Contribution Of Soil Water Research Articles

Assessing the ecohydrological separation hypothesis and seasonal variations in water use by Ginkgo biloba L. in a subtropical riparian area

In this study we used a dual stable isotope approach (δ18O and δ2H) to assess the ecohydrological separation hypothesis and to identify the seasonal variation in water sources of Ginkgo biloba L. in the riparian zone in the Taihu Lake basin, China. Three study sites located at 5, 10, and 30m from a river bank were established. From August 2014 to July 2015, samples of rainwater, river water, groundwater, bulk soil water at five soil depths (i.e. 0–30, 30–60, 60–90, 90–120, 120–150cm), and xylem water of G. biloba, were collected and their δ18O and δ2H values were measured. Generally, the δ18O and δ2H values for xylem water, groundwater, and soil water clustered together and separated from those of river water, suggesting the possible occurrence of ecohydrological separation. However, the line-conditioned excess (lc-excess) values of most xylem water were positive, indicating a mixture of different water sources. Significant correlations were observed between the contributions of precipitation, soil water, and groundwater to water uptake by G. biloba, further supporting ecohydrological connectivity rather than ecohydrological separation. G. biloba switched its major water sources from soil water at 0–60cm depth and precipitation in the wet summer, to soil water from >90cm depth and groundwater in the dry winter. The river water was a minor water source for G. biloba, but its contribution was comparatively greater at the site closest to the river bank. Our findings contribute to understanding of plant–soil–water relationships and the water balance, and may provide important information for investigations of nutrient sources and sinks in riparian zones. The present study suggests the need to rethink the application of ecohydrological connectivity and separation in different biomes, especially where river water and groundwater recharge each other over time.

Contribution of soil water at various depths to water consumption of rainfed winter wheat in the Loess tableland, China

Soil water and stem water were collected in jointing and heading stages of the rainfed winter wheat in the Changwu Loess tableland, and the stable isotopic compositions of hydrogen and oxygen in water samples were measured to analyze the contribution of soil water at various depths to water consumption of winter wheat. The results showed that the isotopes were enriched in soil and wheat stem water in comparison with that in precipitation. Under the condition of no dry layer in soil profile, the contributions to wheat water consumption in jointing and heading stages were 5.4% and 2.6% from soil water at 0-30 cm depth, 73.4% and 67.3% at 60-90 cm depth (the main water source for winter wheat), and 7.9% and 13.5% below 120 cm depth, respectively. With the wheat growth, the contribution of soil water below the depth of 90 cm increased. It was concluded that soil evaporation mainly consumed soil water in 0-30 cm depth and wheat transpiration mainly consumed soil water below 60 cm depth in the experimental period. In the production practice, it is necessary to increase rainwater storage ratio during the summer fallow period, and apply reasonable combination of nitrogen and phosphorus fertilizers in order to increase soil moisture before wheat sowing, promote the wheat root developing deep downwards and raise the deep soil water utilization ratio.

Assessment of Dimensionless Form of Kostiakov Model

Infiltration is the process by which water enters the earth's surface. Although contribution of soil water to total water present on earth surface is negligible, but it is of prime importance for plant life. Very recently, dimensionless form of Kostiakov equation was proposed for precise estimation of Kostiakov model parameters. The present paper is an assessment of accuracy of proposed dimensionless form of Kostiakov model. However, it has been found that the cumulative infiltration estimated by nonlinear optimization technique are more accurate than those estimated by dimensionless form of Kostiakov equation and the graphical method. Therefore, the present paper proposes application of Generalized Reduced Gradient (GRG), a nonlinear optimization technique to estimate Kostiakov model parameters precisely rather than using dimensionless form of Kostiakov equation. Infiltration data of various types of soil have been considered in the present study for the generalization of the results.

Assessment of Dimensionless Form of Kostiakov Model

Infiltration is the process by which water enters the earth's surface. Although contribution of soil water to total water present on earth surface is negligible, but it is of prime importance for plant life. Very recently, dimensionless form of Kostiakov equation was proposed for precise estimation of Kostiakov model parameters. The present paper is an assessment of accuracy of proposed dimensionless form of Kostiakov model. However, it has been found that the cumulative infiltration estimated by nonlinear optimization technique are more accurate than those estimated by dimensionless form of Kostiakov equation and the graphical method. Therefore, the present paper proposes application of Generalized Reduced Gradient (GRG), a nonlinear optimization technique to estimate Kostiakov model parameters precisely rather than using dimensionless form of Kostiakov equation. Infiltration data of various types of soil have been considered in the present study for the generalization of the results.

