High-resolution simulation of the spatial pattern of water use in continental China

ABSTRACT

High-resolution data on the spatial pattern of water use are a prerequisite for appropriate and sustainable water management. Based on one well-validated hydrological model, the Distributed Time Variant Gains Model (DTVGM), this paper obtains reliable high-resolution spatial patterns of irrigation, industrial and domestic water use in continental China. During the validation periods, ranges of correlation coefficient (R) and Nash-Sutcliffe efficiency (NSE) coefficient are 0.67–0.96 and 0.51–0.84, respectively, between the observed and simulated streamflow of six hydrological stations, indicating model applicability to simulate the distribution of water use. The simulated water use quantities have relative errors (RE) less than 5% compared with the observed. In addition, the changes in streamflow discharge were also correctly simulated by our model, such as the Zhangjiafen station in the Hai River basin with a dramatic decrease in streamflow, and the Makou station in the Pearl River basin with no significant changes. These changes are combined results of basin available water resources and water use. The obtained high-resolution spatial pattern of water use could decrease uncertainty of hydrological simulation and guide water management efficiently.

Editor M.C. Acreman; Associate editor X. Fang     

The impact of land use change on water balance in Zhangye city,China

Land use change has a significant effect on water balance, especially in arid region, such as Northwest China. In this paper, we analyze the effect of land use change on water balance in terms of the amount of water supply and demand from economic perspective. It's the first time to extend the basic 48 sectors input-output table to include water and land accounts that involved into multiple production processes for Zhangye city. We then perform the improved ORANI-G model, a single region Computable General Equilibrium (CGE) model, to analyze the effect of land use change on water balance under three scenarios. Subsequently, scenario-based simulation results are interpreted through selected sectors from agricultural, industrial, and service sectors respectively. Finally, the effect of land use change on water balance is analyzed through the difference between business-as-usual and land use unchanged scenarios. The results show that the extent of effect on water balance is different among sectors. Specifically, from the perspective of absolute value, service sectors are the largest, followed by industrial sectors, and the agricultural sectors are the least. Conversely, in terms of percentage change of land use, the largest extent of effect occurs in agricultural sectors. Additionally, with the rapid urbanization and the development of social economy, water balance in industrial sectors and service sectors will be stricken and reconstructed to a new high level. Simulation results also show that agricultural land shrinking will mitigate water scarcity distinctly, which indicates that balance the relationship among different stakeholders is imperative to guarantee water transformation from agricultural sectors to industrial and service sectors.     

Effects of land use on water chemistry in a river draining karst terrain,southwest China

Abstract

The effects of land use on river water chemistry in a typical karst watershed (Wujiang River) of southwest China have been evaluated. Dissolved major ions and Sr isotopic compositions were determined in 11 independent sub-watersheds of the Wujiang River to investigate the spatio-temporal variations in river water chemistry and their relationship to land use. The results show significant spatial variability in pH, major ions, total dissolved solids (TDS), and Sr isotopic compositions throughout the basin. Correlation analysis indicates that nitrogen content is significantly related to forest coverage. Nitrogen and potassium generally have higher values in the rainy season, and the percentage of agricultural land controlled NO₃^- levels, which originate from anthropogenic sources. Forest cover, which varies between 35% and 71%, has no statistically significant impact on river solute concentrations, but the TDS flux is low in sub-watersheds with greater forest cover. Geological sources have a significant influence on pH and Sr isotopic compositions in river water throughout the basin.

Editor D. Koutsoyiannis

Citation Han, G., Li, F., and Tan, Q., 2014. Effects of land use on water chemistry in a river draining karst terrain, southwest China. Hydrological Sciences Journal, 59 (5), 1063–1073.     

