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1.
《Geomorphology》2006,73(1-2):33-49
Intertidal bar systems are ubiquitous features on wave-dominated beaches in coastal settings with a significant (> 1 m) tidal range. Depending primarily on the wave conditions and the tidal range, and to a lesser extent on the nearshore gradient, they can assume a variety of forms. Slip-face bars represent the most pronounced and dynamic intertidal bar morphology, and are generally found on their own around the mean high tide level. They usually form low on the intertidal beach after storm-induced beach erosion and develop into a berm under prolonged calm wave conditions. Low-amplitude ridges and sand waves represent multiple bar morphologies. The bars occur across the entire intertidal profile and they remain present throughout the year. Multiple intertidal bars tend to be rather subdued and relatively static, especially sand waves, and their origin remains unclear.The morphological response of intertidal bars to changing wave conditions is largely forced: bars build up and migrate onshore under calm waves, and are flattened and may migrate offshore during storms. The morphological response is, however, significantly affected by relaxation time effects and morphological feedback, particularly on beaches with multiple intertidal bars. Despite their morphological differences, the intertidal bar types exhibit pronounced similarities in their morphodynamics. Sediment transport processes and morphological response are principally controlled by the tidal water levels on the beach, because these, together with the offshore wave energy level and the beach morphology, determine the type, intensity and duration of the wave processes operating on the cross-shore profile.It is the dominant importance of tidal water level variations and wave processes in shallow water depths (swash and surf zone bores), rather than wave height variability and deeper water wave processes (breaking and shoaling waves), that constitutes the main difference between intertidal and subtidal bar morphodynamics.  相似文献   

2.
Representative rainfall thresholds for landslides in the Nepal Himalaya   总被引：14，自引：0，他引：14  
Measuring some 2400 km in length, the Himalaya accommodate millions of people in northern India and Pakistan, Nepal, Bhutan, and parts of other Asian nations. Every year, especially during monsoon rains, landslides and related natural events in these mountains cause tremendous damage to lives, property, infrastructure, and environment. In the context of the Himalaya, however, the rainfall thresholds for landslide initiation are not well understood. This paper describes regional aspects of rainfall thresholds for landslides in the Himalaya. Some 677 landslides occurring from 1951 to 2006 were studied to analyze rainfall thresholds. Out of the 677 landslides, however, only 193 associated with rainfall data were analyzed to yield a threshold relationship between rainfall intensity, rainfall duration, and landslide initiation. The threshold relationship fitted to the lower boundary of the field defined by landslide-triggering rainfall events is I = 73.90D− 0.79 (I = rainfall intensity in mm h− 1 and D = duration in hours), revealing that when the daily precipitation exceeds 144 mm, the risk of landslides on Himalayan mountain slopes is high. Normalized rainfall intensity–duration relationships and landslide initiation thresholds were established from the data after normalizing rainfall-intensity data with respect to mean annual precipitation (MAP) as an index in which NI = 1.10D− 0.59 (NI = normalized intensity in h− 1). Finally, the role of antecedent rainfall in causing landslides was also investigated by considering daily rainfall during failure and the cumulative rainfall to discover at what point antecedent rainfall plays an important role in Himalayan landslide processes. Rainfall thresholds presented in this paper are generalized so they can be used in landslide warning systems in the Nepal Himalaya.  相似文献   

