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Mean Annual Rainfall (mean + annual_rainfall)
Selected AbstractsSpatial patterns of suspended sediment yields in a humid tropical watershed in Costa RicaHYDROLOGICAL PROCESSES, Issue 12 2001Jagdish Krishnaswamy Abstract An Erratum has been published for this article in Hydrological Processes 16(5) 2002, 1130,1131. Humid tropical regions are often characterized by extreme variability of fluvial processes. The Rio Terraba drains the largest river basin, covering 4767 km2, in Costa Rica. Mean annual rainfall is 3139±419sd mm and mean annual discharge is 2168±492sd mm (1971,88). Loss of forest cover, high rainfall erosivity and geomorphologic instability all have led to considerable degradation of soil and water resources at local to basin scales. Parametric and non-parametric statistical methods were used to estimate sediment yields. In the Terraba basin, sediment yields per unit area increase from the headwaters to the basin mouth, and the trend is generally robust towards choice of methods (parametric and LOESS) used. This is in contrast to a general view that deposition typically exceeds sediment delivery with increase in basin size. The specific sediment yield increases from 112±11·4sd t km,2 year,1 (at 317·9 km2 on a major headwater tributary) to 404±141·7sd t km,2 year,1 (at 4766·7 km2) at the basin mouth (1971,92). The analyses of relationships between sediment yields and basin parameters for the Terraba sub-basins and for a total of 29 basins all over Costa Rica indicate a strong land use effect related to intensive agriculture besides hydro-climatology. The best explanation for the observed pattern in the Terraba basin is a combined spatial pattern of land use and rainfall erosivity. These were integrated in a soil erosion index that is related to the observed patterns of sediment yield. Estimated sediment delivery ratios increase with basin area. Intensive agriculture in lower-lying alluvial fans exposed to highly erosive rainfall contributes a large part of the sediment load. The higher elevation regions, although steep in slope, largely remain under forest, pasture, or tree-crops. High rainfall erosivity (>7400 MJ mm ha,1 h,1 year ,1) is associated with land uses that provide inadequate soil protection. It is also associated with steep, unstable slopes near the basin mouth. Improvements in land use and soil management in the lower-lying regions exposed to highly erosive rainfall are recommended, and are especially important to basins in which sediment delivery ratio increases downstream with increasing basin area. Copyright © 2001 John Wiley & Sons, Ltd. [source] Twenty years of rest returns grazing potential, but not palatable plant diversity, to Karoo rangeland, South AfricaJOURNAL OF APPLIED ECOLOGY, Issue 4 2010Colleen L. Seymour Summary 1.,Up to 73% of the world's rangelands are degraded, and increasing demand for meat in developing countries and a growing human population are likely to exert even greater pressures on rangelands in the next 20,50 years. Restoration of rangeland grazing potential and resilience is therefore important, particularly in the face of climate change. 2.,We investigated the influence of past stocking rates (from 1910 to 1987), rainfall, and current grazing regimes (from 1988 to 2008) on plant assemblages, grazing potential, and diversity of palatable species in southern Karoo rangelands, South Africa. 3.,We used herbivore exclusion experiments to test whether resting rangeland for 20 years enables recovery of plant assemblages (where seed sources are present within 50 m), regardless of previous grazing history. Mean annual rainfall over this period was 15% higher than the mean annual rainfall for the preceding 80 years and included two exceptionally wet years. 4.,While rainfall was a primary driver of total vegetation cover, grazing history explained differences in plant species composition: plots with shared historical grazing intensity were more similar than plots with the same grazing regimes between 1988 and 2008. 5.,In historically heavily-grazed exclusion plots, cover of the palatable species Tripteris sinuata (formerly Osteospermum sinuatum) returned to levels comparable to that in both exclusion and lightly-grazed plots with a moderate grazing history. Five palatable species (Pteronia empetrifolia, Tetragonia spicata, Berkheya spinosa, Hereroa latipetala and Ruschia spinosa) failed to re-establish, however, despite the presence of seed-producing plants nearby. Furthermore, only cover of P. empetrifolia increased significantly in historically moderately-grazed plots. Cover of unpalatable plants (e.g. Pteronia pallens) increased in all plots over time. 6.Synthesis and applications. These findings suggest that present species composition of arid shrublands reflects historical management at time scales greater than 20 years. Despite high rainfall enabling the return of grazing potential through recovery of a single forage species, rest alone