Ecosystem water use efficiency in an irrigated cropland in the North China Plain

The eddy covariance technique and the cuvette method were used to investigate water use efficiency in an irrigated winter wheat (Triticum asetivum L.)/summer maize (Zea mays L.) rotation system in the North China Plain. The results show that ecosystem water use efficiency (WUE_e) changed diurnally and seasonally. Daily maximal WUE_e appeared in the morning. WUE_e generally peaked in late April in wheat field and in late July/early August in maize field. From 2003 to 2006, seasonal mean WUE_e was 6.7–7.4 mg CO₂ g⁻¹ H₂O for wheat and 8.4–12.1 mg CO₂ g⁻¹ H₂O for maize. WUE_e was much lower than canopy water use efficiency (WUE_c) under small leaf area index (LAI) but very close to WUE_c under large LAI. With the increase in LAI, WUE_e enlarged rapidly under low LAI but slowly when LAI was higher than one. WUE_e was greater on the cloudy days than on the sunny days. Under the same solar radiation, WUE_e was higher in the morning than in the afternoon. The ratio of internal to ambient CO₂ partial pressure (C_i/C_a) decreased significantly with the increase in photosynthetically active radiation (PAR) when PAR was lower than the critical values (around 500 and 1000 μmol m⁻² s⁻¹ for wheat and maize, respectively). Beyond critical PAR, C_i/C_a was approximately constant at 0.69 for wheat and 0.42 for maize. Therefore, when LAI and solar radiation was large enough, WUE_e has negative correlation with vapor pressure deficit in both of irrigated wheat and maize fields.     

Rainwater and reclaimed wastewater for sustainable urban water use

Concern about the sustainability of urban water use is the strong motivation to understand the potential of rainwater use and water recycling in urbanized cities. The history of water supply in Tokyo and its experience may provide useful information to develop sustainable urban water use and find future possible tasks in rapidly growing cities. Besides, various innovative strategies to meet the current and future water demand in Tokyo may help us to consider new approaches adjusting to the developing mega cities in Asia. In this paper, the past and current practices on utilization of latent water resources such as rainwater and reclaimed wastewater in Tokyo are summarized from the viewpoint of sustainable water use.The storage of rainwater is a useful measure for water demand in emergency cases. In addition, the rainwater use can work as a kind of environmental education to make citizens aware of sustainable urban water use. There are 850 facilities for rainwater use in Tokyo. Since reclaimed wastewater use has several benefits, a huge water volume has been utilized for various purposes such as washing, water-cooling, toilet flushing, waterway restoration and creation of recreational waterfront. From the viewpoint of human health risk, new micropollutants such as estrogens, endocrine disrupters and surfactants should be considered as quality guideline parameter besides the conventional ones. Importance of infiltration facilities should be also highlighted to secure the sound water cycle. Groundwater recharge through the infiltration facilities provide a potential storage of water resource which can be withdrawn in the future if necessary.     

Impacts of climate change and land use change on runoff of forest catchment in northeast China

Hydrological processes change from the impacts of climate variability and human activities. Runoff in the upper reaches of the Hun‐Taizi River basin, which is mainly covered by forests in northeast China, decreased from 1960 to 2006. The data used in this study were based on runoff records from six hydrological stations in the upper reaches of the Hun‐Taizi River basin. Nonparametric Mann–Kendall statistic was used to identify change trends and abrupt change points and consequently analyze the change characteristics in hydrological processes. The abrupt change in the annual runoff in most subcatchments appeared after 1975. Finally, the effects of climate change and land cover change on water resources were identified using regression analysis and a hydrology model. Results of the regression analysis suggest that the correlation coefficients between precipitation and runoff prior to the abrupt change were higher compared with those after the abrupt change. Moreover, using hydrology model analysis, the water yield was found to increase because of the decrease in forest land. Copyright © 2012 John Wiley & Sons, Ltd.     