3.
Debris flows generated during rain storms on recently burned areas have destroyed lives and property throughout the Western U.S. Field evidence indicate that unlike landslide-triggered debris flows, these events have no identifiable initiation source and can occur with little or no antecedent moisture. Using rain gage and response data from five fires in Colorado and southern California, we document the rainfall conditions that have triggered post-fire debris flows and develop empirical rainfall intensity–duration thresholds for the occurrence of debris flows and floods following wildfires in these settings. This information can provide guidance for warning systems and planning for emergency response in similar settings.Debris flows were produced from 25 recently burned basins in Colorado in response to 13 short-duration, high-intensity convective storms. Debris flows were triggered after as little as six to 10 min of storm rainfall. About 80% of the storms that generated debris flows lasted less than 3 h, with most of the rain falling in less than 1 h. The storms triggering debris flows ranged in average intensity between 1.0 and 32.0 mm/h, and had recurrence intervals of two years or less. Threshold rainfall conditions for floods and debris flows sufficiently large to pose threats to life and property from recently burned areas in south-central, and southwestern, Colorado are defined by: I = 6.5D− 0.7 and I = 9.5D− 0.7, respectively, where I = rainfall intensity (in mm/h) and D = duration (in hours).Debris flows were generated from 68 recently burned areas in southern California in response to long-duration frontal storms. The flows occurred after as little as two hours, and up to 16 h, of low-intensity (2–10 mm/h) rainfall. The storms lasted between 5.5 and 33 h, with average intensities between 1.3 and 20.4 mm/h, and had recurrence intervals of two years or less. Threshold rainfall conditions for life- and property-threatening floods and debris flows during the first winter season following fires in Ventura County, and in the San Bernardino, San Gabriel and San Jacinto Mountains of southern California are defined by I = 12.5D−0.4, and I = 7.2D−0.4, respectively. A threshold defined for flood and debris-flow conditions following a year of vegetative recovery and sediment removal for the San Bernardino, San Gabriel and San Jacinto Mountains of I = 14.0D−0.5 is approximately 25 mm/h higher than that developed for the first year following fires.The thresholds defined here are significantly lower than most identified for unburned settings, perhaps because of the difference between extremely rapid, runoff-dominated processes acting in burned areas and longer-term, infiltration-dominated processes on unburned hillslopes.  相似文献   

4.
We set up an automatic weather station over a playa (the flat floor of an undrained desert basin that becomes at times a shallow lake), approximately 65 km east–west by 130 km north–south, located at the U.S. Army Dugway Proving Ground (40°08′N, 113°27′W, 1124 m above mean sea level) in north-western Utah, U.S.A., in 1999. This station measured the incoming (Rsi) and outgoing (Rso) solar or shortwave radiation using two CM21 Kipp & Zonen pyranometers (one inverted), the incoming (Rli or atmospheric) and outgoing (Rlo or terrestrial) longwave radiation, using two CG1 KippZonen pyrgeometers (one inverted), and the net (Rn) radiation using a Q*7 net radiometer (Radiation Energy Balance System, REBS). We also measured the 10-m wind speed (U10) and direction (R.M. Young wind monitor) and precipitation (Campbell Sci., Inc.). The measurements were taken every 2 s, averaged into 20-min, continuously, throughout the year. The annual (August 1999 – August 2000) comparisons of global or solar radiation and windiness with two other stations in central (Hunter) and northern (Logan) Utah, indicate higher solar radiation (Rsi,Dugway=7797 MJ m−2 period−1vs. Rsi, Hunter=7021 MJ m−2 period−1 and Rsi, Logan=6865 MJ m−2 period−1) and much higher annual mean windiness (UDugway=387 km day−1vs. UHunter=275 km day−1 and ULogan=174 km day−1) throughout the period over the playa. These data reveal the possibility of simultaneously harvesting these two sources of clean energies at this vast and uniform playa.  相似文献   

5.
Cactus seedlings often establish under nurse plants which modify environmental conditions by increasing moisture and decreasing solar radiation, which may cause beneficial and detrimental effects, respectively, on seedling growth. Three soil moisture treatments (5%, 25% and 60%) and two solar radiation levels (100% exposure=243 μmol m−2 s−1, and 40%=102 μmol m−2 s−1) were used in a factorial design to analyze seedling growth response of three rare cactus species (Mammillaria pectinifera, Obregonia denegrii and Coryphantha werdermannii). The variables evaluated were relative growth rate (RGR), root/shoot ratio (R/S), and K (RGRroots/RGRshoot), obtained from an initial seedling harvest (6-month-old seedlings) and a final harvest 6 months after treatment application. All three species had slow RGRs (0.002–0.012 g g−1 day−1). O. denegrii had the lowest RGR values, but was the only species for which R/S and K varied with soil moisture. While all seedlings responded markedly to soil moisture, no response was observed to radiation treatments. The latter might have been related to the relatively low solar radiation levels present in the greenhouse. Yet, our results suggest that the main benefit nurse plants offer to seedlings is the increase in soil moisture.  相似文献   