Using stable isotopes to determine seasonal variations in water uptake of summer maize under different fertilization treatments

Fertilization and water both affect root water uptake in the nutrient and water cycle of the Soil-Plant-Atmosphere-Continuum (SPAC). In this study, dual stable isotopes (D and 18O) were used to determine seasonal variations in water uptake patterns of summer maize under different fertilization treatments in Beijing, China during 2013–2014. The contributions of soil water at different depths to water uptake were quantified by the MixSIAR Bayesian mixing model. Water uptake was mainly sourced from soil water in the 0–20cm depth at the seeding (67.7%), jointing (60.5%), tasseling (47.5%), dough (41.4%), and harvest (43.9%) stages, and the 20–50cm depth at the milk stage (32.8%). Different levels of fertilization application led to considerable differences in the proportional contribution of soil water at 0–20cm (6.0–58.5%) and 20–50cm (6.1–26.3%). There was little difference of contributions in the deep layers (50–200cm) among treatments in 2013, whereas differences were observed in 50–90cm at the milk stage and 50–200cm at the dough stage during 2014. The main water uptake depth was concentrated in the upper soil layers (0–50cm) during the wet season (2013), whereas a seasonal drought in 2014 promoted the contribution of soil water in deep layers. The contribution of soil water was significantly and positively correlated with the proportions of root length (r=0.753, p<0.01). The changes of soil water distribution were consistent with the seasonal variation in water uptake patterns. The present study identified water sources for summer maize under varying fertilization treatments and provided scientific implications for fertilization and irrigation management.

Water circulation and governing factors in humid tropical river basins in the central Western Ghats, Karnataka, India.

The small river basins in the narrow stretch of the Arabian Sea coast of southwest India experience high annual rainfall (800-8000 mm), with a higher proportion (85 %) during the summer monsoon period between June and September. This is due to a unique orographic barrier provided by the Western Ghats mountain belt (600-2600 m) for the summer monsoon brought by the southwesterly winds. This study is the first of a kind focusing on the water cycle with an intensive stable isotopes approach (samples of river water, groundwater, rainwater; seasonal and spatial sampling) in this part of the Western Ghats in Karnataka and also in the highest rainfall-receiving region (with places like Agumbe receiving 7000-8000 mm annual rainfall) in South India. In addition, the region lacks sustainable water budgeting due to high demographic pressure and a dry pre-monsoon season as the monsoon is mainly unimodal in this part of India, particularly close to the coast. The stable isotopic compositions of groundwater, river water and rainwater in two tropical river basins situated approximately 60 km apart, namely the Swarna near Udupi and the Nethravati near Mangalore, were studied from 2010 to 2013. The δ(18)O and δ(2)H values of the water samples were measured by isotope ratio mass spectrometry, and the d-excess values calculated to better understand the dominant source of the water and the influence of evaporation/recycling processes. The water in the smaller area basin (Swarna basin) does not show seasonal variability in the δ(18)O values for groundwater and river water, having a similar mean value of -3.1 ‰. The d-excess value remains higher in both wet and dry seasons suggesting strong water vapor recycling along the foothills of the Western Ghats. In contrast, the larger tropical basin (Nethravati basin) displays specific seasonal isotopic variability. The observation of higher d-excess values in winter with lower δ(18)O values suggests an influence of northeast winter monsoon water in the larger basin. The narrow coastal strip to the west of the Western Ghats displays unique water characteristics in both tropical river basins investigated. For the smaller and hilly Swarna basin, the dense vegetation (wet canopies) could largely re-evaporate the (intercepted) rain, leading to no marked seasonal or altitude effect on the water isotope values within the basin. The larger Nethravati basin, which stretches farther into the foothills of the Western Ghats, receives winter monsoon water, and thus exhibits a clear seasonal variability in rainfall moisture sources. The degree of water vapor recycling in these wet tropical basins dominates the isotopic composition in this narrow coastal stretch of South India. An insight into the soil water contribution to the river water and groundwater, even in the rainfall-dependent tropical basins of South India, is provided in this study.

