湿地系统蒸腾蒸发损失量及污染物去除规律研究

在人工湿地中试系统和实际规模的湿地系统中分别进行了冬季和春季土壤水分蒸腾蒸发损失量(ET)和污染物去除规律的试验。研究结果表明,春季的ET约为冬季的2倍,且自由水面蒸发量远远小于有植物存在时的ET。在相同条件下,有植物存在时湿地系统的ET量排序为:芦苇(挺水植物)>茭草(挺水植物)>浮萍(浮水植物),且芦苇表面流湿地的ET要大于芦苇潜流湿地的。春季湿地系统的总氮、硝态氮、氨氮、总磷和COD去除率均高于冬季的,这主要与植物的生长阶段和湿地的蒸腾蒸发损失量有关。若进水污染物浓度高,则相应的去除率就高,这是春季有机氮的去除率低于冬季的主要原因。

Evapotranspiration and Soil Moisture Balance for Vegetative Restoration in a Gully Catchment on the Loess Plateau, China

Evapotranspiration, soil moisture balance and the dynamics in a gully catchment of the Loess Plateau in China were determined with 6 land use treatments including natural grassland, shrubs （Caragana rnicrophylla）, two woodlands （Prunus armeniaca var. ansu and Pinus tabulaeformis）, cultivated fallow, and farmland （Triticum aestiuum L.） in order to obtain a better understanding of soil moisture balance principles and to improve vegetation restoration efficiency for ecological rebuilding on the plateau. Average runoff from cultivated fallow was very high, reaching 10.3% of the seasonal rainfall. Evapotranspiration under T. aestivurn was not significantly different from natural grasslands. Compared with natural grass, evapotranspiration was significantly greater （P 〈 0.05） in 2002 and there was an increase in soil moisture depleted in the 1-3 m soil under P. armeniaca, P. tabulaeformis and C. microphylla. During the two years of the study the average soil moisture （0-100 cm soil profile） of T. aestivurn was generally the highest, with P. armeniaca, P. tabulaeformis and C. rnicrophylla usually the lowest. Thus, according to the soil moisture balance principle for this area the planned reforestation project was not ecologically reasonable. Reducing human disturbance and restoration with grass could be more effective.

Evapotranspiration, Yield and Crop Coefficient of Irrigated Maize Under Straw Mulch

Maize (Zea mays L.), a staple crop grown from June to September during the rainy season on the North China Plain,is usually inter-planted in winter wheat (Triticum aestivum L.) fields about one week before harvesting of the winterwheat. In order to improve irrigation efficiency in this region of serious water shortage, field studies in 1999 and 2001, twodry seasons with less than average seasonal rainfall, were conducted with up to five irrigation applications to determineevapotranspiration, calculate the crop coefficient, and optimize the irrigation schedule with maize under mulch, as well asto establish the effects of irrigation timing and the number of applications on grain yield and water use efficiency (WUE)of maize. Results showed that with grain production at about 8 000 kg ha-1 the total evapotranspiration and WUE ofirrigated maize under mulch were about 380-400 mm and 2.0-2.2 kg m-3, respectively. Also in 2001 WUE of maizewith mulch for the treatment with three irrigations was 11.8% better than that without mulch. In the 1999 and 2001seasons, maize yield significantly improved (P = 0.05) with four irrigation applications, however, further increases werenot significant. At the same time there were no significant differences for WUE with two to four irrigation applications.In the 2001 season mulch lead to a decrease of 50 mm in the total soil evaporation, and the maize crop coefficient undermulch varied between 0.3-1.3 with a seasonal average of 1.0.