6.
We explore the response of bedrock streams to eustatic and tectonically induced fluctuations in base level. A numerical model coupling onshore fluvial erosion with offshore wave‐base erosion is developed. The results of a series of simulations for simple transgressions with constant rate of sea‐level change (SLR) show that response depends on the relative rates of rock uplift (U) and wave‐base erosion (?w). Simple regression runs highlight the importance of nearshore bathymetry. Shoreline position during sea‐level fall is set by the relative rate of base‐level fall (U‐SLR) and ?w, and is constant horizontally when these two quantities are equal. The results of models forced by a realistic Late Quaternary sea‐level curve are presented. These runs show that a stable shoreline position cannot be obtained if offshore uplift rates exceed ?w. Only in the presence of a relatively stable shoreline position, fluvial profiles can begin to approximate a steady‐state condition, with U balanced by fluvial erosion rate (?f). In the presence of a rapid offshore decrease in rock‐uplift rate (U), short (～5 km) fluvial channels respond to significant changes in rock‐uplift rate in just a few eustatic cycles. The results of the model are compared to real stream‐profile data from the Mendocino triple junction region of northern California. The late Holocene sea‐level stillstand response exhibited by the simulated channels is similar to the low‐gradient mouths seen in the California streams.  相似文献   

7.
As an approach to understand how diurnal and seasonal plant water potentials (Ψ) are related to soil water-content and evaporative demand components, the responses of six thornscrub shrubs species (Havardia pallens, Acacia rigidula, Eysenhardtia texana, Diospyros texana, Randia rhagocarpa, and Bernardia myricaefolia) of the north-eastern region of Mexico to drought stress were investigated during the course of 1 year. All study species showed the typical diurnal pattern of variation in Ψ. That is, Ψ decreased gradually from predawn (Ψpd) maximal values to reach minima at midday (Ψmd) and began to recover in the late afternoon. On a diurnal basis and with adequate soil water-content (>0.20 kg kg−1), diurnal Ψ values differed among shrub species and were negatively and significantly (p<0.001) correlated with air temperature (r=−0.741 to −0.883) and vapor pressure deficit (r=−0.750 to −0.817); in contrast, a positive and significant (p<0.001) relationship was found to exist with relative humidity (r=0.758–0.842). On a seasonal basis, during the wettest period (soil water-content>0.20 kg kg−1), higher Ψpd (−0.10 MPa) and Ψmd (−1.16 MPa) values were observed in R. rhagocarpa, whereas lower figures (−0.26 and −2.73 MPa, respectively) were detected in A. rigidula. On the other hand, during the driest period (soil water-content<0.15 kg kg−1), Ψpd and Ψmd values were below −7.3 MPa; i.e. when shrubs species faced severe water deficit. Soil water-content at different soil layers, monthly mean relative humidity and monthly precipitation were significantly correlated with both Ψpd (r=0.538–0.953; p<0.01) and Ψmd (r=0.431–0.906; p<0.05). Average soil water-content in the 0–50 cm soil depth profile explained between 70% and 87% of the variation in Ψpd. Results have shown that when gravimetric soil water-content values were above 0.15 kg kg−1, Ψpd values were high and constant; below this threshold value, Ψ declined gradually. Among all shrub species, A. rigidula appeared to be the most drought tolerant of the six species since during dry periods it tends to sustain significantly higher Ψpd in relation to B. myricaefolia. The remaining species showed an intermediate pattern. It is concluded that the ability of shrub species to cope with drought stress depends on the pattern of water uptake and the extent to control water loss through the transpirational flux.  相似文献   

