Variation In Amount Of Rainfall Research Articles

Anti-phase variation of long eccentricity and precipitation in inland Asia during the Middle Miocene Climatic Optimum

The mechanisms and pace of orbital forcing on precipitation in inland Asia during the Cenozoic remain poorly understood. Many previous studies using magnetic proxies for precipitation have shown a consistent signal of long eccentricity (405 kyr) in hydrological records of central China that are younger than ca. 11 Ma. Most studies suggest that variations in rainfall amount were controlled by Northern Hemisphere summer insolation, and the peak of precipitation is associated with eccentricity maxima. Here, we report multiple magnetic records dating back to the Middle Miocene Climatic Optimum (MMCO, ∼14−17 m.y. ago), including a detailed record from a new section in the Qaidam Basin, which was dated using magnetostratigraphy, U-Pb geochronology, and apatite low-temperature thermochronology. Our records show the exact opposite: 405 kyr wet-dry cycles dominated, but the wetter intervals correspond to eccentricity minima and ice-volume maxima during the MMCO. Taken at face value, these results question the origin—monsoonal or westerly-derived—of the precipitation that reached central China during the middle Miocene and the mechanisms that enhanced monsoonal penetration of inland Asia. We also suggest that this anti-phase relationship could also reflect biases in magnetic proxies for precipitation during the wettest climatic phases, which can result in the dissolution of magnetic minerals and a significant underestimation of past rainfall.

Sub-daily scale variations and trends in southwest monsoon rainfall over the West coasts of India and Myanmar

The West Coast of India (WCI) and the West Coast of Myanmar (WCM) receive a good amount of monsoon rainfall due to the orographic lifting and moisture transport from the neighbouring sea. Here, diurnal variation in rainfall amount, frequency, intensity, and contribution during the summer monsoon period was studied using TRMM 3 hourly precipitation data. The analysis was carried out over lowland, midland and highland regions of both the coasts. The subregions exhibit a similar diurnal pattern even though the intensity and frequency of rainfall vary from region to region. Generally, bimodal variations with late afternoon and early morning peaks are found over lowland and midland regions of both WCI and WCM in rainfall frequency, amount and contribution. However, highland regions exhibit unimodal variation with a single late afternoon peak. Over WCI, lowland regions exhibit primary peak during early morning, whereas lowlands of WCM exhibit primary peak during afternoon hours. Rainfall intensity exhibits a different pattern compared to other parameters. Rainfall intensity increases from south to north over both the coasts. Among the subregions, maximum rainfall intensity is found over midlands and minimum over highlands. In general, more rainfall is observed over subregions of WCM than subregions of WCI. Most of the lowland and midland regions, exhibit a significant increasing trend in rainfall frequency and contribution at morning hours and a significant decreasing trend at afternoon hours. Rainfall frequency exhibit different periodicities during different time indicating interannual heterogeneity in rainfall. The diurnal pattern of summer monsoon rainfall over the Indian and Myanmar West coasts shows remarkable latitudinal and topographical variations.

Climatological features of high temporal resolution rainfall over the Hyderabad city, India

High spatial variability and continuous evolution of a city's land use and land cover makes the city's rainfall mechanism more complex and dynamic, thus climatological rainfall features have both scientific and engineering relevance. A high-resolution hourly rainfall data from 37 Automatic Weather Stations (AWSs) of the Hyderabad city (an area of 650 km2) was considered. Rainfall analysis was carried out on an event-based approach from a total of 118 rainfall events. Rainfall frequency, depths and intensities were higher in the oldest region of the city for all seasons with exception of rainfall depth in summer. Diurnal variations in rainfall amount suggest large monsoon rainfall in the evening to midnight in fewer 1-hour spells. An increasing trend in rainfall depths and intensities highlights non-stationarity. In addition, decreased association among stations with increasing distance and increased spatial extent of precipitation with decreased spatial variability were observed. Further, enhanced rainfall activity in the city's region was observed as compared to its surrounding suburban region (has 33 AWSs). Results suggest significant rainfall variability across space- and time- scales, which may be associated with urbanization, and have great implications in a city's planning and development particularly under climate change and exponential urbanization context.

