Annual Monsoon Research Articles

Isotopic investigation of river water mixing around the confluence of the Tonle Sap and Mekong rivers

AbstractThe Great Lake (Tonle Sap), located in mid‐western Cambodia, is the largest lake in southeast Asia and joins the Mekong through the Tonle Sap River. During the annual monsoon peak from about the end of May, the flow direction in the Tonle Sap reverses and river water from the Mekong flows to the Great Lake. When the water level of the Mekong decreases sufficiently, usually in October, the Tonle Sap again flows in its normal direction from the Great Lake to the Mekong. This study quantitatively investigated interactions between the Tonle Sap River and the Mekong River using the stable isotope ratios of oxygen (δ18O) in waters from both rivers, and estimated the contribution rate of Tonle Sap River water to Mekong River water during the peak flow period of the Tonle Sap River. During this period, the Tonle Sap River contributed approximately 70–97% along the right bank and 0–5% in the middle and left banks of the Mekong. We also observed δ18O of the Mekong River along a longitudinal section on the right bank side, where the contribution rate of the Tonle Sap River water remained high (80%) even 21 km downstream from the confluence. This study clearly demonstrates that we can determine the mixing rate of these rivers at each point after they converge by using stable oxygen isotopes as tracers, when the Tonle Sap flows in its normal direction. Copyright © 2008 John Wiley & Sons, Ltd.

The impact of Eurasian dust storms and anthropogenic emissions on atmospheric nutrient deposition rates in forested Japanese catchments and adjacent regional seas

Bulk precipitation and stream water chemistry data from 1993 to 2005 are used to analyze the relationship between Eurasian dust storms and nutrient deposition rates in the Kutsuki experimental forest (near Lake Biwa). From 2000 to 2005, atmospheric deposition, total nitrogen (TN), total phosphorus (TP) and dissolved silica (DSi) deposition rates increased by 26%, 132%, and 38%, respectively in the Kutsuki experimental forest. These TN and TP increases are associated with three seasonal factors: the increasing frequency and intensity of Eurasian spring dust events (March/April); the annual typhoon period (late August/September); and autumn/early winter (October to December) monsoons. The annual typhoon and monsoon winter periods are drivers for atmospheric TP and DSi deposition due to the correlation between the deposition and precipitation. In addition, increased spring dust deposition is a primarily driver for TN deposition changes. Increased emissions from urbanized areas in China (and likely Korea) affect the chemical properties of aerosols reaching downwind Japanese regions. Aerosol processes are responsible for increasing TN in aerosols, which are affected primarily by anthropogenic emissions. From 2000 to 2005, coal burning emissions from East Asia have contributed to an increase in TP (and possibly DSi) deposition rates. The observed increase in nutrient deposition did not noticeably impact short-term (5 year) stream water fluxes in the Kutsuki experimental forest. Due to plant uptake, the forest ecosystem retained atmospherically deposited N and P. Finally, the observed increases in nutrient deposition rates over the East China Sea and the Sea of Japan may significantly influence intra-annual net primary production. It is recommended that earth system modelling incorporate changes in atmospheric nutrient deposition rates and their impacts on the regional carbon cycle as well as aquatic and terrestrial ecosystems.

