Standing Water Depth Research Articles

Performance evaluation of alternate wetting and drying irrigation for rice cultivation

An experiment was conducted during the 2009-2010 Boro season at the Shahjalal Science and Technology University campus, Sylhet, Bangladesh, to investigate the impact of alternate wetting and drying irrigation (AWDI) on rice production. Using a randomized complete block design (RCBD), four irrigation treatments were applied to a modern variety (MV) of rice (BRRIdhan 28). One control treatment, T0, maintained a continuous standing water depth of 1-5 cm. Water was irrigated to three AWD treatments, T1, T2, and T3 when the water level dropped 10, 20, and 30 cm below ground level, respectively. In spite of the fact that treatment T0 produced the highest grain yield (4.90 t/ha), its water use efficiency was 38.64 kg/ha/cm. Compared to treatment T1, which produced 4.68 t/ha, treatment T1 had a water use efficiency of 41.86 kg/ha/cm. Treatments T2 and T3 yielded 3.96 t/ha and 3.63 t/ha, respectively. When water levels fall below 10 cm below ground level, treatment T1 may be the best option for rice cultivation in conditions of limited water availability.

CRITICAL VARIABLES OF MORPHO-PHYSIOLOGICAL RICE PLANT UNDER STAGNANT FLOODING CONDITIONS

<p class="abstrakinggris"><span lang="EN-US">Stagnant flooding (SF) stress has contributed decreasing rice production in Indonesia. The study aimed to explore critical variables of rice growth that contribute to the decreasing grain yield under SF conditions and a common irrigation system (control). The experiment was arranged in a complete randomized block design with four replications to test 10 rice genotypes (Inpari 30 Ciherang Sub-1, Inpara 3, Inpara 4, Inpara 8, IRRI119, IRRI154, IR42, IR14D121, IR14D157, and Tapus). The water depth was managed according to the farmer’s practices for control, while for SF plots the standing water depth was gradually increased from 35 days after transplanting and was maintained at 50 cm until harvest. Results showed that plant height, tillering ability, leaf greenness, panicle number per hill and grain filling percentage were critical growth variables that affect grain yield at optimal conditions. The yield of the 10 genotypes decreased by 25–50% under SF conditions. Inpara 3 had the stable yield in those two watering conditions. Therefore, it could be used as a check variety for SF condition. Inpara 9 and IRRI119 experienced decreased yield of more than 50% under SF conditions. The key factors determining the decreased yield were tillering ability and green leaf level. Optimization of the two variables at SF conditions will largely determine rice yield associated with panicle number per hill and grain number per panicle. Results of the study are useful as preliminary recommendations for designing new variety and cultivation techniques to reduce the impact of SF stress on rice yield.</span></p>

Inundation depth affects ecosystem CO2 and CH4 exchange by changing plant productivity in a freshwater wetland in the Yellow River Estuary

Climate change (extreme rainfall) and water management activities have led to variation in hydrological regimes, especially inundation, which may alter the function and structure of wetlands as well as wetland-atmosphere carbon (C) exchange. However, the degree to which different inundation depths (standing water depth above the soil surface) affect ecosystem CH4 fluxes, ecosystem respiration (Reco) and net ecosystem CO2 exchange (NEE) remains uncertain in wetland ecosystems. We conducted a field inundation depth manipulation experiment (no inundation, i.e. only natural precipitation; 0, water-saturated; 5, 10, 20, 30 and 40 cm inundation depth) in a freshwater wetland of the Yellow River Delta, China. The CH4 fluxes, Reco and NEE were measured with a static chamber technique during the growing seasons (May–October) of 2018 and 2019. Inundation depth significantly increased plant shoot density, above-water level leaf area index (WLAI), above-water level plant shoot height (WHeight), aboveground and belowground biomass of the dominant grass Phragmites australis in both years. Meanwhile, inundation depth increased the CH4 fluxes, Reco (except for 0 cm) and NEE compared to no inundation, which could be attributed partly to the increased plant productivity (shoot density, WLAI, WHeight, biomass). Additionally, the CH4 fluxes, Reco or NEE exhibited parabolic responses to inundation depth. Furthermore, global warming potential (GWP) was significantly decreased under different inundation depths during the growing season, especially from 5 to 40 cm inundation depth in 2019. NEE was the largest contributor to the seasonal GWP, which indicates that the inundated wetlands are a net sink of C and have a cooling climate effect in the Yellow River Delta. Inundation depth substantially affects the magnitude of CH4 fluxes, Reco and NEE, which were correlated with altered plant traits in wetland ecosystems. Inundation depth could mitigate greenhouse gas emissions in the P. australis wetlands during the growing season. Inundation depth-induced ecosystem C exchange should be considered when estimating C sequestration capacity of wetlands due to climate change and water management activities, which will assist to accurately predict the impact of hydrological regimes on C cycles in future climate change scenarios.