Litter production rates and soil moisture influences interannual variability in litter respiration in the semi-arid Loess Plateau, China

A better understanding of the factors affecting interannual variability in litter respiration is critical to precisely understand local carbon cycling, especially under the changing climate. In this study, litter respiration was obtained by subtracting in situ soil respiration in a control (LCK) treatment by that in a litter removal (LR) treatment for the period 2009–2013 in a 30-year-old black locust plantation (Robinia pseudoacacia L.) on a ridge slope in a small watershed of Loess Plateau, China. Annual cumulative litter respiration ranged from 48 ± 15 to 165 ± 36 g C m−2 y−1, with mean value of 113 ± 45 g C m−2 y−1 and coefficient of variation (CV) of 40%; annual contribution rate of litter respiration to total soil respiration (hereafter refer to as litter contribution rate) also exhibited a similar interannual variability (ranged from 8 ± 3% to 20 ± 7%; mean = 15 ± 5%; CV = 31%). Additionally, annual mean soil moisture was highest in 2010 (53.2 ± 8.4% WFPS) and lowest in 2013 (31.4 ± 9.5% WFPS), with mean value of 41.6 ± 7.9% WFPS and CV of 19%; annual litter production rates also exhibited a similar interannual variability (ranged from 379 ± 34 to 565 ± 69 g m−2 y−1; mean = 477 ± 71 g m−2 y−1; CV = 15%). Annual mean soil moisture was mainly affected by the frequency and distribution of precipitation, and annual litter production rates varied with summer precipitation. Annual cumulative litter respiration and litter contribution rate increased linearly with both annual litter production rates and mean soil moisture content. The contribution of soil water to litter respiration was larger than that of litter production rates. Therefore, litter production rates and soil moisture resulted from precipitation needs to be taken into account for precisely predicting litter respiration in the dryland ecosystems, especially under the changing climate.

Seasonal changes in runoff generation in a small forested mountain catchment

AbstractThis study aimed to investigate the seasonal variability of runoff generation processes, the sources of stream water, and the controls on the contribution of event water to streamflow for a small forested catchment in the Italian pre‐Alps. Hydrometric, isotopic, and electrical conductivity data collected between August 2012 and August 2013 revealed a marked seasonal variability in runoff responses. Noticeable differences in runoff coefficients and hydrological dynamics between summer and fall/spring rainfall events were related to antecedent moisture conditions and event size. Two‐component and three‐component hydrograph separation and end‐member mixing analysis showed an increase in event water contributions to streamflow with event size and average rainfall intensity. Event water fractions were larger during dry conditions in the summer, suggesting that stormflow generation in the summer consisted predominantly of direct channel precipitation and some saturated overland flow from the riparian zone. On the contrary, groundwater and hillslope soil water contributions dominated the streamflow response during wet conditions in fall. Seasonal differences were also noted between event water fractions computed based on isotopic and electrical conductivity data, likely because of the dilution effect during the wetter months. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Contributions of precipitation, irrigation and soil water to evapotranspiration in (semi)‐arid regions

ABSTRACTEvapotranspiration (ET) is the largest mode of water loss in cultivated (semi)‐arid regions. Isolating the fractions of ET contributed by various components of the water budget is critical not only for water resources management, but also for sustainable crop production, food security and social stability. This study quantifies the fractions of ET from precipitation, irrigation and soil water in the North China Plain (NCP) – a major crop production base in China. About 24 consecutive years (288 months) of hydroclimatic data (spanning from January 1985 to December 2008) are used in the study. The data are derived from Global Land Data Assimilation System (GLDAS) input/output products and field‐measured data in the region. Error analysis suggests that uncertainties/biases in the data products are low, with good correlations among the data products. In the NCP study area, precipitation is the highest contributor to ET (39.0%, 318.2 mm), followed by soil water (36.3%, 296.2 mm) and then irrigation (24.7%, 201.5 mm). This is respectively the equivalent of 29.7, 43.9 and 66.4% of average soil water, irrigation and precipitation in the region. Precipitation (as the highest contributor) drives the overall trend, amplitude and phase of ET in the study area. While the contribution of precipitation is highest in the wet summer cropping season, the contributions of irrigation and soil water are highest in the relatively dry spring cropping season. It is vital to devise more efficient ET‐reduction strategies to ensure sustainability in water use, food security, ecological protection and social stability in the region and beyond.

