Semiarid Northwest China Research Articles

Undervalued dry riverbeds: A key factor in equating intermittent river CO2 emissions to perennial rivers

Intermittent rivers in semiarid and arid regions, constituting over half of the world's rivers, alternate the carbon cycle interactions among the biosphere, hydrosphere, and atmosphere. Inadequate quantification of flow duration and river water surface area, along with overlooked CO2 emissions from dry riverbeds, result in notable inaccuracies in global carbon cycle assessments. High-resolution remote sensing images combined with intensive field measurements and hydrological modelling were used to estimate and extract the flow duration, river water surface area and dry riverbed area of Huangfuchuan, an intermittent river watershed that acts as a major tributary of the Yellow River in semiarid Northwest China. CO2 emission rates and partial pressures in water and air across the watershed were in-situ measured. In 2018, the flow duration of Huangfuchuan increased from less than 5 days in the first-order tributary to 150 days in the sixth-order mainstream. River water surface area estimated by remote sensing extraction plus the hydrodynamic model simulation varied from 3.9 to 88.6 km2 under 5 %–95 % discharge frequencies. CO2 emissions from the water-air interface and dry riverbed in 2018 were estimated at 582.3 × 103 and 355.2 × 103 ton, respectively. The estimated total annual emission (937.5 × 103 ton) aligns closely with the range of emissions (67.3 × 103–1377.2 × 103 ton) calculated for the water-air interface alone, derived using DEM river length and hydraulic geometry method. This similarity can be attributed to the overestimation of flow duration and flow velocity, as well as the over- or under-estimation of river water surface area and slope. The new method proposed in this study has large potential to be applied in estimating CO2 emissions from data-scarce intermittent rivers located in mountainous regions and provides a standardized solution in the estimation of CO2 emission. Results of this research reveal the spatiotemporal distribution of CO2 emissions along an intermittent river system and highlight the substantial role of dry riverbed in carbon cycle.

Long term response and adaptation of farmland water, carbon and nitrogen balances to climate change in arid to semi-arid regions

Climate change poses a challenge for resource utilization and environmental pollution issues caused by agricultural production, especially in arid to semi-arid regions. Farmland water, carbon and nitrogen balances are closely related to these resource and environmental issues. Thus, the Agro-Hydrological & chemical and Crop systems simulator was used to assess the response of water, carbon and nitrogen balances to climate change in a spring wheat farmland of arid to semi-arid Northwest China and to propose adaptation strategies. Five Global Climate Models from the Coupled Model Intercomparison Project 6 and two Shared Socioeconomic Pathways (SSP1–2.6 and SSP5–8.5) were used to establish scenarios with the Agro-Hydrological & chemical and Crop systems simulator to simulate farmland water, carbon and nitrogen balances for the 2025–2100 period. Various irrigation amounts and nitrogen fertilization rates were tested as compensation strategies. Results indicated that climate change could negatively affect farmland water, carbon and nitrogen balances, especially under the SSP5–8.5 scenario. Precipitation showed an increasing trend, thus percolation increased and soil water consumption decreased from 2025 to 2100. However, for the carbon budget, although the soil carbon dioxide emissions tend to decrease, the net primary production was also significantly reduced, which resulted in declining the net ecosystem carbon budget under future climatic conditions. In addition, higher temperature and increased precipitation enhanced soil inorganic nitrogen leaching and nitrous oxide emissions but reduced ammonia volatilization from 2025 to 2100. Overall, the soil total nitrogen loss was increased over time, whereas crop nitrogen uptake was significantly reduced. In relation to the SSP1–2.6 scenario, the SSP5–8.5 scenario accelerated the increase rates of soil water percolation and total nitrogen loss over time, as well as the decrease rates of crop nitrogen uptake and net primary production over time. The negative effects caused by climate change can be mitigated by reducing irrigation and increasing nitrogen fertilization. For the SSP1–2.6 scenario, 30% irrigation reduction and 30% nitrogen fertilization increase can effectively decrease soil water percolation and the related nitrogen losses while crop nitrogen uptake, net primary production and net ecosystem carbon budget increase in relation to the current management (irrigation = 240 mm and nitrogen fertilization = 200 kg ha–1). For SSP5–8.5 the strategy with 45% irrigation reduction and 45% nitrogen fertilization increase can also decrease nitrogen losses and increase crop nitrogen uptake, net primary production and net ecosystem carbon budget.

