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This paper presents a detailed analysis of how future climate change may affect water availability in a typical arid endorheic river basin, the Heihe River basin (HRB), in northwest China. The analysis is based on the improved Soil Water Assessment Tool (SWAT), which is calibrated and validated with historical streamflow data from the upper HRB and is used to predict future hydrological responses. Six general circulation models (GCMs), under two emission scenarios (RCP4.5 and RCP8.5), are downscaled to construct future climate change scenarios. The results suggest that the climate of the upper HRB will likely become warmer and wetter in the near future (2021-50), with the largest increase in precipitation occurring in the summer. Correspondingly, the basinwide evapotranspiration, snowmelt, and runoff are projected to increase over the same period. The mean temperature in the near future is projected to rise, relative to the recent 30 years (1981-2010), by 1.2°-1.7°C under scenario RCP4.5 and by 1.4°-2.1°C under scenario RCP8.5. The mean precipitation is projected to increase by 10.0%-16.6% under scenario RCP4.5, and by 10.5%-22.0% under scenario RCP8.5. The mean values of evapotranspiration, snowmelt, and runoff are expected to increase by 14.2%, 4.3%, and 11.4%, respectively, under scenario RCP4.5 and to increase by 18.7%, 5.8%, and 12.8%, respectively, under scenario RCP8.5. Though the model simulations forecast an increase in streamflows in the headwater region of the HRB, future water availability varies significantly over space and time. The findings of this study will help to frame more effective water management strategies for the HRB under changing climatic conditions. [ABSTRACT FROM AUTHOR]

Net radiation is an important component of the earth's surface energy balance, which plays a vital role in the evolution of regional climate or climate change. The estimation of this component at regional or global scales is critical and challenging due to the sparse and limited ground-based observations. This paper made an attempt to analyze the feasibility of a remote sensing-based surface energy balance model using satellite (TERRA/MODIS) data to derive the net radiation (Rn). In the present study, MODIS data at 15 different days of the year (DOY) were utilized to visualize the spatial pattern of net radiation flux over three versatile and heterogeneous ecohydrological land surfaces (upstream, midstream, and downstream) of northwest China (Heihe river basin). The results revealed that the estimated net radiation from the satellite data agrees well with the ground-based measurements over three different surfaces, with a mean relative error of 9.33% over the upstream superstation (grasslands), 13.95% over the middle stream superstation (croplands), and 11.63% over the downstream superstation (mixed forests), where the overall relative error was 11.64% with an overall rmse of 29.36 W/m2 in the study area. The regional distribution of net radiation over the versatile land surfaces was validated well at a large scale during the five-month period and over different land surfaces. It was also observed that the spatial pattern of net radiation varies spatially over three different landscape regions during four different days of the year, which might be associated with different climatic conditions and landscape features in these regions. The overall findings of this study concluded that satellite-derived net radiation can rationally be obtained using a single-source remote sensing model over different land surfaces. [ABSTRACT FROM AUTHOR]

