Response of water and energy exchange to the environmental variable in a desert‐oasis wetland of Northwest China

A case study on a desert‐oasis wetland ecosystem in the arid region of Northwest China measured the seasonal and interannual variation in energy partitioning and evapotranspiration to analyse the response of water and energy exchange on soil moisture, groundwater, and environmental variables. Energy partitioning showed a clear seasonal and interannual variability, and the process of water and energy exchange differed significantly in the monthly and interannual scales. The net radiation was 7.31 MJ m^?2· day^?1, and sensible heat flux accounted for 50.42% of net radiation in energy fluxes, 40.56% for latent heat flux, and 9.02% for ground heat flux. The parameters in energy fluxes were best described by a unimodal curve, whereas sensible heat flux followed a bimodal curve. Variations in the daily evapotranspiration and crop evapotranspiration also exhibited a single peak curve with annual values of 569.84 and 644.47 mm, respectively. Canopy conductance averaged 20.77 ± 13.75 mm s^?1 and varied from 0.16 to 83.96 mm s^?1 during the two hydrological years. The variation in water and energy exchange reflected environmental conditions and depended primarily on vapour pressure deficit, net radiation, soil moisture, and water depth. Although the effects of precipitation on evapotranspiration showed that the response of this ecosystem to climate changes was not obvious, the variation of air temperatures had a strong influence on evapotranspiration, resulting in a significant increase in evapotranspiration (R = 0.730; P < 0.01). Copyright © 2013 John Wiley & Sons, Ltd.     

Xylem sap flows of irrigated Tamarix elongata Ledeb and the influence of environmental factors in the desert region of Northwest China

Tamarix elongata Ledeb is a desert shrub found in the desert region of Northwest China and is commonly cultivated as a sand‐holding plant in this region. To understand its water requirement and the effects of climate conditions on its growth, trunk xylem sap flows of irrigated 8‐year‐old Tamarix elongata Ledeb plants were monitored continuously with heat‐pulse sap flow meters for the entire season. Soil moisture contents at 0–300 cm layer depth were also measured with a tube type time domain reflectometry (Tube‐TDR). Meteorological factors, i.e. solar radiation, air temperature, relative humidity and wind speed were simultaneously monitored by an automatic weather station at the site. Daily and seasonal variations of the trunk sap fluxes and their correlations with the meteorological factors, reference evapotranspiration and soil moisture contents in the root‐zone were analysed. The results indicated that frost influenced the trunk sap flux greatly under irrigated conditions, although the flux generally fluctuated with the variation of environmental factors and showed a mean trunk sap flux of 4·18 l d^?1. There was a significantly exponential relationship between sap flux and the reference value of crop evapotranspiration, with a correlation coefficient of R² = 0·7172. The sap flux also had a significant correlation with the soil water contents at a depth of 150–300 cm from soil surface (R² = 0·5014). The order of the main meteorological factors affecting the sap flux of Tamarix elongata Ledeb trees was solar radiation > air temperature > vapour pressure deficit > relative humidity > wind speed. Copyright © 2007 John Wiley & Sons, Ltd.     

CO2 absorption of sandy soil induced by rainfall pulses in a desert ecosystem

Soil CO₂ flux is strongly influenced by precipitation in many ecosystem types, yet knowledge of the effects of precipitation on soil CO₂ flux in semi‐arid desert ecosystems remains insufficient, particularly for sandy soils. To address this, we investigated the response of sandy soil CO₂ flux to rainfall pulses in a desert ecosystem in northern China during August–September 2011. Significant changes (P < 0.05) were found in diel patterns of soil CO₂ flux induced by small (2.1 mm), moderate (12.4 mm) and large (19.7 mm) precipitation events. Further analysis indicated that rainfall pulses modified the response of soil CO₂ flux to soil temperature, including hysteresis between soil CO₂ flux and soil temperature, with F_s higher when T_s was increasing than when T_s was decreasing, and the linear relationship between them. Moreover, our results showed that rainfall could result in absorption of atmospheric CO₂ by soil, possibly owing to mass flow of CO₂ induced by a gradient of gas pressure between atmosphere and soil. After each precipitation event, soil CO₂ flux recovered exponentially to pre‐rainfall levels with time, with the recovery times exhibiting a positive correlation with precipitation amount. On the basis of the amounts of precipitation that occurred at our site during the measurement period (August–September), the accumulated rain‐induced carbon absorption evaluated for rainy days was 1.068 g C m^?2; this corresponds approximately to 0.5–2.1% of the net primary production of a typical desert ecosystem. Thus, our results suggest that rainfall pulses can strongly influence carbon fluxes in desert ecosystems. Copyright © 2014 John Wiley & Sons, Ltd.     