Sharp decline of dust events induces regional wetting over arid and semi-arid Northwest China in the NCAR Community atmosphere model

Multiple lines of observational evidence have indicated a significant wetting over the arid and semi-arid Northwest China (NWC) during recent decades, coinciding with a simultaneous sharp decline of dust events. Although recent studies have attributed NWC wetting to different anthropogenic and natural forcings, the mechanisms are not definitive and the regional wetting has been greatly underestimated in the Coupled Model Intercomparison Project historical simulations. Based on sensitivity experiments with different dust emission amounts using the NCAR Community Atmospheric Model version 5 (CAM5), here we find that decreasing dusts exert significant impacts on mixed-phase clouds through reducing the concentration of ice nucleating particles, increase the NWC precipitation and thus induce regional wetting through enhancing convection precipitation. A possible convection invigoration mechanism whereby the atmospheric vertical temperature gradient and convective instability are strengthened by reduced dusts, leading to convection invigoration and increased precipitation. These results are reinforced by simulations over the dust region in North Africa where mixed-phase and ice clouds are rare and reduced dusts do not increase precipitation. This study highlights the possible mechanism of dust-ice cloud interactions in recent NWC wetting and future regional climate change.

Effect of Inter-Row Peanut Growing in the Vineyard on the Quality of 'Cabernet Sauvignon' Grape Fruits and Wines in Northwest China.

In order to solve the problem of premature grape ripening due to global warming, inter-row peanut growing in viticulture was applied. In this two-year (2018-2019) study, the peanut (Arachis hypogaea L.) was used to cover the ground between rows in the vineyards located in the semi-arid Northwest China, Xinjiang. The results showed that reflected solar radiation and temperature around the fruit zone with the peanuts growing were decreased. Compared with clean tillage, the grapes with covering peanuts had lower total soluble solids (TSS) and higher titratable acidity (TA) in the berries. Lower alcohol content and higher total acid (TA) was also found in their corresponding wines. Inter-row peanut growing treatment significantly decreased the contents of flavonols in the grapes and their wines in the two consecutive years, but no significant effect on flavanols was observed in the resulting wines. Norisoprenoids and esters in the grapes and the wines were increased with the peanut growing treatment, respectively. Additionally, compared to clean tillage, the peanut covering significantly improved the sensory value of the wines, especially the aroma complexity of the wines. This study helps us to better understand the feasibility of applying inter-row peanut growing in the viticulture of ground management in the semi-arid climate of Northwest China.

Spatiotemporal variation of household water consumption and coping strategies in water-stressed city of arid and semiarid Northwest China

Increasing water demand due to population growth and urbanization causes cities in arid and semiarid regions in China to face increasing water scarcity. Understanding the spatiotemporal patterns of water consumption and its driving factors is of considerable importance in formulating public policy and alleviating the urban water crisis. In this study, spatiotemporal variations of water consumption in 42 cities in Northwest China were identified, and their influencing factors were determined by cluster tree and factor analyses. Results indicate that per capita household water consumption decreased by 9.5% from 2003 to 2020, with significant spatiotemporal variations decreasing in cities with medium consumption (86.87–193.66 L/d per capita per day) but increasing in cities with low (<86.87 L/d per capita per day) and high consumption (>193.66 L/d per capita per day). The influencing factors vary across the three types of cities. An increase in the capacity of water supply and popularization of washing machines are the main reasons for the rise in water consumption for low and high-consumption cities, whereas the implementation of water-saving activities and improvements of reuse rate are the main reasons for reduction of water consumption for medium-consumption cities. Findings suggest that improving water-saving appliances and water-use efficiency in cities with high-water consumption and increasing water supply capacity in cities with low-water consumption requires consideration in urban sustainable water management strategies in Northwest China.