Seawater intrusion in the Salalah plain aquifer, Oman

Salalah is situated on a fresh water aquifer that is replenished during the annual monsoon season. The aquifer is the only source of water in Salalah city. The rainfall and mist precipitation in the Jabal AlQara recharges the plain with significant renewable fresh groundwater that has allowed agricultural and industrial development to occur. In Salalah city where groundwater has been used extensively since the early 1980s for agricultural, industrial and municipal purposes, the groundwater has been withdrawn from the aquifer more rapidly than it can be replenished by natural recharge. The heavy withdrawal of large quantities of the groundwater from the aquifer has led to the intrusion of seawater. Agricultural activities utilize over 70% of the groundwater. For the study of the saltwater intrusion, the area has been divided into four strips, A, B, C and D, on the basis of land-use in the area. Water samples were collected from 18 water wells. Chemical analysis of major ions and pollution parameters in the groundwater was carried out and compared to the previous observed values. The electrical conductivity and chloride concentrations were highest in the agricultural and residential strips and Garziz grass farm. Before 1992 the aquifer was in a steady state, but presently (2005) the groundwater quality in most of the agricultural and residential strips does not meet drinking water standards. In addition, model simulations were developed with the computer code MODFLOW and MT3DMS for solute transport to determine the movement of the freshwater/saltwater interface. The study proposes the protection of the groundwater in Salalah plain aquifer from further encroachment by artificial recharge with reclaimed water, preferably along the Salalah coastal agricultural strip. This scheme can also be applied to other regions with similar conditions.

The presence of ladderane lipids in the oxygen minimum zone of the Arabian Sea indicates nitrogen loss through anammox

Distributions of ladderane lipids, which are characteristic of membranes of bacteria performing anaerobic ammonium oxidation (anammox), were determined in the northwestern Arabian Sea with respect to season, depth, and distance to the coast of Oman. Ladderane lipids were detected and quantified in suspended particulate matter (SPM) obtained from various water depths along a northwest‐to‐southeast transect during the spring intermonsoon period. Maximum concentrations of 5‐8 pg L−1 generally occurred at 500 m in the upper part of the oxygen minimum zone (OMZ). Fluxes of ladderane lipids obtained from sediment trap material sampled at 500‐m water depth ~350 km off the coast reveal a strong seasonal pattern apparently related to the annual monsoon cycle in the northern Arabian Sea, with highest fluxes of 125 ng m−2 d−1 observed during the southwest monsoon. This fourfold increase in flux during the SW monsoon compared to the spring intermonsoon period may indicate higher anammox bacterial productivity or enhanced export of ladderanes during a period of high particulate matter flux or both. Anammox, in addition to denitrification, seems to be responsible for a significant loss of nutrient nitrogen from OMZ waters in the Arabian Sea.

Daytime cover, diet and space-use of golden jackals (Canis aureus) in agro-ecosystems of Bangladesh

Golden jackals are locally common in Bangladesh despite intensive cultivation and high human densities. We studied the relative importance of seasonal flooding, rodent prey-base, and daytime cover on the occurrence of golden jackals in the two major agro-ecosystems in Bangladesh, one with annual monsoon flooding and the other without. Jackals were less common throughout the year where floodwaters occurred that would have excluded them for 1–3 months during their pup-rearing season. Diets of jackals were similar in the two agro-ecosystems. Rodents were the most common food type in scats throughout the year. The occurrence of burrowing rats in scats peaked seasonally when these rats were most concentrated in ripening cereals, suggesting that jackals are beneficial for rat control. Radiotelemetry of seven jackals in the non-flooded agro-ecosystem over an 11-month period indicated that sugarcane was the preferred type of daytime cover, despite representing only 2–4% of the area. There was a day-to-day return rate of 67% to the same 1-ha patch of cover. Evidently, sugarcane provides daytime cover for avoiding humans and for feeding on roof rats (Rattus rattus), which concentrate in this crop. Evidence suggests that breeding pairs of jackals were annual residents that defended cover (average of 37.3 ha) but not foraging areas beyond.

Climate and the Evolution of Mountains

This article examines the effects of climate on the shape of the earth's surface. It focuses on research conducted in Himalayas in Nepal that displayed a strong relationship between mountain size, plate tectonics, and climactic conditions. The annual Indian monsoon season causes erosion on the east side of the mountains, which, in turn, may cause a flow of crustal material from underneath the southern Tibetan Plateau to other areas of the range, rebuilding it.