The characteristics of soil water cycle and water balance on steep grassland under natural and simulated rainfall conditions in the Loess Plateau of China

Large-scale vegetation restoration has been helpful to prevent serious soil erosion, but also has aggravated water scarcity and resulted in soil desiccation below a depth of 200 cm in the Loess Plateau of China. To understand the dynamic mechanism of soil desiccation formation is very important for sustainable development of agriculture in the Loess Plateau. Based on natural and simulated rainfall, the characteristics of soil water cycle and water balance in the 0–400 cm soil layer on a steep grassland hillslope in Changwu County of Shaanxi Loess Plateau were investigated from June to November in 2002, a drought year with annual rainfall of 460 mm. It was similarly considered to represent a rainy year with annual rainfall of 850 mm under simulated rainfall conditions. The results showed that the temporal variability of water contents would decrease in the upper 0–200 cm soil layer with the increase in rainfall. The depth of soil affected by rainfall infiltration was 0–200 cm in the drought year and 0–300 cm in the rainy year. During the period of water consumption under natural conditions, the deepest layer of soil influenced by evapotranspiration (ET) rapidly reached a depth of 200 cm on July 21, 2002, and soil water storage decreased by 48 mm from the whole 0–200 cm soil layer. However, during the same investigation period under simulated rainfall conditions, soil water storage in the 0–400 cm soil layer increased by only 71 mm, although the corresponding rainfall was about 640 mm. The extra-simulated rainfall of 458 mm from May 29 to August 10 did not result in the disappearance of soil desiccation in the 200–400 cm deep soil layer. Most infiltrated rainwater retained in the top 0–200 cm soil layer, and it was subsequently depleted by ET in the rainy season. Because very little water moved below the 200 cm depth, there was desiccation in the deep soil layer in drought and normal rainfall years.     

Long time-series spatiotemporal variations of NPP and water use efficiency in the lower Heihe River Basin with serious water scarcity

It is of great significance to analyze the long time-series spatiotemporal dynamics of water use efficiency (WUE) to formulating appropriate management measures in response to the growing water scarcity in arid and semi-arid regions. This study analyzed the long time-series variations of WUE in the Lower Heihe River Basin, a typical arid and semi-arid region in China. The net primary productivity (NPP) was first estimated with the C-fix model, then WUE during 2001–2010 was calculated with the NPP and evapotranspiration (ET) data, and the accumulative WUE was further calculated. The results showed that the annual NPP and WUE in the study area ranged from zero to 448.70 gC/(m² a) and from zero to 2.20 gC kg⁻¹ H₂O, respectively, both of which showed an overall increasing trend during 2001–2010. Besides, the spatial pattern of WUE kept overall unchanged during 2001–2010, but with remarkable change in some part of the study area. In addition, the accumulative WUE of the whole study area showed a first sharply decreasing and then gradually increasing trend, but there was still some scope to improve the WUE, and it is necessary to carry out some more specific policies to further improve the water allocation and WUE within the Lower Heihe River Basin. Although with some uncertainties, these results still can provide valuable reference information for improving the water resource management and ecological conservation to guarantee provision of essential ecosystem services in arid and semi-arid regions.     

Sustainability of groundwater usage in northern China: dependence on palaeowaters and effects on water quality,quantity and ecosystem health

A synthesis of groundwater ages, recharge rates and information on processes affecting groundwater quality in northern China highlights the major challenges faced for sustainable management of the region's groundwater. Direct recharge rates range from hundreds of millimetres per year in the North China Plain, to tens of millimetres per year in the Loess Plateau to less than 4 mm/year in the arid northwest. Recharge rates and mechanisms to deep semiconfined and confined aquifers are poorly constrained; however, on the basis of available data, these are likely to be mostly negligible. Severe groundwater level declines (0.5–3 m/year) have occurred throughout northern China in the last three to four decades, particularly in deep aquifers. Radiocarbon dating, stable isotope and noble gas data show that the most intensively extracted deep groundwater is palaeowater, recharged under different climate and land cover conditions to the present. Reservoir construction has reduced surface runoff in mountain‐front areas that would naturally recharge regional Quaternary aquifers in many basins. In combination with intensive irrigation practices, this has resulted in the main recharge source shifting from surface runoff and mountain‐front recharge to irrigation returns. This has reduced infiltration of fresh recharge at basin margins and rapidly increased nitrate concentrations and overall mineralisation in phreatic groundwater over wide areas (in some cases to >400 mg/l and >10 g/l, respectively). In some basins, there is evidence that poor quality shallow water has leaked into deep layers (>200 m) via preferential flow, mixing with palaeowaters stored in semiconfined aquifers. High concentrations of naturally occurring fluoride and arsenic (locally >8.5 and >4 mg/l, respectively) have recently lead to the abandonment of numerous supply wells in northern China, creating further pressure on stressed water resources. Increasing water demand from direct and indirect consumption poses major challenges for water management in northern China, which must consider the full water cycle. Copyright © 2012 John Wiley & Sons, Ltd.     