Variation and attribution of water use efficiency in sunflower and maize fields in an irrigated semi-arid area

Water use efficiency (WUE) links carbon and water exchanges between farmlands and the atmosphere. Understanding the variation and attribution of WUE is essential to reveal the physiological and ecological adaptation mechanisms of crops to the changing environment, and to better allocate, regulate and conserve water resources. However, few studies on the variation and attribution of WUE have been conducted in irrigated arid or semi-arid farmlands. Therefore, in this study, water and carbon fluxes were measured using eddy covariance systems in two farmlands (one sunflower field and one maize field) in a semi-arid irrigation district in China. It was found that the average WUE of sunflower during its full growth period was 1.72 g C kg⁻¹ H₂O, much lower than that of maize (4.07 g C kg⁻¹ H₂O). At each growth stage, the WUE of both crops were negatively correlated with vapour pressure deficit (VPD), net radiation (R_n) and soil water content (SWC). The negative correlations could be attributed to the arid meteorological condition and the relatively abundant soil moisture due to irrigation and shallow groundwater levels. VPD was the main factor affecting WUE, followed by R_n and SWC. It was also found that the response of WUE to crop leaf area index (LAI) and to canopy conductance (g_c) depended on the VPD ranges: when VPD increased, the response of WUE to LAI and to g_c decreased. Our findings could improve the understanding of the coupling effect of water and carbon fluxes over farmland ecosystems in arid and semi-arid irrigation areas and help improve agricultural production and save water resources in such areas.     

Seasonal variations in energy exchange and evapotranspiration of an oasis-desert ecotone in an arid region

Knowledge of exchanges of energy and water over terrestrial surfaces is the first step towards understand the ecohydrological mechanisms, particularly in water-limited ecosystems in dryland environments. However, patterns of energy exchange and evapotranspiration (ET) are not well understood in the oasis-desert ecotone, which plays an important role in protecting oases against the threat of desertification in arid regions of northwestern China. Here, the continuous measurements of surface energy fluxes were made using eddy covariance in conjunction with auxiliary measurements for 2 years (2014–2015) in an oasis-desert ecotone mainly covered by phreatophyte shrubs Haloxylon ammodendron, Nitraria tangutorum/sphaerocarpa, and Calligonum mongolicum in arid northwestern China. Based on the collated data for 2 years, statistical analysis on a 30-min time scale indicated that approximately 50% of daytime net radiation (R_n) in the ecotone was dissipated as H on average, and one-third of R_n was consumed by soil heat flux (G). Only 9% of R_n was consumed for latent heat flux (λE), which peaked in summer (21% in 2014 and 16% in 2015), corresponding to the highest rainfall season. Daily mean ET was approximately 1 mm days⁻¹ during the growing season of the shrub species. Accumulated annual ET was 195 and 181 mm in 2014 and 2015, respectively, exceeding the corresponding precipitation (P) by approximately 87 and 77 mm, indicating that groundwater may be another important source of water for ET in the ecotone aside from rainfall. Results within provide valuable insights into the mechanisms responsible for sustaining energy and water balance in the ecotone, a potentially groundwater-dependent ecosystem. These results also offer a foremost ecohydrological implication for water and land resources management and ecotone conservation, such as avoiding heavy groundwater pumping for extensive agricultural irrigation use to sustain groundwater availability for these shrub species in the ecotone.     