did not ensure the return of all palatable species, with implications for rangeland resilience. Restoring the full suite of palatable species over management timeframes will require more complex interventions such as reseeding or selective clearing. [source] Effects of Rainfall and Ground-Water Pumping on Streamflow in M,kaha, O'ahu, Hawai'i,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 1 2007Alan Mair Abstract:, Land-use/land-cover changes in M,kaha valley have included the development of agriculture, residential dwellings, golf courses, potable water supply facilities, and the introduction of alien species. The impact of these changes on surface water and ground water resources in the valley is of concern. In this study, streamflow, rainfall, and ground-water pumping data for the upper part of the M,kaha valley watershed were evaluated to identify corresponding trends and relationships. The results of this study indicate that streamflow declined during the ground-water pumping period. Mean and median annual streamflow have declined by 42% (135 mm) and 56% (175 mm), respectively, and the mean number of dry stream days per year has increased from 8 to 125. Rainfall across the study area appears to have also declined though it is not clear whether the reduction in rainfall is responsible for all or part of the observed streamflow decline. Mean annual rainfall at one location in the study area declined by 14% (179 mm) and increased by 2% (48 mm) at a second location. Further study is needed to assess the effect of ground-water pumping and to characterize the hydrologic cycle with respect to rainfall, infiltration, ground-water recharge and flow in the study area, and stream base flow and storm flow. [source] Bilby distribution and fire: a test of alternative models of habitat suitability in the Tanami Desert, AustraliaECOGRAPHY, Issue 6 2007Richard Southgate The distribution of the bilby Macrotis lagotis was assessed in the Tanami Desert using stratified random plots, repetitively sampled transects, aerial survey transects, and ground truth plots. Compared to a previous assessment of distribution, the extent of occurrence has changed little in the last 20 yr. However, the area of occupancy is small relative to the extent of occurrence and <25% of the current geographic range has bilby sign <20 km apart. Generalised linear modelling was used to determine the strength of association between bilby occurrence and habitat variables and identify refugia characteristics. Four competing candidate models were examined to determine whether bilby occurrence associated significantly with productive substrates and introduced herbivores, the distribution of key predator species, the pattern of fire, and climatic gradients including rainfall and temperature. For the entire study area, bilby presence associated most strongly with variables of mean annual rainfall, substrate type and the probability of dingo occurrence. Proximity to recently burnt habitat formed a significant predictor of bilby occurrence in a model derived for a reduced part of the study area where most sign was found. The work suggested that the current frameworks underpinning understanding of biotic distributions in arid Australia are deficient, and that climatic gradients, lateritic and rocky systems, and predators need to be incorporated into our thinking in the future. The extent of occurrence based on outlier records from opportunistic reports provided a misleading indication of the true status of the bilby. [source] Canopy structure in savannas along a moisture gradient on Kalahari sandsGLOBAL CHANGE BIOLOGY, Issue 3 2004Robert J. Scholes Abstract Measurements of tree canopy architecture were made at six savanna sites on deep, sandy soils, along a gradient of increasing aridity. There was substantial variation in the leaf area estimated within each site, using the same sample frame, but different measurement techniques. The trends in canopy properties in relation to the aridity gradient were consistent, regardless of the technique used for estimating the properties. The effective plant area index for the tree canopy (the sum of the stem area index and the leaf area index (LAI)) declined from around 2 to around 0.8 m2 m,2 over a gradient of mean annual rainfall from 1000 to 350 mm. Stems contributed 2,5% of the tree canopy plant area index. Since the tree canopy cover decreased from 50% to 20% over this aridity range, the leaf area index within the area covered by tree canopies remained fairly constant at 3,4 m2 m,2. Tree leaves tended from a horizontal orientation to a more random orientation as the aridity increased. On the same gradient, the leaf minor axis dimension decreased from around 30 mm to around 3 mm, and the mean specific leaf area decreased from 14 to 5 m2 kgha,1. There was good agreement between LAI observed in the field using a line ceptometer and the LAI inferred by the MODIS sensor on the Terra satellite platform, 2 months later in the same season. [source] The