Evaluating contribution of soil water to paddy rice by stable isotopes of hydrogen and oxygen

Rice is a highly water-consumptive crop under frequent flooding. However, the contributions of soil water and ponded water to rice are unclear, complicating efforts to water management in paddy ecosystem efficiently. Using stable isotopes of the oxygen (δ18O) and hydrogen (δD), we aimed to determine isotopic compositions of ponded water, soil water, and groundwater, and then to evaluate their contributions to rice crop. Two types of paddy water management, one under continuous flooding (CF) and the other under alternate flooding and drying (AFD), were investigated. The isotope ratios of rice stem water, ponded water, soil water along soil profile of 0–50 cm, irrigation water, groundwater, and rain water were determined by isotope ratio infrared spectroscopy. The direct inference method and Iso-source model approach showed that rice appeared to use soil water from 90 % of rice water was 0–15 cm from soil depth. The contribution of ponded water to rice crop was negligible (<1.1 %). Water use efficiency under the AFD condition was significantly greater than that under CF condition. This study provides direct evidence that water can be saved considerably if the ponded water is reduced.

Dynamics of water usage in Haloxylon ammodendron in the southern edge of the Gurbant&amp;uuml;ngg&amp;uuml;t Desert

Aims Plant water use is an important aspect of plant-water relations in desert regions, and is vital in understanding the adaptation of desert species to arid environment. Haloxylon ammodendron is a dominant plant species in the Gurbantunggut Desert, China. Its water use pattern has an important effect on water balance of the local ecosystem and plant community composition. This study aims to investigate the dynamics of water usage in H. ammodendron and its response to soil water fluctuations resulting from precipitation pulses. Methods Oxygen stable isotope ratios (δO) were measured for xylem water, soil water in different soil layers (0–300 cm depth), precipitation and groundwater. Four potential water sources were classified: shallow (0–40 cm), middle (40–100 cm) and deep soil water (100–300 cm), and groundwater. The possible ranges of potential water sources used by H. ammodendron were calculated using the IsoSource model. Important findings Main water sources for H. ammodendron shifted from the shallow soil water in April to the groundwater during May to September. In April, the contributions of shallow soil water were in the range of 62%–95%, and the possible ranges of middle and deep soil water and groundwater were 0–8%, 0–15% and 0–38%, respectively. However, during May to September, the contribution of shallow soil water decreased drastically while that of groundwater increased rapidly. Contributions of groundwater were in the range of 68%–100%. Haloxylon ammodendron responded differently to two similar precipitation pulses occurred in different months. After the 6.7 mm precipitation pulse on May 22, the usage of soil water increased from 9.8% prior to the event to maximum at 40.4% one day after rainfall (May 23), while the usage of groundwater decreased from 83%–98% to 42%–81%. After 7 mm precipitation pulse on August 31, the usage of soil water was almost unchanged and the usage of groundwater was still up to 71%–98%, implying that roots of H. ammodendron in the upper soil layer are 戴岳等: 古尔班通古特沙漠南缘梭梭水分利用动态 1215 doi: 10.3724/SP.J.1258.2014.00117 inactive due to the long-lasting low soil water content. Hence, H. ammodendron seemed to be insensitive to the August precipitation pulse. The shallow soil water recharged by snowmelt and precipitation in spring and groundwater are two important water sources for H. ammodendron. Dynamics of water usage in H. ammodendron reflects its adaptation to this water-limiting desert environment.

Evidence to support the significance of rapid lateral flow contributions from a subsurface palaeochannel to a stream during event flows

Debate over water movement processes and pathway contributions through the landscape into streams remains contested, as often paradox evidence between published studies demonstrate major differences in reported component contributions. The objectives of this paper are to add further evidence to this ongoing debate through using multidisciplinary field-based measurement techniques and a focus around the impact of ancient palaeochannels for directing significant contributions of soil water and solutes into streams in relatively short and reactive time frames during stream flow events. The project involved afield-based study in the Livingstone Creek subcatchment located within the Murrumbidgee valley of inland southern New South Wales, where dryland salinity problems are of major concern. The project investigates the processes influencing salt movement from the landscape to the stream in a upland catchment that also has large areas of river flats and valleys. The major finding of the study was that during rainfall events rapid infiltration of recharge occurred in the unsaturated zones across the alluvial landform.