Effects of climate change and anthropogenic activities on lake environmental dynamics: A case study in Lake Bosten Catchment, NW China

Arid and semiarid regions account for ∼ 40% of the world's land area. Rivers and lakes in these regions provide sparse, but valuable, water resources for the fragile environments, and play a vital role in the development and sustainability of local societies. During the late 1980s, the climate of arid and semiarid northwest China dramatically changed from “warm-dry” to “warm-wet”. Understanding how these environmental changes and anthropogenic activities affect water quantity and quality is critically important for protecting aquatic ecosystems and determining the best use of freshwater resources. Lake Bosten is the largest inland freshwater lake in NW China and has experienced inter-conversion between freshwater and brackish status. Herein, we explored the long-term water level and salinity trends in Lake Bosten from 1958 to 2019. During the past 62 years, the water level and salinity of Lake Bosten exhibited inverse “W-shaped” and “M-shaped” patterns, respectively. Partial least squares path modeling (PLS-PM) suggested that the decreasing water level and salinization during 1958–1986 were mainly caused by anthropogenic activities, while the variations in water level and salinity during 1987–2019 were mainly affected by climate change. The transformation of anthropogenic activities and climate change is beneficial for sustainable freshwater management in the Lake Bosten Catchment. Our findings highlight the benefit of monitoring aquatic environmental changes in arid and semi-arid regions over the long-term for the purpose of fostering a balance between socioeconomic development and ecological protection of the lake environment.

Effect of Covering Crops between Rows on the Vineyard Microclimate, Berry Composition and Wine Sensory Attributes of ‘Cabernet Sauvignon’ (Vitis vinifera L. cv.) Grapes in a Semi-Arid Climate of Northwest China

Covering crops was a commonly used viticultural technique to adjust the vineyard microclimate, thus affecting the grape and wine quality. In this two-year study, the purslane (Portulaca oleracea L.) was used to cover the lands between rows in the vineyards located in the semi-arid Northwest China, Xinjiang. Results showed that the photosynthetically active radiation around the fruit zone and the temperature with the purslane covering treatment decreased. Compared with the clean tillage, covering purslane had lower TSS and higher TA in the grape berries, while lower alcohol content and higher TA was also found in their corresponding wines. Covering purslane treatment significantly increased the contents of anthocyanin and flavonol in the grapes and wines in the year 2018, but no significant effect on flavanols was observed in the wines. Norisoprenoids, esters, and C6 alcohols in the grapes and wines were increased in the purslane covering treatment, respectively. Additionally, compared to the clean tillage, the purslane covering treatment significantly improved the sensory value of the wines, especially the floral aroma and the complexity of the wines. This study helped us to better understand the feasibility of applying covering purslane in viticulture in the semi-arid climate of Northwest China.

Effect of drip-line layout and irrigation amount on yield, irrigation water use efficiency, and quality of short-season tomato in Northwest China

Greenhouse tomato cropping with under-mulch drip irrigation is common in arid and semi-arid Northwest China. To investigate the influence of drip-line layout and irrigation amount on yield, fruit quality, irrigation water use efficiency, and comprehensive benefit of tomato production, the experiment was conducted during two cropping seasons of spring to summer in 2015 and winter in 2015 to spring in 2016. In these two cropping seasons, two drip-line layout types were conducted, including a one drip-line for each plant-row and a one drip-line for two plant-rows, with four levels of irrigation amount, i.e., 0.6, 0.8, 1.0, and 1.2 times of accumulated evaporation of a 20 cm standard pan. The results showed that one drip-line for two plant-rows outperformed one drip-line for each plant-row in achieving higher yield and irrigation water use efficiency with the same irrigation amount. In these two drip-line layouts, tomato yield increased at first and then decreased with the increase of irrigation amount; while irrigation water use efficiency decreased all the way with the increase of irrigation amount. A principal component analysis demonstrated that the combination of one drip-line for each plant-row and 1.0 times of pan evaporation obtained the highest comprehensive fruit nutritional quality among all combinations of drip-line layout and irrigation amount in spring-summer season; while the combination of one drip-line for two plant-rows and 0.8 times of pan evaporation had the highest comprehensive fruit quality in winter-spring season. To purse high comprehensive benefits, combinations of one drip-line for two plant-rows and 0.8 times of pan evaporation, and one drip-line for each plant-row and 0.6 times of pan evaporation should be recommended for spring-summer and winter spring cropping seasons, respectively. The findings of this study can provide irrigation guidance for greenhouse tomato growing in Northwest China.