8.
Terrestrial Laser Scanning of grain roughness in a gravel-bed river   总被引：2，自引：1，他引：1  
This paper demonstrates the application of Terrestrial Laser Scanning (TLS) to determine the full population of grain roughness in gravel-bed rivers. The technique has the potential to completely replace the need for complex, time-consuming manual sampling methods. Using TLS, a total of 3.8 million data points (mean spacing 0.01 m) were retrieved from a gravel bar surface at Lambley on the River South Tyne, UK. Grain roughness was extracted through determination of twice the local standard deviation (2σz) of all the elevations in a 0.15 m radius moving window over the data cloud. 2σz values were then designated to each node on a 5 cm regular grid, allowing fine resolution DEMs to be produced, where the elevation is equivalent to the grain roughness height. Comparisons are made between TLS-derived grain roughness and grid-by-number sampling for eight 2 m2 patches on the bar surface. Strong relationships exist between percentiles from the population of 2σz heights with measured a-, b-, and c-axes, with the closest matches appearing for the c-axis. Although strong relationships exist between TLS-derived grain roughness (2σz), variations in the degree of burial, packing and imbrication, results in very different slope and intercept exponents. This highlights that conventional roughness measurement using gravel axis length should be used with caution as measured axes do not necessarily represent the actual extent to which the grain protrudes into the flow. The sampling error inherent in conventional sampling is also highlighted through undertaking Monte Carlo simulation on a population of 2000 clasts measured using the grid-by-number method and comparing this with the TLS-derived population of grain roughness heights. Underestimates of up to − 23% and overestimates of up to + 50% were found to occur when considering the D84, and − 20% and overestimates of up to + 36% were found to occur when considering the D50.  相似文献   

9.
The migration of a small slipfaceless dome dune close to the northern edge of the Namib Sand Sea has been measured by topographic survey. The dune dimensions are 45-m wide and 1-m high with a volume of 551 m3 that has been calculated as the difference between the dune's surface elevation and an interdune surface extrapolated from measurements around the edge of the dune. The migration direction, 64°, and distance moved, 90 m, are measured against stakes set out in 1976. The dune has moved about 90 m between 1976 and 1999. This is an average linear migration rate of around 4 m year−1, and is equivalent to an annual sand transport rate of about 1.2 tonnes m−1 year−1. The calculated total potential sand flow in this part of the Namib Desert is 119 tonnes m−1 year−1, and the resultant potential sand flow is 63 tonnes m−1 year−1. The dune migration is therefore about 1% of the total potential sandflow and 2% of the resultant indicating that dune migration is only a small part of total potential sand transport. The results suggest that small slipfaceless dome dunes are very inefficient at trapping sand, and that winds blowing across the interdune in this area are undersaturated with sand.  相似文献   