Mechanisms associated with tiller suppression under stagnant flooding in rice

AbstractStagnant flooding (SF) during vegetative growth triggers stem elongation usually at the cost of tiller production in rice, reducing grain yield. To explore physiological mechanisms associated with tillering suppression under SF, three contrasting genotypes (Swarna and Swarna‐Sub1, both sensitive and IRRI154, tolerant) were evaluated under standing water depths of 0, 5, 30 and 50 cm. SF significantly suppressed tiller formation but increased plant height, root biomass, shoot elongation (ratio of plant height before and after flooding), leaf emergency and non‐structural carbohydrate (NSC) concentration (in root–shoot junction) in all genotypes at the early stage of development. Chlorophyll concentration in the upper leaves (upper most fully expanded leaf at top) was higher than in lower leaves (lowest green leaf at base), but decreased under SF in both. SF increased hydrogen peroxide (H2O2) at the early stage of treatment, with concomitant increase in malondialdehyde (MDA) production by stems and leaves. MDA concentration in root–shoot junction increased but delayed. Tiller number correlated negatively with plant height, shoot elongation, leaf emergency, MDA concentration in leaves and root–shoot junction, root biomass, and NSC concentration in the root–shoot junction. The results suggested existence of compensatory mechanisms between tiller growth and shoot elongation in rice for resilience under SF, where energy is mainly diverted for shoot elongation to escape flooding. The SF‐tolerant genotype produced less H2O2 and maintained energy balance for higher survival and better growth under stagnant flooding.

Effects of irrigation water regime, soil clay content and their combination on growth, yield, and water use efficiency of rice grown in South China

To investigate the effect of irrigation regime, soil clay content and their combination on growth, yield, and water productivity of rice, a shelter experiment was conduct using Randomized Complete Block Design (RCBD) with a factorial arrangement of treatments with four replications. Irrigation regime was the main treatment investigated, set in three levels as R(30 mm-100%) (100% of saturation and 30 mm flooded), R(30 mm-90%) (90% of saturation and 30 mm flooded) and R(30 mm-70%) (70% saturation and 30 mm flooded), respectively. The sub-treatment was soil type, set in three levels as 40%, 50% and 60% clay content, respectively. Results showed that irrigation regime and soil clay content had significant effects on growth, yield and water productivity of rice. However, their combination showed no significant impact on panicles number, root biomass, harvest index and irrigation water productivity. Higher soil clay content results in increase in growth, yield, and water productivity of rice. The total water consumption during R(30 mm-100%) was higher than that during R(30 mm-90%) and R(30 mm-70%) because the latter two saturation levels led to the cracking of soil and decrease of total number of irrigations. Cracks were consistently getting more serious with the reduction in soil water content and the increase in soil clay content. Cracks in soil will preferentially become the major routes of water losses, thus water percolation during R(30 mm-70%) was higher than that during R(30 mm-90%) and R(30 mm-100%) after each irrigation event. The total water use under R(30 mm-70%) exceeded the water consumption under R(30 mm-90%) due to the great amount of soil cracking as well as the excessive volume of standing water depth. Considering water consumption and grain yield, the following conclusion can be reached: (i) The reduction in water consumption was greater than the reduction in grain yield in the case of drying soil 10% below saturation before reflooding. (ii) The reduction in water consumption was less than the reduction in grain yield in the case of drying soil 30% below saturation before reflooding; (iii) The increase in water use was greater than the increase in grain yield in the case of maintaining soil moisture at 100% of saturation before reflooding. Therefore, the water use efficiency was recorded in the order of R(30 mm-90%) >R(30 mm-100%) >R(30 mm-70%). Keywords: irrigation regime, clay content, combination, growth, yield, water productivity, rice DOI: 10.25165/j.ijabe.20181104.3895 Citation: Hamoud Y A, Guo X P, Wang Z C, Chen S, Rasool G. Effects of irrigation water regime, soil clay content and their combination on growth, yield, and water use efficiency of rice grown in South China. Int J Agric & Biol Eng, 2018; 11(4): 144-155.