Wetland dynamics using a suite of satellite observations: A case study of application and evaluation for the Indian Subcontinent

Using a suite of satellite observations, including passive and active microwave along with visible and infrared observations developed to estimate wetlands on a global scale, the present study examine wetlands spatial and temporal dynamics over the Indian subcontinent, a region subject to an annual monsoon. Monthly wetlands extent estimates over a 2‐year (1993–1994) period and their consistency are analyzed using independent data sets, such as rainfall rate in‐situ rain‐gauge observations and the Global Precipitation Climatology Project product, as well as river water levels derived from radar altimeter observations. The extent variations show a good agreement with the rain‐gauge data, a high correlation with the GPCP rain data and a similar seasonal cycle with the altimeter estimates over the 2 years. This case study evaluation shows the potential of the remote sensing to give spatial‐temporal consistent information on the variability of flooded areas and encourages the development of longer satellite wetlands estimates.

Effects of monsoon-driven wave action on coral reefs of Guam and implications for coral recruitment

Benthic cyanobacteria can respond rapidly to favorable environmental conditions, overgrow a variety of reef organisms, and dominate benthic marine communities; however, little is known about the dynamics and consequences of such cyanobacterial blooms in coral reef ecosystems. In this study, the benthic community was quantified at the time of coral spawnings in Guam to assess the substrate that coral larvae would encounter when attempting settlement. Transects at 9, 18, and 25-m depths were surveyed at two reef sites before and after heavy wave action driven by westerly monsoon winds. Communities differed significantly between sites and depths, but major changes in benthic community structure were associated with wave action driven by monsoon winds. A shift from cyanobacteria to crustose coralline algae (CCA) accounted for 44% of this change. Coral recruitment on Guam may be limited by substrate availability if cyanobacteria cover large areas of the reef at the time of settlement, and consequently recruitment may in part depend upon wave action from annual monsoon winds and tropical storms which remove cyanobacteria, thereby exposing underlying CCA and other substrate suitable for coral settlement.

A method for defining monsoon onset and demise in the southwestern USA

AbstractThe southwestern USA is subject annually to an inflow of atmospheric moisture in association with the North American monsoon, which is a summertime shift in the atmospheric circulation stretching from the Caribbean Sea, to Mexico, and into the southwestern USA. There are currently no regional criteria used to define the temporal aspect of the annual monsoon season in the southwestern USA, and only a single local definition. A regionalized definition of the annual timing of the monsoon season would seem to be a greater representation of the synoptic‐scale effects of the monsoon rather than individual definitions at point locations across the region. The research presented here outlines a method for defining the annual onset and demise of the regional monsoon season of the southwestern USA, and assesses the results through comparisons with a method currently used at one location within the region and with atmospheric composites from historic data.Using hourly dew‐point temperature data at five surface stations and daily precipitation data for 193 stations across the southwestern USA for June through to October for a 52 year period, daily humidity and precipitation threshold values indicative of monsoon moisture are established. The first and last occurrences of both humidity and precipitation sustained for a synoptic period of 3 days mark the annual onset and demise of the monsoon season, whereas all days in between that meet the humidity and precipitation thresholds are categorized as ‘monsoon days’. Results show good agreement with sample historic data for the sole local definition within the region, and atmospheric composites indicate a likelihood of accurate representation of monsoon onset and demise across the region. The significance of accurate onset and demise dates is apparent in the intimate relationship between seasonal precipitation across the region and the length of the monsoon season. Based upon the procedure, a historic monsoon season database was constructed for the southwestern USA for use in climatological, meteorological, and case‐study analyses. Copyright © 2004 Royal Meteorological Society

Factors controlling atmospheric fluxes of 7Be and 210Pb in northern Taiwan

Fluxes of 7Be and 210Pb were continuously monitored at two sites in northern Taiwan during 1996–2001. The time series of both nuclides show seasonality and dependence on precipitation, annual monsoon cycle and episodic events (typhoons and Asian dust storms). Besides, anomalously high 7Be and 210Pb fluxes occurred in 1998, which are most likely caused by enhanced atmospheric convection in the north Pacific resulting from significantly increased SST in the western Pacific warm pool.