Simulation of ambient parameters using Grell convective scheme to study rainfall heterogeneity over western India

Regional climate variability is considered to be the major factor causing the advancement in global warming. Its effect influences precipitation patterns, which are more sensitive for monsoon dominated region like India. High-resolution (0.5°x0.5°) climate models play a significant role to understand variation in rainfall amount through simulations of various types of ambient parameters such as an atmospheric parameter-relative humidity (RH) (at 850 hPa), a cloud macrophysical parameter-cloud fraction (CF) (total column), a radiance parameter-outgoing longwave radiation (OLR) (total column) and a precipitation parameter-precipitation rate (PR) (total column). These parameters are studied over two different regions-an inland region which has more heterogeneity (central India-M.P.) as well as over coastal region which has less heterogeneity (western India-Gujarat) to understand spatial heterogeneity. The present study show daily averaged data of monsoon (June-September) from 2003 to 2017 for selected parameters over these regions with the help of linear correlation method with their uncertainties, which uses satellite (AIRS and TRMM) and model (RegCM 4.4) data. In this study, simulations are carried out by implementing Grell convective scheme during different rainfall scenarios. Results show that the correlation values are ∼ 0.35, ∼ 0.68, ∼ 0.58 and ∼ 0.18 for RH, CF, OLR and PR respectively, which signify good correlation coefficients during deficit years over coastal region. Also, these correlation values are almost same but their uncertainties are minimum for deficit as compared to normal and excess rainfall years. It shows that deficit rainfall scenarios are captured well over coastal regions as compared to excess rainfall scenarios, which is may be due to the rainfall departure as deficit rainfall years are less extreme (∼−22.34%) than excess rainfall years (∼42.12%). Further study continues to understand regional heterogeneity with respect to rainfall departure during excess and normal rainfall years. Further statistics can be improved with the simulation of more number of atmospheric/cloud parameters. Studies may be useful to determine heterogeneity and can be utilized in climate models, which further can help to improve weather predictability.

Effects of straw and biochar amendment on hydrological fluxes of dissolved organic carbon in a subtropical montane agricultural landscape

Straw and biochar amendments have been shown to increase soil organic carbon (SOC) stocks in arable land; however, their effects on hydrological fluxes of dissolved organic carbon (DOC), which may offset the benefits of C sequestration amounts remain uncertain. Therefore, we conducted a three-year field study that included four treatments (CK, control with no fertilizer; NPK, synthetic N fertilizer; RSDNPK, synthetic N fertilizer plus crop residues; BCNPK, synthetic N fertilizer plus biochar of crop straw) to investigate the effects of straw and biochar amendment on DOC losses through hydrological pathways of overland flow and interflow from a wheat-maize rotation system in the subtropical montane agricultural landscape. We detected substantial intra- and inter-annual variations in runoff discharge, DOC concentration, and DOC fluxes for both overland flow and interflow pathways, which were primarily attributed to variations in rainfall amount and intensity. On average, the DOC concentrations for interflow (2.98 mg C L−1) were comparable with those for overland flow (2.71 mg C L−1) throughout the three-year experiment. However, average annual DOC fluxes for interflow were approximately 2.60 times greater than those for overland flow, which probably related to higher runoff discharges of interflow than overland flow. Compared to the control, on average, the N fertilization treatments significantly decreased the annual DOC fluxes of overland flow and significantly increased annual DOC fluxes of interflow. Relative to the application of synthetic N fertilizer only, on average, crop straw amendment practice significantly increased annual DOC fluxes of interflow by 28.7%, while decreasing annual DOC fluxes of overland flow by 12.0%; in contrast, biochar amendment practice decreased annual DOC fluxes of interflow by 25.3% while increasing annual DOC fluxes of overland flow by 44.6%. Overall, considering both overland flow and interflow, crop straw amendment significantly increased hydrological DOC fluxes, whereas biochar had no significant effects on hydrological DOC fluxes throughout the three-year experiment. We conclude that crop straw incorporation strategies that aim to increase SOC stocks may enhance hydrological losses of DOC, thereby in turn offsetting its benefits in the subtropical montane agricultural landscapes.