influence of hydroelectrical development on the flow regime of the karstic river CetinaHYDROLOGICAL PROCESSES, Issue 1 2003Ognjen Bonacci Abstract The Cetina River is a typical karst watercourse in the deep and well-developed Dinaric karst. The total length of the Cetina River open streamflow from its spring to the mouth is about 105 km. Estimated mean annual rainfall is 1380 mm. The Cetina catchment is built of Triassic, Jurassic, and Cretaceous carbonate strata. The western part of the catchment by the Cetina River is referred to as the ,direct' or topographic catchment. It was defined based on surface morphologic forms, by connection between mountain chain peaks. This part of the catchment is almost entirely situated in the Republic of Croatia. The eastern part of the catchment is referred to as the ,indirect' catchment, and is mainly situated in Bosnia-Herzegovina. Water from the ,indirect' catchment emerges from the western ,direct' catchment in numerous permanent and temporary karst springs. Since 1960, numerous hydrotechnical works have been carried out on the Cetina River and within its catchment. Five hydroelectric power plants (HEPPs), five reservoirs, and three long tunnels and pipelines have been built. Their operation has significantly altered the natural hydrological regime. The Cetina River is divided into two hydrological reaches. In the 65 km upstream, the hydrological regime was redistributed within the year: low flows had increased and high flows had decreased, although the mean annual discharge remained the same. Part of the Cetina watercourse downstream from the Pran,evi,i Reservoir lost the majority of its flow. The mean annual discharges dropped from 100 m3 s,1 to less than 10 m3 s,1 because of the Zaku,ac HEPP development. Copyright © 2003 John Wiley & Sons, Ltd. [source] Modelling climate change in West African Sahel rainfall (1931,90) as an artifact of changing station locationsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2004Adrian Chappell Abstract Since the major droughts in the West African Sahel during the 1970s, it has been widely asserted that mean annual summer rainfall has declined since the late 1960s. Explanation of this persistent regional drying trend was important for famine early-warning and global climate models. However, the network of rainfall stations changed considerably during that recent period of desiccation. Furthermore, it was difficult to reconcile the calculation of a simple mean value for a region known to have a complex spatial and temporal rainfall pattern. A simple model separated the Sahel into ,wet' and ,dry' regions. This model was inverted against mean annual summer rainfall for the Sahel between 1931 and 1990. Model predictions were found to be insensitive to initial starting conditions. The optimized parameters explained 87% of the variation in observed mean annual summer rainfall. The model predicted the mean annual rainfall in the wet ,coastal' and dry ,continental' regions of the Sahel to be 973 mm and 142 mm respectively. Consequently, the predicted long-term mean annual summer rainfall was 558 mm, 15% greater than that of the observed long-term mean (417 mm). The mean annual summer rainfall for the region was corrected by removing the influence of changing station locations over the study period. No persistent decline was found in mean annual summer rainfall, which suggested that the perceived drying trend was an artifact of the crude statistical aggregation of the data and historical changes in the climate station networks. The absence of a decline in rainfall questioned the validity of the hypotheses and speculations for the causes of the drying trend in the region and its effects on global climate change. It also increased the likelihood that changes over time in other regional and global climate station networks have influenced the performance and interpretation of global climate models. Copyright © 2004 Royal Meteorological Society [source] Short-term transformation of matrix into hospitable habitat facilitates gene flow and mitigates fragmentationJOURNAL OF ANIMAL ECOLOGY, Issue 6 2007NIELS BLAUM Summary 1Habitat fragmentation has major implications for demography and genetic structure of natural plant and animal populations as small and isolated populations are more prone to extinction. Therefore, many recent studies focus on spatial fragmentation. 2However, the temporal configuration of suitable habitat may also influence dispersal and gene flow in fragmented landscapes. We hypothesize that short-term switching of inhospitable matrix areas into suitable habitat can mitigate effects of spatial fragmentation in natural and seminatural ecosystems. 3To test our hypothesis, we investigated the hairy-footed gerbil (Gerbillurus paeba, Smith 1836), a ground-dwelling rodent, in fragmented Kalahari savannah areas. Here, rare events of high above mean annual rainfall suggest short-term matrix suitability. 