Going beyond the flood insurance rate map: insights from flood hazard map co-production

Abstract. Flood hazard mapping in the United States (US) is deeply tied to the National Flood Insurance Program (NFIP). Consequently, publicly available flood maps provide essential information for insurance purposes, but they do not necessarily provide relevant information for non-insurance aspects of flood risk management (FRM) such as public education and emergency planning. Recent calls for flood hazard maps that support a wider variety of FRM tasks highlight the need to deepen our understanding about the factors that make flood maps useful and understandable for local end users. In this study, social scientists and engineers explore opportunities for improving the utility and relevance of flood hazard maps through the co-production of maps responsive to end users' FRM needs. Specifically, two-dimensional flood modeling produced a set of baseline hazard maps for stakeholders of the Tijuana River valley, US, and Los Laureles Canyon in Tijuana, Mexico. Focus groups with natural resource managers, city planners, emergency managers, academia, non-profit, and community leaders refined the baseline hazard maps by triggering additional modeling scenarios and map revisions. Several important end user preferences emerged, such as (1) legends that frame flood intensity both qualitatively and quantitatively, and (2) flood scenario descriptions that report flood magnitude in terms of rainfall, streamflow, and its relation to an historic event. Regarding desired hazard map content, end users' requests revealed general consistency with mapping needs reported in European studies and guidelines published in Australia. However, requested map content that is not commonly produced included (1) standing water depths following the flood, (2) the erosive potential of flowing water, and (3) pluvial flood hazards, or flooding caused directly by rainfall. We conclude that the relevance and utility of commonly produced flood hazard maps can be most improved by illustrating pluvial flood hazards and by using concrete reference points to describe flooding scenarios rather than exceedance probabilities or frequencies.

A two-year field measurement of methane and nitrous oxide fluxes from rice paddies under contrasting climate conditions.

The effects of three irrigation levels (traditional normal amount of irrigation [NA100%], 70%, and 30% of the normal amount [NA70% and NA30%]) and two rice varieties (Oryza sativa L. Huayou14 and Hanyou8) on CH4 and N2O emissions were investigated over two years under contrasting climate conditions (a ‘warm and dry’ season in 2013 and a normal season in 2014). Hanyou8 was developed as a drought-resistant variety. The mean seasonal air temperature in 2013 was 2.3 °C higher than in 2014, while the amount of precipitation from transplanting to the grain-filling stage in 2013 was only 36% of that in 2014. CH4 emission rose by 93–161%, but rice grain yield fell by 7–13% in 2013, compared to 2014 under the NA100% conditions. Surface standing water depths (SSWD) were higher in Hanyou8 than in Huayou14 due to the lower water demand by Hanyou8. A reduction in the amount of irrigation water applied can effectively reduce the CH4 emissions regardless of the rice variety and climate condition. However, less irrigation during the ‘warm and dry’ season greatly decreased Huayou14 grain yield, but had little impact on Hanyou8. In contrast, N2O emission depended more on fertilization and SSWD than on rice variety.

