Small Rainfall Events Research Articles

Soil Moisture Dynamics in Response to Precipitation and Thinning in a Semi-Dry Forest in Northern Mexico

Understanding soil moisture behavior in semi-dry forests is essential for evaluating the impact of forest management on water availability. The objective of the study was to analyze soil moisture based in storm observations in three micro-catchments (0.19, 0.20, and 0.27 ha) with similar tree densities, and subject to different thinning intensities in a semi-dry forest in Chihuahua, Mexico. Vegetation, soil characteristics, precipitation, and volumetric water content were measured before thinning (2018), and after 0%, 40%, and 80% thinning for each micro-catchment (2019). Soil moisture was low and relatively similar among the three micro-catchments in 2018 (mean = 8.5%), and only large rainfall events (>30 mm) increased soil moisture significantly (29–52%). After thinning, soil moisture was higher and significantly different among the micro-catchments only during small rainfall events (<10 mm), while a difference was not noted during large events. The difference before–after during small rainfall events was not significant for the control (0% thinning); whereas 40% and 80% thinning increased soil moisture significantly by 40% and 53%, respectively. Knowledge of the response of soil moisture as a result of thinning and rainfall characteristics has important implications, especially for evaluating the impact of forest management on water availability.

Catchment based estimation of pollutant event mean concentration (EMC) and implications for first flush assessment

The Event Mean Concentration (EMC) is considered as a key analytical parameter for assessing the quality of stormwater. The conventional estimation methods to determine EMC do not necessarily address the variability associated with the hydrologic characteristics. Accordingly, this study was conducted to identify the potential hydrologic variables that can influence EMC and thereby to create a mathematical model to determine EMC using the hydrologic variables while incorporating the catchment as an influential factor. This paper introduces an innovative approach to estimate EMC of a runoff event using a stepwise multiple linear regression model. The model incorporates hydrologic variables together with their two-way interaction terms. The catchment was included in the model as a dummy variable. This allows identifying the variability of EMC between catchments. Model can reasonably predict the EMC with an overall prediction error of 0.811. The regression coefficients of the model specify that, maximum rainfall intensity is the most influential variable having a coefficient of 1.008, followed by the average intensity with a coefficient −0.586. The interaction term of rainfall depth and the antecedent dry period indicates that for a relatively small rainfall event (<5 mm), an optimum value of antecedent dry period exists that maximises the EMC. Subsequently, EMC was employed to define the first flush runoff as an alternative approach to the conventional approaches for determining the first flush. The dynamic mean concentration (DMCt), was introduced as a parameter for estimating the first flush using EMC. The maximum accumulated runoff volume such that, DMCt≥EMC was defined as the first flush runoff. It was found that residential catchments generate more intense first flush compared to catchments with totally impervious surface areas and thereby a significant pollutant load is transported within a small initial fraction of the runoff.f

Modelling Hydrological Performance of a Bauxite Residue Profile for Deposition Management of a Storage Facility

Accurate scheduling of bauxite residue (red mud) deposition time is required in order to prevent the risk of storage facility failure. This study was conducted to precisely determine the hydraulic parameters of bauxite residue and investigate the capability of HYDRUS to accurately estimate the residue moisture profile and the timing for its deposition. The hydraulic properties of the bauxite residue profile were determined by solving an inverse problem. A one-dimensional hydrological model (HYDRUS-1D) was validated using a 300 mm long column filled with bauxite residue and exposed to a dynamic lower boundary condition. After numerical validation, the model was used to simulate the moisture profile of bauxite residue under the climatic conditions of an alumina refinery site in Queensland, Australia, as well as other scenarios (i.e., high (300 mm) and small (1.7 mm) rainfall events of the site). This study showed that the HYDRUS model can be used as a predictive tool to precisely estimate the moisture profile of the bauxite residue and that the timing for the re-deposition of the bauxite residue can be estimated by understanding the moisture profile and desired shear strength of the residue. This study revealed that the examined bauxite residue approaches field capacity (water potential −10 kPa) after three days from a low rainfall event (&lt;1.7 mm) and after eight days from an intense rainfall event (300 mm) at the time of disposal. This suggests that the bauxite residue can be deposited every four days after low rainfall events (as low as 1.7 mm) and every nine days after high rainfall events (as high as 300 mm) at the time of deposition, if bauxite residue experiences an initial drying period following deposition.

