Variable Rainfall Patterns Research Articles

Spatiotemporal Variability of West African Rainfall. Part I: Regionalizations and Typings

Abstract Space and time scales for analysis of the interannual variability of Sahelian rainfall are determined. Regionalizations of annual and monthly rainfall fields are performed in West Africa for the period 1948–78. Four coherent regions and a less coherent one are identified. Different type classifications derived from the regionalization results are built. The monthly type based on the rainfall anomaly signs north and south of 10°N suggests two major causes of the rainfall pattern variability, one resulting from an anomaly of rainfall amount and the other from a displacement of the intertropical convergence zone (ITCZ). The type based solely on the anomaly sign north of 10°N blends these factors and may give misleading analyses. The use of monthly rainfall fields over all of West Africa is then recommended.

Sugar‐beet treatments, water supply and depth of sowing

SummaryWith variation in rainfall patterns, evaporation, seedbed soil structure and depth of sowing, sugar‐beet seed and seedlings can be subjected to damaging extremes of water availability. Under controlled conditions, seed given progressively longer pre‐sowing treatments (steeped for 12 h at 25°C or advanced for 4 days at 25°C) was more tolerant of non‐optimal conditions than seed given a brief steep (20 min at 15°C). The prolonged steep treatment accelerated emergence and increased establishment (3% on average) in the field, compared with the brief steep. Advanced seed consistently gave the quickest emergence, produced as many plants as the prolonged steep, and was less affected by depth of sowing so its introduction should improve the establishment phase of the sugar‐beet crop.

Annual and seasonal patterns of rainfall variability over Venezuela

Annual and seasonal patterns of rainfall variability over Venezuela

Influence of Geographic Region on Jimsonweed (Datura stramonium) Interference in Soybeans (Glycine max) and Cotton (Gossypium hirsutum)

Experiments with similar experimental design, soil, and planting date were conducted between 1975 and 1979 to evaluate the interference of jimsonweed on soybeans and cotton in Alabama (AL), Arkansas (AR), and Mississippi (MS). Jimsonweed densities were 0, 4, 8, 16, 32, and 64 plants 12 m−1of row. Because of variation in rainfall patterns, geographic region influenced jimsonweed interference in cotton but not in soybeans. Even though jimsonweed interference was reduced by lack of rainfall, jimsonweed was more competitive in the less competitive cotton than in soybeans. Jimsonweed at 64 plants 12 m−1of row was more competitive with cotton than soybeans, causing a 56 and 16% yield reduction, respectively. Position of jimsonweed to the soybean row did not influence competitiveness. Interspecific interference with soybeans caused one-third more reduction in jimsonweed fresh weight and capsules per plant than interspecific interference with cotton. The potential for jimsonweed to become a problem in southern soybeans is less than its potential in cotton because of the competitive ability of soybeans to reduce the growth and reproductive potential of jimsonweed.

The temporal distribution of amazonian rainfall and its implications for forest interception

AbstractA model for the temporal distribution of rainfall is derived from measurements made in central Amazonian rainforest. A contrast is made with a rainfall distribution model which has been used in a stand‐alone comparison of land‐surface sub‐models of two General Circulation Models (GCMs). In terms of the rainfall interception loss, it is shown that the central Amazonian model produces an overestimate of 35%, but is within the error bars of the measured interception loss, whereas the latter model overestimates the interception loss by more than 100%. Two other time distributions of rainfall from contrasting regions of the Amazon are derived from published data in order to investigate the possible interception variation due to rainfall pattern variability. the results illustrate the need for GCM modellers to monitor their synthesized rainfall regime against real data and to be aware of the sensitivity of the forest interception loss to rainfall distribution.

The temporal distribution of Amazonian rainfall and its implications for forest interception

A model for the temporal distribution of rainfall is derived from measurements made in central Amazonian rainforest. A contrast is made with a rainfall distribution model which has been used in a stand-alone comparison of land-surface sub-models of two General Circulation Models (GCMs). In terms of the rainfall interception loss, it is shown that the central Amazonian model produces an overestimate of 35%, but is within the error bars of the measured interception loss, whereas the latter model overestimates the interception loss by more than 100%. Two other time distributions of rainfall from contrasting regions of the Amazon are derived from published data in order to investigate the possible interception variation due to rainfall pattern variability. the results illustrate the need for GCM modellers to monitor their synthesized rainfall regime against real data and to be aware of the sensitivity of the forest interception loss to rainfall distribution.

