Evapotranspiration Trend and Its Relationship with Precipitation over the Loess Plateau during the Last Three Decades

There have been few studies conducted on the changes in actual ET over the Loess Plateau, due to the lack of reliable ET data. Based on ET data simulated by the Community Land Model, the present study analyzed the changes in ET over the Loess Plateau. The results showed the domain-average ET to have decreased in the past 31 years, at a rate of 0.78 mm year−1. ET fluctuated much more strongly in the 1990s than in the 1980s and 2000s, and, apart from in autumn, ET decreased in all seasons. In particular, ET in summer comprised about half of the annual ET trend and had the sharpest trend, dominating the interannual decline. ET also decreased more sharply in the semiarid than semihumid regions. The declining trend of ET was attributed to declining precipitation and air humidity. Locally, the ET trend was closely related to local mean annual precipitation: in areas with precipitation less than 400 mm, ET showed a decreasing trend; in areas with precipitation larger than 600 mm, ET showed an increasing trend; and in areas with precipitation in the range of 400–600 mm could be classified as a transitional zone

Hydrothermal Factors Influence on Spatial-Temporal Variation of Evapotranspiration-Precipitation Coupling over Climate Transition Zone of North China

As a land–atmosphere coupling “hot spot”, the northern China climate transition zone has a sharp spatial gradient of hydrothermal conditions, which plays an essential role in shaping the spatial and temporal pattern of evapotranspiration-precipitation coupling, but whose mechanisms still remain unclear. This study analyzes the spatial and temporal variation in land–atmosphere coupling strength (CS) in the climate transitional zone of northern China and its relationship with soil moisture and air temperature. Results show that CS gradually transitions from strong positive in the northwest to negative in the southeast and northeast corners. The spatial distribution of CS is closely related to climatic hydrothermal conditions, where soil moisture plays a more dominant role: CS increases first, and then decreases with increasing soil moisture, with the threshold of soil moisture at 0.2; CS gradually transitions from positive to negative at soil moisture between 0.25 and 0.35; CS shows an exponential decreasing trend with increasing temperature. In terms of temporal variation, CS is strongest in spring and weakens sequentially in summer, autumn, and winter, and has significant interdecadal fluctuations. The trend in CS shifts gradually from significantly negative in the west to a non-significant positive in the east. Soil moisture variability dominates the intra-annual variability of CS in the study regions, and determines the interannual variation of CS in arid and semi-arid areas. Moreover, the main reason for the positive and negative spatial differences in CS in the study area is the different driving regime of evapotranspiration (ET). ET is energy-limited in the southern part of the study area, leading to a positive correlation between ET and lifting condensation level (LCL), while in most of the northern part, ET is water-limited and is negatively correlated with LCL; LCL has a negative correlation with P across the study area, thus leading to a negative ET-P coupling in the south and a positive coupling in the north

Hydrothermal Factors Influence on Spatial-Temporal Variation of Evapotranspiration-Precipitation Coupling over Climate Transition Zone of North China

As a land–atmosphere coupling “hot spot”, the northern China climate transition zone has a sharp spatial gradient of hydrothermal conditions, which plays an essential role in shaping the spatial and temporal pattern of evapotranspiration-precipitation coupling, but whose mechanisms still remain unclear. This study analyzes the spatial and temporal variation in land–atmosphere coupling strength (CS) in the climate transitional zone of northern China and its relationship with soil moisture and air temperature. Results show that CS gradually transitions from strong positive in the northwest to negative in the southeast and northeast corners. The spatial distribution of CS is closely related to climatic hydrothermal conditions, where soil moisture plays a more dominant role: CS increases first, and then decreases with increasing soil moisture, with the threshold of soil moisture at 0.2; CS gradually transitions from positive to negative at soil moisture between 0.25 and 0.35; CS shows an exponential decreasing trend with increasing temperature. In terms of temporal variation, CS is strongest in spring and weakens sequentially in summer, autumn, and winter, and has significant interdecadal fluctuations. The trend in CS shifts gradually from significantly negative in the west to a non-significant positive in the east. Soil moisture variability dominates the intra-annual variability of CS in the study regions, and determines the interannual variation of CS in arid and semi-arid areas. Moreover, the main reason for the positive and negative spatial differences in CS in the study area is the different driving regime of evapotranspiration (ET). ET is energy-limited in the southern part of the study area, leading to a positive correlation between ET and lifting condensation level (LCL), while in most of the northern part, ET is water-limited and is negatively correlated with LCL; LCL has a negative correlation with P across the study area, thus leading to a negative ET-P coupling in the south and a positive coupling in the north

A SIMPLE MODEL FOR THE CHLORIDE DIFFUSION COEFFICIENT OF STEEL FIBRE REINFORCED CONCRETE

The chloride diffusion coefficient is the most important parameter when describing the chloride ingress in concrete. This paper proposes a simple model to predict the chloride diffusion coefficient of steel fibre reinforced concrete (SFRC). The chloride diffusion coefficients of a cement paste and an interfacial transition zone (ITZ) were calculated with their porosities, respectively. The ITZ porosity was obtained with a w/b ratio based on a porosity model. The chloride diffusion coefficient of the matrix was then determined according to the general self-consistent scheme, and the influence of the steel fibre was simplified with its volume fraction. The calculated chloride diffusion coefficient of the SFRC was verified with the results from the bulk diffusion tests and the accuracy was acceptable

Enhanced Impact of Vegetation on Evapotranspiration in the Northern Drought-Prone Belt of China

Evapotranspiration (ET) is an essential component of the land–atmosphere water cycle. In this work, the trend of ET and its dominant factors during 1982 to 2011 are investigated in the northern drought-prone belt of China (NDPB) based on five datasets, including the gridded FLUXNET, using the Pearson correlation and linear regression methods. Specially, we focus on the increasing contribution of vegetation in the change of ET. During 1982–2011, summer ET significantly increased at the rate of 0.33 mm/year (p p p < 0.01) and positive correlation with ET before and after the fluctuation in the NDPB, and it reflected over 60% of the change in ET. Moreover, the LAI in NDPB increased by 19.6% which was more than double of the global-mean increase. The ET increase due to rising LAI offset the ET decrease due to reduction of soil moisture, and vegetation became the primary constraint on ET during 2001–2011. The expansion of vegetation may intensify the risk of drought and cause conflicting demands for water between the ecosystem and humans in the NDPB, especially in the case of weak summer monsoon