This paper investigates reference evapotranspiration (ET0) changes, sensitivities to and contributions of meteorological factors in the Heihe River Basin (arid and inland region). Results show that annual ET0 over the whole basin has increasing trend (2.01 mm·10 yr−2) and there are significant increasing spatial variations from the upper (753 mm yr−1) to the lower (1553 mm yr−1) regions. Sensitivity analysis indicates that relative humidity is the most sensitive factor for seasonal and annual ET0 change, and the influence is negative. The sensitivity of minimum temperature is the weakest and negative. Contribution analysis shows that the main contributors to ET0 changes are aerodynamic factors rather than radiative factors. This study could be helpful to understand the response of ecoenvironment to the meteorological factors changes in the Heihe River Basin. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, the Populus euphratica Oliv population has shown signs of degradation that are hypothesized to reflect decreases in surface water. Based on the analysis of tree-ring data and field inventories for P. euphratica populations in 14 study sites in the lower reach of the Heihe River, China, this paper assesses (1) age–height growth relationships, (2) tree growth levels, and (3) effects of stand type, soil type, water condition and river runoff on rate of change in tree height growth. There is a close relationship between tree height and age in P. euphratica, but the extent of this relationship varies at different growth stages and at different locations in the lower reach of the Heihe River. In this area, most of the natural P. euphratica forest is currently at a suppressed growth level with an unfavorable growing condition. The height growth in particular appears to be quite sensitive and variable in terms influences of internal and external factors over the tree lifespan, with changes in the water–soil conditions, wind action and gravity being particularly detrimental. In general, tree height growth rate was higher when growth conditions were favorable than in poorer growth conditions; however, the actual differences in growth between trees in the same area was more variable under good growth conditions and could be less variable for trees growing in poor conditions. In addition, the correlation between P. euphratica height growth and river runoff was not significant. These findings should be helpful in determining forestry management measures for the sustainable development and maintenance of natural P. euphratica forests in this study area and for overall environmental evaluation and monitoring of ecosystems of inland river basins of arid areas. [Copyright &y& Elsevier]

Surface soil heat flux ( G) is an indispensable component of the surface energy balance and plays an important role in the estimation of surface evapotranspiration (ET). This study calculated G in the Heihe River Basin based on the thermal diffusion equation, using the observed soil temperature and moisture profiles, with the aim to analyze the spatial-temporal variations of G over the heterogeneous area (with alpine grassland, farmland, and forest). The soil ice content was estimated by the difference in liquid soil water content before and after the melting of the frozen soil and its impact on the calculation of G was further analyzed. The results show that: (1) the diurnal variation of G is obvious under different underlying surfaces in the Heihe River Basin, and the time when the daily maximum value of G occurs is a few minutes to several hours earlier than that of the net radiation flux, which is related to the soil texture, soil moisture, soil thermal properties, and the vegetation coverage; (2) the net radiation flux varies with season and reaches the maximum in summer and the minimum in winter, whereas G reaches the maximum in spring rather than in summer, because more vegetation in summer hinders energy transfer into the soil; (3) the proportions of G to the net radiation flux are different with seasons and surface types, and the mean values in January are 25.6% at the Arou site, 22.9% at the Yingke site and 4.3% at the Guantan site, whereas the values in July are 2.3%, 1.6% and 0.3%, respectively; and (4) G increases when the soil ice content is included in thermal diffusion equation, which improves the surface energy balance closure by 4.3%. [ABSTRACT FROM AUTHOR]

Snow is an important water resource and greatly influences water availability in the downstream areas. In this study, snow cover variations of the Upper Heihe River Basin (UHRB) during hydrological years (HY) 2003-2013 (September through August) is examined using the flexible multiday-combined MODIS snow cover products. Spatial distribution and pattern of snow cover from year to year for the basin is found to be relatively stable, with maximum snow cover area (SCA) and snow cover days occurring in HY2004, HY2008 and HY2012. A method, based on correlation coefficients between SCA and climate factors (mainly air temperature and precipitation), is presented to identify the threshold altitude that determines contributions of climate factors to SCA. A threshold altitude of 3650 ± 150 m is found for the UHRB, where below this altitude, both air temperature (Tair) and precipitation are negative factors on SCA, except in the winter season when both are positive factors. Above the threshold altitude, precipitation acts as a positive factor except in summer, while Tair is a negative factor except in autumn. Overall, Tair is the primary controlling factor on SCA below the threshold altitude, while precipitation is the primary controlling factor on SCA above the threshold altitude. [ABSTRACT FROM AUTHOR]