10.
The natural abundance15N/14N method was used to estimate the influence of silvicultural and P fertilization treatments on N accretion, N2fixation and N partitioning among tissues in a mature mesquiteProsopis glandulosavar.glandulosastand in Texas. The silvicultural treatments consisted of understory removal, herbicide treatment of brushy resprouts, thinning trees to single stems and 100 kg ha−1P fertilization. The trees had a mean basal diameter of 17·8 cm with 8 to 35 cm range. The stand was slow growing with the increase in dry matter ranging from 0·465 Mg ha−1year−1to 0·701 Mg ha−1year−1for the 8 years after the treatments were applied. N accretion after 8 years ranged from 3·1 kg ha−1year−1to 4·4 kg ha−1year−1.Due to the range in δ15N of the leaves, twigs, branches and trunk, we used the weighted (by biomass) average δ15N per tree in calculations of the percent N derived from N2fixation (%Ndfa). There was considerable variability in δ15N of the reference plants, i.e. from 3·3 to 5·9. In contrast there was low variability in the background δ15N of nearby soils (7·0±1·0). As the total above-ground biomass δ15N of a grass grown outside the influence of mesquite (7·8±0·58) had the same δ15N as the soil (7·5±1·0), we used the grass outside the influence of mesquite and the weighted tree mean δ15N to calculate % of N derived from N2fixation.The decrease in intraspecific competition by thinning multistemed trees to single stemmed trees was the only treatment that significantly (p= 0·0001) increased growth. Interspecific competition, i.e. understory removal, did not increase growth. There were no significant differences in total N production or N fixation among treatment means. The most striking result was the highly positive correlation between tree δ15N and total N per tree and biomass per tree (R2= 0·90,F= 164·4, df. = 18, mean square error (MSE) = 0·155,p= 0·0001). This implies that the younger trees colonizing infertile soils relied more heavily on N2fixation than larger trees which accumulated 1200 kg ha−1more N under their canopies. The percentage N derived from N2fixation ranged from 63 to 73% in the various treatments. Despite the high percentage of N derived from N2fixation, the N2fixation of the stand was very low, i.e. 1·98 to 2·80 kg N ha−1year−1, due to the low growth of the stand. We believe that comparisons of the whole tree weighted δ15N to background soil δ15N provides a more reasonable approach to estimate % N2fixation than comparisons of leaves of fixers and reference plants.  相似文献   

11.
The formation of lahars and a debris avalanche during Holocene eruptions of the Spurr volcanic complex in south-central Alaska have led to the development of volcanic debris dams in the Chakachatna River valley. Debris dams composed of lahar and debris-avalanche deposits formed at least five times in the last 8000–10,000 years and most recently during eruptions of Crater Peak vent in 1953 and 1992. Water impounded by a large debris avalanche of early Holocene (?) age may have destabilized an upstream glacier-dammed lake causing a catastrophic flood on the Chakachatna River. A large alluvial fan just downstream of the debris-avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood. Application of a physically based dam-break model yields estimates of peak discharge (Qp) attained during failure of the debris-avalanche dam in the range 104<Qp<106 m3 s−1 for plausible breach erosion rates of 10–100 m h−1. Smaller, short-lived, lahar dams that formed during historical eruptions in 1953, and 1992, impounded smaller lakes in the upper Chakachatna River valley and peak flows attained during failure of these volcanic debris dams were in the range 103<Qp<104 m3 s−1 for plausible breach erosion rates.Volcanic debris dams have formed at other volcanoes in the Cook Inlet region, Aleutian arc, and Wrangell Mountains but apparently did not fail rapidly or result in large or catastrophic outflows. Steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams.  相似文献   

12.
This paper presents a detailed cost accounting of the processes involved in the production of flours from the mesocarps of the fruits of Prosopis alba and P. pallida to be used in human foods. The flours from both species are similar and contain 7–8% protein, 0·8–2% fat, 26–32% total dietary fiber and 48–59% sucrose. The data were based on actual production rates in use at the Universidad de Piura pilot plant using simple machinery. Machinery hourly charges were based on equivalent rental costs in order to avoid interest charge calculations. As opposed to other analyses which only included processing costs delivered to the plant, our analysis ascribed value to the owner of the trees in order to provide incentives to plant and care for the trees. The sensitivity analyses showed that the most economically important factor was the conversion efficiency of pods into flour. When a 40% conversion efficiency, such as that which has been obtained in both Peru and Argentina was obtained, and by-product credits for high fiber containing fraction were assumed, the wholesale selling price with a 5% marketing cost and a 40% profit direct costs was estimated to be $0·66 kg−1 for Peru (using a $0·40 h−1 labor rate ) and $1·76 kg−1 for Argentina (using a $1·5 h−1 labor rate).  相似文献   