Quantifying the influence of rainfall, vegetation and animals on soil erosion and hillslope connectivity in the monsoonal tropics of northern Australia

AbstractErosion of soil by water is facilitated by both diffusive and fluvial processes. Here we examine three different soil redistribution processes operating at very different spatial and temporal scales in the monsoonal tropics of northern Australia. The first process, rainsplash, operates across the entire catchment. This process, while subject to annual and seasonal variations in rainfall amount and intensity, can be considered a constant forcing and redistributes on average 9 t ha−1 year−1 (range −0.9 to 19 t ha−1 year−1). The second process, bioturbation, where in this study soil is disturbed by feral pigs (wild boar), occurs in selected areas throughout each year. Pigs exhume 3 to 36.0 t ha−1 year−1 (average ~11 t ha−1 year−1). The effect of this disturbance may last for many years afterwards. The third process is the disturbance of the soil surface by tree throw and creation of pit–mound topography (also a form of bioturbation), together with the resultant placement of the tree superstructure (above ground biomass) on the ground, which may form debris dams. Tree throw at the scale examined here is likely to occur only once every 50–100 years, with the influence of this single event lasting for at least 10 years post event. Tree throw in a single event exhumed ~5 t ha−1 (1.1–9.5 t ha−1) of soil. In contrast to rainsplash, pig disturbance and tree throw events are largely point‐based phenomena. Field observation suggests that it takes many years for the disturbance from both pigs and tree throw to be removed. We find here that in terms of relative soil redistribution, rainsplash has the largest influence, with any erosional disturbance by pigs and tree throw being within the variability of rainsplash. However, the disruption of surface flow by the pig digs and tree throw disrupts sedimentological and hydrological connectivity.

Investigation of methane fluxes from temporary cover of Xi’an Jiangcungou landfill, China

Methane emissions from the Jiangcungou municipal solid waste landfill in China were examined using a static chamber. The monitoring results were compared firstly with those obtained from the California Landfill Methane Inventory Model (Calmim). The monitoring results show that average methane emission flux from the temporary cover is one to two orders of magnitude greater than those from the final cover and the working face. This is because the methane generation rate reaches its maximum at less than 1 year after the wastes were landfilled. The observed methane fluxes are comparable with those obtained by Calmim. The observed results are lower than those predicted by Calmim, which were 74.98 and 98.64 (g/m2)/day, respectively. The difference is reasonable because the results obtained by field monitoring are transient, while those simulated by Calmim are the average daily flux. Moreover, Calmim considers only one-dimensional gas transport through the cover and neglects lateral gas migration. The model results indicate that there is a strong seasonal variability due to the variation in rainfall amount in different months. The methane emission amount was greatly reduced with the increase in vegetation coverage due to the increase in the volumetric water content of the soil induced by the water retention capability of vegetation.

İKLİM DEĞİŞİKLİĞİ: YAĞIŞLARIN EKONOMİK BÜYÜME ÜZERİNE ETKİSİ

The variation in rainfall amount is found to be more forceful in poorer or agriculture-dependent countries and seems to be less effective for industrial economies. As Turkey is a newly industrialised country with a growing service economy, it is categorized as an emerging economy in the world. However, agricultural industry still plays a significant role in the economy especially in terms of the level of employment. The aim of the study is to examine the impact of rainfall on economy through economic outcomes of drought on agricultural industry. There seems to be a positive correlation between rainfall and agricultural industry in over the investigated period. As the growth in agricultural industry is highly correlated, similarly rainfall and overall economic growth appear to be coinciding and moving together. When there is drought in the country, economic crises follow this pattern. Besides rainfall, another variable, temperature, is allowed to identify whether it has any effect on the growth. Contrary to rain, temperature has no influence on agricultural output. The descriptive figures note that while agricultural industry involves a small share of the whole economy, the results of the study imply rainfall amount in Turkey has more impacts on economic growth than thought.

