Impact of land use/land cover types on surface humidity in northern China in the early 21^(st)century

In the context of global change,it is essential to promote the rational development and utilization of land resources,improve the quality of regional ecological environment,and promote the harmonious development of human and nature for the regional sustainability.We identified land use/land cover types in northern China from 2001 to 2018 with ENVI images and ArcGIS software.Meteorological data were selected from 292 stations in northern China,the potential evapotranspiration was calculated with the Penman-Monteith formula,and reanalysis humidity and observed humidity data were obtained.The reanalysis minus observation(RMO,i.e.,the difference between reanalysis humidity and observed humidity)can effectively characterize the impact of different land use/land cover types(forestland,grassland,cultivated land,construction land,water body and unused land)on surface humidity in northern China in the early 21^(st) century.The results showed that from 2001 to 2018,the area of forestland expanded(increasing by approximately 1.80×10^(4) km^(2)),while that of unused land reduced(decreasing by approximately 5.15×10^(4) km^(2)),and the regional ecological environment was improved.Consequently,land surface in most areas of northern China tended to be wetter.The contributions of land use/land cover types to surface humidity changes were related to the quality of the regional ecological environment.The contributions of the six land use/land cover types to surface humidity were the highest in northeastern region of northern China,with a better ecological environment,and the lowest in northwestern region,with a fragile ecological environment.Surface humidity was closely related to the variation in regional vegetation coverage;when the regional vegetation coverage with positive(negative)contributions expanded(reduced),the land surface became wetter.The positive contributions of forestland and water body to surface humidity were the greatest.Unused land and construction land were associated with the most serious negative contributions to surface humidity.Affected by the regional distribution pattern of vegetation,surface humidity in different seasons decreased from east to west in northern China.The seasonal variation in surface humidity was closely related to the growth of vegetation:surface humidity was the highest in summer,followed by autumn and spring,and the lowest in winter.According to the results,surface humidity is expected to increase in northeastern region of northern China,decrease in northern region,and likely increase in northwestern region.

Evaluation of Surface Relative Humidity in China from the CRA-40 and Current Reanalyses

Recently,the China Meteorological Administration(CMA)released a new Global Atmospheric Reanalysis(CRA-40)dataset for the period 1979−2018.In this study,surface relative humidity(RH)from CRA-40 and other current reanalyses(e.g.,CFSR,ERA5,ERA-Interim,JRA-55,and MERRA-2)is comprehensively evaluated against homogenized observations over China.The results suggest that most reanalyses overestimate the observations by 15%−30%(absolute difference)over the Tibetan Plateau but underestimate the observations by 5%−10%over most of northern China.The CRA-40 performs relatively well in describing the long-term change and variance seen in the observed surface RH over China.Most of the reanalyses reproduce the observed surface RH climatology and interannual variations well,while few reanalyses can capture the observed long-term RH trends over China.Among these reanalyses,the CFSR does poorly in describing the interannual changes in the observed RH,especially in Southwest China.An empirical orthogonal function(EOF)analysis also suggests that the CRA-40 performs better than other reanalyses to capture the first two leading EOF modes revealed by the observations.The results of this study are expected to improve understanding of the strengths and weaknesses of the current reanalysis products and thus facilitate their application.

Surface Acoustic Wave Humidity Sensors Based on(1120) ZnO Piezoelectric Films Sputtered on R-Sapphire Substrates

ZnO films on R-sapphire substrates are prepared and characterized by x-ray diffraction and scanning electron microscopy, which indicate that the thin films are well crystallized with （1120） texture. Love wave and Rayleigh wave are used for fabrications of humidity sensors, which are excited in [1100] and [0001] directions of the （1120） ZnO piezoelectric films, respectively. The experimental results show that both kinds of sensors have good humidity response and repeatability, and the performances of the Love wave sensors are better than those of the Rayleigh wave sensors at room temperature. Moreover, the theoretical calculations of the mass sensitivity of the sensors are a/so carried out and the calculated results are in good agreement with the experimental measurements.

Reciprocal Analysis of Sensible and Latent Heat Fluxes in a Forest Region Using Single Height Temperature and Humidity Based on the Bowen Ratio Concept

Evapotranspiration in forests has been researched for a long time because it serves an important role in water resource issues and biomass production. By applying the reciprocal analysis based on the Bowen ratio concept to the canopy surface, the sum result of sensible and latent heat fluxes, i.e., actual evapotranspiration (ET), is estimated from engineering aspect using the net radiation (Rn) and heat flux into the ground (G). The new method uses air temperature and humidity at a single height by determining the relative humidity (rehs) using the canopy temperature (Ts). The validity of the method is confirmed by the latent heat flux (lE) and sensible heat flux (H) observed by mean of eddy covariance method. The heat imbalance is corrected by multiple regression analysis. The temporal change of lE and H at the canopy surface is clarified using hourly and yearly data. Furthermore, the observed and estimated monthly evapotranspiration of the sites are compared. The research is conducted using hourly data and the validation of the method is conducted using observed covariance at five sites in the world using FLUXNET.

The extreme dry/wet events in northern China during recent 100 years

Using monthly precipitation and monthly mean temperature, a surface humid index was proposed. According to the index, the distributed characteristics of extreme dryness has been fully analyzed. The results indicated that there is an obvious increasing trend of extreme dryness in the central part of northern China and northeastern China in the last 10 years, which shows a high frequency period of extreme dryness; while a low frequency period in the regions during the last 100 years. Compared with variation trend of the temperature in these regions, the region of high frequent extreme dryness is consistent with the warming trend in the same region.

Application of the Reciprocal Analysis for Sensible and Latent Heat Fluxes with Evapotranspiration at a Humid Region

Evapotranspiration acts an important role in hydrologic cycle and water resources planning. But the estimation issue still remains until nowadays. This research attempts to make clear this problem by the following way. In a humid region, by applying the Bowen ratio concept and optimum procedure on the soil surface, sensible and latent heat fluxes are estimated using net radiation (Rn) and heat flux into the ground (G). The method uses air temperature and humidity at a single height by reciprocally determining the soil surface temperature (Ts) and the relative humidity (rehs). This feature can be remarkably extended to the utilization. The validity of the method is confirmed by comparing of observed and estimated latent (lE) and sensible heat flux (H) using the eddy covariance method. The hourly change of the lE, H, Ts and rehs on the soil surface, yearly change of lE and H and relationship of estimated lE and H versus observed are clarified. Furthermore, monthly evapotranspiration is estimated from the lE. The research was conducted using hourly data of FLUXNET at a site of Japan, three sites of the United States and two sites of Europe in humid regions having over 1000 mm of annual precipitation.