4During the field survey in ,matrix' areas in the Kalahari (shrub encroachment by heavy grazing) we never observed the hairy-footed gerbil in years of average rainfall, but observed mass occurrences of this species during rare events of exceptionally high rainfall. 5In a second step, we developed an agent-based model simulating subpopulations in two neighbouring habitats and the separating matrix. Our mechanistic model reproduces the mass occurrences as observed in the field and thus suggests the possibly underlying processes. In particular, the temporary improvement in matrix quality allows reproduction in the matrix, thereby causing a substantial increase in population size. 6The model demonstrates further how the environmental trigger (rainfall) impacts genetic connectivity of two separated subpopulations. We identified seasonality as a driver of fragmentation but stochasticity leading to higher connectivity. 7We found that our concept of temporal fragmentation can be applied to numerous other fragmented populations in various ecological systems and provide examples from recent literature. We conclude that temporal aspects of fragmentation must be considered in both ecological research and conservation management. [source] Twenty years of rest returns grazing potential, but not palatable plant diversity, to Karoo rangeland, South AfricaJOURNAL OF APPLIED ECOLOGY, Issue 4 2010Colleen L. Seymour Summary 1.,Up to 73% of the world's rangelands are degraded, and increasing demand for meat in developing countries and a growing human population are likely to exert even greater pressures on rangelands in the next 20,50 years. Restoration of rangeland grazing potential and resilience is therefore important, particularly in the face of climate change. 2.,We investigated the influence of past stocking rates (from 1910 to 1987), rainfall, and current grazing regimes (from 1988 to 2008) on plant assemblages, grazing potential, and diversity of palatable species in southern Karoo rangelands, South Africa. 3.,We used herbivore exclusion experiments to test whether resting rangeland for 20 years enables recovery of plant assemblages (where seed sources are present within 50 m), regardless of previous grazing history. Mean annual rainfall over this period was 15% higher than the mean annual rainfall for the preceding 80 years and included two exceptionally wet years. 4.,While rainfall was a primary driver of total vegetation cover, grazing history explained differences in plant species composition: plots with shared historical grazing intensity were more similar than plots with the same grazing regimes between 1988 and 2008. 5.,In historically heavily-grazed exclusion plots, cover of the palatable species Tripteris sinuata (formerly Osteospermum sinuatum) returned to levels comparable to that in both exclusion and lightly-grazed plots with a moderate grazing history. Five palatable species (Pteronia empetrifolia, Tetragonia spicata, Berkheya spinosa, Hereroa latipetala and Ruschia spinosa) failed to re-establish, however, despite the presence of seed-producing plants nearby. Furthermore, only cover of P. empetrifolia increased significantly in historically moderately-grazed plots. Cover of unpalatable plants (e.g. Pteronia pallens) increased in all plots over time. 6.Synthesis and applications. These findings suggest that present species composition of arid shrublands reflects historical management at time scales greater than 20 years. Despite high rainfall enabling the return of grazing potential through recovery of a single forage species, rest alone did not ensure the return of all palatable species, with implications for rangeland resilience. Restoring the full suite of palatable species over management timeframes will require more complex interventions such as reseeding or selective clearing. [source] Effects of climate and local aridity on the latitudinal and habitat distribution of Arvicanthis niloticus and Arvicanthis ansorgei (Rodentia, Murinae) in MaliJOURNAL OF BIOGEOGRAPHY, Issue 1 2004B. Sicard Abstract Introduction, The genus Arvicanthis (Lesson 1842) (Rodentia: Murinae), usually referred to as the unstriped grass rat, is mainly distributed in savanna and grassland habitats of Sub-Saharan Africa. Among the four chromosomal forms of Arvicanthis recently differentiated in Western and Central Africa, the one with a diploid chromosomal number (2n) of 62 and an autosomal fundamental number (NFa) of 62 or 64 is ascribed to Arvicanthis niloticus (Demarest 1822), while the one with 2n = 62 and a NFa between 74 and 76 is referred to A. ansorgei (Thomas 1910). Despite the broad area of sympatry recently uncovered along the inner delta of the Niger river in Mali [details in Volobouev et al. (2002) Cytogenetics and Genome Research, 96, 250,260], the distribution of the two species is largely parapatric and follows the latitudinal patterns of the West-African biogeographical domains, which are related to the latitudinal patterns of annual rainfall in this region. Here, we analyse the suggestion