Identifying the source of atmospheric moisture over arid deserts using stable isotopes (2H and 18O) in precipitation

Precipitation is a major component of the hydrologic cycle in arid desert areas. To date, however, few studies have been conducted on investigating the isotope characteristics and moisture sources of precipitation in arid desert environments. The Alxa Desert Plateau is a critical arid desert area in North China. This study is the first to analyse the stable isotopic composition of precipitation to identify the sources of atmospheric moisture over this plateau. Our results show that the δD and δ¹⁸O values of precipitation across the plateau change greatly at both daily and monthly timescales, and exhibit seasonal variations. Among the main meteorological parameters, atmospheric temperature is the most predominant factor controlling the isotopic composition and the δD–δ¹⁸O relationship of local precipitation. Analyses of the precipitation isotopes with the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model reveal that (a) the westerly and polar moisture sources are the dominant controls on summer and winter precipitation and (b) the evaporation of local lake water significantly affects winter precipitation even though it only represents a small amount. Based on the isotope data of 2013–2016 precipitation, a local meteoric water line (LMWL) is derived: δD = (8.20 ± 0.22)·δ¹⁸O + (8.15 ± 2.16)‰ for the study site. Compared to the global meteoric water line, the LMWL has a greater slope and lower d‐excess. This can be explained by admixing of atmospheric moisture resulting from the evaporation of local lake water. Based on this LMWL, we are able to trace that groundwater of the Badain Jaran Desert originates from the surrounding mountains with altitudes of <4,000 m. The newly derived LMWL shows that the recharge altitudes of desert groundwater are overestimated on the basis of the previous LMWLs. This study not only provides insights into the hydrological cycle but also offers guidance for water resource management in arid desert areas of China. Additionally, this study provides techniques that can be applied to the analyses of precipitation isotopes in similar arid regions of the world.     

Mercury cycling during acid rain recovery at the forested Lesní potok catchment,Czech Republic

From 2011 to 2019, mercury (Hg) stores and fluxes were studied in the small forested catchment Lesní potok (LES) in the central Czech Republic using the watershed mass balance approach together with internal measurements. Mean input fluxes of Hg via open bulk deposition, beech throughfall and spruce throughfall during the periodwere 2.9, 3.9 and 7.6 μg m⁻² year⁻¹, respectively. These values were considerably lower than corresponding deposition Hg fluxes reported in the early years of the 21st century from catchments in Germany. Current bulk precipitation inputs at unimpacted Czech mountainous sites were lower than those in Germany. The largest Hg inputs to the catchment were via litterfall, averaging 22.6 and 17.8 μg m⁻² year⁻¹ for beech and spruce stands. The average Hg input, based on the sum of mean litterfall and throughfall deposition, was 23.0 μg m⁻² year⁻¹, compared to the estimated Hg output in runoff of 0.5 μg m⁻² year⁻¹, which is low compared to other reported values. Thus, only ~2% of Hg input is exported in stream runoff. Stream water Hg was only weakly related to dissolved organic carbon (DOC) but both concentrations were positively correlated with water temperature. The estimated total soil Hg pool averaged 47.5 mg m⁻², only 4% of which was in the O-horizon. Thus Hg in the O-horizon pool represents 72 years of deposition at the current input flux and 3800 years of export at the current runoff flux. Age-dating by ¹⁴C suggested that organic soil contains Hg from recent deposition, while mineral soil at 40–80 cm depth contained 4400-year old carbon, suggesting the soil had accumulated atmospheric Hg inputs through millennia to reach the highest soil Hg pool of the soil profile. These findings suggest that industrial era intensification of the Hg cycle is superimposed on a slower-paced Hg cycle during most of the Holocene.     