Performance assessment of coupled green-grey-blue systems for Sponge City construction

In recent years, Sponge City has gained significant interests as a way of urban water management. The kernel of Sponge City is to develop a coupled green-grey-blue system which consists of green infrastructure at the source, grey infrastructure (i.e. drainage system) at the midway and receiving water bodies as the blue part at the terminal. However, the current approaches for assessing the performance of Sponge City construction are confined to green-grey systems and do not adequately reflect the effectiveness in runoff reduction and the impacts on receiving water bodies. This paper proposes an integrated assessment framework of coupled green-grey-blue systems on compliance of water quantity and quality control targets in Sponge City construction. Rainfall runoff and river system models are coupled to provide quantitative simulation evaluations of a number of indicators of land-based and river quality. A multi-criteria decision-making method, i.e., Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) is adopted to rank design alternatives and identify the optimal alternative for Sponge City construction. The effectiveness of this framework is demonstrated in a typical plain river network area of Suzhou, China. The results demonstrate that the performance of Sponge City strategies increases with large scale deployment under smaller rainfall events. In addition, though surface runoff has a dilution effect on the river water quality, the control of surface pollutants can play a significant role in the river water quality improvement. This framework can be applied to Sponge City projects to achieve the enhancement of urban water management.

Stormflow threshold behaviour in a subtropical mountainous headwater catchment during forest recovery period

AbstractForest ecohydrological feedbacks complicate the threshold behaviour of stormflow response to precipitation or wetting conditions on a long‐term scale (e.g. several years). In this study, the threshold behaviours in an evergreen‐deciduous mixed forested headwater catchment in southern China were examined during 2009–2015, when damaged vegetation was recovering after the great 2008 Chinese ice and snowstorm. The non‐uniqueness of the thresholds and the slow and rapid responses of stormflow at the outlet of the catchment in different hydro‐climate datasets with different maximum values of gross precipitation (P) and sums of precipitation and antecedent soil moisture index (P + ASI) were assessed. The thresholds of P and P + ASI required to trigger stormflows (i.e. ‘generation thresholds’) and the transition from slow to rapid responses of stormflow (i.e. ‘rise thresholds’) were compared both seasonally and annually. The results indicated significant differences in the analysed datasets, highlighting the need to compare thresholds with care to avoid misinterpretation. Seasonal variations in threshold behaviours in the catchment suggested that vegetation canopy interception contributed to higher rise thresholds, and wetter conditions resulted in higher runoff sensitivity to precipitation during the growing and rainy seasons. Furthermore, the generation thresholds were higher in the dormant season, possibly due to drier soil moisture conditions in the near‐channel areas. During the vegetation recovery period, the annual generation thresholds increased, however the rise thresholds did not exhibit a similar trend. The rapid stormflow response above the threshold decreased, possibly due to transpiration and interception of the recovered vegetation. However, the slow stormflow response to small rainfall events below the thresholds was higher in wetter years but lower in drier years, suggesting that the total water input dominated the stormflow response during small rainfall events. In conclusion, the seasonal and annual variations in threshold behaviours highlight that vegetation recovery and hydro‐climatic conditions had a notable impact on the stormflow response.

Hydrological behaviour of vertisols in the Brazilian semi-arid region: the importance of rainfall of less than 30 mm

The greater probability of small rainfall events occurring in semi-arid regions, and the little understanding of their role in hydrological processes, has led to this investigation of surface runoff generated during these events in two adjacent micro basins in the Brazilian semi-arid region. The types of plant cover to be investigated were the Phytogeographical Caatinga Domain under regeneration for 35 years (CPDReg) and thinned CPD (CPDThin), which consisted of the elimination of trees with a diameter of <10 cm. Two historical series were considered, one of 40 years (1974-2013) with 2,259 events and the other consisting of 247 rainfall events of <30 mm from 2009 to 2013. The cumulative frequency distribution showed that the series of 247 24-hour events proved to be statistically representative in investigating hydrological processes in the Brazilian semi-arid region, compared to the long series of 2,259 events. Irrespective of the pattern, rainfall with an intensity of less than 17 mm h-1 generated effective precipitation with small depths (<2 mm). Regardless of the rainfall pattern, the lowest effective precipitation was registered for the plant cover of thinned CPD, both on an annual scale and per event. The occurrence of 3 or 4 consecutive dry days is enough for events of <30 mm to generate no runoff, due to the appearance of micro-cracks formed in vertisols during the drying process. It is believed, therefore, that the expansion and contraction of vertic soils is the main determinant for the start of Pe in areas with a water source (micro basins of <2 ha) in the CPD.