Lipid biomarker fluxes in the Arabian Sea, with a comparison to the equatorial Pacific Ocean

Abstract Fluxes and distributions of organic carbon (OC) and lipid biomarkers were measured in the western Arabian Sea as a function of season, depth, and distance from the coast of Oman during the US JGOFS Arabian Sea Process Study in 1994–1995. A strong seasonal pattern in OC and lipid flux is related to the annual monsoon cycle in the western Arabian Sea, with the highest fluxes measured during the Southwest Monsoon. Fluxes were greatly attenuated with depth in the water column and in surface sediments as remineralization effectively consumed particulate organic matter. A comparison of water-column fluxes with OC and lipid accumulation rates in sediments confirms that the water–sediment interface is a “hot-spot” for organic matter degradation. Biomarker compositions also varied seasonally and with distance offshore, reflecting seasonal and spatial succession of their biological sources and their subsequent export through the water column. Degradation of OC and biomarkers was extremely efficient, with only a tiny fraction (

Annual monsoon rains recorded by Jurassic dunes.

Pangaea, the largest landmass in the Earth's history, was nearly bisected by the Equator during the late Palaeozoic and early Mesozoic eras. Modelling experiments and stratigraphic studies have suggested that the supercontinent generated a monsoonal atmospheric circulation that led to extreme seasonality, but direct evidence for annual rainfall periodicity has been lacking. In the Mesozoic era, about 190 million years ago, thick deposits of wind-blown sand accumulated in dunes of a vast, low-latitude desert at Pangaea's western margin. These deposits are now situated in the southwestern USA. Here we analyse slump masses in the annual depositional cycles within these deposits, which have been described for some outcrops of the Navajo Sandstone. Twenty-four slumps, which were generated by heavy rainfall, appear within one interval representing 36 years of dune migration. We interpret the positions of 20 of these masses to indicate slumping during summer monsoon rains, with the other four having been the result of winter storms. The slumped lee faces of these Jurassic dunes therefore represent a prehistoric record of yearly rain events.

An apparent atmospheric teleconnection between snow cover and the North American Monsoon

Snow cover has been shown to reduce lower‐atmospheric temperatures thereby modifying atmospheric circulation patterns over time scales of days to months. Over the past three decades an inverse relationship between Eurasian winter snow cover extent and subsequent summer rainfall across India has been studied. Very little research has addressed the potential relationship between North American snow cover and the North American monsoon. The work presented here identifies an apparent inverse relationship between July and August snow cover across western North America and August precipitation across a portion of the southwestern U.S. that is susceptible to an annual monsoon circulation. Results suggest that during a summer season characterized by low snow cover across western North America, the typical subtropical ridge across the southern U.S. intensifies. The product appears to be an atmosphere that is conducive to advection of low‐level moisture northward across areas of the southwestern U.S.

Bank robbery: the real losers in the 1998 Bangladesh flood

AbstractDuring July, August and September 1998, Bangladesh was subjected to the most extensive flood in living memory. At peak flood, approximately two‐thirds of the country was under water. The media showed submerged houses, drowned livestock, and women and children wading neck deep in water carrying water jars or food on their heads. Such images reinforce many people's perceptions of Bangladesh as a country incessantly subjected to devastating floods that destroy crops and cause untold misery to the nation's people. Yet, beyond this media‐generated view, there is a country where agricultural production is still the mainstay of the rural population's livelihood system. The nation's water resources, both above and below ground, provide a multitude of services to the population: water to drink and in which to bathe, fish to eat and water to irrigate crops. This paper outlines the local‐level research indicating that most rural Bangladeshis living on the central Bangladesh floodplain, view the annual monsoon flood season not as an inconvenience but as a necessity. It suggests that many people in the research areas recognize the benefits associated with extreme flood events. The 1998 flood provided plentiful amounts of fish, a staple diet and labouring work in the form of building up flood defences. It also flushed out stagnant water in ponds and pagars (open system ponds), and in time, led to a bumper harvest of rice. The research highlights the lives of those whose livelihoods are devastated by such events, describes the nature of the monsoon of 1998, analyses the effects on the community's livelihood systems and describes the nature of their coping strategies. It also suggests ways in which the government and aid agencies could better assist those whose livelihoods are most vulnerable. Copyright © 2001 John Wiley & Sons, Ltd.