Developing an empirical model of canopy water flux describing the common response of transpiration to solar radiation and VPD across five contrasting woodlands and forests

A modified Jarvis–Stewart model of canopy transpiration (E_c) was tested over five ecosystems differing in climate, soil type and species composition. The aims of this study were to investigate the model's applicability over multiple ecosystems; to determine whether the number of model parameters could be reduced by assuming that site‐specific responses of E_c to solar radiation, vapour pressure deficit and soil moisture content vary little between sites; and to examine convergence of behaviour of canopy water‐use across multiple sites. This was accomplished by the following: (i) calibrating the model for each site to determine a set of site‐specific (SS) parameters, and (ii) calibrating the model for all sites simultaneously to determine a set of combined sites (CS) parameters. The performance of both models was compared with measured E_c data and a statistical benchmark using an artificial neural network (ANN). Both the CS and SS models performed well, explaining hourly and daily variation in E_c. The SS model produced slightly better model statistics [R² = 0.75–0.91; model efficiency (ME) = 0.53–0.81; root mean square error (RMSE) = 0.0015–0.0280 mm h^‐1] than the CS model (R² = 0.68–0.87; ME = 0.45–0.72; RMSE = 0.0023–0.0164 mm h^‐1). Both were highly comparable with the ANN (R² = 0.77–0.90; ME = 0.58–0.80; RMSE = 0.0007–0.0122 mm h^‐1). These results indicate that the response of canopy water‐use to abiotic drivers displayed significant convergence across sites, but the absolute magnitude of E_c was site specific. Period totals estimated with the modified Jarvis–Stewart model provided close approximations of observed totals, demonstrating the effectiveness of this model as a tool aiding water resource management. Analysis of the measured diel patterns of water use revealed significant nocturnal transpiration (9–18% of total water use by the canopy), but no Jarvis–Stewart formulations are able to capture this because of the dependence of water‐use on solar radiation, which is zero at night. Copyright © 2012 John Wiley & Sons, Ltd.     

Eddy covariance imaging of diffuse volcanic CO<Subscript>2</Subscript> emissions at Mammoth Mountain,CA, USA

Use of eddy covariance (EC) techniques to map the spatial distribution of diffuse volcanic CO₂ fluxes and quantify CO₂ emission rate was tested at the Horseshoe Lake tree-kill area on Mammoth Mountain, California, USA. EC measurements of CO₂ flux were made during September–October 2010 and ranged from 85 to 1,766 g m⁻² day⁻¹. Comparative maps of soil CO₂ flux were simulated and CO₂ emission rates estimated from three accumulation chamber (AC) CO₂ flux surveys. Least-squares inversion of measured eddy covariance CO₂ fluxes and corresponding modeled source weight functions recovered 58–77% of the CO₂ emission rates estimated based on simulated AC soil CO₂ fluxes. Spatial distributions of modeled surface CO₂ fluxes based on EC and AC observations showed moderate to good correspondence (R ² = 0.36 to 0.70). Results provide a framework for automated monitoring of volcanic CO₂ emissions over relatively large areas.     

Aquifer recharge from flash floods in the arid environment: A mass balance approach at the catchment scale

Estimation of the infiltration/natural recharge to groundwater from rainfall is an important issue in hydrology, particularly in arid regions. This paper proposes the application of The Natural Resources Conservation Service (NRCS) mass balance model to develop infiltration (F)–rainfall (P) relationship from flash flood events. Moreover, the NRCS method is compared with the rational and the Ф-index methods to investigate the discrepancies between these methods. The methods have been applied to five gauged basins and their 19 sub-basins (representative basins with detailed measurements) in the southwestern part of Saudi Arabia with 161 storms recorded in 4 years. The F–P relationships developed in this study based on NRCS method are: F = 39% P with R² = 0.932 for the initial abstraction factor, λ = 0.2. However, F = 77% P with R² = 0.986 for λ = 0.01. The model at λ = 0.01 is the best to fit the data, therefore, it is recommended to use the formula at λ = 0.01. The results show that the NRCS model is appropriate for the estimation of the F–P relationships in arid regions when compared with the rational and the Ф index methods. The latter overestimates the infiltration because they do not take λ into account. There is no significant difference between F–P relationships at different time scales. This helps the prediction of infiltration rates for aquifer recharge at ungauged basins from monthly and annual rainfall data with a single formula.     