that the two species show specific adaptations to differences in climate aridity. Methods, Karyologically screened animals were sampled in 19 localities in seasonally flooded regions located along the ,Niger' river in Mali and extending from 1100 to 200 mm of mean annual rainfall. The analysis of trapping success (TS) data allowed us to investigate the respective effects of climate (i.e. annual rainfall) and local (i.e. duration of the green herbaceous vegetation) aridity on the latitudinal and habitat distribution of the two species. Conclusions, The broad zone of sympatry was found to correspond to a northward expansion of the recognized distribution area of A. ansorgei. TS values indicated that the two species responded very differently to climatic and local conditions of aridity. Arvicanthis ansorgei decreased in TS as regional conditions became more arid; a similar trend was also observed within regions where habitat occupancy decreased with local aridity. The higher TS observed in the most humid habitat relative to the others persisted throughout the latitudinal rainfall gradient. In contrast, TS of A. niloticus increased with latitudinal aridity. This species was present in more arid habitats than A. ansorgei from 1000 mm down to 400 mm of mean annual rainfall where a shift to the most humid habitat occurred. These opposite trends in TS distribution between species suggest that A. ansorgei is less adapted than A. niloticus to arid environments at both a regional and habitat level; thus, A. ansorgei would be able to invade dry regions only along the extensive floodplains bordering the inner delta of the ,Niger' river. Several biological traits that may be involved in limiting the southward distribution of A. niloticus are discussed. [source] Variation in vegetative water use in the savannas of the North Australian Tropical TransectJOURNAL OF VEGETATION SCIENCE, Issue 3 2002Garry D. Cook Abstract. The decline in tree density on sandy soils in savannas is highly correlated with declining mean annual rainfall along the North Australian Tropical Transect (NATT). We reanalyse various data on water use by individual trees and argue that a common relationship can be used to estimate annual water use by tree stands along the NATT from ca. 600 mm mean annual rainfall to at least 1600 mm. Where rainfall is less than 600 mm, trees of a given size use less water than at sites where rainfall is higher. We use these relationships to relate water use at the stand scale with mean annual rainfall along the NATT. From this we show that the empirical data imply that the minimum depth of sandy soil that needs to be exploited by trees declines with increasing aridity along the NATT from more than 5 m to less than 1 m. This finding is consistent with other observations and the pattern that with increasing aridity, an increasing proportion of rainfall coming from isolated storms rather than from periods of extended monsoon activity. [source] Drivers of megaherbivore demographic fluctuations: inference from elephantsJOURNAL OF ZOOLOGY, Issue 1 2009M. J. Trimble Abstract Environmentally induced variation in survival and fecundity generates demographic fluctuations that affect population growth rate. However, a general pattern of the comparative influence of variation in fecundity and juvenile survival on elephant population dynamics has not been investigated at a broad scale. We evaluated the relative importance of conception, gestation, first year survival and subsequent survivorship for controlling demographic variation by exploring the relationship between past environmental conditions determined by integrated normalized difference vegetation index (INDVI) and the shape of age distributions at 17 sites across Africa. We showed that, generally, INDVI during gestation best explained anomalies in age structure. However, in areas with low mean annual rainfall, INDVI during the first year of life was critical. The results challenge Eberhardt's paradigm for population analysis that suggests that populations respond to limited resource availability through a sequential decrease in juvenile survival, reproductive rate and adult survival. Contrastingly, elephants appear to respond first through a reduction in reproductive rate. We conclude that this discrepancy is likely due to the evolutionary significance of extremely large body size , an adaptation that increases survival rate but decreases reproductive potential. Other megaherbivores may respond similarly to resource limitation due to similarities in population dynamics. Knowing how vital rates vary with changing environmental conditions will permit better forecasts of the trajectories of megaherbivore populations. [source] Contractile roots in succulent monocots: convergence, divergence and adaptation to limited rainfallPLANT CELL & ENVIRONMENT, Issue 8 2008GRETCHEN B. NORTH ABSTRACT Contractile roots (CRs) that pull shoots further down in the soil are a possible example of convergent evolution in two