Plastic film mulching with ridge planting alters soil chemical and biological properties to increase potato yields in semiarid Northwest China

BackgroundThe growth of potato (Solanum tuberosum L.) is severely affected by the complex and variable soil environment, and film mulching has been widely used for potato growth in semiarid areas of western China. However, there are few studies on the effects of film mulching on soil quality and tuber yield in potato fields.MethodsThe effects of four mulching patterns (flat plot without film mulching, FP; flat plot with film mulching, FPM; ridge planting with half mulch, RPHM; ridge planting with full mulch, RPFM) on soil chemical and biological properties and potato tuber yield were investigated during two growing seasons (2018 and 2019) in the field.ResultsThe results showed that compared with FP, the mulching treatments significantly increased the tuber yield of potato, with an increase of 3.7–20.77% and 7.89–26.35% in 2018 and 2019, respectively, and the yield of RPFM was higher than that of other treatments. In both growing seasons, RPFM significantly increased the contents of alkali-hydrolyzed N and available P, and the activities of soil urease, catalase, alkaline phosphatase, and sucrase. Bacterial and actinomycete counts were significantly higher in RPFM than those in the other treatments. Furthermore, RPFM significantly increased large potatoes and decreased small potatoes. Pearson’s correlation analysis revealed that soil alkaline dissolved nitrogen and actinomycete populations were the main factors affecting potato yield formation.ConclusionsThese results indicate that RPFM can improve the soil environment and further increase potato tuber yield, which is a viable option for potato production in semiarid areas.Graphical

Plastic film mulching increases yield, water productivity, and net income of rain-fed winter wheat compared with no mulching in semiarid Northwest China

Whole-ground plastic film mulching with thin soil mulching above the film (PMS) and no mulching (NM) are the two dominant planting patterns for winter wheat in the semiarid rain-fed region (mean annual precipitation <450 mm) of Northwest China. In this study, differences in water consumption and production performance between PMS and NM were analyzed, as well as reasons for differences in wheat yield, in 24 field experiments conducted from 2008 to 2019 at three sites. Compared with NM, PMS had higher average grain yield (23.2%), crop water productivity based on grain yield (25.6%), and net income (603 ¥ ha−1) across all 24 experiments. In PMS, yield stability also improved under different environments. Yield differences between planting patterns and among environments primarily depended on differences in spike number ha−1. Average consumption of pre-sowing stored water in soil from 0 to 200 cm in PMS and NM accounted for 38% and 35%, respectively, of total evapotranspiration (ET). In early (sowing–jointing), middle (jointing–flowering), and late (flowering–maturity) phases, average water consumption in PMS was 36.4%, 39.3%, and 24.3%, respectively. In the middle phase, PMS increased water consumption by 29 mm and the proportion of water consumption relative to total ET by 5.6% compared with NM. In early and late phases, water consumption was similar in PMS and NM. Increased water consumption in the middle phase could increase vegetative growth and sink capacity (grain yield ha−1). ET, yield, and vegetative growth were highly positively correlated with one another. Compared with NM, PMS significantly increased yield and crop water productivity by maximizing transpiration, increasing water consumption in the middle phase, and improving use of pre-sowing stored water. In conclusion, PMS is suitable to cultivate wheat for improving water productivity and economic benefits in the semiarid rain-fed agricultural region of Northwest China.