A comparative study of daytime-based methane emission from two wetlands of Nepal Himalaya

Natural wetlands constitute one of the major sources of methane emission to the atmosphere. Data on methane emission from wetlands on southern slopes of the Himalaya (SSH) have not been reported so far. Such data are very valuable for filling the gap and generating the whole emission patterns at regional or even global scale. We selected two wetlands at different altitudinal locations in Nepal, i.e. Beeshazar Lake (286 m a.s.l.) and Dhaap Lake (2089 m a.s.l.), to monitor the daytime methane emissions in monsoon season and dry season separately. Daytime methane emission varied between monsoon and dry seasons and also across different plant communities. The daytime methane emission variations were stronger in dry season than in monsoon season. The source/sink strengths of the two selected plant communities in each wetland were significantly different, presenting the strong spatial variation of methane emission within wetland. The methane emissions recorded in monsoon season were significantly higher (7.74 ± 6.49 mg CH4m−2 h−1 and 1.00 ± 1.23 mg CH4m−2 h−1 in low and high altitude wetlands, respectively) than those in dry season (1.84 ± 4.57 mg CH4m−2 h−1 and 0.27 ± 0.71 mg CH4m−2 h−1 in low and high altitude wetlands, respectively). Methane emissions from the low altitude wetland were significantly higher than those from the high altitude wetland in both of the seasons. Plant community height, standing water depth and soil temperature correlated to the methane emission from wetlands in this region.

Analysis of Impact of Transverse Slope on Hydroplaning Risk Level

Hydroplaning has become a major inducement of wet-weather road accidents. By the way of CFD (Computational Fluid Dynamics) approach, this paper frames a finite-element meta-models for rib tires with the rang of different road transverse slopes, aiming to figure out the water pressures on tires under different traveling conditions and to find out the theoretical variational condition and regularities of water pressure of the different parts of the tire tread. Analysis shows that there is not an apparent impact of transverse slope on the action coverage of the high pressure induced by hydrodynamic pressure, but with the increase of the standing water depth, the area of high pressure zone will be larger and the variation is the most apparent when the standing water depth ranges from 5mm to 8mm. Therefore, the consequence of this paper can give recommendations on pavement construction, especially on the construction of drainage.

Experimental Investigation of Tsunami Bore Forces on Vertical Walls

A series of experiments have been carried out in the large wave flume (LWF) at Oregon State University to quantify tsunami bore forces on structures. These tests included “offshore” solitary waves, with heights up to 1.3 m, that traveled over a flat bottom, up a sloping beach, and breaking onto a flat reef. Standing water depths on the reef varied from 0.05 m to 0.3 m. Resulting bores on the reef measured up to approximately 0.8 m. After propagating along the reef, the bores struck a vertical wall. The resulting forces and pressures on the wall were measured. The test setup in the LWF is described, and the experimental results are reported. The results include forces and pressure distributions. Results show that the bores propagated with a Froude number of approximately 2 and that the forces follow Froude scaling. Finally, a design formula for the maximum impact force is given. The formula is shown to be an improvement over existing formulas found in the literature.

Inter-Annual Variations of Methane Emission from an Open Fen on the Qinghai-Tibetan Plateau: A Three-Year Study

The study aimed to understand the inter-annual variations of methane (CH4) emissions from an open fen on the Qinghai-Tibetan Plateau (QTP) from 2005 to 2007. The weighted mean CH4 emission rate was 8.37±11.32 mg CH4 m−2 h−1 during the summers from 2005 to 2007, falling in the range of CH4 fluxes reported by other studies, with significant inter-annual and spatial variations. The CH4 emissions of the year of 2006 (2.11±3.48 mg CH4 m−2 h−1) were 82% lower than the mean value of the years 2005 and 2007 (13.91±17.80 mg CH4 m−2 h−1 and 9.44±14.32 mg CH4 m−2 h−1, respectively), responding to the inter-annual changes of standing water depths during the growing season of the three years. Significant drawdown of standing water depth is believed to cause such significant reduction in CH4 emissions from wetlands in the year 2006, probably through changing the methanogen composition and decreasing its community size as well as activating methanotrophs to enhance CH4 oxidation. Our results are helpful to understand the inter-annual variations of CH4 emission and provide a more reasonable regional budget of CH4 emission from wetlands on the QTP and even for world-wide natural wetlands under climate change.

