Abstract. The zero-flow phenomenon appeared frequently in the lower reaches of the Yellow River in China in the 1990s, whereas it has almost disappeared in recent years. The disappearance of the zero-flow phenomenon should be mainly attributed to the recent water management practices. However, little is known about the effects of recent climatic variations on natural runoff. In this study, we investigated the impacts of climatic variations on natural runoff above the Huayuankou station. The results indicate that there was little increase in precipitation, but substantial recovery of natural runoff in the recent period (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011) compared with the low-flow period (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002). The recent precipitation was slightly greater (∼ 2 % of the baseline precipitation in than precipitation in the low-flow period. However, the recent natural runoff was much larger (∼ 14 % baseline runoff) than runoff in the low-flow period. The runoff reduction in the low-flow period was mainly caused by precipitation decrease. In the recent period, precipitation accounted for a runoff reduction (∼ 21 % baseline runoff), whereas net radiation, wind speed, air temperature, and relative humidity accounted for a runoff increase (∼ 7.5 % baseline runoff). The spatial pattern of the climatic variation is a factor influencing the response of runoff to climatic variations. The reduction in runoff induced by precipitation change was offset up to half by the impacts of changes in net radiation and wind speed at most sub-basins in the recent period.

Abstract. Model calibration is essential for hydrologic modeling of large watersheds in a heterogeneous mountain environment. Little guidance is available for model calibration protocols for distributed models that aim at capturing the spatial variability of hydrologic processes. This study used the physically-based distributed hydrologic model, MIKE SHE, to contrast a lumped calibration protocol that used streamflow measured at one single watershed outlet to a multi-site calibration method which employed streamflow measurements at three stations within the large Chaohe River basin in northern China. Simulation results showed that the singlesite calibrated model was able to sufficiently simulate the hydrographs for two of the three stations (Nash-Sutcliffe coefficient of 0.65-0.75, and correlation coefficient 0.81-0.87 during the testing period), but the model performed poorly for the third station (Nash-Sutcliffe coefficient only 0.44). Sensitivity analysis suggested that streamflow of upstream area of the watershed was dominated by slow groundwater, whilst streamflow of middle-and down-stream areas by relatively quick interflow. Therefore, a multi-site calibration protocol was deemed necessary. Due to the potential errors and uncertainties with respect to the representation of spatial variability, performance measures from the multi-site calibration protocol slightly decreased for two of the three stations, whereas it was improved greatly for the third station. We concluded that multi-site calibration protocol reached a compromise in term of model performance for the three stations, reasonably representing the hydrographs of all three stations with Nash-Sutcliffe coefficient ranging from 0.59-072. The multi-site calibration protocol applied in the analysis generally has advantages to the single site calibration protocol.

Abstract. Poplar (Populus sp.) plantations have been, on the one hand, broadly used in northern China for urban greening, combating desertification, as well as for paper and wood production. On the other hand, such plantations have been questioned occasionally for their possible negative impacts on water availability due to the higher water-use nature of poplar trees compared with other tree species in water-limited dryland regions. To further understand the acclimation of poplar species to semiarid environments and to evaluate the potential impacts of these plantations on the broader context of the region's water supply, we examine the variability of bulk resistance parameters and energy partitioning in a poplar (Populus euramericana cv. "74/76") plantation located in northern China over a 4-year period, encompassing both dry and wet conditions. The partitioning of available energy to latent heat flux (LE) decreased from 0.62 to 0.53 under mediated meteorological drought by irrigation applications. A concomitant increase in sensible heat flux (H ) resulted in the increase of a Bowen ratio from 0.83 to 1.57. Partial correlation analysis indicated that surface resistance (R s ) normalized by leaf area index (LAI; R s :LAI) increased by 50 % under drought conditions and was the dominant factor controlling the Bowen ratio. Furthermore, R s was the main factor controlling LE during the growing season, even in wet years, as indicated by the decoupling coefficient ( = 0.45 and 0.39 in wet and dry years, respectively). R s was also a major regulator of the LE / LE eq ratio, which decreased from 0.81 in wet years to 0.68 in dry years. All physiological and bioclimatological metrics indicated that the water demands of the poplar plantation were greater than the amount available through precipitation, highlighting the poor match of a water-intensive species like poplar for this water-limited region.

No abstract

Liupan Mountains are an important region in China in the context of forest cover and vegetation due to huge afforestation and plantation practices, which brought changes in soil physio-chemical properties, soil stocks, and soil stoichiometries are rarely been understood. The study aims to explore the distribution of soil nutrients at 1-m soil depth in the plantation forest region. The soil samples at five depth increments (0-20, 20-40, 40-60, 60-80, and 80-100 cm) were collected and analyzed for different soil physio-chemical characteristics. The results showed a significant variation in soil bulk density (BD), soil porosity, pH, cation exchange capacity (CEC), and electric conductivity (EC) values. More soil BD (1.41 g cm-3) and pH (6.97) were noticed in the deep soil layer (80-100 cm), while the highest values of porosity (60.6%), EC (0.09 mS cm-1), and CEC (32.9 c mol kg-1) were reflected in the uppermost soil layer (0-20 cm). Similarly, the highest contents of soil organic carbon (SOC), total phosphorus (TP), available phosphorus (AP), total nitrogen (TN), and available potassium (AK) were calculated in the surface soil layer (0-20 cm). With increasing soil depth increment a decreasing trend in the SOC and other nutrient concentration were found, whereas the soil total potassium (TK) produced a negative correlation with soil layer depth. The entire results produced the distribution of SOCs and TNs (stocks) at various soil depths in forestland patterns were 0→20cm > 20→40cm > 40→60cm ≥ 60→80cm ≥ 80→100 cm. Furthermore, the stoichiometric ratios of C, N, and P, the C/P, and N/P ratios showed maximum values (66.49 and 5.46) in 0-20 cm and lowest values (23.78 and 1.91) in 80-100 cm soil layer depth. Though the C/N ratio was statistically similar across the whole soil profile (0-100 cm). These results highlighted that the soil depth increments might largely be attributed to fluctuations in soil physio-chemical properties, soil stocks, and soil stoichiometries. Further study is needed to draw more conclusions on nutrient dynamics, soil stocks, and soil stoichiometry in these forests.

