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1.
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   

2.
Field and laboratory studies were conducted during the period May 1993–April 1994 on the cocoons and hatchlings ofAporrectodea calignosa(Savigny 1826), a common earthworm found in Benghazi, Libya. Cocoon production started in August 1993 and continued until April 1994. A positive correlation between soil moisture (r=0.74;p< 0.05) and cocoon production, and a negative one between soil temperature (r=−0.85;p< 0.05) on cocoon production were found. Mean incubation time was 47 days. Some cocoons contained a single worm (single hatchling) whereas the majority of them (68%) contained two, the first hatchling and second hatchling. The rate of change of body mass (F=2.18;p>0.05) and the average body mass (F=1.25;p>0.05) of the single, first and second hatchlings were similar.  相似文献   

3.
The selective use by cattle of range resources in the Sahel was assessed in terms of quantity and nutritional quality of the herbage grazed and diet selected. Peak standing crop was 1951 kg ha−1dry matter (DM) with 47·9 kg nitrogen (N) ha−1and 7·4 kg phosphorus (P) ha−1. Esophageally fistulated cattle diets were consistently (p<0·05) higher in N, P and organic matter digestibility (OMD) than the herbage grazed. Cattle consumed 48% of the mean annual herbage production. The results indicate grazing ruminants select a higher quality diet from Sahelian rangelands than can be predicted on the basis of pasture evaluation alone. Therefore, the foraging behaviour of the animals needs to be considered in the evaluation of Sahelian rangelands for animal production.  相似文献   

4.
The responses of desert rams to diet composition (concentrate vs. lucerne hay) in unshaded and shaded environments were investigated. The rectal temperature was higher (p < 0·01) with the concentrate diet but only in the afternoon, whereas the respiration rate was higher (p < 0·01) with the concentrate diet in unshaded and shaded environments during both morning and afternoon. Dry matter intake (DMI) was higher (p < 0·05) with the concentrate diet, while the ratio of water intake to DMI was higher (p < 0·01) with lucerne hay. Water gain through preformed and metabolic avenues and water loss through faeces and evaporative avenues were higher (p < 0·01) with the concentrate diet. With both diets, water turnover rate was higher (p < 0·05) in the unshaded environment. Feeding lucerne hay increased plasma total protein (p < 0·01) and albumin (p < 0·05) in unshaded and shaded environments. Plasma urea was higher (p < 0·01) with the concentrate diet. Plasma glucose level and plasma osmalality were higher (p < 0.01) and level of free fatty acids was lower (p < 0·01) with the concentrate diet in both thermal environments. Plasma cholesterol level was higher (p < 0·05) with the concentrate diet in the shaded environment, but lower (p < 0·05) in the unshaded environment. With lucerne hay, plasma cholesterol level remained the same in both environments.  相似文献   

5.
Forage availability for mule deer (Odocoileus hemionus) in the Sonoran Desert depends on plant biomass, which is influenced by rainfall. We determined how rainfall, temperature and plant characteristics affected biomass of deer forage. We measured forage biomass, rainfall and temperature every 3 months from April 2000 to December 2002. Quarterly rainfall ranged from <1 to 57 mm, and forage biomass in desert washes fluctuated between 6 and 34 g m−2. There was a positive relationship between forage biomass and rainfall the previous six months (p<0.001), and a negative relationship between biomass and average temperature the previous 3 months (p<0.001). Quarterly forage growth was positively influenced by rainfall (p<0.001) and negatively influenced by forage biomass (p<0.001). The relationships between deer forage and environmental factors established here will be useful in understanding population ecology of mule deer as part of an interactive model of plant–herbivore dynamics in arid environments.  相似文献   