13.
Piñon (Pinus edulis)-juniper (Juniperus monosperma)-ecosystems increased substantially in the western USA during the 20th century. Sustainability of these ecosystems primarily depends on soil quality and water availability. This study was undertaken with the objective of assessing the effect of tree species on soil physical quality in a semi-arid region in the western part of Sugarite Canyon, northeast of Raton, Colfax County, NM (37°56′32″N and 104°23′00″W) USA. Three cores and three bulk soil samples were obtained from the site under the canopy of three juniper, Gambel oak (Quercus gambelii) and piñon trees for 0–10 and 10–20 cm depths. These samples were analyzed for particle size distribution, soil bulk density (ρb), water stable aggregation (WSA), mean weight diameter (MWD) of aggregates, pH, electrical conductivity (EC) and soil organic carbon (SOC) and total nitrogen (TN) concentrations and stocks. Sand content was greater under juniper (48%) than oak (32%), whereas clay content followed the opposite trend. The ρb, WSA, MWD, pH and EC were similar under juniper, piñon, oak canopies for both depths. Estimated (from Philip and Green and Ampt infiltration models) and measured water infiltration parameters did not vary among these sites and were in accord with the values for ρb, WSA and MWD. The SOC concentrations and stocks were greater under oak (43.1 Mg ha−1 for 0–10 and 37.5 Mg ha−1 for 10–20 cm depths) than piñon (23.3 Mg ha−1 for 0–10 and 18.5 Mg ha−1 for 10–20 cm depths). The TN concentrations were greater under oak (3.4 g kg−1) than piñon (1.7 g kg−1) for the 0–10 cm depth only. Accumulation of detritus material under tree canopies reduced soil compaction and crusting caused by raindrop impact and increased SOC, and TN concentrations, and water infiltration. Coefficients of variation ranged from low to moderate for most soil properties except infiltration rate at 2.5 h, which was highly variable. Overall, soil quality for each site was good and soil aggregation, water infiltration and SOC concentrations were high, and soil ρb was low.  相似文献   

14.
Wind erodibility of major soils in the farming-pastoral ecotone of China   总被引：2，自引：0，他引：2  
Wind erosion and desertification are severe problems in China's farming-pastoral ecotone. In this study, wind erodibility of five major soils in both uncultivated and simulated cultivated conditions, were determined through wind tunnel tests at nine wind speeds ranging from 10 to 26 m s−1. The average wind erosion rate (g m−2 min−1) under the uncultivated condition (q0) for the five soils could be set in the order: chestnut soil (28.5)>brown soil (24.8)>sierozem (21.8)>chernozem (19.9)>fixed sandy soil (11.4). The highest natural wind erosion might take place in the semi-arid steppe zone where the Chestnut soils predominate. Cultivation can significantly accelerate wind erosion, the mean wind erosion rate under the cultivated condition (qc) for all five soils was 743.7 g m−2 min−1 in the following order: sandy soil (3313.2)>brown soil (227.2)>chernozem (221.8)>sierozem (85.1)>chestnut soil (81.2). For both the uncultivated and cultivated soil samples, the relationship between wind erosion rate (q) and wind speed (U) could be expressed in general as q=A eBU (A and B are constant coefficients). There was a critical wind speed for each soil type except for the sandy soil. Below the critical wind speed, cultivation reduced wind erosion rate possibly due to soil clodiness and roughness effects. Above the critical speed, cultivation greatly intensified wind erosion rates due to the break down of the original soil structure. The critical wind speed measured at 20 cm above the soil surface was 20 m s−1 for the brown soil, 14 m s−1 for chernozem and the chestnut soils, and 10 m s−1 for the sierozem. Among the five tested soils, the high wind erosion rate of the cultivated sandy soil showed its extreme sensitivity to cultivation, possibly because of the structureless nature of the loose sand. The “effect of cultivation on wind erosion” index, η (=qc/q0), increased exponentially with the increase of wind speed, indicating that under higher wind speed conditions, cultivation could result in more severe wind erosion.  相似文献   