Effects of diffuse groundwater discharge, internal metabolism and carbonate buffering on headwater stream CO2 evasion

Groundwater discharge to headwater streams and concomitant terrestrially derived CO2 input play a key role in stream CO2 evasion. However, previous studies rarely examined the transport and transformation of terrestrially derived CO2 in headwater streams, thereby limiting our understanding of stream CO2 evasion. Here, we firstly used a numerical flow model to quantify groundwater discharge rates along a 43 km headwater stream (semiarid northwest China) by combining stream flow rate, radon-222 isotopic data, and then estimated internal metabolism through a reach-scale dissolved O2 mass balance and carbonate buffering through carbon budget for the stream. Modeling results indicate that groundwater discharge rates were highly variable (2.17–7.03 m2 d−1, mean ± 1 standard deviation: 4.48 ± 1.25 m2 d−1). Groundwater was supersaturated with CO2 at all sites (5.45 ± 3.87 mg C L−1). Our results also show that diffuse groundwater discharge was a major control on terrestrial carbon input to the stream. Terrestrial CO2 input (3.92 ± 2.03 g C m−2 d−1) contributed more CO2 to the stream than internal metabolism (1.74 ± 2.33 g C m−2 d−1). However, most terrestrially derived CO2 was transformed into HCO3− through carbonate buffering after entering the stream, inhibiting stream CO2 evasion. This carbonate buffering process depends on the hydrochemistry and proportion of mixing waters. Overall, our study provides a bottom-up and holistic perspective to understand the transport and transformation of terrestrially derived CO2 in headwater streams.

Interannual Variation of Transpiration and Its Modeling of a Larch Plantation in Semiarid Northwest China

Quantifying the variation of forest transpiration (T) is important not only for understanding the water and energy budget of forest ecosystems but also for the prediction, evaluation, and management of hydrological effects as well as many other ecosystem services of forests under the changes of climate, vegetation, and anthropological impacts. The accurate prediction of T, a key component of water used by forests, requires mechanism-based models describing the T response to environmental and canopy conditions. The daily T of a larch (Larix principis-rupprechtti) plantation was measured through monitoring the sap flow in the growing season (from May to September) of a dry year (2010), a normal year (2012), and a wet year (2014) at a shady slope in the semi-arid area of Liupan Mountains in northwest China. Meanwhile, the meteorological conditions, soil moisture, and forest canopy leaf area index (LAI) were monitored. To get a simple and easily applicable T model, the numerous influencing parameters were grouped into three factors: the atmospheric evapotranspiration demand indicated by the potential evapotranspiration (PET), the soil water supply ability indicated by the relative extractable soil water content (REW), and the vegetation transpiration capacity indicated by the forest canopy LAI. The T model was established as a continuous multiplication of the T response equations to individual factors, which were determined using the upper boundary lines of measured data. The effect of each factor on the T in a dry year (2010) or normal year (2012) was assessed by comparing the measured T in the baseline of the wet year (2014) and the model predicted T, which was calculated through inputting the actual data of the factor (i.e., PET) to be assessed in the dry or normal year and the measured data of other two factors (i.e., REW, LAI) in the baseline of the wet year. The results showed that the mean daily T was 0.92, 1.05, and 1.02 mm; and the maximum daily T was 1.78, 1.92, and 1.89 mm in 2010, 2012, and 2014, respectively. The T response follows a parabolic equation to PET, but a saturated exponential equation to REW and LAI. The T model parameters were calibrated using measured data in 2010 and 2012 (R2 = 0.89, Nash coefficient = 0.88) and validated using measured data in 2014 satisfactorily (R2 = 0.89, Nash coefficient = 0.79). It showed a T limitation in the dry year 2010 for all factors (18.5 mm by PET, 11.5 mm by REW, and 17.8 mm by LAI); while a promotion for PET (1.4 mm) and a limitation for REW (4.2 mm) and LAI (14.3 mm) in the normal year 2012. The daily T model established in this study can be helpful to assess the individual factor impact on T and improve the daily T prediction under changing environmental and canopy conditions.