6.
The objectives of this study were to determine the composition, density and spatial distribution of the soil seed bank of woody species, as well as their regeneration pattern in two different land use systems, controlled (ranch) and open grazing, in an Acacia woodland of the Rift Valley in Ethiopia. We also compared the species composition of the soil seed bank and the above-ground vegetation to find out if differences exist in the soil seed bank and advance regeneration between the two land use systems. The germination requirements of seeds of the woody species were also investigated under laboratory conditions. Acacia senegal, Acacia seyal, Acacia tortilis, Dichrostacys cinerea and Balanites aegyptiaca were encountered in the above-ground vegetation in both systems. Seeds of only A. tortilis (90±32 seeds m−2) from the ranch, and A. senegal (5±3 seeds m−2) and A. tortilis (72±34 seeds m−2) from the open system were found in the soil seed bank along transects with patchy horizontal pattern. The two systems were not significantly different in density of soil seed banks of A. senegal andA. tortilis . Jaccard's similarity index showed that only a few woody species were common in the soil seed flora and above ground vegetation. However, all of the species accumulated seeds (58±43–331±130 seeds m−2) in/on the soil under the canopy. Very large numbers of seeds of A. tortilis (19382±9722 seeds m−2) and D. cinerea (1278±494 seeds m−2) were also found in barns. Most of the seeds recovered from the soil samples (60–80%) were found in the litter layer. Acid and mechanical scarification improved legume germination (36–99% and 60–99%, respectively) over boiling water (0–48%). Treatment means differed significantly for all the legumes (p<0·001) but not for Balanites. Height and diameter class distribution of regeneration of A. tortilis and D. cinerea in both systems and A. senegal in the open system had a negative exponential correlation (rs=−0·5, −0·25 and −0·86, respectively). A. seyal and B. aegyptiaca showed poor regeneration. Horizontal distribution of advance regeneration of all the species was patchy. Advance regeneration of A. seyal, A. tortilis and B. aegyptiaca were not significantly different, while that of A. senegal and D. cinerea were significantly different between the two systems. Poor representation of species in the soil seed bank along transects and in the different height and diameter classes may be attributed to the low density of mature trees as well as the mode and strategy of seed dispersal. Ungulate and wind dispersed species (e.g. A. senegal, A. tortilis and D. cinerea) were highly favoured. Patchiness in the distribution of seeds and advance regeneration was also a result of endozoochory. Dispersal of non-ungulate dispersed seeds (A. senegal, A. seyal and B. aegyptiaca) was restricted to the canopy zone. Piles of seeds ofA. tortilis and D. cinerea that were found in barns were a result of consumption of their pods by cattle. High concentration of seeds in the litter layer may be due to low soil disturbance and larger size of seeds. The height and diameter class distribution of A. senegal (in the open system), A. tortilis and D. cinerea also indicated that the species have good regeneration. Results from the germination tests indicated that seeds of the legumes require pre-sowing treatments to give a rapid, uniform and improved germination. Intervention through artificial regeneration should be employed to improve the density and regeneration capacity of those species with hampered regeneration at both systems.  相似文献   

7.
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   

8.
Three-week old soybean (Glycine max) plants were subjected to a factorial combination of four regimes of soil matric water potential (ψm=−0·03, −0·5, −1·0 and −1·5 MPa), two levels of supplementary Zn (O and 20 mgl−1) and two levels of foliar IAA application (O and 10 mgl−1). Under control conditions (no Zn, no IAA), increasing soil drying progressively retarded shoot and root growth (length and dry mass production), reduced leaf relative water content (RWC) and decreased the contents of chlorophyll (Chl) and shoot soluble sugars (SS), but increased soluble sugar content of roots and lowered osmotic water potential of shoots and roots (osmotic adjustment). Total free amino acid (TAA) content increased in shoots but decreased in roots whereas contents of soluble proteins (SP) decreased in shoots and roots. The effect of water stress was statistically significant (p<0·05) and had a major effect (as indicated by η2values) on leaf RWC, shoot and root dry masses and osmotic potential. Supplementary Zn improved root growth at all levels of stress and shoot growth under severe stress. Improvement of growth was positively correlated with the internal tissue Zn concentrations (r=0·91 and 0·86 for shoot and 0·94 and 0·82 for root length and dry mass respectively). Exogenous IAA raised (p<0·05) RWC, Chl, DM (slightly), root SS, and SP, whereas shoot TAA was lowered. Effects on root TAA and shoot SS were more complex: they were lowered at zero stress and raised under severe stress. IAA and Zn in combination had additive effects on Chl, growth and osmotic potential, but their combined effects on SP and TAA were more complex. It is concluded that the treatment of soybean plants grown under conditions of low soil water potentials and Zn deficiency with Zn and IAA solutions counteracted the deleterious effects of stress, especially at high stress levels, and helped stressed plants to grow successfully under these adverse unfavourable conditions.  相似文献   