Spatiotemporal Variations in Nitrous Oxide Emissions from an Open Fen on the Qinghai–Tibetan Plateau: a 3-Year Study

To understand spatial and temporal variations of nitrous oxide (N2O) fluxes, we chose to measure N2O emissions from three plant stands (Kobresia tibetica, Carex muliensis, and Eleocharis valleculosa stands) in an open fen on the northeastern Qinghai-Tibetan plateau during the growing seasons from 2005 to 2007. The overall mean N2O emission rate was about 0.018 +/- 0.056 mgN m(-2)h(-1) during the growing seasons from 2005 to 2007, with highly spatiotemporal variations. The hummock (K. tibetica stand) emitted N2O at the highest rate about 0.025 +/- 0.051 mgNm(-2)h(-1), followed by the hollow stands: the E. valleculosa stand about 0.012 +/- 0.046 mgNm(-2)h(-1) and the C. muliensis stand about 0.017 +/- 0.068 mgNm(-2)h(-1). Within each stand, we also noted significant variations of N2O emission. We also observed the significant seasonal and inter-annual variation of N2O fluxes during the study period. The highest N2O emission rate was all recorded in July or August in each year from 2005 to 2007. Compared with the mean value of 2005, we found the drought of 2006 significantly increased N2O emissions by 104 times in the E. valleculosa stand, 45 times in K. tibetica stand, and 18 times in the C. muliensis stand. Though there was no significant relation between standing water depths and N2O emissions, we still considered it related to the spatiotemporal dynamics of soil water regime under climate change.

Analysis of Impact of Tire Tread Groove Depth on Hydroplaning Risk Level

Hydroplaning ranks a maior inducement of wet-weather road accidents. By way of CFD (Computational Fluid Dynamics) approach, this paper frames a finite-element meta-models for rib tires ranging in groove depth, aiming to figure out the wheel water pressures on tires under different traveling conditions, and to find out water velocities of the different parts of the tire tread. Analysis shows that the action coverage of the high pressure induced by hydradynamic pressure is correlated with the thickness of standing water, and the grooves fail to drain as designed in case of total hydraplaning. Based on the outcomes of the experiments, this paper frames the relation formula between hydradynamic pressure and traveling velocity, standing water depth and tire tread groove depth, and thus gives recommendations on speed range in wet weather driving, and also on measures for safe driving in wet weather conditions.

Analysis of Impact of Tire Tread Groove Depth on Hydroplaning Risk Level

Hydroplaning ranks a maior inducement of wet-weather road accidents. By way of CFD (Computational Fluid Dynamics) approach, this paper frames a finite-element meta-models for rib tires ranging in groove depth, aiming to figure out the wheel water pressures on tires under different traveling conditions, and to find out water velocities of the different parts of the tire tread. Analysis shows that the action coverage of the high pressure induced by hydradynamic pressure is correlated with the thickness of standing water, and the grooves fail to drain as designed in case of total hydraplaning. Based on the outcomes of the experiments, this paper frames the relation formula between hydradynamic pressure and traveling velocity, standing water depth and tire tread groove depth, and thus gives recommendations on speed range in wet weather driving, and also on measures for safe driving in wet weather conditions.