monocot families, the Agavaceae and the Asphodelaceae. The association between CRs, water uptake and habitat aridity was investigated for agaves, yuccas and aloes by assessing the occurrence of CRs and the amount of root contraction for glasshouse-grown plants with respect to mean annual rainfall of their native habitats. Structural features of CRs as well as root hydraulic conductance were compared with those of non-contractile roots (NCRs). CRs occurred in 55% of the 73 species examined, including 64% of the agaves and 85% of the yuccas, but in none of the aloes despite the occurrence of CRs in related genera. The phylogenetic distribution of CRs was consistent with multiple acquisitions or losses of the trait. The amount of root contraction showed a highly significant negative relationship with mean annual rainfall, although other environmental factors may also be important. Radial hydraulic conductance of the basal (contractile) zone exceeded that of the midroot zone for CRs; for NCRs, the opposite was true. Thus, CRs in the species examined may provide a mechanism for greater water uptake near the soil surface in regions with limited rainfall. [source] Decadal dynamics of tree cover in an Australian tropical savannaAUSTRAL ECOLOGY, Issue 6 2009CAROLINE E. R. LEHMANN Abstract Spatio-temporal variation in tropical savanna tree cover remains poorly understood. We aimed to quantify the drivers of tree cover in tropical mesic savannas in Kakadu National Park by relating changes in tree cover over 40 years to: mean annual rainfall, fire activity, initial tree cover and prior changes in tree cover. Aerial photography, acquired in 1964, 1984 and 2004, was obtained for fifty sites in Kakadu that spanned a rainfall gradient from approximately 1200 to 1600 mm. The remotely sensed estimates of tree cover were validated via field survey. Linear mixed effects modelling and multi-model inference were used to assess the strength and form of the relationships between tree cover and predictor variables. Over the 40 years, tree cover across these savannas increased on average by 4.94 ± 0.88%, but was spatio-temporally variable. Tree cover showed a positive albeit weak trend across the rainfall gradient. The strength of this positive relationship varied over the three measurement times, and this suggests that other factors are important in controlling tree cover. Tree cover was positively related to prior tree cover, and negatively correlated with fire activity. Over 20 years tree cover was more likely to increase if (i) tree cover was initially low or (ii) had decreased in the previous 20-year interval or (iii) there had been fewer fires. Across the examined rainfall gradient, the greater variability in fire activity and inherently higher average tree cover at the wetter latitudes resulted in greater dynamism of tree cover compared with the drier latitudes. This is consistent with savanna tree cover being determined by interactions between mean annual rainfall, tree competition and frequent fire in these tropical mesic savannas. [source] Climate Change and Pastoral Economy in Kenya: A Blinking FutureACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 5 2009Julius M. HUHO Abstract: The present paper examines the changing climatic scenarios and associated effects on livestock farming (pastoralism) in the arid and semi arid lands (ASAL) of Kenya, which cover over 80% of the country. The study was carried out in the semi arid Mukogodo Division of Laikipia District in Kenya. This division received a mean annual rainfall of approximately 507.8 mm and the main source of livelihood was pastoralism. Questionnaire, structured interview, observation and literature review were the main methods of data collection. Rainfall was used in delineating changes in climate. Standardized precipitation index (SPI) and Markov process were used in analyzing drought severity and persistence, respectively. Approximately 38% of all droughts between 1975 and 2005 were prolonged and extremely severe, with cumulative severity indices ranging between ,2.54 and ,6.49. The probability that normal climatic conditions persisted for two or more consecutive years in Mukogodo Division remained constant at approximately 52%. However, the probability of wet years persisting for two or more years showed a declining trend, while persistence of dry years increased with duration. A drying climatic trend was established. This drying trend in the area led to increased land degradation and encroachment of invasive nonpalatable bushes. The net effect on pastoralism was large-scale livestock loss through starvation, disease and cattle rustling. Proper drought monitoring and accurate forecasts, community participation in all government interventions, infrastructural development in the ASAL and allocation of adequate resources for livestock development are some of the measures necessary for mitigating the dwindling pastoral economy in Kenya and other parts of the world. [source] | ||||||||||||||||