Enhancement of no-tillage, crop straw return and manure application on field organic matter content overweigh the adverse effects of climate change in the arid and semi-arid Northwest China

Soil organic matter (SOM) is important for sustainable agricultural production. Agricultural management can directly or indirectly affect SOM content and associated soil organic carbon (SOC) stocks. The SOM content and crop productivity are generally low in the farmlands in the arid and semi-arid regions of China, and are highly sensitive to climate changes. An improvement in SOM content may greatly enhance the efficiency of fertilizer and water use, thereby increasing the resilience of agricultural systems to climate change. However, little information is available on the response of SOM to the interactions of cultivation measures and changing climate in the regions. Here we analyzed the effects of climate and long-term management practices on SOM content at three sites in the arid and semi-arid regions of China using the SOM data monitored under a range of management practices. We showed that the manure application, crop residue return and no-tillage had significantly increased SOM content from 12.1% to 127.5% in the long-term experiment at three sites. We also used the DayCent model to simulate the effects of these management practices in combination with future climate changes on the SOM content, and found that climate changes in the near future would not increase SOM content, and even decrease SOM content under conventional agronomic management, but the possible adverse impact of climate change on SOM can be outweighed by the enhancement of agronomic management. Our results provide important information for improving management to maintain SOM content and sustainable production under future climate changes.

Farmers’ Adoption Preferences for Sustainable Agriculture Practices in Northwest China

In the highest soil erosion regions of arid and semiarid northwest China, water resource deficits and farmland misuse have further exacerbated soil degradation. Therefore, understanding how farmers in diverse agroclimatic zones perceive and respond to different conservation practices is important to the implementation of sustainable agriculture practices (SAPs). To this end, this study uses a best–worst scaling approach to examine the adoption preferences for nine SAPs among grain and cash crop farmers and investigates the influence of farm and climatic characteristics on adoption preferences based on a face to face survey of 554 households in Gansu province, which is classified as an arid and semiarid area in northwest China. Both grain and cash crop farmers had stronger preferences for the practices of using organic instead of chemical fertilizers and of improving irrigation practices. In addition, while cash crop farmers also had strong preferences for cover crop-related practices, they preferred long-term fallows least. Household income, livestock, and precipitation influence the potential perceived importance of SAPs. The different perceived importance of these practices suggests new possible combinations or packages for a sustainable agriculture program during the cropping structure adjustment in Gansu.

Interactive Effects of Water and Fertilizer on Yield, Soil Water and Nitrate Dynamics of Young Apple Tree in Semiarid Region of Northwest China

Exploring the interactive effect of water and fertilizer on yield, soil water and nitrate dynamics of young apple tree is of great importance to improve the management of irrigation and fertilization in the apple-growing region of semiarid northwest China. A two-year pot experiment was conducted in a mobile rainproof shelter of the water-saving irrigation experimental station in Northwest A&amp;F University, and the investigation evaluated the response of soil water and fertilizer migration, crop water productivity (CWP), irrigation water use efficiency (IWUE), partial factor productivity (PFP) of young apple tree to different water and fertilizer regimes (four levels of soil water: 75%–85%, 65%–75%, 55%–65% and 45%–55% of field capacity, designated W1, W2, W3 and W4, respectively; three levels of N-P2O5-K2O fertilizer, 30-30-10, 20-20-10 and 10-10-10 g plant−1, designated F1, F2 and F3, respectively). Results showed that F1W1, F2W1 and F3W1 had the highest average soil water content at 0~90 cm compared with the other treatments. When fertilizer level was fixed, the average soil water content was gradually increased with increasing irrigation amount. For W1, W2, W3 and W4, high levels of water content were mainly distributed at 50~80 cm, 40~70 cm, 30~50 cm and 10~30 cm, respectively. There was no significant difference in soil water content at all fertilizer treatments. However, F1 and F2 significantly increased soil nitrate-N content by 146.3%~246.4% and 75.3%~151.5% compared with F3. The highest yield appeared at F1W1 treatment, but there was little difference between F1W1 and F2W2 treatment. F2W2 treatment decreased yield by 7.5%, but increased IWUE by 11.2% compared with F1W1 treatment. Meanwhile, the highest CWP appeared at F2W2 treatment in the two years. Thus, F2W2 treatment (soil moisture was controlled in 65–75% of field capacity, N-P2O5-K2O were controlled at 20-20-10 g·tree−1) reached the best water and fertilizer coupling mode and it was the optimum combinations of water and fertilizer saving.