Paleocurrent patterns in braid-bar and point-bar deposits; examples from the Ganga River, India

ABSTRACT A braid bar at Bithur and a point bar at Saidpur, both in the Ganga River, were studied for their paleocurrent patterns by making trenches in a grid system and documenting the azimuthal direction of planar and trough cross-bedding. The bars are made up of fine sand and subjected to annual monsoon discharge fluctuations. The paleocurrent trends in the Bithur braid bar show changes from the upstream to the downstream parts, viz., the upcurrent part has a unimodal pattern with high variance, the middle part shows a bimodal pattern with very high variance, and the downstream part shows a unimodal pattern with high variance. There is large variance between the paleocurrent directions of planar and trough cross-bedding and a very high variance in the overall paleocurrent trend. The Saidpur point bar shows a polymodal pattern with very high variance in the upstream part, unimodal distribution with low variance in the middle part, and unimodal distribution with high variance in the downstream part. There is no difference in the paleocurrent direction of planar and trough cross-bedding; the variance value for the overall paleocurrent is slightly lower than in the Bithur braid bar. Contrary to the existing models, the value of azimuthal dispersion is higher for the Bithur braid bar and lower for the Saidpur point bar. Reconstruction of bedform movement on both Bithur and Saidpur bars shows a prominent movement direction of bedforms obliquely into the channel in the marginal parts of the bars. This trend of bedform movement is in contrast to the trends of the theoretical models, where in the marginal parts of bars bedforms move obliquely on the bars.

Molecular Systematics of Australian Gossypium Section Grandicalyx (Malvaceae)

Australian Gossypium consists of 17 species classified into three sections. The largest and most poorly understood is the group of 12 species in sect. Grandicalyx that occur in the Kimberley and Cobourg regions of NW Australia. These areas are characterized by annual monsoon rains and dry-season fires. Species in sect. Grandicalyx exhibit a suite of morphological and ecological features that are otherwise unknown in the genus and that apparently evolved in response to seasonal fires and ant mutualism. These features include an herbaceous perennial habit, the ability to regenerate from thickened rootstocks following fires and ex-

Lacustrine sedimentation in a monsoon environment: the record from Phewa Tal, middle mountain region of Nepal

A watershed-scale approach to the sedimentary environment of the Phewa Tal drainage basin is used to assess rates and patterns of sediment accumulation within Phewa Tal reservoir. The susceptibility of surficial materials to erosion and transportation is highest in the early part of the monsoon season but major erosional events also occur during the monsoon proper. Thus, sediment delivery to the reservoir is at least two orders of magnitude higher during the monsoon than the dry season. In the reservoir, suspended sediment is distributed mainly by interflows at 4 to 6 m and 8 to 12 m depth. Phewa Tal has four distinct sedimentary environments as interpreted from the sedimentary record in cores. (1) The deltaic environment, which receives sediment directly from the main inflowing stream, the Harpan Khola, and from up-valley wetlands and agricultural fields, experiences rates of sedimentation of up to 1 m a−1. (2) The delta-proximal region where clearly varved sediments are deposited in response to the annual monsoon, has a mean rate of sedimentation is 23.5 mm a−1. (3) The distal environment is beyond the influence of sediment-charged interflows, and is characterised by accumulation from suspension to form largely massive deposits, so that annual rates of sedimentation cannot be determined stratigraphically. (4) The region near the Pardi Dam is dominated by seasonal flushing and erosion of fine sediments during the monsoon. The sedimentary record documents the geomorphic processes of the drainage basin, especially as controlled by the monsoon, and indicates the impact of human occupance in the drainage basin in the twentieth century. The useable life of the reservoir is estimated as 360 years.