Nitrous oxide fluxes from the littoral zone of a lake on the Qinghai-Tibetan Plateau

Nitrous oxide (N(2)O) fluxes were measured in six littoral mirco-zones of Lake Huahu on Qinghai-Tibetan Plateau in the peak growing season of years of 2006 and 2007. The weighted mean N(2)O flux rate was 0.08 mg N m(-2) h(-1) (ranged from -0.07 to 0.35 mg N m(-2) h(-1)). The result was relatively high in the scope of N(2)O fluxes from boreal and temperate lakes. Emergent plant zones (Hippuris vulgaris and Glyceria maxima stands) recorded the highest N(2)O flux rate (0.11 ± 0.24 and 0.08 ± 0.17 mg N m(-2) h(-1), respectively). Non-vegetated lakeshore recorded the lowest N(2)O flux (0.03 ± 0.11 mg N m(-2) h(-1)), lower than that from the floating mat zone of Carex muliensis (0.05 ± 0.18 mg N m(-2) h(-1)), the floating-leaved plant zone of Polygonum amphibium (0.07 ± 0.11 mg N m(-2) h(-1)), and the wet meadow (0.07 ± 0.15 mg N m(-2) h(-1)). Standing water depths were important factors to explain such spatial variations in N(2)O fluxes. Significant temporal variations in N(2)O fluxes were also found. Such temporal variation in N(2)O flux in the littoral zone may be dependent on the interaction of water regime and thermal conditions, instead of the latter solely. These results showed the importance of the littoral zone of lake, especially the emergent plant zone, as a hotspot of N(2)O fluxes in such grazing meadows.

Farmers’ Management Practices and Grain Yield of Rice in Response to Different Water Environments in Kamping Puoy Irrigation Rehabilitation Area in Northwest Cambodia

The farmers’ management practices and grain yield were examined in the consecutive 4 cropping seasons from wet season rice (WSR) in 2008 to dry season rice (DSR) in 2010 across upstream, midstream and downstream fields, along two secondary drainage canals (located either upstream or downstream side along the main canal) in the Kamping Puoy Irrigation Rehabilitation area (KPIR). In WSR, standing water depth was much deeper in downstream fields where medium and late maturing varieties were planted from May than in upstream fields where early and early medium maturing varieties were planted later (mostly in July and August). In DSR there was less difference in water conditions between upstream and downstream fields and variation in planting and harvesting time was small. As the area percentage of fields where DSR was introduced increased from 2008 (54%) to 2010 (100%), planting time in WSR was later (e.g., from May to July) with declining proportion of dry seeding method and mid-season tillage. Grain yield was low in DSR, particularly in 2010 (287 and 247 g m-2 in 2009 and 2010 on average, respectively), due to insufficient weed control and small amount of fertilizer, and the yield was lowest in fields which practiced DSR for the first time. Grain yield in WSR (286 and 291 g m-2 in 2008 and 2009 respectively) increased by transplanting, use of high yielding Raing Chey variety, and application of a larger amount of N chemical fertilizer. These findings indicated that the agriculture extension support to farmers, particularly in DSR, is a key important factor for rice yield improvement in KPIR.

Dynamics of dissolved organic carbon in the mires in the Sanjiang Plain, Northeast China

Mires in boreal area had proved to be an important dissolved organic carbon (DOC) reserve for the sensitivity to climate change and human interfering. The study was focused on the temporal and spatial dynamics and controlling factors of DOC in a seasonally-waterlogged mire (SLM) and perennially-waterlogged mire (PLM) in the Sanjiang Plain, Northest China. In the two mires, DOC concentrations in both surface water and upper soil strata experienced pronounced seasonal variation. DOC concentrations in the surface waters were the greatest and averagely was 47.82 in SLM and 34.84 mg/L and PLM, whereas that in soil water at 0.3-m depth had little difference (20.25 mg/L in SLM and 26.51 mg/L in PLM). Results revealed that DOC concentrations declined 5–8 times vertically from the surface down to groundwater. DOC in the groundwater only was in a very small part with the average concentration of 5.18 mg/L. In relation to the surface water, DOC concentrations varied positively with temperature just before 8 August, and only in early spring and later autumn DOC concentrations exhibited identifiable spatial trends along with standing water depths in PLM. It was supposed that the influences from standing water depth took effect only in conditions of low temperature, and temperature should be the most powerful factor controlling DOC dynamics in the mires. Redox potential (Red) showed negative relationship with DOC values while total nitrogen (TN) and the majority of free ions in the soil solution exhibited no relationship. High soil TOC/TN ratio and low redox potentials also led to DOC accumulation in the mires in the Sanjiang Plain.