Spatiotemporal partitioning of savanna plant functional type productivity along NATT

Realistic representations and simulation of mass and energy exchanges across heterogeneous landscapes can be a challenge in land surface and dynamic vegetation models. For mixed life-form biomes such as savannas, plant function is very difficult to parameterise due to the distinct physiological characteristics of tree and grass plant functional types (PFTs) that vary dramatically across space and time. The partitioning of their fractional contributions to ecosystem gross primary production (GPP) remains to be achieved at regional scale using remote sensing. The objective of this study was to partition savanna gross primary production (GPP) into tree and grass functional components based on their distinctive phenological characteristics. Comparison of the remote sensing partitioned GPPtree and GPPgrass against field measurements from eddy covariance (EC) towers showed an overall good agreement in terms of both GPP seasonality and magnitude. We found total GPP, as well as its tree and grass components, decreased dramatically with rainfall over the North Australian Tropical Transect (NATT), from the Eucalyptus forest and woodland in the northern humid coast to the grasslands, Acacia woodlands and shrublands in the southern xeric interior. Spatially, GPPtree showed a steeper decrease with precipitation along the NATT compared to GPPgrass, thus tree/grass GPP ratios also decreased from the northern mesic region to the arid south region of the NATT. However, results also showed a second trend at the southern part of the transect, where tree-grass ratios and total GPP increased with decreasing mean annual precipitation, and this occurred in the physiognomic transition from hummock grasslands to Acacia woodland savannas. Total GPP and tree-grass GPP ratios across climate extremes were found to be primarily driven by grass layer response to rainfall dynamics. The grass-containing xeric savannas exhibited a higher hydroclimatic sensitivity, whereas GPP in the northern mesic savannas was fairly stable across years despite large variations in rainfall amount. The pronounced spatiotemporal variations in savanna vegetation productivity encountered along the NATT study area suggests that the savanna biome is particularly sensitive and vulnerable to predicted future climate change and hydroclimatic variability.

Diurnal variation of stable isotopes in rainfall observed at Bengkulu for the YMC-Sumatra 2017

Diurnal variability of stable isotopes in precipitation and its controlling factors are poorly understood, especially in the tropics. This study investigated the diurnal variability of stable isotopes in rainfall observed at Bengkulu in southern Sumatra, Indonesia. Rainfall samples were collected at 6-h intervals from 1 December 2017 to 15 January 2018 for the Year of the Maritime Continent (YMC-Sumatra 2017). The oxygen and hydrogen isotopic compositions δ18O and δH) of the rainwater varied from –15.1%o to –1.8%o and from –108.9%o to –1.3%o, respectively, and the d-excess varied from 1.9%o to 17.1%o. The observations at Bengkulu revealed that diurnal variation in rainfall amount was not significant for the entire period; however, the δ18O was lighter in both the early morning and the afternoon and heavier before noon and overnight. The δ18O became gradually depleted in the periods of continuous rain that occurred during 1–4 and 21–24 December. The amount of morning rain was much greater than evening rain in these periods, whereas the amount of evening rain was greater than morning rain in other periods. Based on satellite-derived CMORPH data, it is assumed that morning rain and evening rain corresponded to weak rainfall over surrounding sea region and strong rainfall over inland regions of Sumatra, respectively. The differences in moisture source and ratio of stratiform/convective rain rate between morning and evening might affect the isotopic composition of rainfall in the tropics.

