threshold of temperature were more suitable for chilling injury in most regions, whereas the indices based on absolute threshold were more suitable for heat damage. The findings could help the Chinese government and insurance companies to design effective insurance products.

Energy partitioning and environmental influence factors in different vegetation types in the GEWEX Asian Monsoon Experiment

Environmental influences upon energy balance in areas of different vegetation types （i.e., forest at Kog-Ma in Thailand and at Yakutsk in Russia, grassland at Amdo in Chinese Tibet and at Arvaikheer in Mongolia, and mixed farmland at Tak in Thailand） in the GEWEX Asian Monsoon Experiment were investigated. The sites we investigated are geographically and climatologically different; and consequently had quite large variations in temperature （T）, water vapor pressure deficit （VPD）, soil moisture （SM）, and precipitation （PPT）. During May- October, the net radiation flux （Rn） （in W·m^-2） was 406.21 at Tak, 365.57 at Kog-Ma, 390.97 at Amdo, 316.65 at Arvaikheer, and 287.10 at Yakutsk. During the growing period, the Rn partitioned into latent heat flux （2E/Rn） was greater than that partitioned into sensible heat flux （H/Rn） at Tak and at Kog-Ma. In contrast, 2E/Rn was lower than H/Rn at Arvaikheer, H/Rn was less than 2E/Rn between DOY 149 and DOY 270 at Amdo, and between DOY 165 and DOY 235 at Yakutsk. The R, partitioned into ground heat flux was generally less than 0.15. The short-wave albedo was 0.12, 0.18, and 0.20 at the forest, mixed land, and grass sites, respectively. At an hourly scale, energy partitions had no correlation with environmental factors, based on average summer half- hourly values. At a seasonal scale energy partitions were linearly correlated （usually p 〈 0.05） with T, VPD, and SM. The 2E/Rn increased with increases in SM, T, and VPD at forest areas. At mixed farmlands, ）λE/Rn generally had positive correlations with SM, T, and VPD, but was restrained at extremely high values of VPD and T. At grasslands, λE/Rn was enhanced with increases of SM and T, but was decreased with VPD.

Modeling the rice phenology and production in China with SIMRIW： sensitivity analysis and parameter estimation

Crop models are robust tools for simulating the impact of climate change on rice development and production, but are usually designed for specific stations and varieties. This study focuses on a more adaptable model called Simulation Model for Rice-Weather Relations （SIMRIW）. The model was calibrated and validated in major rice production regions over China, and the parameters that most affect the model＇s output were determined in sensitivity analyses. These sensitive parameters were estimated in different ecological zones. The simulated results of single and double rice cropping systems in different ecological zones were then compared. The accuracy of SIMRIW was found to depend on a few crucial parameters. Using optimized parameters, SIMRIW properly simulated the rice phenology and yield in single and double cropping systems in different ecological zones. Some of the parameters were largely dependent on ecological zone and rice type, and may reflect the different climate conditions and rice varieties among ecological zones.

Effects of climate and land use changes on runoff,sediment,nitrogen and phosphorus losses in the Haihe River Basin

Investigating the impacts of climate and land use changes on the hydrological cycle and water environment at the basin scale is important for providing scientific evidence to manage the trade-offs and synergies among water resources,agricultural production and environmental protection.We used the Soil and Water Assessment Tool(SWAT)with various spatiotemporal data to quantify the contributions of climate and land use changes to runoff,sediment,nitrogen(N)and phosphorus(P)losses in the Haihe River Basin since the 1980s.The results showed that 1)climate and land use changes significantly increased evapotranspiration(ET),transport loss,sediment input and output,and organic N and P production,with ET,sediment input and organic N affected the most;2)runoff,sediment and ammonia N were most affected by climate and land use changes in the Daqing River Basin(217.3 mm),Nanyun River Basin(3917.3 tons)and Chaobai River Basin(87.6 kg/ha),respectively;3)the impacts of climate and land use changes showed explicit spatiotemporal patterns.In the Daqing,Yongding and Nanyun River Basins,the contribution of climate change to runoff and sediment kept increasing,reaching 88.6%-98.2%and 63%-77.2%,respectively.In the Ziya and Chaobai River Basins,the contribution of land use was larger,reaching 88.6%-92.8%and 59.8%-92.7%,respectively.In the Yongding,Chaobai,Ziya and Daqing River Basins,the contribution of land use to N and P losses showed an increasing trend over the past 40 years(maximum 89.7%).By contrast,in Nanyun and Luanhe River Basins,the contribution of climate change to N and P losses increased more(maximum 92.1%).Our evaluation of the impacts of climate and land use changes on runoff,sediment,and N and P losses will help to support the optimization of land and water resources in the Haihe River Basin.