Land use effects on climate in China as simulated by a regional climate model

A regional climate model (RegCM3) nested within ERA40 re-analyzed data is used to investigate the climate effects of land use change over China. Two 15-year simulations (1987―2001), one with current land use and the other with potential vegetation cover without human intervention, are conducted for a domain encompassing China. The climate impacts of land use change are assessed from the difference between the two simulations. Results show that the current land use (modified by anthropogenic ac- tivities) influences local climate as simulated by the model through the reinforcement of the monsoon circulation in both the winter and summer seasons and through changes of the surface energy budget. In winter, land use change leads to reduced precipitation and decreased surface air temperature south of the Yangtze River, and increased precipitation north of the Yangtze River. Land use change signifi- cantly affects summer climate in southern China, yielding increased precipitation over the region, de- creased temperature along the Yangtze River and increased temperature in the South China area (south-end of China). In summer, a reduction of precipitation over northern China and a temperature rise over Northwest China are also simulated. Both daily maximum and minimum temperatures are affected in the simulations. In general, the current land use in China leads to enhanced mean annual precipitation and decreased annual temperature over south China along with decreased precipitation over North China.     

Water, heat fluxes and water use efficiency measurement and modeling above a farmland in the North China Plain

Net radiation (Rn), water vapor flux (LE), sensible heat flux (Hs) and soil heat flux (G)were measured above a summer maize field with the eddy-covariance technique, simulation and analysis of water, heat fluxes and crop water use efficiency were made with the RZ-SHAW model at the same time in this study. The results revealed significant diurnal and seasonal variability of water vapor flux for summer maize. Most part of Rn was consumed by the evapotranspiration of the summer maize. The proportion of water vapor flux to net radiation ((LE/Rn) increased with the crop development and peaked around milk-filling stage with a value of 60%, a slightly lower than that obtained by the RZ-SHAW model. Daily evapotranspiration estimated by the model agreed with the results measured with the eddy-covariance technique, indices of agreement (IA) for hourly water vapor fluxes simulated and measured were above 0.75, root mean square errors (RMSE) were no more than 1.0. Diurnal patterns of Hs showed the shape of inverted "U" shifted to the forenoon with a maximum value around 11:30 (Beijing time), while LE exhibited an inverted "V" with a maximum value at around 13:00, about an hour later than Hs. Diurnal change of CO2showed an asymmetrical "V" curve and its maximal rates occurred at about 11:30. Variations of water use efficiency during the phonological stages of the summer maize showed a rapid increase with the photosynthetic photon flux density (PPFD) after sunrise, a state of equilibrium around 10:00 followed a decrease. Maximum values of water use efficiency were 24.3, and its average value ranged from 7.6 to 10.3 g kg-1.     