9.
We investigate the risk that supplemental feeding of white-tailed deer (Odocoileus virginianus) to improve trophy quality may degrade rangeland through overutilization of forage plants near the feeders. We divided a South Texas ranch into 6 areas (each 1100 ha). Supplemental feed was provided year-round in a centrally placed feeder in 3 areas, no supplement was provided in the 3 control areas. Radio-telemetry collars were fitted on 2 bucks and 2 does in each area. Spatial distribution of these deer was assessed using the CALHOME program. Feed supplementation did not alter home range size of does but did reduce their 50% core area. Bucks without supplemental feed showed 2 spatial strategies; sedentary bucks had home range distributions similar to supplemented bucks, while commuter bucks moved to supplemented areas during the breeding season. Browsing pressure near feeders and control sites was assessed by measuring consumption of seedlings set in arrays radiating from feeders and control points. Deer browsing was heavier near feeders than at the control sites, and the probability of a seedling being browsed increased with proximity to the feeder. We caution against long-term supplemental feeding in fixed locations because of the potential for localized range degradation around the feeders.  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.
A principal task of evaluating large wildfires is to assess fire's effect on the soil in order to predict the potential watershed response. Two types of soil water repellency tests, the water drop penetration time (WDPT) test and the mini-disk infiltrometer (MDI) test, were performed after the Hayman Fire in Colorado, in the summer of 2002 to assess the infiltration potential of the soil. Remotely sensed hyperspectral imagery was also collected to map post-wildfire ground cover and soil condition. Detailed ground cover measurements were collected to validate the remotely sensed imagery and to examine the relationship between ground cover and soil water repellency. Percent ash cover measured on the ground was significantly correlated to WDPT (r = 0.42; p-value < 0.0001), and the MDI test (r = − 0.37; p-value < 0.0001). A Mixture Tuned Matched Filter (MTMF) spectral unmixing algorithm was applied to the hyperspectral imagery, which produced fractional cover maps of ash, soil, and scorched and green vegetation. The remotely sensed ash image had significant correlations to the water repellency tests, WDPT (r = 0.24; p-value = 0.001), and the MDI test (r = − 0.21; p-value = 0.005). An iterative threshold analysis was also applied to the ash and water repellency data to evaluate the relationship at increasingly higher levels of ash cover. Regression analysis between the means of grouped data: MDI time vs. ash cover data (R2 =0.75) and vs. Ash MTMF scores (R2 = 0.63) yielded significantly stronger relationships. From these results we found on-the-ground ash cover greater than 49% and remotely sensed ash cover greater than 33% to be indicative of strongly water repellent soils. Combining these results with geostatistical analyses indicated a spatial autocorrelation range of 15 to 40 m. Image pixels with high ash cover (> 33%), including pixels within 15 m of these pixel patches, were used to create a likelihood map of soil water repellency. This map is a good indicator of areas where soil experienced severe fire effects—areas that likely have strong water repellent soil conditions and higher potential for post-fire erosion.  相似文献   