Observations on changes in Korean Changma rain associated with climate warming in 2017 and 2018

The period of prolonged summer rain in Korea called Changma was studied. Changma, also known as Meiyu in China and Baiu in Japan, is an integral part of the East Asian Summer Monsoon System. Until the 1980s, the Changma rainy season in Korea occurred from late June and July for 30 to 40 days or approximately 4 to 5 weeks. This study investigated whether changes have been observed in the timing and amounts of precipitation associated with Changma. From the analysis of meteorological data obtained in 2017 and 2018, it is observed that the length of the rainy season has shortened to 2 to 3 weeks in agreement with the earlier studies. Furthermore, during the rainy season, there were many days of no rain and partly cloudy days. Although an active elongated-linear Changma front was common in the past, it was found that an inactive Changma front with a large meridional amplitude has caused intermittent rain showers for several hours from linear convective cloud streaks. In recent years, this has produced large variations in rainfall amounts among regional measuring stations in Korea. Climate warming in the north side of the Changma front has resulted in less contrast with the warm-moist air in the south side of it from the Pacific Ocean. This has resulted in a weaker and inactive quasi-stationary front which has caused a discontinuous broken Changma front and sporadic showery days. It is also observed that maritime polar air mass, mP, over the Okhotsk Sea of far eastern Russia has not affected the Changma front over the Korean Peninsula. Overall, this study found that the characteristics of past Changma fronts have changed significantly in recent years. The present observations imply the need of further studies on climate change and summer rainfall including water management.

Understanding post-mining landforms: Utilising pre-mine geomorphology to improve rehabilitation outcomes

Post-mining landscapes are required to geomorphically and ecologically integrate with the surrounding undisturbed environment. To understand the pre- and post-mining landscape, models can be used to guide rehabilitation strategies. This is particularly important where an absence exists of pre-mine (predisturbance) data regarding the landscape system. In particular, geomorphic evolution may be simulated with tools such as landform evolution models (LEMs). In this study, we use the CAESAR-Lisflood LEM to assess how a pre-mine landscape may evolve over 1000 years and compare it to the evolution of a catchment representing a rehabilitated post-mining landscape. Environmental conditions such as rainfall, vegetation cover, and surface particle size characteristics were varied through a series of model simulations to determine their impact on the different landforms. We compared the outputs of the model simulations – specifically, sediment loads, denudation rates, and extent, depth, and distribution of gullies – from the undisturbed, pre-mine landform and the disturbed, post-mine landform. Under all scenarios simulated, much greater erosion occurred on the post-mine landform, producing sediment yields and denudation rates that were higher than the undisturbed pre-mine landform by up to an order of magnitude. The results showed that the presence/absence of vegetation and variations in rainfall amount and intensity could influence erosion rates, sediment yields, and gully depths between the two primary scenarios. However, the parameters that most influenced landscape evolution over centennial time scales were the particle size classes used to represent the surface conditions in the simulations, and the topographic characteristics of the catchments on the pre- and post-mine landforms. The model outputs reinforce the expectation that a pre-mining landscape will experience less erosion and sediment transport than a post-mining landscape.

Isotopic and geochemical assessment of the sensitivity of groundwater resources of Guam, Mariana Islands, to intra- and inter-annual variations in hydroclimate

Assessing the sensitivity of groundwater systems to hydroclimate variability is critical to sustainable management of the water resources of Guam, US territory. We assess spatial and temporal variability of isotopic and geochemical compositions of vadose and phreatic groundwater sampled from cave drip sites and production wells, respectively, to better understand the vulnerability of the freshwater lens on Guam to variability in hydroclimate. We independently evaluate the existing conceptual model of the Northern Guam Lens Aquifer that is largely based on physical, as opposed to geochemical, observations. Sampling was conducted from 2008 to 2015, over which rainfall gradually increased. Major ion geochemistry and Sr isotope values of groundwater show varying influence from soil, limestone bedrock, and seawater. Geochemical modeling that can explain spatial variability in groundwater Na+ and Mg2+ concentrations and Sr/Ca and 87Sr/86Sr values indicates that groundwater compositions are dominantly controlled by mixing of freshwater with seawater and water-rock interaction. Differences between amount-weighted annual average precipitation δ18O values and groundwater δ18O values indicate a recharge bias toward the wet season, consistent with other tropical carbonate island aquifer settings. Intra- and inter-annual variations in Na+ concentrations and δ18O values in groundwater reflect sensitivity of recharge to seasonal variations in rainfall amount and changes in annual rainfall amounts. Our results indicate the influence of multiple modes of recharge on groundwater compositions and spatial variability in the sensitivity of groundwater to seawater mixing. This sensitivity of the freshwater lens points to the vulnerability of groundwater resources to changes in recharge associated with climate, land-use change, and increases in population.