Responses of soil moisture to vegetation restoration type and slope length on the loess hillslope

Soil moisture, a critical variable in the hydrologic cycle, is highly influenced by vegetation restoration type. However, the relationship between spatial variation of soil moisture, vegetation restoration type and slope length is controversial. Therefore, soil moisture across soil layers（0-400 cm depth） was measured before and after the rainy season in severe drought（2015） and normal hydrological year（2016） in three vegetation restoration areas（artificial forestland, natural forestland and grassland）, on the hillslopes of the Caijiachuan Catchment in the Loess area, China. The results showed that artificial forestland had the lowest soil moisture and most severe water deficit in 100-200 cm soil layers. Water depletion was higher in artificial and natural forestlands than in natural grassland. Moreover, soil moisture in the shallow soil layers（0-100 cm） under the three vegetation restoration types did not significantly vary with slope length, but a significant increase with slope length was observed in deep soil layers（below 100 cm）. In2015, a severe drought hydrological year, higher water depletion was observed at lower slope positions under three vegetation restoration types due to higher transpiration and evapotranspiration and unlikely recharge from upslope runoff. However, in 2016, a normal hydrological year, there was lower water depletion, even infiltration recharge at lower slope positions, indicating receiving a large amount of water from upslope. Vegetation restoration type, precipitation, slope length and soil depth during a rainy season, in descending order of influence, had significant effects on soil moisture. Generally, natural grassland is more beneficial for vegetation restoration than natural and artificial forestlands, and the results can provide useful information for understanding hydrological processes and improving vegetation restoration practices on the Loess Plateau

Daily SPEI Reveals Long-term Change in Drought Characteristics in Southwest China

Drought is the most widespread and insidious natural hazard, presenting serious challenges to ecosystems and human society. The daily Standardized Precipitation Evapotranspiration Index(SPEI) has been developed to identify the regional spatiotemporal characteristics of drought conditions from 1960 to 2016, revealing the variability in drought characteristics across Southwest China. Daily data from142 meteorological stations across the region were used to calculate the daily SPEI at the annual and seasonal time scale. The Mann-Kendall test and the trend statistics were then applied to quantify the significance of drought trends, with the following results. 1) The regionally averaged intensity and duration of all-drought and severe drought showed increasing trends, while the intensity and duration of extreme drought exhibited decreasing trends. 2) Mixed(increasing/decreasing) trends were detected, in terms of intensity and duration, in the three types of drought events. In general, no evidence of significant trends(P < 0.05) was detected in the drought intensity and duration over the last 55 years at the annual timescale. Seasonally, spring was characterized by a severe drought trend for all drought and severe drought conditions, while extreme drought events in spring and summer were very severe. All drought intensities and durations showed an increasing trend across most regions, except in the northwestern parts of Sichuan Province. However, the areal extent of regions suffering increasing trends in severe and extreme drought became relatively smaller. 3) We identified the following drought hotspots: Guangxi Zhuang Autonomous Region from the 1960 s to the 1990 s, respectively. Guangxi Zhuang Autonomous Region and Guizhou Province in the 1970 s and 1980 s, and Yunnan Province in the 2000 s. Finally, this paper can benefit operational drought characterization with a day-to-day drought monitoring index, enabling a more risk-based drought management strategy in the context of global warming.

Drought and Spatiotemporal Variability of Forest Fires Across Mexico

Understanding the spatiotemporal links between drought and forest fire occurrence is crucial for improving decision-making in fire management under current and future climatic conditions. We quantified forest fire activity in Mexico using georeferenced fire records for the period of 2005–2015 and examined its spatial and temporal relationships with a multiscalar drought index, the Standardized Precipitation-Evapotranspiration Index(SPEI). A total of 47975 fire counts were recorded in the 11-year long study period, with the peak in fire frequency occurring in 2011. We identified four fire clusters, i.e., regions where there is a high density of fire records in Mexico using the Getis-Ord G spatial statistic. Then, we examined fire frequency data in the clustered regions and assessed how fire activity related to the SPEI for the entire study period and also for the year 2011. Associations between the SPEI and fire frequency varied across Mexico and fire-SPEI relationships also varied across the months of major fire occurrence and related SPEI temporal scales. In particular, in the two fire clusters located in northern Mexico(Chihuahua, northern Baja California), drier conditions over the previous 5 months triggered fire occurrence. In contrast, we did not observe a significant relationship between drought severity and fire frequency in the central Mexico cluster, which exhibited the highest fire frequency. We also found moderate fire-drought associations in the cluster situated in the tropical southern Chiapas where agriculture activities are the main causes of forest fire occurrence. These results are useful for improving our understanding of the spatiotemporal patterns of fire occurrence as related to drought severity in megadiverse countries hosting many forest types as Mexico.