Hydrograph separation to improve understanding of Dissolved Organic Carbon Dynamics in Headwater catchments

AbstractDissolved organic carbon (DOC) is a key component of the global carbon cycle, but, to date, large uncertainties still exist on its source and fate in first‐order streams. In a 23 ha rangeland and steep‐slope headwater of South Africa, our aim was to quantify the contribution of overland flow (OF), soil water (SW) and ground water (GW) to DOC fluxes (DOCF), and to interpret the results in terms of DOC sources and fate. The average 2010–2011 DOC concentration (DOCC) at the catchment outlet was 4.7 mg C l−1 with a standard error of ±2.5 mg C l−1, which was significantly lower than in SW (15.2 ± 1.6 mg C l−1) and OF (11.9 ± 0.8 mg C l−1), but higher than in GW (2.3 ± 0.6 mg C l−1). Based on end‐member mixing using Si and Na concentration in the water compartments, the average SW contribution to DOCF was 66.4%, followed by OF (30.0%) and GW (3.6%). The resulting estimated DOCF at the catchment outlet was 8.05 g C m2 y−1. This was much higher than the observed value of 2.80 g C m2 y−1, meaning that 5.25 g C m2 y−1 or 65% of the DOC is lost during its downslope and/or downstream transport to the catchment outlet. Complementary investigations revealed that the DOCC in SW dropped from 15.2 ± 1.6 to 2.6 ± 0.3 mg C l−1 during its downslope transport to the river system, which corresponded to a net loss of 5.10 g C m2 y−1, or 97% of the catchment DOC losses. These results on DOC sources and potential fate in headwaters are expected to improve our understanding of the impact of hydrology on the global C‐cycle. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Root functioning, tree water use and hydraulic redistribution in Quercus suber trees: A modeling approach based on root sap flow

Mediterranean evergreen oaks have to survive a long summer drought. Roots may play a relevant role under these conditions. We studied their structure and function in a mature Quercus suber L. tree in central Portugal. The root system was mapped till the lowest water table level (4.5m depth). Xylem anatomy was analyzed in a vertical profile belowground. Sap flow was continuously monitored for 1.5yrs in the stem and roots of this intensively studied tree (heat field deformation method) and in the stem of four trees (Granier method), in relation to environmental variables and predawn leaf water potential. The sources of water uptake were assessed by stable isotope analyses in summer. Results showed a dimorphic root system with a network of superficial roots linked to sinker roots, and a taproot diverting into tangles of deep fine roots submerged for long periods, with parenchyma aerenchyma. Transpiration was not restricted in summer due to root access to groundwater. The isotopic δ18O signature of twig xylem water was similar to that of groundwater in the dry season. Two functional types of superficial roots were identified: shallow connected and deep connected roots. A modeling approach was built considering that each superficial root was linked to a sinker, with part of the root deep connected (between the stem and the sinker) and part shallow connected (between the sinker and topsoil). This conceptual framework simulated tree stem sap flow from root sap flow with a high efficiency (R2=0.85) in four plot trees.On an annual basis, soil water and groundwater contributions were 69.5% and 30.5% of stem flow, respectively. Annual hydraulic lift and hydraulic descent were 0.9% and 37.0% of stem flow, respectively. The trees maximize the exploitation of the environmental resources by using the topsoil water during most of the year, and groundwater together with hydraulic lift (nutrient supply) in the dry summer. This study shows that a dimorphic root system, with roots reaching groundwater, is an efficient strategy of Q. suber trees to cope with seasonal drought. Knowledge of the functional behavior of Q. suber trees under shallow water table conditions may contribute to the definition of better adapted management practices and to anticipate their responses to climate change.

Stable Isotopic Study on Water Utilization Sources of &lt;i&gt;Pinus yunnanensis &lt;/i&gt;Plantations in the Central Yunnan Karst Plateau

In order to explain the water utilization mechanism of Pinus Yunnanensis plantations in the Central Yunnan Karst Plateau, SW China, and their adaptive strategies to arid karst habitat, the authors collected and analyzed the stable 18O isotope values of stem water of Pinus Yunnanensis in Bajiang valley, and their accompanying trees and bushes and their potential water sources. The IsoSource model was applied to compute the probable contribution of multiple potential water sources to total plant. The results show that in the ecosystem of Pinus Yunnanensis plantations, there exists significant evaporative isotopic fractionation during the process of hydraulic infiltration from precipitation to ground water and soil water and that of hydraulic redistribution in the soil profile. Water utilization positions and proportions of Pinus Yunnanensis and its accompanying trees and bushes vary from season to season, and they are evident competitors for water. During dry seasons, mature Pinus Yunnanensis uses 68.6% of 0-55cm soil water while the contribution of 30-110cm soil water and groundwater to the young Pinus Yunnanensis reaches 55.8% and 31.7% respectively. In rainy seasons, the use ratio of 0-55cm surface soil water by young Pinus Yunnanensis, Pistacia weinmannifolia and Rhododendron spiciferum are 63.1%, 56.3% and 86.2% respectively while the mature Pinus Yunnanensis uses rainfall, groundwater and fog droplets of canopy interception.