Direct measurement of dissolved N2 and denitrification along a subtropical river-estuary gradient,China

The spatial pattern and seasonal variation of denitrification were investigated during 2010–2011 in the Jiulong River Estuary (JRE) in southeast China. Dissolved N₂ was directly measured by changes in the N₂:Ar ratio. The results showed that excess dissolved N₂ ranged from ?9.9 to 76.4 μmol L^?1. Tidal mixing leads to a seaward decline of dissolved gaseous concentrations and water–air fluxes along the river-estuary gradient. Denitrification at freshwater sites varied between seasons, associated with changes in N input and water temperature. The denitrification process was controlled by the nitrate level at freshwater sites, and the excess dissolved N₂ observed at the tidal zone largely originated from upstream water transport. Compared to other estuaries, JRE has a relative low gaseous removal efficiency (E_d = 12% of [DIN]; annual N removal = 24% of DIN load), a fact ascribed to strong tidal mixing, coarse-textured sediment with shallow depth before bedrock and high riverine DIN input.     

Modeling gross primary production of a temperate grassland ecosystem in Inner Mongolia,China, using MODIS imagery and climate data

Carbon fluxes in temperate grassland ecosystems are characterized by large inter-annual variations due to fluctuations in precipitation and land water availability. Since an eddy flux tower has been in operation in the Xilin Gol grassland, which belongs to typical temperate grassland in North China, in this study, observed eddy covariance flux data were used to critically evaluate the biophysical per- formance of different remote sensing vegetation indices in relation to carbon fluxes. Furthermore, vegetation photosynthesis model (VPM) was introduced to estimate gross primary production (GPP) of the grassland ecosystem for assessing its dependability. As defined by the input variables of VPM, Moderate Resolution Imaging Spectroradimeter (MODIS) and standard data product MOD09A1 were downloaded for calculating enhanced vegetation index (EVI) and land surface water index (LSWI). Measured air temperature (Ta) and photosynthetically active radiation (PAR) data were also included for model simulating. Field CO2 flux data, during the period from May, 2003 to September, 2005, were used to estimate the "observed" GPP (GPPobs) for validation. The seasonal dynamics of GPP predicted from VPM (GPPVPM) was compared quite well (R2=0.903, N=111, p<0.0001) with the observed GPP. The ag- gregate GPPVPM for the study period was 641.5 g C·m?2, representing a ~6% over-estimation, compared with GPPobs. Additionally, GPP predicted from other two typical production efficiency model (PEM) represents either higher overestimation or lower underestimation to GPPobs. Results of this study demonstrate that VPM has potential for estimating site-level or regional grassland GPP, and might be an effective tool for scaling-up carbon fluxes.     

Prediction of river temperature surges is dependent on precipitation method

Urban river systems are particularly sensitive to precipitation‐driven water temperature surges and fluctuations. These result from rapid heat transfer from low‐specific heat capacity surfaces to precipitation, which can cause thermally polluted surface run‐off to enter urban streams. This can lead to additional ecological stress on these already precarious ecosystems. Although precipitation is a first‐order driver of hydrological response, water temperature studies rarely characterize rain event dynamics and typically rely on single gauge data that yield only partial estimates of catchment precipitation. This paper examines three precipitation measuring methods (a statutory automatic weather station, citizen science gauges, and radar estimates) and investigates relationships between estimated rainfall inputs and subhourly surges and diurnal fluctuations in urban river water temperature. Water temperatures were monitored at 12 sites in summer 2016 in the River Rea, in Birmingham, UK. Generalized additive models were used to model the relationship between subhourly water temperature surges and precipitation intensity and subsequently the relationship between daily precipitation totals and standardized mean water temperature. The different precipitation measurement sources give highly variable precipitation estimates that relate differently to water temperature fluctuations. The radar catchment‐averaged method produced the best model fit (generalized cross‐validation score [GCV] = 0.30) and was the only model to show a significant relationship between water temperature surges and precipitation intensity (P < 0.001, R² = 0.69). With respect to daily metrics, catchment‐averaged precipitation estimates from citizen science data yielded the best model fit (GCV score = 0.20). All precipitation measurement and calculation methods successfully modelled the relationship between standardized mean water temperature and daily precipitation (P < 0.001). This research highlights the potential for the use of alternative precipitation datasets to enhance understanding of event‐based variability in water quality studies. We conclude by recommending the use of spatially distributed precipitation data operating at high spatial (<1 km²) and temporal (<15 min) resolutions to improve the analysis of event‐based water temperature and water quality studies.     