Evapotranspiration and humidity variations in response to land cover conversions in the Three Gorges Reservoir Region

A new land cover classification system was established for the Three Gorges Reservoir Region(TGRR) after considering the continuity of inundation and the natural characteristics of land cover. The potential evapotranspiration(PET) was predicted using a modified Penman-Monteith(P-M) model. The region's ratio of precipitation to evapotranspiration was calculated as the humidity index(HI). The data obtained was used to analyze climatic responses to land cover conversions from the perspectives of evapotranspiration and humidity variations. The results show that, from 1997 to 2009, the average annual PET increased in the early years and decreased later. In terms of overall spatial distribution, a significant reciprocal relationship appeared between annual PET and annual HI. In 1997,the annual PET was higher in the lower reaches than in the upper reaches of the TGRR, but the areas with high PET shifted substantially westward by 2003. The annual PET continued to increase in 2006, but the areas with high PET shrank by 2009. In contrast, the annual HI showed varying degrees of localized spatial variability. Over the three periods, the dominantforms of land cover conversions occurred from evergreen cover to seasonal green cover, from seasonal green cover to evergreen cover, and from seasonal green cover to seasonally inundated areas, respectively. These accounted for 48.0%, 38.4%, and 23.8% of the total areas of converted land covers in the three periods, respectively. During the period between 1997 and 2003, the main forms of land cover conversions resulted in both positive and negative growths in the average annual PET, while all of them pushed down the average annual HI. From 2003 to 2006, the reservoir region experienced neither a decrease in the annual PET nor an increase in the annual HI. The period between 2006 and 2009 saw a consistent downward trend in the annual PET and a consistent upward trend in the annual HI.

Validation of Global Evapotranspiration Product(MOD16) Using Flux Tower Data from Panjin Coastal Wetland,Northeast China

Recent advances in remote sensing technology and methods have resulted in the development of an evapotranspiration(ET) product from the Moderate Resolution Imaging Spectrometer(MOD16). The accuracy of this product however has not been tested for coastal wetland ecosystems. The objective of this study therefore is to validate the MOD16 ET product using data from one eddy covariance flux tower situated in the Panjin coastal wetland ecosystem within the Liaohe River Delta, Northeast China. Cumulative ET data over an eight-day period in 2005 from the flux tower was calculated to coincide with the MOD16 products across the same period. Results showed that data from the flux tower were inconsistent with that gained form the MOD16 ET. In general, results from Panjin showed that there was an underestimation of MOD16 ET in the spring and fall, with Biases of -2.27 and -3.53 mm/8 d, respectively(–40.58% and -49.13% of the observed mean). Results for Bias during the summer had a range of 1.77 mm/8 d(7.82% of the observed mean), indicating an overestimation of MOD16 ET. According to the RMSE, summer(6.14 mm/8 d) achieved the lowest value, indicating low accuracy of the MOD16 ET product. However, RMSE(2.09 mm/8 d) in spring was the same as that in the fall. Relationship between ET and its relevant meteorological parameters were analyzed. Results indicated a very good relationship between surface air temperature and ET. Meanwhile a significant relationship between wind speed and ET also existed. The inconsistent comparison of MOD16 and flux tower-based ET are mainly attributed to the parameterization of the Penman-Monteith model, flux tower measurement errors, and flux tower footprint vs. MODIS pixels.

Study of the temporal and spatial patterns of drought in the Yellow River basin based on SPEI

Drought is one of the severe natural disasters to impact human society and occurs widely and frequently in China,causing considerable damage to the living environment of humans.The Yellow River basin(YRB)of China shows great vulnerability to drought in the major basins;thus,drought monitoring in the YRB is particularly important.Based on monthly data of 124 meteorological stations from 1961 to 2015,the Standardized Precipitation Evapotranspiration Index(SPEI)was used to explore the temporal and spatial patterns of drought in the YRB.The periods and trends of drought were identified by Extreme-point Symmetric Mode Decomposition(ESMD),and the research stages were determined by Bernaola-Galvan Segmentation Algorithm(BGSA).The annual and seasonal variation,frequency and intensity of drought were studied in the YRB.The results indicated that(1)for the past 55 years,the drought in the YRB has increased significantly with a tendency rate of-0.148(10 a)^(-1),in which the area Lanzhou to Hekou was the most vulnerable affected(-0.214(10 a)^(-1));(2)the drought periods(2.9,5,10.2 and 18.3 years)and stages(1961–1996,1997–2002 and 2003–2015)were characterized and detected by ESMD and BGSA;(3)the sequence of drought frequency was summer,spring,autumn and winter with mean values of 71.0%,47.2%,10.2%and 6.9%,respectively;and(4)the sequence of drought intensity was summer,spring,winter and autumn with mean values of 0.93,0.40,0.05 and 0.04,respectively.