Water uptake from different soil depths for halophytic shrubs grown in Northern area of Ningxia plain (China) in contrasted water regimes

In order to understand the contributions of groundwater and deep soil water to the growth of halophytes in salinity-affected area, water use strategies of four shrubes, i.e. 20-year-old Tamarix ramosissima Ledeb., three-year-old T. ramosissima., Lycium barbarum L., and Atriplex canescens (Pursh) Nutt. were studied under contrasted water regimes in Northwest China. The result showed that there was a vertical gradient in soil δ18O and δD profiles resulted from evaporation and irrigation. The 20-year-old T. ramosissima mainly used water from middle (40–140 cm) and deep (140–200 cm) under both water regimes indicating its phreatophytic nature. Soil water in upper profile (0–40 cm) was the dominant water source for the three-year-old T. ramosissima before irrigation. After irrigation, the three-year-old T. ramosissima and L. barbarum switched their water sources to middle soil profile. Our experiment revealed phreatophytic tendency for the three-year-old A. canescens, which was not responsive to irrigation enlighten by photosynthetic parameters and stem water potentials.

Optimizing the yield of winter wheat by regulating water consumption during vegetative and reproductive stages under limited water supply

To ensure sustainable agricultural water use in water shortage regions, practices of deficit irrigation should be adopted. This study investigated the performance of winter wheat (Triticum aestivum L.) under limited water supply from 2005 to 2011, a six-season field test on the North China Plain. The test was comprised of four treatments: rain-fed, single irrigation applied at sowing to obtain a good level of soil moisture at the start of crop growth (I1s), single irrigation applied during recovery to jointing (I1r), and full irrigation supplied as three irrigations (control, I3). The results showed that grain yield was significantly correlated with rainfall before heading and with evapotranspiration (ET) after heading (P < 0.01) under rain-fed conditions. The average contribution of soil water stored before sowing to seasonal ET was 90, 103, and 145 mm for rain-fed, I1s, and I1r, respectively, during the six seasons. A smaller root length density (RLD), which restricted utilization of deep soil water by the crop, was one of the reasons for the lower yield with rain-fed and I1s treatments compared with the I1r treatment in dry seasons. The results also showed that the limited irrigation applied from recovery to jointing stage (Treatment I1r) significantly promoted vegetative growth and more efficient soil water use during the reproductive (post-heading) stage, resulting in a 21.6 % yield increase compared with that of the I1s treatment. And although the average yield of the I1r treatment was 14 % lower than that of the full irrigation treatment, seasonal irrigation was reduced by 120–140 mm. With smaller penalties in yield and a larger reduction in applied irrigation, I1r could be considered a feasible irrigation practice that could be used in the NCP for conservation of groundwater resources.