15.
Studies addressing within-lake variability of fossil chironomid assemblages are very few, and all deal with hydrologically stable temperate lakes where the question of spatial integration mostly relates to the mixing of faunal assemblages associated with shallow, warm-water habitat and those associated with deeper, cold-water habitat. Here we study within-lake variability of surface-sediment chironomid assemblages in the fairly large (∼100–170 km2 since 1983) and shallow (Z max = 5–8 m) fluctuating tropical lake basin of Lake Naivasha, Kenya, and compare the patterns observed with those in two smaller adjacent basins, one similarly shallow (Lake Oloidien, 5.1–5.7 km2, 5–8 m), the other deep and stratified (Crescent Island Crater, 1.9 km2, 14–17 m). Chironomid assemblages were analysed in core-top samples and surface sediments along inshore to offshore transects, and how well individual samples represented the total (basin-wide mean) subfossil assemblage was considered both in terms of taxon richness and taxon percent composition. Within-lake variability of subfossil chironomid concentrations (with generally higher absolute values in nearshore samples) could be explained by effects of sediment winnowing and focusing, whereas between-lake variability reflected their relative susceptibility to wind-driven sediment disturbance or bottom anoxia. In all study lakes, but most significantly in lakes Naivasha and Oloidien, species distribution in the subfossil chironomid assemblages showed a strong nearshore to offshore gradient, which in these shallow lakes, reflects the dominant control of substrate and food quality on species distribution in the living community. Particularly in the larger basins, nearshore samples better represented the total lake assemblage than offshore samples, because the former always contained a component of mud-dwelling species whereas the latter often lacked a component of macrophyte-dwelling species. Our results show that although sedimentation dynamics in the shallow, wind-stressed Lake Naivasha is dominated by frequent resuspension and random sediment redistribution, the near- to offshore gradient in chironomid habitat remains imprinted on subfossil assemblages. We conclude that also in shallow fluctuating lakes, given sufficient size, incomplete pre-burial spatial integration of habitat-specific chironomid assemblages can be exploited for within-lake calibration of environmental gradients.  相似文献   

16.
We investigated the effects of soil water potential (SWP) and temperature on seed germination of six coexisting species of an inter-tropical desert. These species include three life-forms; the shrubs Cercidium praecox andProsopis laevigata ; the columnar succulents Neobuxbaumia tetetzo and Pachycereus hollianus; and the arborescent semi-succulents Beaucarnea gracilis and Yucca periculosa. In the six species germinability increased and germination time and speed of germination (t50) decreased as SWP decreased. The SWP treatments were 0 MPa, −0·12 MPa, −0·2 MPa, −0·41 MPa and −0·66 MPa. There was, however, a SWP threshold below which germination time and t50increased and germinability decreased. The shrubs had the highest germinability whereas the columnar succulents had the lowest. The shrubs also had shorter germination time andt50 than arborescent semi-succulents whereas seeds of the columnar succulents were indeterminate. In all species except P. laevigata germinability increased and the germination time and t50decreased as temperature increased. The temperature treatments were 12°C, 20°C and 26°C. The shrubs had the shortest t50and germination time and the highest germinability at all temperatures. Arborescent semi-succulents had the lowest germinability and longest germination time and t50at the three temperatures treatments. Our results support the hypothesis that in desert environments different plant life-forms utilise different germination strategies to persist.  相似文献   

17.
We designed, constructed, calibrated and field-tested a lightweight (30 kg), 4.2 m diameter, 16.4 m3 polyethylene-covered dome static chamber ecosystem gas exchange cuvette that can quantify ecosystem CO2 and water vapour fluxes as low as 0.1 μmol CO2 m−2 s−1 and 0.1 mmol H2O m−2 s−1 with little impact on environmental conditions. Fluxes measured in May 2001 in an intact Great Basin sagebrush ecosystem at midday were significantly higher than in an adjacent post-wildfire successional ecosystem, with observed ranges from –0.71 to 1.49 μmol CO2 m−2 s−1 for CO2 and from –0.09 to 0.53 mmol H2O m−2 s−1 for water vapour.  相似文献   