Simultaneous water and vapour transfer in an unsaturated mudstone in badlands terrain,southwestern Taiwan

The simultaneous transfer of pore fluid and vapour was studied in the unsaturated shallow subsurface of a Plio-Pleistocene marine mudstone badland slope in southwestern Taiwan during the dry season using field monitoring data and numerical simulations. Data from field monitoring show mass-basis water contents of ~0.05 to ~0.10 that decrease towards the unsaturated ground surface and were invariant during the middle part of the dry season, except for daily fluctuations. In addition, the observed daily fluctuations in water content correlate with fluctuations in bedrock temperature, especially at depths of 2.5–5.0 cm. Periodic increases in water content occurred most notably during the day, when the bedrock temperature showed the greatest increase. Water contents then decreased to the previous state as bedrock temperature decreased during the night. Calculated vapour fluxes within the mudstone during the day increased up to 6 × 10⁻⁶–1 × 10⁻⁵ kg m⁻² s⁻¹, deriving a 0.01–0.02 increase in mass-basis water content at 2.5 cm depth for a 12-h period. This agrees with field monitoring data, suggesting that increases in water content occurred due to vapour intrusions into the bedrock. Pore water electrical conductivity (EC) showed periodic variations due to vapour intrusion, and gradually increased between the ground surface and depths of 2.5–5.0 cm. In contrast, pore water EC gradually decreased between 15 and 40 cm depth. Calculated water fluxes at depths of 2.5–40.0 cm varied from −4 × 10⁻⁶ to −2 × 10⁻⁹ kg m⁻² s⁻¹. These fluxes generated an increase in solute concentrations at the ground surface, with negative values of water flux indicating an upwards movement of water towards the surface. We show that the increase in solute content due to solute transfer from depth is highly dependent on variations in water flux with depth. © 2020 John Wiley & Sons, Ltd.     

Comparison of five snow water equivalent estimation methods across categories

Snow water equivalent (SWE) is an important indicator used in hydrology, water resources, and climate change impact. There are various methods of estimating SWE (falling in 3 categories: indirect sensors, empirical models, and process‐based models), but few studies that provide comparison across these different categories to help users make decisions on monitoring site design or method selection. Five SWE estimation methods were compared against manual snow course data collected over 2 years (2015–2016) from the Dorset Environmental Science Centre, including the gamma‐radiation‐based CS725 sensor, 3 empirical estimation models (Sexstone snow density model, McCreight & Small snow density model, and a meteorology‐based model), and the University of British Columbia Watershed Model snow energy‐balance model. Snow depth, density, and SWE were measured at the Dorset Environmental Science Centre weather station in south‐central Ontario, on a daily basis over 6 winters from 2011 to 2016. The 2 snow density‐based models, requiring daily snow depth as input, gave the best performance (R² of .92 and .92 for McCreight & Small and Sexstone models, respectively). The CS725 sensor that receives radiation coming from soil penetrating the snowpack provided the same performance (R² = .92), proving that the sensor is an applicable method, although it is expensive. The meteorology‐based empirical model, requiring daily climate data including temperature, precipitation and solar radiation, gave the poorest performance (R² = .77). The energy‐balance‐based University of British Columbia Watershed Model snow module, only requiring climate data, worked better than the empirical meteorology‐based model (R² = .9) but performed worse than the density models or CS725 sensor. Given differences in application objectives, site conditions, and budget, this comparison across SWE estimation methods may help users choose a suitable method. For ongoing and new monitoring sites, installation of a CS725 sensor coupled with intermittent manual snow course measurements (e.g., weekly) is recommended for further SWE method estimation testing and development of a snow density model.     