Unique stable isotope signatures of large cyclonic events as a tracer of soil moisture dynamics in the semiarid subtropics

Evaporative flux from soils in arid and semi-arid climates can be very high and may substantially reduce soil moisture retained between infrequent rainfall events. Direct measurement of the evaporative losses from soils is technically challenging. However, environmental tracers such as water stable hydrogen and oxygen isotope composition can be used to calculate evaporation rates if the initial signature of the infiltrating rainwater is distinct from the signature of residual soil moisture. Large tropical cyclones typically result in rainfall events of large volume and very negative δ18O signatures that are significantly lower than the signatures of more frequent and smaller rainfall events. These very negative stable isotope signatures are retained in the soil and can be used to understand the depth of water infiltration, retention and subsequent rate of evaporation from the soil. At our study site in dry subtropical northwest Australia, we repeatedly sampled rainwater and soil moisture prior to, during and after tropical Cyclones Heidi and Lua in 2012. Site inundation from Cyclone Heidi (rainfall 213 mm, δ18O −17.6‰) replenished soil moisture in the unsaturated zone for several months, completely replacing soil moisture down to depths of ~3.5 m and contributing to groundwater recharge. The transient momentary evaporative losses from wet soil at the time of sampling varied between 0.21 and 0.60 mm × day−1 (equivalent to 76 to 220 mm × yr−1 recalculated as an annual rate). During the prolonged dry period between cyclones, evaporative losses decreased to between 8 and 30 mm × yr−1. Mean long-term groundwater recharge for the study site was low (<6 mm × yr−1). Recharge is primarily driven by infrequent but high-volume cyclones that are an important source of soil moisture and an essential water source for vegetation in this semi-arid environment. However, variation in lithology, position in the landscape and time since the last inundation contribute to highly heterogeneous patterns of δ18O in the vadose zone, which complicates upscaling observations from a local to a regional scale model of evaporative demand.

Characteristics of biochemical oxygen demand and chemical oxygen demand export from paddy fields during rainfall and non-rainfall periods

Rice growing in paddy fields equipped with irrigation and drainage systems is less affected by unfavorable weather patterns such as prolonged dry periods and severe storms, but for the same reason, the fields discharge water to maintain suitable water depths even between rainfall events. Studies have been done to examine the quality of water drained from rice paddy fields during or after rainfall events. However, it is not clear how drainage affects the water quality of downstream waterbodies, especially during non-rainfall periods. We investigated the water quality characteristics of rainfall and non-rainfall rice paddy drainage, which was characterized by biological oxygen demand (BOD) and chemical oxygen demand (COD). In this study, hydrological variables including water inflow/outflow rates, depth, BOD, and COD were monitored at a paddy field in Korea from 2008 to 2012. This study then quantified how BOD and COD loadings are controlled by irrigation and drainage management practices. Water quality measurements showed that rainfall events that resulted in rainfall depths greater than 50 mm are responsible for half of the overall BOD and COD loads discharged by rainfall events of the fields. On the other hand, a quarter of the total BOD and COD was attributed to events that provided rainfall depths less than 10 mm. The results suggested that the BOD and COD loads of rainfall events can be reduced by up to 25% if water at paddy fields is carefully managed in small rainfall events. Water samples taken during rainfall events had greater nutrient loads only by 6 kg BOD ha−1 (17 kg BOD ha−1 vs. 11 kg BOD ha−1) and 7 kg COD ha−1 (41 kg COD ha−1 vs. 34 kg COD ha−1) compared to those of the non-rainfall periods in the cropping season. Such a finding indicates the significant impacts of paddy field drainage practices on downstream water quality between rainfall events, particularly when considering a long period of non-rainfall. The monitoring data also showed that the standard unit load of 15.5 kg BOD ha−1 established for rice paddy fields by the Ministry of Environment, the environmental authority of Korea, is substantially different from the actual BOD load, because the non-rainfall BOD loads were not considered in the standard. Thus, this study suggests that non-rainfall BOD loads should be counted and managed for successful implementations of total maximum daily load programs.