Particulate organic carbon fluxes: compilation of results from the 1995 US JGOFS Arabian Sea Process Study: By the Arabian Sea Carbon Flux Group

Organic carbon fluxes in the Arabian Sea were measured as a function of depth, season and distance from the coast of Oman. We present here a compilation of measurements of primary production, water-column export flux and sediment accumulation of organic carbon over a full annual monsoon cycle on a 1500-km transect from the coast of Oman toward the central Arabian Sea. This represents an integration of measurements spanning one day (primary production) to 1000 yr (sediments) and gives a broad overview of organic carbon removal and remineralization in the highly productive, seasonally varying region of the northern Indian Ocean. Organic carbon fluxes decreased from the surface to the sediments by a factor of 500–10,000, with the largest rates of change in the upper ocean and at the sediment–seawater interface. Organic carbon fluxes generally decreased with distance offshore, with the largest gradient between surface and seafloor being at the offshore station. Sediment accumulation rates of organic carbon differed by a factor of 40 between nearshore and offshore, while primary productivity varied only by a factor of 2. The decrease in carbon flux with depth that occurs between the deepest traps and the sediment becomes a greater proportion of the total loss with increasing distance from shore. Thus, the influence of processes at the sediment–water interface on the proportion of primary productivity preserved in the sediment increases offshore relative to upper water column processes. Carbon fluxes changed greatly with season, with highest fluxes during the Southwest Monsoon. Export fluxes varied more with season than primary productivity or mid-water fluxes.

Wave Trains Excited by Cross-Equatorial Passage of the Monsoon Annual Cycle

Abstract In this paper the authors illustrate wave trains that are excited during the equatorial passage of the annual cycle of monsoonal convection. Twice a year, during roughly the months of December–January and March–April, the annual cycle of monsoonal convection crosses the equator. A principle axis of annual cycle monsoon precipitation extends from the Java Sea to the eastern Himalayas. Monsoonal convection makes a north–south seesaw roughly along this axis each year. Near-equatorial convection provides a tropospheric heat source somewhat akin to that of El Nino over the equatorial Pacific Ocean. This equatorial passage of the monsoonal heat source excites a wave train, somewhat similar to the familiar Pacific–North American pattern. Monsoonal wave trains were extracted from a 9-yr dataset, and a composite geometry was constructed. This note also illustrates excitation of short-period wet and dry spells associated with excitation of this wave train. This is illustrated for several trough and ridge l...

Agroforestry for soil and water conservation in the western Himalayan Valley Region of India. 1. Runoff, soil and nutrient losses

Erosion losses were investigated in a field experiment, from 90 × 15 m erosion plots at 4% slope in the western Himalayan valley region of India. The plots were planted with Leucaena leucocephala and Eucalyptus hybrid, either as block plantation or in alley farming with maize (Zea mays), Chrysopogon fulvus grass or turmeric (Curcuma longa). The treatments also included contour planting of sole maize, sole Chrysopogon fulvus grass and a clean weeded cultivated fallow. During the nine year study period, the average annual monsoon rainfall was about 1000 mm and it caused 347 mm runoff and 39 Mg ha-1 soil loss per year from fallow plots. The runoff and soil loss were reduced by 27% and 45% by contour cultivation of maize. Contour tree-rows or leucaena hedges reduced the runoff and soil loss by 40% and 48%, respectively, over the maize plot, reducing soil loss to about 12.5 Mg ha-1. This reduction in erosion was primarily due to the barrier effect of tree or hedgerows and micro-terraces formed through sediment deposition along the contour barriers. Such vegetative measures, that are productive while being protective, offer viable alternative for erosion control in areas with gentile slopes of the valley region. High density block plantations of eucalyptus and leucaena almost completely controlled erosional losses and can be recommended for steeper slopes that are vulnerable to heavy erosion.