Diurnal Variations of Presummer Rainfall over Southern China

AbstractIn this study, the presummer diurnal cycle of rainfall (DCR) over southern China is examined using the merged 0.1°-resolution gridded hourly rain gauge and satellite rainfall dataset and the National Centers for Environmental Prediction Final Global Analysis during April to June of 2008–2015. Results show pronounced diurnal variations in rainfall amount, frequency, and intensity over southern China, with substantially different amplitudes from southwestern to southeastern China, and from the pre- to postmonsoon-onset period. Southwestern China often encounters significant nocturnal-to-morning rainfall under the influence of enhanced nocturnal low-level southwesterly winds. Southeastern China is dominated by afternoon rainfall, as a result of surface heating, likely aided by local topographical lifting. Both the pre- and postmonsoon-onset periods exhibit two diurnal rainfall peaks: one in the early morning and the other in the late afternoon. But the latter shows the two peaks with nearly equal amplitude whereas the former displays a much larger early morning peak than that in the late afternoon. Three propagating modes accounting for the presummer DCR are found: (i) an eastward- or southeastward-propagating mode occurs mostly over southwestern China that is associated with enhanced transport of warm and moist air from tropical origin and the induced low-level convergence, (ii) a quasi-stationary mode over southeastern China appears locally in the warm sector with weak-gradient flows, and (iii) an inland-propagating mode occurs during the daytime in association with sea breezes along the southern coastal regions, especially evident throughout the postmonsoon-onset period.

Suspended Solids Export by the Outflowing Water from Irrigation Paddy Field during Rice Growing Season of Korea

This experiment was conducted in Suwon and Iksan city from 2012 to 2014 to evaluate soil erosion and nutrient loss from irrigated paddy fields during cropping period. Rainfall amount and rainfall erosivity of <TEX>$EI_{30}$</TEX> were, on average, 1,026 mm and <TEX>$3,922mm\;ha^{-1}yr^{-1}hr^{-1}$</TEX> for the cropping period, respectively, and the rainfall event with maximum <TEX>$EI_{30}$</TEX> occurred in July. Annual average of runoff was <TEX>$2,508MT\;ha^{-1}yr^{-1}$</TEX> in Suwon and <TEX>$3,375MT\;ha^{-1}yr^{-1}$</TEX> in Iksan, accounting for 36% of rainfall of the cropping period. Nutrient loss by runoff, on average, was <TEX>$7.0kg\;N\;ha^{-1}yr^{-1}$</TEX>, <TEX>$1.3kg\;P\;ha^{-1}yr^{-1}$</TEX>, and <TEX>$16.6kg\;K\;ha^{-1}yr^{-1}$</TEX>; N, P, and K loss were 5.0, 0.6, and <TEX>$8.3kg\;ha^{-1}yr^{-1}$</TEX>, respectively, in Suwon and 8.9, 1.9, and <TEX>$16.7kg\;ha^{-1}yr^{-1}$</TEX> in Iksan. Soil loss in Korean paddy rice was evaluated as <TEX>$0.33MT\;ha^{-1}yr^{-1}$</TEX> ranging from <TEX>$0.05MT\;ha^{-1}yr^{-1}$</TEX> to <TEX>$0.88MT\;ha^{-1}yr^{-1}$</TEX>. Amount of soil loss, however, depended on areas and year influenced by variation of rainfall amount and intensity. Interestingly, soil erosion in Iksan in 2012 was remarkably greater than those in other periods due to heavy rainfall between late May and June with soil flake dispersion right after the rice-planting season.