Spatiotemporal variations of aridity in China during 1961–2015: decomposition and attribution

Changes in global climate intensify the hydrological cycle, directly influence precipitation, evaporation, runoff, and cause the re-distribution of water resources in time and space. The aridity index （AI）, defined as the ratio of annual precipitation to annual potential evapotranspiration, is a widely used numerical indicator to quantify the degree of dryness at a given location. This study examined the effects of climate change on Al in China during 1961-2015. The results showed that the nationally averaged AI experienced a notable interdecadal transition in 1993, characterized by increasing AI （wetter） between 1961 and 1993, and decreasing AI （drier） after 1993. Overall, the decreased solar radiation （solar dimming） was the main factor affected the nationally averaged AI during 1961-1993, while the relative humidity dominated the variations of nationally averaged AI during 1993-2015. However, the roles of individual factors on the changes in AI vary in different subregions. Precipitation is one of the important contributing factors for the changes orAl in almost all subregions, except the Mid-Lower Yangtze and Huaihe basins. Solar radiation has been significantly decreased during 1961-1993 in South China, Southwest China, Mid-Lower Yangtze and Huaihe basins, and the Tibetan Plateau. Therefore, it dominated the trends of AI in these subregions. The relative humidity mainly affected the Mid-Lower Yangtze and Huaihe basins, Southwest China, and the Tibetan Plateau during 1993-2015, hence dominated the trends of Al in these subregions. The changes of temperature and wind speed, however, played a relatively weak role in the variations of AI.

Influence of land evapotranspiration on climate variations

A coupled numerical model of the global atmosphere with a qualified biosphere (GOALS/LASG) has been used to assess the nature of the physical mechanisms for land-atmosphere interactions, and the impacts of the Asian/North American land-surface evapotranspiration on the regional and global climate. This sensitivity study suggests that the simulated climate would be relatively sensitive to land surface evapotranspiration, especially over the Asian regions. The removal of evapotranspiration in Asia would create a warmer and drier climate to a certain degree. Furthermore, the surface evapotranspiration anomalies would make a substantial contribution to the formation and variation of subtropical anticyclones through the changes in monsoon precipitation and the β -effect, but also make a large contribution to the variations of the atmospheric circulation in the Northern Hemisphere and even the globe. Therefore, besides the traditional perception that we have generally emphasized on the influence of subtropical anticyclones activities on the boreal summer precipitation over the regions of eastern China, the surface evapotranspiration anomalies, however, also have substantial impacts on the subtropical anticyclones through the changes in monsoon precipitation. For this reason, the variation in the internal heating sources of the atmosphere caused by the land surface evapotranspiration and the vapor phase change during the boreal summer is an important external factor forcing the weather and climate.

Impacts of hydraulic redistribution on eco-hydrological cycles： A case study over the Amazon basin

Hydraulic redistribution(HR)refers to the process of soil water transport through the low-resistance pathway provided by plant roots.It has been observed in field studies and proposed to be one of the processes that enable plants to resist water limitations.However,most land-surface models(LSMs)currently do not include this underground root process.In this study,a HR scheme was incorporated into the Community Land Model version 4.5(CLM4.5)to investigate the effect of HR on the eco-hydrological cycle.Two paired numerical simulations(with and without the new HR scheme)were conducted for the Tapajos National Forest km83(BRSa3)site and the Amazon.Simulations for the BRSa3 site in the Amazon showed that HR during the wet season was small,<0.1 mm day^(–1),transferring water from shallow wet layers to deep dry layers at night;however,HR in the dry season was more obvious,up to 0.3 mm day^(–1),transferring water from deep wet layers to shallow dry layers at night.By incorporating HR into CLM4.5,the new model increased gross primary production(GPP)and evapotranspiration(ET)by 10%and 15%,respectively,at the BRSa3 site,partly overcoming the underestimation.For the Amazon,regional analysis also revealed that vegetation responses(including GPP and ET)to seasonal drought and the severe drought of 2005 were better captured with the HR scheme incorporated.