12.
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   

13.
The profile distribution of total, diethylene triamine penta-acetic acid (DTPA)- and 0·1 HCl-extractable Fe and Mn were determined in 12 pedons formed on three contiguous sand dunes in the semi-arid savanna of Nigeria. The total Fe and Mn contents varied from 100 to 3750 and 40 to 11,375 μg g−1, respectively. Values of 0·1 HCl-extractable Fe and Mn varied from 8·0 to 123 and 1·5 to 43·5 μg g−1, respectively. The corresponding values of DTPA-extractable Fe and Mn were 4·5 to 16·0 and 1·0 to 38·8 μg g−1. Total Fe and Mn correlated significantly with clay in nearly all the dunes (p≤ 0·01) but were not significantly correlated with organic matter. The 0·1 HCl-extractable Mn had a positive significant correlation with soil pH (r= 0·58*) in Illela dunes but a negative significant correlation with pH in the Sangiwa dunes (r= −0·75***). The values of extractable Fe and Mn in the sand dune soils are above the critical limits reported in other literature, thus deficiencies of these micronutrients do not pose a problem to crop production in the study area.  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.
Sediment rating parameters and their implications: Yangtze River, China   总被引：4，自引：0，他引：4  
This study examines the characteristics of sediment rating parameters recorded at various gauging stations in the Yangtze Basin in relation to their controls. Our findings indicate that the parameters are associated with river channel morphology of the selected reaches. High b-values (> 1.600) and low log(a) values (< − 4.000) occur in the upper course of the steep rock-confined river, characterizing high unit stream power flows. Low b-values (< 0.900) and high log(a) values (> − 1.000) occur in the middle and lower Yangtze River associated with meandering reaches over low gradients, and can be taken to imply aggradation in these reaches with low stream power. Higher b-values (0.900–1.600) and lower log(a)-values (− 4.000 to − 1.000) characterize the reaches between Yichang and Xinchang, immediately below the Three Gorges. These values indicate channel erosion and bed instability that result from changes in channel gradient from the upstream steep valley to downstream low slope flood plain settings. Differences in channel morphology accompany these changes. Confined, V-shaped valleys occur upstream and are replaced downstream by broad U-shaped channels. The middle and lower Yangtze shows an apparent increase in channel instability over the past 40 years. This inference is based on sediment rating parameters from various gauging stations that record increasing b-values against decreasing log(a)-values over that time. Analysis of the sediment load data also reveals a strong correlation between changes in sediment rating curve parameters and reduction of annual sediment budget (4.70 × 108 t to 3.50 × 108 t/year, from the 1950s to 1990s), largely due to the damming of the Yangtze and sediment load depletion through siltation in the Dongting Lake. Short-term deviations from the general trends in the sediment rating parameters are related to hydroclimatic events. Extreme low b-values and high log(a)-values signify the major flood years, while the reverse indicates drought events. When compared with rivers from other climate settings, it is evident that the wide range of values of the Yangtze rating parameters reflects the huge discharge driven by the monsoon precipitation regime of eastern China.  相似文献   

16.
Downward material fluxes under seasonal sea ice were measured using a time-series sediment trap installed at an offshore site in the Okhotsk Sea north of Hokkaido, Japan, from 13 January to 23 March 2005. The maximum fluxes of lithogenic material (753 mg m−2 day−1) and organic matter (mainly detritus; 333 mg m−2 day−1) were recorded during the period in which sea ice drifted ashore and increased in extent, from 13 January to 9 February. Organic matter as fecal pellets (81–93 mg m−2 day−1) and opal as biosilica (51–67 mg m−2 day−1), representing diatom fluxes, were abundant in sediment trap samples obtained during the period of full sea ice coverage from 10 February to 9 March. Microscopic observations revealed that fecal pellets were largely diatom frustules, suggesting that zooplankton actively grazed on ice algae during the period of full sea ice coverage. During the period of retreating sea ice, from 10 to 23 March, the phytoplankton flux showed a rapid increase (from 9.5 to 22.5 × 106 cells m−2 day−1), reflecting their release into the water column as the sea ice melted. Our results demonstrate that the quantity and quality of sinking biogenic and lithogenic materials vary with the seasonal extent of sea ice in mid-winter.  相似文献   

17.
In this study, an attempt has been made to evaluate the temporal variations in specific stream power and the total energy available for geomorphic work during the monsoon season for the Tapi River, in central India. Continuous daily discharge data (1978–1990), hydraulic geometry equations and the relationship between discharge and water surface slope were used to compute the daily specific stream power (ω) for the Savkheda gauging site in the lower Tapi Basin. The total amount of energy generated by all the monsoon flows was estimated by integrating the area under the ω-graph derived for the monsoon season.The analyses of the 13-year daily discharge data reveal that the average and maximum ω values range from 4–20 W m− 2, and 22–964 W m− 2 respectively. Specific stream power duration curve derived for the site shows that for 25% of the time the power per unit area is > 10 W m− 2. Furthermore, unit stream power was found to be above the Williams' [Williams, G.P., 1983. Paleohydrological methods and some examples from Swedish fluvial environments. I. Cobble and boulder deposits. Geografiska Annaler 65A, 227–243.] threshold of pebble-movement (1.5 W m− 2), cobble-movement (16 W m− 2) and boulder-movement (90 W m− 2) for 71%, 15% and 2% of the time, respectively. Computations further indicate that the total amount of energy generated by the flows during the monsoon season is in the range of 37 MJ (deficit monsoon years) to 256 MJ (excess monsoon and/or flood years). Large floods have one-third share in the total monsoon energy expenditure. In the absence of appropriate data on the yearwise geomorphic effects, the geomorphic work was evaluated in terms of the total suspended sediment load transported. The total monsoon sediment load is strongly related to the total monsoon energy. The results of the study indicate that the average flow competence and capacity are remarkably higher during wetter monsoon seasons and flood years than during the shorter and drier monsoon seasons.The present analyses demonstrate that the flows are geomorphically effective for a greater part of the monsoon season, except during the deficient monsoon years, and there is little doubt that large-magnitude floods are effective agents of geomorphic change in monsoonal rivers.  相似文献   