Assessing the impact of climate-change scenarios on landslide occurrence in Umbria Region, Italy

Summary Landslides are frequent and widespread geomorphological phenomena causing loss of human life and damage to property. The main tool for assessing landslide risk relies on rainfall thresholds and thus, many countries established early warning systems aimed to landslide hazard assessment. The Umbria Region Civil Protection Centre developed an operational early warning system for landslide risk assessment, named PRESSCA, based on the soil saturation conditions to identify rainfall thresholds. These thresholds, currently used by the Civil Protection operators for the day-by-day landslide hazard assessment, provided satisfactory results with more than 86% of the landslides events correctly identified during the period 1990–2013. In this study, the PRESSCA system was employed for the assessment of climate change impact on landslide hazard in Central Italy. The outputs of five different Global Circulation Models (GCMs) were downscaled and weather generators were used for obtaining hourly rainfall and temperature time series from daily GCMs projection. Then, PRESSCA system was employed to estimate the number of landslide occurrence per year. By comparing results obtained for three different periods (1990–2013 (baseline), 2040–2069 and 2070–2099), for the Umbria territory a general increase in events occurrence was expected (up to more than 40%) in the future period, mainly during the winter season. The results also revealed that the effect of climate change on landslides was not straightforward to identify and the close interaction between rainfall magnitude/intensity, temperature and soil moisture should be analysed in depth. Overall, soil moisture was projected to decrease throughout the year but during the wet season the variations with respect to the present period were very small. Specifically, it was found that during the warm-dry season, due to the strong decrease of soil moisture, even for a sensible increase in rainfall intensity, the landslide occurrence was unchanged. Conversely, during the cold-wet season, the number of landslide events increased considerably if a positive variation in rainfall amount, more significant than rainfall intensity, was coupled with small negative variations in soil moisture.

A late-Quaternary perspective, on atmospheric pCO2, climate, and fire as drivers of C4-grass abundance.

Various environmental factors, including atmospheric CO2 (pCO2), regional climate, and fire, have been invoked as primary drivers of long-term variation in C4 grass abundance. Evaluating these hypotheses has been difficult because available paleorecords often lack information on past C4 grass abundance or potential environmental drivers. We analyzed carbon isotope ratios (delta13C) of individual grains of grass pollen in the sediments of two East African lakes to infer changes in the relative abundance of C3 vs. C4 grasses during the past 25 000 years. Results were compared with concurrent changes in pCO2, temperature, moisture balance, and fire activity. Our grass-pollen delta13C analysis reveals a dynamic history of grass-dominated vegetation in equatorial East Africa: C4 grasses have not consistently dominated lowland areas, and high-elevation grasses have not always been predominantly C3. On millennial timescales, C4 grass abundance does not correlate with charcoal influx at either site, suggesting that fire was not a major proximate control of the competitive balance between C3 and C4 grasses. Above the present-day treeline on Mt. Kenya, C4 grass abundance declined from an average of approximately 90% during the glacial period to less than approximately 60% throughout the Holocene, coincident with increases in pCO2 and temperature, and shifts in moisture balance. In the lowland savanna southeast of Mt. Kilimanjaro, C4 grass abundance showed no such directional trend, but fluctuated markedly in association with variation in rainfall amount and seasonal-drought severity. These results underscore spatiotemporal variability in the relative influence of pCO2 and climate on the interplay of C3 and C4 grasses and shed light on an emerging conceptual model regarding the expansion of C4-dominated grasslands in Earth's history. They also suggest that future changes in the C3/C4 composition of grass-dominated ecosystems will likely exhibit striking spatiotemporal variability as a result of varying combinations of environmental controls.

Hydroclimate of the western Indo-Pacific Warm Pool during the past 24,000 years.