The modelling of rainfall interception in growing and dormant seasons for a pine plantation and a black locust plantation in semi-arid Northwest China

Interception loss can remove a significant portion of rainwater from forested ecosystems. Therefore, the quantification and modelling of interception loss are of significant importance if human and ecosystem water demands are to be balanced under a future changing climate. This is particularly true for semi-arid/arid regions, where afforestation has become an important ecological restoration measure to tackle desertification, poverty and climate change. However, quantification and modelling of interception loss of plantations in these regions have rarely been reported. In the present study, rainfall interception loss was quantified and modelled over a one-year period (January-December 2016) for a deciduous broad-leafed R. pseudoacacia plantation and an evergreen needle-leaf P. tabuliformis plantation (common afforestation tree species) situated in the semi-arid Loess Plateau of China. The stand age, density, canopy cover and leaf area index of R. pseudoacacia during the study period were 15 years, 2000 tree ha−1, 0.48 and 1.41 m2 m−2, respectively. The corresponding values for Pinus tabuliformis were 17 years, 1200 tree ha−1, 0.62 and 2.53 m2 m−2. The measured throughfall, stemflow and derived estimates of interception loss for R. pseudoacacia were 81.1%, 1.3% and 17.6%, respectively. The corresponding values for P. tabuliformis were 75.4%, 0.7% and 23.9%. Given that the weather conditions experienced by the two forest stands were similar, the observed higher interception loss for P. tabuliformis can be explained by the higher canopy storage capacity and wet canopy evaporation rate of this species. The revised Gash analytical model of rainfall interception was well calibrated and validated against field measurements and was able to simulate the cumulative interception loss at two forest stands accurately, and it also effectively captured the seasonal variation (leafed growing and leafless dormant seasons), provided that the optimized wet-canopy evaporation rates were used. The revised model was highly sensitive to the canopy storage capacity and changes in the ratio of mean wet canopy evaporation to mean rainfall intensity and less sensitive to canopy cover, but it was found to be fairly insensitive to the trunk storage capacity.

An intercomparison of multiple statistical downscaling methods for daily precipitation and temperature over China: present climate evaluations

In this study, we use four statistical downscaling methods to statistically downscale seven Coupled Model Intercomparison Project (CMIP5) Global Climate Models (GCMs) and project the changes in precipitation and temperature over China under RCP4.5 and RCP8.5 emission scenarios. The four statistical downscaling methods are bias-correction and spatial downscaling (BCSD), bias-correction and climate imprint (BCCI), bias correction constructed analogues with quantile mapping reordering (BCCAQ), and cumulative distribution function transform (CDF-t). Though large inter-model variability exists in the distribution and magnitude of changes in projected precipitation, particularly for wet spell length (CWD), all downscaling methods generally project a consistent enhancement of precipitation in both summer and winter over most parts of China. For the arid and semiarid Northwest China, the shortened dry spell length (CDD) is accompanied by the pronouncedly intensified very wet days (R95p), as well as the increase in maximum 5-day precipitation amount (Rx5day). In contrast, southeastern regions may experience more consecutive dry days and more severe wet precipitation extremes. The projected changes from different downscaling techniques are fairly similar for temperature, apart from the diurnal temperature range for BCSD. Warming is projected across the whole domain with larger magnitude over the north and in winter under RCP8.5. More summer days and fewer frost days would appear in the future. The bias correction components of downscaling methods cause a higher degree of agreement among models, and the downscaled results generally retain the main climate change signal of the driving models.