夏季亚洲极涡的长期变化对东亚环流和水汽收支的影响

主要分析了1951～2004年夏季亚洲极涡强度和面积的长期变化趋势及其对东亚夏季环流,水汽输送和降水量的影响,发现1951～2004年,夏季亚洲极涡表现出了明显的强度减弱,面积缩小的变化趋势,并以面积缩小更为显著,这正对应于北极涛动（AO）指数在该时段的显著升高.在这种北半球中高纬大尺度环流变化的影响下,东亚夏季高空西风急流在近54年显著南移,冷空气活动的南侵程度明显增强,从而造成低空偏北风显著增强而偏南风减弱.与此相应,近54年整个中国区域内低空纬向风速呈明显的减小趋势.总的来看,东亚夏季风环流发生了明显减弱.同时,流经中国的中纬度西风水汽输送在近54年也表现出一致减弱的趋势,而南风水汽输送大致以110°E为界,以东的夏季风区呈显著的减弱趋势而以西则有明显的增加趋势.这种水汽输送的变化影响了中国不同区域内水汽输送通量散度的改变,进而使得夏季降水量发生变化.分析表明,夏季亚洲极涡的面积和强度与东北、华北和西北东部的水汽输送通量散度和夏季降水呈正相关,而与长江中下游、华南、西南、青藏高原和西北西部呈显著负相关,夏季亚洲极涡在近几十年的面积缩小和强度减弱是中国夏季降水长期变化的一个可能原因.     

Impacts of riparian zone plant water use on temporal scaling of groundwater systems

In this work, we study groundwater system temporal scaling in relation to plant water use and near‐river‐stage fluctuations in riparian zones where phreatophytes exist. Using detrended fluctuation analysis (DFA), we investigate the influence of regular diurnal fluctuations due to phreatophyte water use on temporal scaling properties of groundwater level variations. We found that groundwater use by phreatophytes, at the field site on the Colorado River, USA, results in distinctive crossovers (slope changes when the plots are fitted with straight lines) in the logarithm plots of root‐mean‐square fluctuations of the detrended water level time series versus time scales of groundwater level dynamics. For groundwater levels monitored at wells close to the river, we identified one crossover at ～1 day in the scaling characteristics of groundwater level variations. When time scale exceeds 1 day, the scaling properties decrease from persistent to close to 1/f noise, where f is the frequency. For groundwater levels recorded at wells further away from the river, the slope of the straight line fit (i.e. scaling exponent) is smallest when the time scale is between 1 and 3 days. When the time scale is < 1 day, groundwater variations become persistent. When the time scale is between 1 and 3 days, the variations are close to white noise, but return to persistent when the time scale is > 3 days. Copyright © 2011 John Wiley & Sons, Ltd.     

利用深井地下水位动态研究大华北地区现今构造应力场状态

在水平层状含水层(一维)模式下, 根据含水层应力与井水位变化之间的定量关系表达式, 利用华北地区40多口深井水位动态变化资料, 探索研究华北地区现今构造应力场状态。 即水位上升代表构造应力场压性增强张性减弱; 水位下降代表构造应力场压性减弱张性增强。 初步研究结果表明, 1995~1998年华北北部地区张家口-渤海构造带和山西构造带北部水位普遍上升, M_S5.0以上地震活动频繁; 1999年以后华北地区水位普遍处于下降状态, 反映了华北地区1999年以来构造应力场状态以张性为主, 整个华北地区M_S5.0以上地震活动处于相对平静状态。 该结果可为华北地区地震活动性预测研究提供应力场背景信息。     