18.
The objective of this study is to outline how the ground-water quality is affected by geological factors, soil characteristics and subsurface structure. The results of 25 ground-water samples, and 30 soil samples of which 10 soil samples analysed chemically provide the basis of this study. Procedures to interpret water quality data are based on a combined use of the traditional graphical methods, ratios of Na+/Cl−and Ca2+/Mg2+, multivariate geostatistical methods and computation of the saturation indices (SI) of minerals. The results of soil analyses show that the grain size ranges from very coarse sand to clayey loams, and from well sorted and extremely poorly sorted. The soil salinity increases in old alluvial plains and decreases in the dissected wadis of limestone plateaux, while vertical distribution decreases with depth. It is found that the ground-water is supersaturated with respect to calcite and dolomite, and undersaturated with respect to gypsum. Calculated values of PCO2for the ground-water samples range from 2×10−4to 7×10−3with a mean value of 3×10−3(atm.). This indicates that the ground-water becomes charged with CO2during infiltration processes.  相似文献   

19.
The objective of this research was to study the relationships between environmental factors and vegetation in order to find the most effective factors in the separation of the vegetation types in Poshtkou rangelands of Yazd province. Sampling of soil and vegetation were performed with randomized-systematic method. Vegetation data including density and cover percentage were estimated quantitatively within each quadrat, and using the two-way indicator species analysis (TWINSPAN), and vegetation was classified into different groups. The topographic conditions were recorded in quadrat locations. Soil samples were taken in 0–30 and 30–60 cm depths in each quadrat. The measured soil variables included texture, lime, saturation moisture, gypsum, acidity (pH), electrical conductivity, sodium absorption ratio, and soluble ions (Na+, K+, Mg2+, Cl−, CO32−, HCO3− and SO42−). Multivariate techniques including principal component analysis (PCA) and canonical correspondence analysis (CCA) were used to analyse the collected data. The results showed that the vegetation distribution pattern was mainly related to soil characteristics such as salinity, texture, soluble potassium, gypsum, and lime. Totally, considering the habitat conditions, ecological needs and tolerance range each plant species has a significant relation with soil properties.  相似文献   

20.
Through field rainfall simulation experiments in an upland mountainous watershed of northern Thailand, we have identified two phenomena that increase the potential for Horton overland flow (HOF) generation on agricultural lands. First, there appears to be a transition period of 12–18 months, extending from the time of abandonment until the formation of a dense vegetation layer capable of intercepting rainfall and ponding surface water, during which HOF generation is accelerated. Simulation data indicate these recently abandoned fields may have runoff coefficients (ROCs) as high as 40% during large seasonal storms with wet antecedent soil moisture conditions. In comparison, actively cultivated lands and advanced (>16–18 months) fallow fields, the land surfaces existing before and after the threshold period, have ROCs≤4%. Secondly, compacted path surfaces initiate HOF within agricultural fields, which have saturated hydraulic conductivity (Ks) values that are 100–200 mm h−1 higher. In the study area, path/furrow networks, comprising 8–24% of field surface areas, are designed to provide walking access within fields and channel excess surface flow from the fields. Compared with hoed surfaces, path/furrows reduce the time to HOF generation by about 85% and have ROCs that are six times higher. Access paths have the lowest Ks values of all watershed surfaces, but conveyance efficiency of HOF generated on these surfaces is low. Even recently created field paths are capable of reducing runoff generation time by 40–90% and producing sixfold increases in ROCs. Collectively, the data suggest that agricultural erosion rates are accelerated during the 12–18-month threshold period following abandonment and during storms where path-generated HOF interacts with adjacent planting surfaces. Despite having periods of increased HOF generation, the total HOF contribution from agricultural fields to the basin stream hydrograph is similar in magnitude to that of unpaved roads, which occupy 95% less land area.  相似文献   

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