Nitrous Oxide Emissions from Newly Created Littoral Marshes in the Drawdown Area of the Three Gorges Reservoir, China

This study aimed to understand the seasonal and spatial variations of N(2)O emissions from newly created littoral marshes in the drawdown area of the Three Gorges Reservoir (TGR), China. We measured N(2)O emissions at 10-day intervals during the growing season (early July to late September) in 2008. N(2)O emissions were measured with static chambers in four typical vegetation stands. The results showed great spatial variations of N(2)O emissions among the four stands. The greatest N(2)O emissions (0.052 +/- 0.063 mg N(2)O m(-2)h(-1)) were from Scirpus triqueter stand, while the lowest N(2)O emissions (0.020 +/- 0.020 mg N(2)O m(-2) h (1)) were from Typha angustifolia stand. To such spatial variations in N(2)O emissions, standing water depths and soil water content may be important explaining factors. Besides spatial variations, we also found significant temporal variations of N(2)O emissions in this area. The temporal variation of N(2)O emissions in the growing season was not found significantly related to any measured factor in the study. However, based on principal component analysis, we consider it partly caused by thermal conditions and the marked temporal variation of the standing water depth in the growing season, which to some degree influenced the process of denitrification and N(2)O emissions. These results about TGR enable us to make a more reasonable estimate of N(2)O emissions from large dam reservoirs, particularly those with a large drawdown area in the growing season in an agricultural landscape.

Methane emissions from newly created marshes in the drawdown area of the Three Gorges Reservoir

The study aimed to understand the methane (CH4) emission and its controlling factors in the Three Gorges Reservoir Region and to explore its implication for large dams. We measured CH4 emissions from four vegetation stands in newly created marshes in the drawdown area of the Three Gorges Reservoir, China, in the summer of 2008. The results showed highly spatial variations of methane emissions among the four stands, with the smallest emission (0.25 ± 0.65 mg CH4 m−2 h−1) in the Juncus amuricus stand, and the greatest (14.9 ± 10.9 mg CH4 m−2 h−1) in the Scirpus triqueter stand. We found that the spatial variations of CH4 emissions are caused by difference in standing water depth and dissolved organic carbon (DOC). Results also showed a special seasonal variation of CH4 emissions in this area, i.e., maximal emissions in early July followed by a low and steady value before the winter flooding. The seasonality of CH4 emissions was found closely related to temperature and standing water depth. Because of the large area of the drawdown zones for global dam reservoirs and a large CH4 emission rate, such newly created marshes should not be neglected when estimating CH4 emissions from reservoirs.

High methane emissions from a littoral zone on the Qinghai-Tibetan Plateau

The littoral zones of lakes have been regarded as hotspots of methane (CH 4) fluxes through several studies. In the present study, we measured CH 4 fluxes in six kinds of littoral zones of Huahu Lake on the Qinghai-Tibetan Plateau in the peak growing season of 2006 and 2007. We found that CH 4 efflux (ranging from −0.1 to 90 mg CH 4 m −2 h −1) from the littoral zones of this lake was relatively high among those of boreal and temperate lakes. Our results also showed that emergent plant zones ( Hippuris vulgaris and Glyceria maxima stands) recorded the highest CH 4 flux rate. The CH 4 flux in the floating mat zone of Carex muliensis was significantly lower than those of the emergent plant zones. CH 4 fluxes in the floating-leaved zone of Polygonum amphibium and bare lakeshore showed no significant difference and ranked last but one, only higher than that of the littoral meadow ( Kobresia tibetica). Plant biomass and standing water depths were important factors to explain such spatial variations in CH 4 fluxes. No significant temporal variations in CH 4 fluxes were found due to the insignificant variations of physical factors in the peak growing season. These results may help in our understanding of the importance of the littoral zone of lakes, especially the emergent plant zone, as a hotspot of CH 4 emission.