Impacts of land use on the hydrological response of tropical Andean catchments

Changes in land use and land cover are major drivers of hydrological alteration in the tropical Andes. However, quantifying their impacts is fraught with difficulties because of the extreme diversity in meteorological boundary conditions, which contrasts strongly with the lack of knowledge about local hydrological processes. Although local studies have reduced data scarcity in certain regions, the complexity of the tropical Andes poses a big challenge to regional hydrological prediction. This study analyses data generated from a participatory monitoring network of 25 headwater catchments covering three of the major Andean biomes (páramo, jalca and puna) and links their hydrological responses to main types of human interventions (cultivation, afforestation and grazing). A paired catchment setup was implemented to evaluate the impacts of change using a ‘trading space‐for‐time’ approach. Catchments were selected based on regional representativeness and contrasting land use types. Precipitation and discharge have been monitored and analysed at high temporal resolution for a time period between 1 and 5 years. The observed catchment responses clearly reflect the extraordinarily wide spectrum of hydrological processes of the tropical Andes. They range from perennially humid páramos in Ecuador and northern Peru with extremely large specific discharge and baseflows, to highly seasonal, flashy catchments in the drier punas of southern Peru and Bolivia. The impacts of land use are similarly diverse and their magnitudes are a function of catchment properties, original and replacement vegetation and management type. Cultivation and afforestation consistently affect the entire range of discharges, particularly low flows. The impacts of grazing are more variable but have the largest effect on the catchment hydrological regulation. Overall, anthropogenic interventions result in increased streamflow variability and significant reductions in catchment regulation capacity and water yield, irrespective of the hydrological properties of the original biome. Copyright © 2016 The Authors. Hydrological Processes. Published by John Wiley & Sons Ltd.     

非点源污染对太湖上游西苕溪流域水环境的影响

水资源短缺是全球性关注的问题,水质恶化更加剧了这一问题的严重性,定量研究水质变化及其影响凶素可为治理水环境提供基础依据．与实测数据及综合污染指数的对比表明,水质指数能够合理反映水质的变化程度和时空变化趋势．利用西苕溪流域1996-2000年水质监测数据的研究结果如下：西苕溪流域水质的空间变化趋势是自上游至下游逐渐恶化,时间变化的总体趋势是逐年转好;点污染源得到有效控制年份(1999年)的水质指数比以前年份(1996-1998年)有所提高但幅度不大,说明非点源污染是影响西苕溪流域水质的重要因素;流域水污染的主要形式是氮、磷污染,其主要非点源是农田、经济竹林和城镇径流及居民生活污水等．     

Varied impact of land use on water and sediment parameters in fish ponds of the Dombes agro-ecosystem,France

Abstract

Agricultural land use in the area of water bodies is generally considered to increase the nutrient status of the water body water and sediments, but is this also the case for already nutrient-rich fish ponds? We studied 83 fish ponds in the Dombes region, France, where 1100 ponds are located in a heterogeneous agricultural landscape. Different water and sediment parameters were analysed for ponds and in ditches after rainfall events. Land use was studied in the primary catchment of ponds and in a 100-m zone around ponds. Soil parameters of different land-use types were analysed and farmers interviewed about agricultural practices. Increasing cropping area in the catchment of the ponds is significantly correlated to higher PO₄ ^3- concentration of pond water and to a lower degree, also to NO₃ ^?, but only in certain years with higher rainfall and with a more uneven distribution in spring. Sediment parameters were not significantly influenced. High NO₃- concentration in the water of a ditch during significant rainfall events was found for a cropland dominated catchment.

Citation Wezel, A., Arthaud, F., Dufloux, C., Renoud, F., Vallod, D., Robin, J., and Sarrazin, B., 2013. Varied impact of land use on water and sediment parameters in fish ponds of the Dombes agro-ecosystem, France. Hydrological Sciences Journal, 58 (4), 854–871.     