The Indo-Pacific Warm Pool (IPWP) is a key site for the global hydrologic cycle, and modern observations indicate that both the Indian Ocean Zonal Mode (IOZM) and the El Niño Southern Oscillation exert strong influence on its regional hydrologic characteristics. Detailed insight into the natural range of IPWP dynamics and underlying climate mechanisms is, however, limited by the spatial and temporal coverage of climate data. In particular, long-term (multimillennial) precipitation patterns of the western IPWP, a key location for IOZM dynamics, are poorly understood. To help rectify this, we have reconstructed rainfall changes over Northwest Sumatra (western IPWP, Indian Ocean) throughout the past 24,000 y based on the stable hydrogen and carbon isotopic compositions (δD and δ(13)C, respectively) of terrestrial plant waxes. As a general feature of western IPWP hydrology, our data suggest similar rainfall amounts during the Last Glacial Maximum and the Holocene, contradicting previous claims that precipitation increased across the IPWP in response to deglacial changes in sea level and/or the position of the Intertropical Convergence Zone. We attribute this discrepancy to regional differences in topography and different responses to glacioeustatically forced changes in coastline position within the continental IPWP. During the Holocene, our data indicate considerable variations in rainfall amount. Comparison of our isotope time series to paleoclimate records from the Indian Ocean realm reveals previously unrecognized fluctuations of the Indian Ocean precipitation dipole during the Holocene, indicating that oscillations of the IOZM mean state have been a constituent of western IPWP rainfall over the past ten thousand years.

Inundation of Playa Wetlands in the Western Great Plains Relative to Landcover Context

Playa wetlands in the western Great Plains of the United States are an essential component of the ecological and hydrological systems in the region. However, these wetlands are threatened through culturally-accelerated sedimentation from the surrounding row-crop dominated landscape. As a consequence, conservation efforts have focused mainly on installing Conservation Reserve Program (CRP) grasslands around playas but these may impede water from reaching playa basins. Thus, better understanding of the factors that influence inundation is needed for effective conservation of playas. The objectives of this project were to investigate the effects of hydrological factors, landcover at local and watershed scales, and thresholds for playa inundation in a variety of landscape conditions. We used generalized linear mixed modeling to determine the landscape and hydrological factors that influence playa inundation. We found playa inundation was positively related to amount of rainfall in the previous 14 days, the variation in amount of rainfall, playa area and the slope of the surrounding landscape. After accounting for hydrological factors, we found CRP adjacent to the playa and CRP in the watershed had a larger influence on playa inundation than the landcover of Pasture or Grassland. There was a negative non-linear effect of CRP on playa inundation and evidence for a negative linear effect at the watershed scale. Playa conservation is an integral component of the Ogallala Initiative, and Farm Bill programs such as CRP are essential conservation tools thus it is imperative that we understand potential impacts of such programs in order to inform improvements.

Evaluation of satellite rainfall products through hydrologic simulation in a fully distributed hydrologic model

The goal of this study is to evaluate the accuracy of four global high‐resolution satellite rainfall products (CMORPH, TMPA 3B42RT, TMPA 3B42, and PERSIANN) through the hydrologic simulation of a 1656 km2 mountainous watershed in the fully distributed MIKE SHE hydrologic model. This study shows that there are significant biases in the satellite rainfall estimates and large variations in rainfall amounts, leading to large variations in hydrologic simulations. The rainfall algorithms that use primarily microwave data (CMORPH and TMPA 3B42RT) show consistent and better performance in streamflow simulation (bias in the order of −53% to −3%, Nash‐Sutcliffe efficiency (NSE) from 0.34 to 0.65); the rainfall algorithm that uses primarily infrared data (PERSIANN) shows lower performance (bias from −82% to −3%, Nash‐Sutcliffe efficiency from −0.39 to 0.43); and the rainfall algorithm that merges the satellite data with rain gage data (TMPA 3B42) shows inconsistencies and the lowest performance (bias from −86% to 0.43%, Nash‐Sutcliffe efficiency from −0.50 to 0.27). A dilemma between calibrating the hydrologic model with rain gage data and calibrating it with the corresponding satellite rainfall data is presented. Calibrating the model with corresponding satellite rainfall data increases the performance of satellite streamflow simulation compared to the model calibrated with rain gage data, but decreases the performance of satellite evapotranspiration simulation.