Although elevation data are globally available and used in many existing hydrological models, their information content is still underexploited. Topography is closely related to geology, soil, climate and land cover. As a result, it may reflect the dominant hydrological processes in a catchment. In this study, we evaluated this hypothesis through four progressively more complex conceptual rainfall-runoff models. The first model (FLEXL) is lumped, and it does not make use of elevation data. The second model (FLEXD) is semidistributed with different parameter sets for different units. This model uses elevation data indirectly, taking spatially variable drivers into account. The third model (FLEXT0), also semi-distributed, makes explicit use of topography information. The structure of FLEXT0 consists of four parallel components representing the distinct hydrological function of different landscape elements. These elements were determined based on a topography-based landscape classification approach. The fourth model (FLEXT) has the same model structure and parameterization as FLEXT0 but uses realism constraints on parameters and fluxes. All models have been calibrated and validated at the catchment outlet. Additionally, the models were evaluated at two sub-catchments. It was found that FLEXT0 and FLEXT perform better than the other models in nested sub-catchment validation and they are therefore better spatially transferable. Among these two models, FLEXT performs better than FLEXT0 in transferability. This supports the following hypotheses: (1) topography can be used as an integrated indicator to distinguish between landscape elements with different hydrological functions; (2) FLEXT0 and FLEXT are much better equipped to represent the heterogeneity of hydrological functions than a lumped or semi-distributed model, and hence they have a more realistic model structure and parameterization; (3) the soft data used to constrain the model parameters and fluxes in FLEXT are useful for improving model transferability. Most of the precipitation on the forested hillslopes evaporates, thus generating relatively little runoff. [ABSTRACT FROM AUTHOR]

Fluxes of carbon dioxide, water vapor, and heat were measured above crop canopy using the eddy covariance method during the 2008 maize growing season, over an agricultural field within an oasis located in the middle reaches of Heihe River basin, northwest China. The values for friction velocity, the Monin-Obukhov stability parameter, and energy balance closure indicated that the eddy covariance system at this study site provided reliable flux estimates. Results from measurements showed that the mean sensible heat flux was 70 W m with a maximum value of 164 W m (May) and a minimum value of 45 W m (July) during the maize growing season. In contrast, the mean latent heat was 278 W m with a maximum value of 383 W m (July) and minimum of 101 W m (May). The mean downward soil heat flux was 55 W m with a maximum value of 127 W m (May) and minimum of 49 W m (July). The magnitude of mean daytime net CO uptake was −11.50 μmol m s with a maximum value of −28.32 μmol m s (18 and 19 July) and a minimum values of −0.32 μmol m s (18 and 19 May). Correlation was observed between daytime half-hourly carbon dioxide flux and canopy conductance. In addition, the relationship between carbon dioxide flux and photosynthetically active radiation for selected days during different stages of maize growing season indicated the carbon dioxide flux uptake by the canopy was controlled by actual photosynthetic activity related to the variation of green leaf area index for the different growing stages. [ABSTRACT FROM AUTHOR]

We investigated the moisture origin and contribution of different water sources to surface runoff entering the headwaters of the Heihe River basin on the basis of NECP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) re-analysis data and variations in the stable hydrogen and oxygen isotope ratios (δD and δ18O) of precipitation, spring, river, and melt water. The similar seasonality in precipitation δ18O at different sites reveals the same moisture origin for water entering the headwaters of the Heihe River basin. The similarity in the seasonality of δ18O and d-excess for precipitation at Yeniugou and Urumchi, which showed more positive δ18O and lower d-excess values in summer and more negative δ18O and higher d-excess values in winter, indicates a dominant effect of westerly air masses in summer and the integrated influence of westerly and polar air masses in winter. Higher d-excess values throughout the year for Yeniugou suggest that in arid inland areas 18 of northwestern China, water is intensively recycled. Temporal changes in δ18O, δD, and d-excess reveal distinct contributions of different bodies of water to surface runoff. For example, there were similar trends for δD, δ18O, and d-excess of precipitation and river water from June to September, similar δ18O trends for river and spring water from December to February, and similar trends for precipitation and runoff volumes. However, there were significant differences in δ18O between melt water and river water in September. Our results show that the recharge of surface runoff by precipitation occurred mainly from June to mid-September, whereas the supply of surface runoff in winter was from base flow (as spring water), mostly with a lower runoff amount. [ABSTRACT FROM AUTHOR]