Effects of Small Rainfall Events on &lt;i&gt;Haloxylon ammodendron&lt;/i&gt; Seedling Establishment in Northwest China

Effects of Small Rainfall Events on &lt;i&gt;Haloxylon ammodendron&lt;/i&gt; Seedling Establishment in Northwest China

Determining hot moments/spots of hillslope soil moisture variations based on high-resolution spatiotemporal soil moisture data

Characterizing soil moisture variation has critical implications for various ecosystem processes and the hydrological cycle. In this study, we identified the hot moments (times with high temporal variation rates) and hot spots (areas with high temporal variation rates) of the soil moisture variation and investigated their controlling factors on a tea garden hillslope in Taihu Lake Basin, China. Daily soil moisture data were calculated for 39 sites, and then, daily soil moisture maps were generated from March 1, 2014 to February 28, 2015. The soil moisture temporal variation rates (VR) at different locations and during different time periods were calculated based on these maps. We found that because soil water content at the subsurface was more spatially varied than that at the surface on this hillslope, spatial heterogeneity of VR was also greater at 0.3-m depth than that at 0.1-m depth. Elevation, sand content and rock fragment content were positively correlated (p < 0.05) with this spatial heterogeneity, while slope and clay content were negatively correlated (p < 0.05) with it. June and July 2014 were the hot moments with high VR at both depths of 0.1- and 0.3-m, while December 2014 was the cold moment with the lowest VR at both depths on this study hillslope. Meteorological factors (precipitation and temperature) explained 59.9% and 56.9% of the total variance in the time series of spatial mean VR at the depths of 0.1- and 0.3-m, respectively. Stronger variations in spatial mean VR were found during representative medium and large rainfall events than during representative small rainfall events. In addition, the response of spatial mean VR to rainfall at 0.3-m depth was delayed compared to that at 0.1-m depth. The findings and methodologies of this study can be useful in determining the hot spots and hot moments of soil moisture variations, which could be potentially useful for investigating various hydrological, ecological, environmental, and agronomic processes.

Responses of soil respiration to rainfall addition in a desert ecosystem: Linking physiological activities and rainfall pattern

The tight linkage between photosynthesis (An) and soil respiration (Rs) has been verified in many terrestrial ecosystems. However, it remains unclear whether this linkage occurs in desert ecosystems, where water is considered an important trigger of carbon cycling. A field experiment was performed under seven simulated rainfall amounts (0, 3, 5, 10, 15, 25, and 40 mm) with two co-existing desert plants (Reaumuria soongorica and Nitraria sphaerocarpa) in June (early growing season, EGS) and August (middle growing season, MGS) in 2016. An, Rs, predawn water potential (Ψpd), soil temperature (Ts) and soil moisture (Swc) were measured for each treatment or control plot for 3 weeks. Our objective was to examine the effects of rainfall pattern on Rs and physiological responses of the two plants and the relationships between Rs and biotic and abiotic factors. No obvious variations in Ψpd or An were found under small rainfall events. However, when the rainfall amount exceeded 10 mm, both plants responded strongly, and the response patterns of Rs showed trends similar to those of An, which varied between species and seasons. Moreover, rain additions of 3–40 mm significantly increased Rs, and the relative changes in Rs (ΔRs) of both species were much larger in the EGS than in the MGS. Importantly, abiotic factors may have controlled the variations in Rs under small rain events while An played a more important role in regulating the variations in Rs when the rainfall amount exceeded 10 mm for both species, suggest that the rainfall pattern–driven changes in Rs composition interact with physiological activity and abiotic factors to regulate the response of Rs to rainfall variability in desert ecosystems. Thus, climate change in the coming decades may lead to carbon sequestration by desert plants, which may cause desert ecosystems to act as carbon sinks.