Abstract:As an important source of income in the region's economy, the jujube plantations are very common in arid north-western China, and their planted areas continue to expand. In the central Heihe River Basin of arid north-western China, Linze jujube (Zizyphus jujuba Mill. var. inermis (Bunge) Rehd.) plantations cover more than 10,000 ha, too. Water use by this species is expected to change or modify catchment hydrological process. To our knowledge, there is no information on the transpiration and canopy conductance of the jujube plantations in arid north-western China. Therefore, Transpiration and canopy conductance were monitored in a 14-year-old Linze jujube orchard. The experiment was carried out in the central Heihe River Basin, near Pingchuan Town (Linze County, Gansu Province, China) during growing season of 2006, from May to the first ten days of October. Eight trees were used to measure sap flow using the heat-pulse-velocity method. The orchard was irrigated adequately during the study. Transpiration was estimated from the sap flow measurements. During the experiment, the transpiration rate of the orchard ranged from 0Ð32 to 1Ð40 mm per day. Canopy conductance was obtained from estimated daily transpiration and climatic variables measured on a half-hour basis, and canopy conductance for water vapour transfer was between 1Ð20 to 82Ð57 mm s 1 , with a mean of 11Ð86 š 6Ð84 mm s 1 during the observation period. Air temperature and vapour-pressure deficit exhibited a linear relationship with sap flow velocity and the relationship between these factors and canopy conductance could be represented by an exponential decay function.

Model calibration is a complex task for large watersheds, especially for those in a heterogeneous mountain environment where multi-objective calibration strategy is essential. That may improve a model's capability to capture the spatial variations of the internal hydrologic variables. This study used the physically-based distributed hydrologic model, MIKESHE, to contrast a lumped calibration protocol that uses data measured at one single outlet to a multi-site calibration method which employed streamflow measurements at three separate stations within the large Chaohe River basin in Northern China. The results showed that, the single-site calibrated model was able to sufficiently simulate the hydrographs for two of the three stations (Nash-sutchliffe coefficient of 0.65–0.75, and correlation coefficient 0.81–0.87 during the testing period), but model performance was poor at the third station (EF only 0.44). By using the multi-site measurements model calibration reached a compromise between the different stations, the model reasonably representing the hydrographs of all three stations with EF ranging from 0.59–0.72. The modeling calibration results suggested that the dominant hydrological processes varied across the large watershed with upstream area dominated by slow groundwater and middle- and down-stream areas dominated by relatively quick interflow. We conclude that to account for the different hydrological process of watershed with large heterogeneity, it is necessary to employ a multi-site calibration protocol to reduce prediction errors

Abstract. Poplar (Populus sp.) plantations have been used broadly for combating desertification, urban greening, and paper and wood production in northern China. However, given the high water use by the species and the regional dry environment, the long-term sustainability of these plantations needs to be evaluated. Currently, the understanding of energy partitioning and canopy resistance to water vapor and CO2 in poplar plantations is limited, impeding an accurate assessment of their true ecosystem functions. This study examined the variability of canopy bulk resistance parameters and energy partitioning over a four-year period encompassing both dry and wet conditions in a poplar (Populus euramericana CV. "74 / 76") plantation ecosystem located in northern China. Available energy (Net radiation Rn minus Soil Heat Flux, G) partitioning to latent (LE) and sensible (H) heat was responsive to climatological drought, with LE/(Rn-G) ranging from 62% in wet years (e.g. 2007 and 2008) to 53% in dry years (e.g. 2006 and 2009), and H/(Rn-G) from 25 to 33% between wet and dry years. Correspondingly, the Bowen ratio (β=H/LE) were 0.83 and 1.57. Surface resistance (Rs) had the greatest response to drought (+43%), but the aerodynamic and climatological resistances did not change significantly (p > 0.05). Partial correlation analysis indicated that Rs was the dominant factor in controlling the Bowen ratio. Furthermore, Rs was the major factor controlling LE during the growing season, even in wet years, as indicated by the decoupling coefficient (Ω = 0.45 and 0.39 in wet and dry years, respectively), and the LE / LEeq ratio ranged from 0.81 and 0.68 in wet and dry years, respectively. In general, the dry surface conditions dominated in this poplar plantation ecosystem regardless of soil water availability suggesting that fast-growing and water use-intensive species like poplar plantations are poorly adapted for the water limited region.