影响中国北方强降雪事件年际变化的典型环流背景和水汽收支特征分析

基于1961-2014年中国台站观测资料和NECP/NCAR再分析资料,对影响中国北方强降雪事件（日降雪量5 mm及以上,包括大到暴雪）年际变化的典型大尺度环流特征和水汽条件进行了综合分析.结果表明：中国北方强降雪事件主要集中在新疆北部和东北两个地区,而且强降雪日数和降雪量具有高度一致的年际变化特征.中国北方强降雪事件偏多时,对应北大西洋涛动（NAO）和北极涛动（AO）负位相;贝加尔湖上空维持异常低槽区,有利于冷空气的爆发南下;热带印度洋至热带西太平洋上空维持一条异常反气旋带,有利于暖湿气流向北输送;中国北方及以北区域高空为异常西风气流,提供有利的动力抬升条件,使得强降雪易于在中国北方发生;反之亦然.水汽收支分析显示,中国北方西边界和南边界水汽入流增强在强降雪偏多中起着主要贡献.异常西风水汽输送利于新疆北部大到暴雪偏多,异常西南风水汽输送则利于东北地区大到暴雪的发生.进一步研究揭示：与小雪相比,影响中国北方大到暴雪年际偏多的中高纬环流特征相类似,但环流经向度更大;而且大到暴雪与NAO和AO的关系更密切,并更多的受到来自中低纬地区的水汽输送影响.

     

Impacts of forest conversion and agriculture practices on water pathways in Southern Brazil

Land‐use/cover change (LUCC), and more specifically deforestation and multidecadal agriculture, is one of the various controlling factors of water fluxes at the hillslope or catchment scale. We investigated the impact of LUCC on water pathways and stream stormflow generation processes in a subtropical region in southern Brazil. We monitored, sampled and analysed stream water, pore water, subsurface water, and rainwater for dissolved silicon concentration (DSi) and ¹⁸O/¹⁶O (δ¹⁸O) signature to identify contributing sources to the streamflow under forest and under agriculture. Both forested and agricultural catchments were highly responsive to rainfall events in terms of discharge and shallow groundwater level. DSi versus δ¹⁸O scatter plots indicated that for both land‐use types, two run‐off components contributed to the stream discharge. The presence of a dense macropore network, combined with the presence of a compact and impeding B‐horizon, led to rapid subsurface flow in the forested catchment. In the agricultural catchment, the rapid response to rainfall was mostly due to surface run‐off. A 2‐component isotopic hydrograph separation indicated a larger contribution of rainfall water to run‐off during rainfall event in the agricultural catchments. We attributed this higher contribution to a decrease in topsoil hydraulic conductivity associated with agricultural practices. The chemical signature of the old water component in the forested catchment was very similar to that of the shallow groundwater and the pore soil water: It is therefore likely that the shallow groundwater was the main source of old water. This is not the case in the agricultural catchments where the old water component had a much higher DSi concentration than the shallow groundwater and the soil pore water. As the agricultural catchments were larger, this may to some extent simply be a scale effect. However, the higher water yields under agriculture and the high DSi concentration observed in the old water under agriculture suggest a significant contribution of deep groundwater to catchment run‐off under agriculture, suggesting that LUCC may have significant effects on weathering rates and patterns.     

Seasonal changes in the water use strategies of three co‐occurring desert shrubs

Water is a major limiting factor in desert ecosystems. In order to learn how plants cope with changes in water resources over time and space, it is important to understand plant–water relations in desert region. Using the oxygen isotopic tracing method, our study clarified the seasonal changes in the water use strategies of three co‐occurring desert shrubs. During the 2012 growing season, δ¹⁸O values were measured for xylem sap, the soil water in different soil layers between 0 and 300 cm depth and groundwater. Based on the similarities in δ¹⁸O values for the soil water in each layer, three potential water sources were identified: shallow soil water, middle soil water and deep soil water. Then we calculated the percentage utilization of potential water sources by each species in each season using the linear mixing model. The results showed that the δ¹⁸O values of the three species showed a clear seasonal pattern. Reaumuria songarica used shallow soil water when shallow layer was relatively wet in spring, but mostly took up middle soil water in summer and autumn. Nitraria tangutorum mainly utilized shallow and middle soil water in spring, but mostly absorbed deep soil water in summer and autumn. Tamarix ramosissima utilized the three water sources evenly in spring and primarily relied on deep soil water in summer and autumn. R. songarica and N. tangutorum responded quickly to large rainfall pulses during droughts. Differential root systems of the three species resulted in different seasonal water use strategies when the three competed for water. Copyright © 2013 John Wiley & Sons, Ltd.