Retrofitting a grass swale with rock check dams: hydrologic impacts

The hydrologic performance of a grass swale, a common stormwater control measure often utilized to drain roads, may potentially be improved using simple retrofits. Two rock check dams were retrofitted into an existing grass swale located in Knightdale, North Carolina, USA. The swale was monitored before and after check dam installation, and the addition of check dams improved runoff volume reduction (17%), peak flow mitigation, and hydraulic retention time in the swale, particularly for small (< 19 mm) and moderate (19–38 mm) rainfall events. The check dams were effective filters of gross solids, which eventually led to clogging and caused extended inundation and subsequent loss of swale vegetation. Because check dams are relatively inexpensive and simple vis-à-vis other stormwater control measure enhancements, their use for stormwater treatment is encouraged, provided they are adequately maintained.

Quantifying land use influences on event-based flow frequency, timing, magnitude, and rate of change in an urbanizing watershed of the central USA

There is a need for research that advances understanding of flow alterations in contemporary watersheds where natural and anthropogenic interactions can confound mitigation efforts. Event-based flow frequency, timing, magnitude, and rate of change were quantified at five-site nested gauging sites in a representative mixed-land-use watershed of the central USA. Statistically independent storms were paired by site (n = 111 × 5 sites) to test for significant differences in event-based rainfall and flow response variables (n = 17) between gauging sites. Increased frequency of small peak flow events (i.e., 64 more events less than 4.0 m3 s−1) was observed at the rural–urban interface of the watershed. Differences in flow response were apparent during drier periods when small rainfall events resulted in increased flow response at urban sites in the lower reaches. Relationships between rainfall and peak flow were stronger with decreased pasture/crop land use and increased urban land use by approximately 20%. Event-based total rainfall explained 40–68% of the variance in peak flow (p < 0.001). Coefficients of determination (r2) were negatively correlated with pasture/crop land use (r2 = 0.92; p = 0.007; n = 5) and positively correlated with urban land use (r2 = 0.90; p = 0.008; n = 5). Significant differences in flow metrics were observed between rural and urban sites (p < 0.05; n = 111) that were not explained by differences in rainfall variables and drainage area. An urban influence on flow timing was observed using median time lag to peak centroid and time of maximum precipitation to peak flow. Results highlight the need to establish manageable flow targets in rapidly urbanizing mixed-land-use watersheds.

Specific Conductance as a Tracer of Preferential Flow in a Subsurface‐Drained Field

Core Ideas An agricultural subsurface drain was used to quantify preferential flow paths. High‐frequency specific conductance was a tracer of water movement through the soil. Preferential flow contributed up to 75% of the water in the subsurface drain. Most preferential flow events were probably through macropores or surface cracks. Specific conductance (SC), soil volumetric water content (VWC), and discharge were monitored on a subsurface agricultural drain for a 2‐yr period (2007–2008) to differentiate preferential flow paths from matrix flow paths. A major observation from the 2‐yr period was the fast SC decrease after relatively small rainfall events, often &lt;5 mm. A total of 25 paired rainfall–SC events were classified, with an average preferential flow onset time (from the event start) after 1.7 h and maximum preferential flow after 2.4 h. A specific conductance end‐member mixing analysis (SC‐EMMA) was used to determine the volume of water that infiltrated through preferential flow pathways. The SC‐EMMA was used for 20 of the 25 paired rainfall–SC events; of the 20 classified events, the maximum preferential flow ranged from 11 to 75% of the total subsurface drain flow, with a mean maximum preferential flow of 31%. Overall, SC‐EMMA illustrated that a significant portion of the subsurface drain discharge can be attributed to preferential flow, mainly through macropores or other largely open preferential flow pathways. The other primary mechanism, antecedent moisture conditions shifts, could only be shown for four of the 25 classified events. Specific conductance as a tracer of preferential flow was shown to be an effective tool for distinguishing preferential flow to subsurface drains. Even during relatively dry periods, the SC had a substantial decrease shortly after a rainfall event contrary to the conventional idea that macropore flow starts only after all the smaller pores are saturated and surface ponding begins to occur.