18.
An experiment was carried out at Nahshala Farm, north west of Al-Ain City, U.A.E. during the 1998–2000 growing seasons, using six halophytes (Batis maritima, Distichlis spicata, Juncus roemerianus, Paspalum vaginatum, Salicornia bigelovii and Spartina alterniflora) and two levels of leaching fraction (0·25 and 0·50) under three irrigation salinity levels (10, 20, and 40 gL−1) in a randomized complete block design arranged in split plots. The purpose of the research was to apply the theory of crop salt tolerance on aboveground yields and agronomic characteristics of halophytes to determine their thresholds for salinity and to determine leaching requirements. The results indicated that the halophyte species tested can grow with minimum reduction in the growth potential at <20 gL−1of mean salinity of soil solution. Leaching fraction (LF) of 0·25 at the highest salinity of irrigation water (40 gL−1) was inadequate to attain the steady-state salt balance during the growth period, although the drainage salinity reached more than 90 g L−1. Furthermore, if the same level of LF is used for longer period, soil salinity under this high salt treatment will continue to rise, and plant growth may deteriorate. Leaching fraction of 0·50 is preferable if salinity of irrigation water was more than 20 gL−1and dry matter production is considered, although the amount of water use will be excessive  相似文献   

19.
Cultivation, overgrazing, and overharvesting are seriously degrading forest and grassland ecosystems in the Taurus Mountains of the southern Mediterranean region of Turkey. This study investigated the effects of changes on soil organic carbon (SOC) content and other physical soil properties over a 12-year period in three adjacent ecosystems in a Mediterranean plateau. The ecosystems were cropland (converted from grasslands in 1990), open forest, and grassland. Soil samples from two depths, 0–10 and 10–20 cm, were collected for chemical and physical analyses at each of cropland, open forest, and grassland ecosystems. SOC pools at the 0–20 cm depth of cropland, forest, and grassland ecosystems were estimated at 32,636, 56,480, and 57,317 kg ha−1, respectively. Conversion of grassland into cropland during the 12-year period increased the bulk density by 10.5% and soil erodibility by 46.2%; it decreased SOM by 48.8%, SOC content by 43%, available water capacity (AWC) by 30.5%, and total porosity by 9.1% for the 0–20 cm soil depth (p<0.001). The correlation matrix revealed that SOC content was positively correlated with AWC, total porosity, mean weight diameter (MWD), forest, and grassland, and negatively with bulk density, pH, soil erodibility factor, and cropland. The multiple regression (MLR) models indicated that any two of the three ecosystems and one of the two soil depths accounted for 86.5% of variation in mean SOC values ((p<0.001).  相似文献   

20.
Impacts of six initial treatments and rainfall on maximum above-ground herbaceous standing crop were assessed in 1978 and annually from 1984 to 1992 in rangelands of east-central semi-arid Argentina. Treatments applied in 1977–1978 were: (1) untreated control; (2) burning; (3) herbicide application (shrub control); (4) Old Field 1 or (5) Old Field 2 (areas previously exposed to different degrees of mechanical soil disturbance for 25 years); and (6) overgrazing. Domestic herbivores were excluded thereafter from all treatments until 1993.During 1984–1992, total above-ground herbaceous standing crop often remained greater in both Old Fields than in the other treatments; highest mean values were 2336 and 1640 kg ha−1for Old Field 1 and Old Field 2 treatments, respectively. Lowest total above-ground standing crops in all treatments (M = 296–475 kg ha−1) occurred in 1989, the year with the lowest annual rainfall (257·5 mm). Desirable perennial grasses contributed most (44–100%) to total herbaceous standing crop in all treatments during the study period. Most of this standing crop was made up of the cool-season grassesPiptochaetium napostaense, Poa ligularis, Stipa clarazii, S. papposaandS. tenuis. Annual rainfall was closely related (p< 0·05) to total herbaceous and desirable perennial grass standing crops in most treatments, and accounted for most of the variation in herbage production between years.  相似文献   

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