Distribution of surface imperviousness in small urban catchments predicts runoff peak flows and stream flashiness

Urban growth is a global phenomenon, and the associated impacts on hydrology from land development are expected to increase, especially in peri‐urban catchments. It is well understood that greater peak flows and higher stream flashiness are associated with increased surface imperviousness and storm location. However, the effect of the distribution of impervious areas on runoff peak flow response and stream flashiness of peri‐urban catchments has not been well studied. In this study, a new geometric index, Relative Nearness of Imperviousness to the Catchment Outlet (RNICO), is defined to correlate imperviousness distribution of peri‐urban catchments with runoff peak flows and stream flashiness. Study sites include 21 suburban catchments in New York representing a range of drainage area from 5 to 189 km² and average imperviousness from 10% to 48%. On the basis of RNICO, all development patterns are divided into 3 classes: upstream, centralized, and downstream. Results showed an obvious increase in runoff peak flows and decrease in time to peak when moving from upstream to centralized and downstream urbanization classes. This indicates that RNICO is an effective tool for classifying urban development patterns and for macroscale understanding of the hydrologic behavior of small peri‐urban catchments, despite the complexity of urban drainage systems. We also found that the impact of impervious distribution on runoff peak flows and stream flashiness decreases with catchment scale. For small catchments (A < 40 km²), RNICO was strongly correlated with the average (R² = .95) and maximum (R² = .91) gaged peak flows due to the relatively efficient subsurface routing through stormwater and sewer networks. Furthermore, the Richards–Baker stream flashiness index in small catchments was positively correlated with fractional impervious area (R² = .84) and RNICO (R² = .87). For large catchments (A > 40 km²), the impact of impervious surface distribution on peak flows and stream flashiness was negligible due to the complex drainage network and great variability in travel times. This study emphasizes the need for greater monitoring of discharge in small peri‐urban catchments to support flood prediction at the local scale.     

Hydrometeorological behaviour of pine and larch forests in eastern Siberia

Seasonal changes in the water and energy exchanges over a pine forest in eastern Siberia were investigated and compared with published data from a nearby larch forest. Continuous observations (April to August 2000) were made of the eddy‐correlation sensible heat flux and latent heat flux above the canopy. The energy balance was almost closed, although the sum of the turbulent fluxes sometimes exceeded the available energy flux (R_n ? G) when the latent heat flux was large; this was related to the wind direction. We examined the seasonal variation in energy balance components at this site. The seasonal variation and magnitude of the sensible heat flux (H) was similar to that of the latent heat flux (λE), with maximum values occurring in mid‐June. Consequently, the Bowen ratio was around 1·0 on many days during the study period. On some clear days just after rainfall, λE was very large and the sum of H and λE exceeded R_n ? G. The evapotranspiration rate above the dry canopy from May to August was 2·2 mm day^?1. The contributions of understory evapotranspiration (E_u) and overstory transpiration (E_o) to the evapotranspiration of the entire ecosystem (E_t) were both from 25 to 50% throughout the period analysed. These results suggest that E_u plays a very important role in the water cycle at this site. From snowmelt through the tree growth season (23 April to 19 August 2000), the total incoming water, comprised of the sum of precipitation and the water equivalent of the snow at the beginning of the melt season, was 228 mm. Total evapotranspiration from the forest, including interception loss and evaporation from the soil when the canopy was wet, was 208–254 mm. The difference between the incoming and outgoing amounts in the water balance was from +20 to ?26 mm. The water and energy exchanges of the pine and larch forest differed in that λE and H increased slowly in the pine forest, whereas λE increased rapidly in the larch forest and H decreased sharply after the melting season. Consequently, the shape of the Bowen ratio curves at the two sites differed over the period analysed, as a result of the differences in the species in each forest and in soil thawing. Copyright © 2003 John Wiley & Sons, Ltd.     