Water storage at the Panola Mountain Research Watershed, Georgia, USA

AbstractStorage is a major component of a catchment water balance particularly when the water balance components are evaluated on short time scales, that is, less than annual. We propose a method of determining the storage–discharge relation using an exponential function and daily precipitation, potential evapotranspiration (PET) and baseflow during the dormant season when evapotranspiration (ET) is low. The method was applied to the 22‐year data series of the 0.41‐ha forested Panola Mountain Research Watershed, Georgia. The relation of cumulative daily precipitation minus daily runoff and PET versus baseflow was highly significant (r2 = 0.92, p &lt; 0.0001), but the initial storage for each year varied markedly. For the 22‐year study period, annual precipitation and runoff averaged 1240 and 380 mm, respectively, whereas the absolute catchment storage range was ~400 mm, averaging 219 mm annually, which is attributed to contributions of soil water and groundwater. The soil moisture of a catchment average 1‐m soil depth was evaluated and suggests that there was an active (changes in soil storage during stormflow) and passive (a longer‐term seasonal cycle) soil water storage with ranges of 40–70 and 100–120 mm, respectively. The active soil water storage was short term on the order of days during and immediately after rainstorms, and the passive or seasonal soil storage was highest during winter when ET was lowest and lowest during summer when ET was highest. An estimate of ET from daily changes in soil moisture (ETSM) during recessions was comparable with PET during the dormant season (1.5 mm day−1) but was much lower during the growing season (June through August); monthly average SMET and PET ranged from 2.8 to 4.0 mm day−1 and from 4.5 to 5.5 mm day−1, respectively. The growing season difference is attributed to the overestimation of PET. ETSM estimates were comparable with those derived from hillslope water balances during sprinkling experiments. Master recession curves derived from the storage–discharge relation adjusted seasonally for ET (1.5 and 4.0 mm day−1 during the dormant and growing seasons, respectively) fit actual recessions extremely well. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Evapotranspiration and soil water relationships in a range of disturbed and undisturbed ecosystems in the semi-arid Inner Mongolia, China

Aims Evapotranspiration (ET) is a key component of water balance and is closely linked to ecosystem productivity. In arid regions, large proportion of precipitation (PPT) is returned to the atmosphere through ET, with only a small amount available to plants. Our objective was to examine the variability in ET–soil water relationship based on a set of ecosystems that are representative for semi-arid Inner Mongolia and its main land use practices. Methods This study used Eddy covariance (EC) data of water vapor (i.e. ET, mm), PPT (mm), soil volumetric water content (VWC, %), root biomass density and soil properties from three paired sites in semi-arid Inner Mongolia: cropland (Cropland-D) versus undisturbed grassland (Steppe-D), grazed grassland (Grazed Steppe-X) versus fenced grassland (Fenced Steppe-X) and poplar plantation (Poplar-K) versus undisturbed shrubland (Shrubland-K). The paired sites experienced similar climate conditions and were equipped with the same monitoring systems. Important Findings The ET/PPT ratio was significantly lower at Cropland-D and Grazed Steppe-X in comparison to the undisturbed grasslands, Steppe-D and Fenced Steppe-X. These differences are in part explained by the lower VWC in the upper soil layers associated with compaction of surface soil in heavily grazed and fallow fields. In contrast, the ET/PPT ratio was much higher at the poplar plantation compared to the undisturbed shrubland because poplar roots tap groundwater. The VWC of different soil layers responded differently to rainfall events across the six study sites. Except for Poplar-K, ET was significantly constrained by VWC at the other five sites, although the correlation coefficients varied among soil layers. The relative contribution of soil water to ET correlated with the density of root biomass in the soil (R 2 =0 .67,P < 0.01). The soil water storage in the upper 50 cm of soil contributed 59, 43, 64 and 23% of total water loss as ET at Steppe-D, Cropland-D, Shrubland-K and Poplar-K, respectively. Our across-site analysis indicates that the site level of soil water for ET differs between land use and land cover type due to altered root distribution and/or soil physical properties. As a result, we recommend that ecosystem models designed to predict the response of a wide variety of vegetation to climatic variation in arid regions include more detail in defining soil layers and interactions between evaporation, infiltration and root distribution patterns.

強雨が森林流域における土壌浸透水の水質へ及ぼす影響

Soil infiltration process greatly contributes to the hydrological process in watershed, thus, soil infiltration water and runoff water were collected to characterize pollutant load from two planted forests. Especially, this study focused on the heavy rain which is reported to be increasing because of climate change. Glass fiber wick samplers were used to collect soil infiltration water samples, while runoff water was collected at the weir. Results showed that the wick sampler successfully collected the soil infiltration water without accumulating excess water above the device. The amounts of pollutant loads in soil infiltration water (total organic carbon; TOC, total nitrogen; TN) were positively correlated with pollutant loads in runoff water, which directly showed the contribution of soil water to runoff water. During the research period, environmental pollution load density in soil infiltration water was negatively correlated with sampled soil water volume. This could be a dilution effect caused by rainfall infiltration. However, after the heavy rainfall, soil infiltration water quality showed higher concentration than other rainfall events. The trend was clear in the long lasting strong rainfall, especially for well conductive forest slope. Because of the high permeable properties, pollutant loads from the soil surface easily reached to the depth of glass fiber wick (50cm depth), without filtering process through the soil body.