Effects of temporal scales and space mismatches on the TRMM 3B42 v7 precipitation product in a remote mountainous area

AbstractThe accurate measurement of precipitation is essential to understanding regional hydrological processes and hydrological cycling. Quantification of precipitation over remote regions such as the Tibetan Plateau is highly unreliable because of the scarcity of rain gauges. The objective of this study is to evaluate the performance of the satellite precipitation product of tropical rainfall measuring mission (TRMM) 3B42 v7 at daily, weekly, monthly, and seasonal scales. Comparison between TRMM grid precipitation and point‐based rain gauge precipitation was conducted using nearest neighbour and bilinear weighted interpolation methods. The results showed that the TRMM product could not capture daily precipitation well due to some rainfall events being missed at short time scales but provided reasonably good precipitation data at weekly, monthly, and seasonal scales. TRMM tended to underestimate the precipitation of small rainfall events (less than 1 mm/day), while it overestimated the precipitation of large rainfall events (greater than 20 mm/day). Consequently, TRMM showed better performance in the summer monsoon season than in the winter season. Through comparison, it was also found that the bilinear weighted interpolation method performs better than the nearest neighbour method in TRMM precipitation extraction.

Characteristics of Carbon Transportation Under Rainfall Events and Associated Carbon Loss Evaluation in Loess Plateau, China: A Case Study of Yangjuangou Dam Watershed

In this study, by monitoring carbon transportation and its rainfall-runoff process during the rainy season in Loess Plateau's Yangjuangou dam watershed, we analyzed changes in carbon transportation driven by rainfall and further evaluated the C loss flux for the dam watershed. Results showed that the monthly C wet deposition flux for the wet and dry seasons were 3.33 kg·hm-2and 2.18 kg·hm-2, respectively, which were only small contributions to C transportation for the watershed. C transportation under the rainfall-runoff process in this watershed can reach 944.89 kg·km-2 and 300.29 kg·km-2 in August and September, respectively. Different intensities of rainfall runoff lead to different C loss processes, wherein dissolved inorganic carbon (DIC) is the main C form. Under small rainfall events, the output of dissolved total carbon (DTC) in this watershed was 156.98 kg·km-2; and the output of moderate rainfall events was 284.60 kg·km-2. Finally, we determined that the C loss modulus of the Yangjuangou watershed was 1.89 kg·(km2·mon)-1 in the rainy season, thus the C loss modulus for the study area could reach 2.70 kg·(km2·a)-1.

Deuterium isotope characteristics of precipitation infiltrated in the West Ordos Desert of Inner Mongolia, China

Understanding the soil-profile temporal and spatial distribution of rainwater in arid and semiarid regions provides a scientific basis for the restoration and maintenance of degraded desert ecosystems in the West Ordos Desert of Inner Mongolia, China. In this study, the deuterium isotope (δD) value of rainwater, soil water, and groundwater were examined in the West Ordos Desert. The contribution of precipitation to soil water in each layer of the soil profile was calculated with two-end linear mixed model. In addition, the temporal and spatial distribution of δD of soil water in the soil profile was analyzed under different-intensity precipitation. The results showed that small rainfall events (0-10 mm) affected the soil moisture and the δD value of soil water in surface soil (0-10 cm). About 30.3% to 87.9% of rainwater was kept in surface soil for nine days following the rainfall event. Medium rainfall events (10-20 mm) influenced the soil moisture and the δD value of soil water at soil depth of 0-40 cm. About 28.2% to 80.8% of rainwater was kept in soil layer of 0-40 cm for nine days following the medium rainfall event. Large (20-30 mm) and extremely large (＞30 mm) rainfall events considerably influenced the soil moisture and δD value of soil water in each of the soil layers, except for the 100-150 cm layer. The δD value of soil water was between those δD values of rainwater and groundwater, which suggested that precipitation and groundwater were the sources of soil water in the West Ordos Desert. Under the same intensity rainfall, the δD value of surface soil water (0-10 cm) was directly affected by δD of rainwater. With increasing soil depth, the variation of soil water δD decreased, and the soil water of 100-150 cm kept stable. With increasing intensity of precipitation, the influence of precipitation on soil water δD lasted for a longer duration and occurred at a deeper soil depth.