Effectiveness of introducing crop coefficient and leaf area index to enhance evapotranspiration simulations in hydrologic models

Evapotranspiration (ET) is an essential component of the hydrological cycle and plays a critical role in water resource management. However, ET is often overlooked in order to transform rainfall to runoff for better streamflow simulation. Hydrological models are commonly used to estimate areal actual evapotranspiration (AET) after calibration against observed discharge. However, classical approaches are often inadequate to appropriately simulate other hydrologic components. Hence, it is important to introduce natural heterogeneity to enhance hydrological processes and reduce water balance errors. In this study, the effectiveness of introducing a constant crop coefficient (K_c), flux tower‐based K_c, and leaf area index (LAI) to three hydrological models (Three‐Parametric Hydrologic Model [TPHM], Génie Rural à 4 paramètres Journalier [GR4J], and Catchment hydrologic cycle Assessment Tool [CAT]) is assessed for the simulation of daily streamflow and AET in a mountainous mixed forest watershed (8.54 km²) in South Korea. The results show that the streamflow simulations after introduction of K_c and LAI to the original models are quite similar. However, the effectiveness of the AET estimation was significantly enhanced after introduction of the flux tower‐based K_c and LAI. The information criterion computed to compare the models reveals that the flux tower‐based K_c‐simulated AET demonstrated better agreement with the observed AET. The Pearson's correlation coefficients (R²) of the TPHM (8%), GR4J (55%), and CAT (55%) models also showed improvements that were greater than the constant based K_c simulation. Similarly, the limitations of the three models with respect to capturing seasonal variation as well as high and low flows were enhanced after the introduction of the flux tower‐based K_c, which adequately reproduced hydrological processes with minimum water balance errors and bias. A regression analysis revealed the potential of estimating K_c as a linear function of LAI (R² = 0.84). The results of this study indicate that introduction of K_c and LAI to the conceptual rainfall–runoff models can be considered an effective approach to reduce water balance errors and uncertainties in hydrological models and improve the reliability of climate change studies and water resource management.     

Soil moisture influenced the variability of air temperature and oasis effect in a large inland basin of an arid region

Soil moisture plays a significant role in land-atmosphere interactions. Changing fractions of latent and sensible heat fluxes caused by soil moisture variations can affect near-surface air temperature, thus influencing the oasis's cooling effect in arid regions. In this study, the framework for the evaporative fraction (EF) dependence on soil moisture is used to analyse the impacts of soil moisture variation on near-surface air temperature and the oasis effect. The result showed that soil moisture's contribution rate to EF was significantly higher than that of EF to temperature. Under the interaction of temperature sensitivity to EF and EF to soil moisture, the ∂T/∂ϴ presented a similar tempo-spatial variation with both of the above. It was most significant in oasis areas during summer (−1.676), while it was weaker in plain desert areas during the autumn (−0.071). In the study region, the effect of soil moisture variation on air temperature can reach 0.018–0.242 K for different land-cover types in summer. The maximum variation of soil moisture in summer can alter air temperature by up to 0.386 K. The difference in temperature variability between the oasis and desert areas promoted the formation of the oasis effect. For different oasis, the multi-year average oasis cold effect index (OCI) ranged from −1.36 to −0.26 K. In comparison, the average summer OCI ranged from −1.38 to −0.29 K. The lower bound of the cooling effect of oasis ranged from −4.97 to −1.69 K. The analysis framework and results of this study will provide a new perspective for further research on the evolution process of the oasis effect and water–heat balance in arid areas.