Influences of environmental changes on water storage variations in Central Asia

The spatio-temporal pattern of the global water resource has significantly changed with climate change and intensified human activities. The regional economy and ecological environment are highly affected by terrestrial water storage(TWS), especially in arid areas. To investigate the response relationships between TWS and changing environments(climate change and human activities) in Central Asia, we used the Gravity Recovery and Climate Experiment(GRACE) data, Climatic Research Unit(CRU) climate data and Moderate Resolution Imaging Spectroradiometer(MODIS) remote sensing data products(MOD16A2, MOD13A3 and MCD12Q1) from 2003 to 2013, as well as the slope and Pearson correlation analysis methods. Results indicate that:(1) TWS in about 77% of the study area decreased from 2003 to 2013. The total change volume of TWS is about 2915.6 × 108 m^3. The areas of decreased TWS are mainly distributed in the middle of Central Asia, while the areas of increased TWS are concentrated in the middle-altitude regions of the Kazakhstan hills and Tarim Basin.(2) TWS in about 5.91% of areas, mainly distributed in the mountain and piedmont zones, is significantly positively correlated with precipitation, while only 3.78% of areas show significant correlation between TWS and temperature. If the response time was delayed by three months, there would be a very good correlation between temperature and TWS.(3) There is a significantly positive relationship between TWS and Normalized Difference Vegetation Index(NDVI) in 13.35% of the study area.(4) The area of significantly positive correlation between TWS and evapotranspiration is about 31.87%, mainly situated in mountainous areas and northwestern Kazakhstan. The reduction of regional TWS is related to precipitation more than evaporation. Increasing farmland area may explain why some areas show increasing precipitation and decreasing evapotranspiration.(5) The influences of land use on TWS are still not very clear. This study could provide scientific data useful for the estimation of changes in TWS with climate change and human activities.

An improved technique for global daily sunshine duration estimation using satellite imagery

This paper presems an improved model for global sunshine duration estimation. The methodology incorporates geostationary satellite images by including snow cover information, sun and satellite angles and a trend correction factor for seasons, for the determination of cloud cover index. The effectiveness of the proposed methodology has been tested using Meteosat geostationary satellite images in the visible band with a temporal resolution of 1 h and spatial resolution of 2.5 km×2.5 km, for the Brue Catchment in the southwest of England. Validation results show a significant improvement in the estimation of global sunshine duration by the proposed method as compared to its predecessor （R2 is improved from 0.68 to 0.83, root mean squared error （RMSE） from 2.37 h/d to 1.19 h/d and the mean biased error （MBE） from 0.21 h/d to 0.08 h/d）. Further studies are needed to test this method in other parts of the world with different climate and geographical conditions.

Hydrologic implications of the isotopic kinetic fractionation of open-water evaporation

The kinetic fractionation of open-water evaporation against the stable water isotope H_2 ^(18)O is an important mechanism underlying many hydrologic studies that use ^(18)O as an isotopic tracer. A recent in-situ measurement of the isotopic water vapor flux over a lake indicates that the kinetic effect is much weaker(kinetic factor 6.2‰) than assumed previously(kinetic factor14.2‰) by lake isotopic budget studies. This study investigates the implications of the weak kinetic effect for studies of deuterium excess-humidity relationships, regional moisture recycling, and global evapotranspiration partitioning. The results indicate that the low kinetic factor is consistent with the deuterium excess-humidity relationships observed over open oceans.The moisture recycling rate in the Great Lakes region derived from the isotopic tracer method with the low kinetic factor is a much better agreement with those from atmospheric modeling studies than if the default kinetic factor of 14.2‰ is used. The ratio of transpiration to evapotranspiration at global scale decreases from 84±9%(with the default kinetic factor) to 76±19%(with the low kinetic factor), the latter of which is in slightly better agreement with other non-isotopic partitioning results.