Differential intra-specific stemflow funnelling efficiencies of Caragana korshinskii within arid desert ecosystems

Abstract Stemflow is known as a highly localized point input of rainwater and solutes around tree/shrub bases where roots are concentrated, thus having considerable effects on hydrology and biogeochemistry of vegetated ecosystems. Stemflow shows a pronounced inter-specific variation due to morphological differences among species, while the intra-specific variation of stemflow has been poorly explored. We systematically examined the effects of shrub morphological metrics on intra-specific funnelling efficiencies by quantifying the stemflow of nine shrubs of Caragana korshinskii within a water-limited arid desert ecosystem of northern China. Stemflow volume was used to compare the absolute amount of stemflow generated by shrubs of varying size, and funnelling ratio was used to assess their funnelling efficiencies. Both rainfall depth and shrub morphological metrics significantly affected stemflow volume, while funnelling ratio was more associated with shrub morphology. Under the same rainfall condition, smaller shrubs produced lower volumes of stemflow, while gaining access to rainfall via higher funnelling ratio than larger shrubs. Our findings highlight a large variation in funnelling efficiency among individual shrubs within the same species, and in particular, smaller shrubs might profit more from sporadic small rainfall events than larger shrubs.

Soil water variability as a function of precipitation, temperature, and vegetation: a case study in the semiarid mountain region of China

Seasonal variations of soil water content over 16 months at three typical sites of the Qilian Mountain were investigated to understand the controlling factors in regulating soil moisture content and dynamics. In this study, we quantified soil moisture response to cumulative rainfall events and freezing and thawing processes. The significant response depth of soil moisture after rainfall was 0–40 cm, and the forms of effective rainfall in this semiarid region: consecutive, with small dry intervals, and a few small rainfall events, with the cumulative amount of no less than 18.5 mm. Not only vegetation but also topography and soil texture were the primary controlling variables for soil moisture dynamics in this mountain ecosystem. The freezing temperature in 0–100-cm soil depth was in the range of −1.66 to 1.73 °C, and the thawing temperature was in the range of −0.54 to 0.84 °C; soil aspect and water content were the main controlling factors in the freezing and thawing points in this mountain region. The results of this study contributed to identifying the major controls on soil moisture content in mountain ecosystems and will provide basis for ecological restoration in arid regions.

First flush characteristics of rainfall runoff from a paddy field in the Taihu Lake watershed, China.

Nonpoint storm runoff remains a major threat to surface water quality in China. As a paddy matures, numerous fertilizers are needed, especially in the rainy seasons; the concentration of nitrogen and phosphorus in rainfall runoff from farmland is much higher than at other times, and this poses a great threat to water bodies and is the main reason for water eutrophication, especially in high concentration drainages. To date, most studies regarding the characteristics of pollutants in rainfall runoff have mainly been concentrated on urban runoff and watershed runoff; therefore, it is particularly important to investigate the characteristics of nitrogen and phosphorus loss in rainfall runoff from paddy fields. To study the characteristics of nitrogen and phosphorus loss and whether the first flush effect exists, continuous monitoring of the rainfall runoff process of six rainfall events was conducted in 2013, of which four rainfall events during storm, high, middle, and low intensity rainfalls were analyzed, and runoff and quality parameters, such as suspended solids (SS), total nitrogen (TN), ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3--N), total phosphorus (TP), and phosphate (PO43--P), were analyzed to determine the relationship between runoff and water quality. The paddy field is located north of Wuxi Lake Basin along the Hejia River upstream in Zhoutie town, Yixing city. An analysis of the load distribution during rainfall runoff was conducted. Event mean concentration (EMC) was used to evaluate the pollution situation of the paddy field's rainfall runoff. A curve of the dimensionless normalized cumulative load (L) vs. normalized cumulative flow (F) (L-F curve), the probability of the mass first flush (MFFn), and the pollutants carried by the initial 25% of runoff (FF25) were used to analyze the first flush effect of the paddy field runoff, and different contaminants show different results: the concentration of nitrogen and phosphorus fluctuate and follow a similar trend as runoff changes, NO3--N concentration is lower in the early part of runoff and higher in the later, and TP mainly occurs in the particle state in storm runoff and mainly in the dissolved state when the rainfall intensity is smaller. Nitrogen and phosphorus losses from paddy fields are closely related to the average rainfall intensity and the max rainfall intensity, and the runoff loss of nitrogen and phosphorus is more severe when the rainfall intensity is large. Based on an analysis of multiple methodologies, TN and NH4+-N show a certain degree of a first flush effect, whereas the first flush effect of TP is not obvious. The first flush effect of SS is obvious in larger intensity rainfall and shows a slight secondary flush effect in smaller rainfall events.