Nocturnal sap flow of Hedysarum scoparium and its response to meteorological factors in semiarid Northwest China

Nocturnal sap flow of shrub may indicate transpiration and water recharge storage at night. Hedysarum scoparium is an important sand fixation shrub in semiarid regions. This paper deals with the water use of Hedysarum scoparium at night in the semiarid Northwest China. Our primary goal was to reveal and understand the nature of nocturnal sap flow and its functional significance. Granier’s stem heat balance method was used to determine the nocturnal sap flow of Hedysarum scoparium (stem of 25, 16, 13 and 9mm). Nocturnal sap flow was substantial and showed seasonal variation similar to the patterns of daytime sap flow. Mean nocturnal sap flow was higher in more precipitation months of year 2011 for more daytime transpiration and high soil water availability which can be the cause of more nocturnal sap flow. And vapour deficit pressure, mean air temperature and wind velocity were significantly correlated with nocturnal sap flow, they could explain a small fraction of the variance in nocturnal sap flow of studied trees. And the meteorological factors have greater impact on nocturnal sap flow rate of Hedysarum scoparium with smaller stem diameter than other diameter classes of Hedysarum scoparium.

Why was the arid and semiarid northwest China getting wetter in the recent decades?

AbstractThe arid and semiarid northwest China has experienced a significant wetting trend in summer during 1961–2010, but the reasons remain ambiguous. In this study, moisture budget analysis is employed to quantify the contributions of different factors to the wetting trend. The results show that more than 50% of the increasing precipitation is balanced by the increased evaporation. The convergence of moisture flux (the sum of horizontal moisture advection and wind convergence terms) has a significant positive contribution to the wetting trend. The increased net surface radiation, which is contributed by the increased downward longwave radiation, supplies more energy to favor the evaporation process of vaporization. The moisture flux convergence is further separated into thermodynamic component in association with changes in specific humidity and dynamic component due to changes in atmospheric circulation. The thermodynamic contribution to the wetting trend is induced by the increased specific humidity which is associated with enhanced evaporation. The dynamic contribution is dominated by an anomalous cyclone over central Asia. The anomalous cyclone is related with intensified horizontal vorticity advection which is associated with a significant southward displacement of Asian subtropical westerly jet. The results indicate that the changes of evaporation against the background of global warming deserve more attention in projecting the climate change in arid and semiarid regions.

Response of sap flux and evapotranspiration to deficit irrigation of greenhouse pear-jujube trees in semi-arid northwest China

The objective of this study was to investigate the response of sap flux (SF) and evapotranspiration (ET) to deficit irrigation (DI) at different growth stages of greenhouse pear-jujube trees (Zizyphus jujube Mill.). Taking full irrigation (FI) as the control treatment (CK), moderate water deficit (50% of FI) at bud burst to leafing stage (stage I) (T2), flowering to fruit set stage (stage II) (T3), fruit growth stage (stage III) (T4), and fruit maturation stage (stage IV) (T5) were treated on greenhouse 8-year old pear-jujube trees in two growing seasons. Water deficit at different growth stages reduced SF, after the re-watering, SF had significant compensatory effects. Therefore, compared with CK, water deficit treatments (T2–T5) didn’t decrease SF significantly for the whole growing seasons, with maximum decrease of 10.5%. Simultaneously, SF under T2–T5 showed typical diurnal patterns on sunny and cloudy days, and nighttime SF accounted for 23.7%–52.3% of total SF for all the weather conditions. There was significantly positive relationship between SF and reference evapotranspiration, with R2 of 0.349–0.594. Principal component analysis (PCA) indicated that 97% of SF of pear-jujube can be characterized by relative humidity (RH), solar radiation (Rs), air temperature (Ta), and vapor pressure deficit (VPD), and VPD was the main variable affecting SF for all the treatments (P<0.001). Averaging the two years, ET of pear-jujube were 258–288mm for all the treatments, compared with CK, T2–T5 could decrease ET by 2.3%–10.3%.