A documentary-derived climatic chronology for South Africa, 1820?1900

There is a paucity of detailed information regarding the climate of South Africa for the period preceding reliable meteorological instrumentation. Documentary evidence of climatic change, including missionary, official and archival material, is a useful surrogate source of information that is used to extend the rainfall record in the southern and south-eastern parts of the country. Chronologies of yearly rainfall variations for these areas are deduced from a detailed study of various documentary sources. The nature and extent of extreme events such as drought and flood episodes are identified. Corroboration of the chronologies with relevant dendrochronological and meteorological data enhances the documentary-derived weather chronology. Frequency analysis of rainfall for both the eastern and southern Cape, for the periods 1820–1900 and 1900–1980, indicates that the pattern of rainfall variability in these areas has not changed greatly over the past two centuries. Documentary evidence is shown to be a viable means of reconstructing past climates of areas in South Africa devoid of long and detailed instrumental records.

Relative Effect of Root and Shoot Genotypes on Yield of Common Bean under Drought Stress

The inherent interactions of roots and shoots make it difficult to determine whether a characteristic such as response to drought is under control of genes expressed in roots, shoots, or throughout the plant. One approach for obtaining such information is to interchange root and shoot genotypes through grafting. In this study, plants of common bean (Phaseolus vulgaris L.) were cleft grafted and then transplanted to the field to evaluate their yield response under drought. Anticipating that the importance of root or shoot genotype would vary with genotypes evaluated, variation in rainfall patterns, and soil conditions, four yield trials with varying sets of genotypes were conducted at two sites with highly contrasting soils. At Palmira, Colombia, the soil was a fertile Mollisol (Aquic Hapludoll, pH = 7.7), and at Quilichao, Colombia, an Oxisol (Plinthic Kandiudox, pH = 5.0), where problem of low pH and high Al saturation were expected to adversely affect root growth. In all trials, root genotype had a significant and usually large effect on seed yield, while shoot genotype had no effect. No root and shoot genotype interaction occurred, and an effect of grafting was only detected in one trial. These results suggest that root characteristics are of primary importance in determining drought response of common bean and, conversely, that shoot characteristics are of much less importance. However, response of specific genotypes varies greatly with environment.

A synoptic climatological analysis of rainfall variability in south‐eastern Australia

AbstractMonthly rainfall data for the state of Victoria in south‐eastern Australia were analyzed using principal component analysis and Varimax orthogonal rotation, with the intention of revealing typical patterns of rainfall variability. Two data sets were used: a spatially detailed set for 1970–1980, and a less spatially detailed set for 1905–1980. The first five rotated principal component patterns (RPCPs) produced from the two data sets showed the same structure. These five RPCPs each dominated rainfall over a particular sub‐region of the study region, and the climatic regionalization that this suggested appeared to be one strongly determined by topography.The anomalies of Australian region mean monthly mean sea‐level (MSL) pressure associated with these five RPCPs of rainfall were determined using correlational techniques. Of the five main RPCPs of rainfall, the pattern based in the south‐west of the state showed the strongest relationship with monthly pressure anomalies, and was associated with the occurrence of onshore westerly winds. The results for the rainfall pattern based in northern Victoria were perhaps the most interesting. Rainfall in this region was related to the occurrence of anomalous northerly winds through southeastern Australia, although through the cooler months of the year these winds accompanied a low pressure anomaly centred over central Australia, whereas in the warmer months they were associated with a high pressure anomaly centred to the south‐east of the continent. This northern rainfall pattern also showed some month to month persistence, and a distinct correlation with the Southern Oscillation.

On the use of cross-correlation analysis in studies of patterns of rainfall variability

Cross-correlation structures based on daily rainfall data recorded during a period of 5 years at 67 stations throughout Tunisia are used to examine spatial rainfall variability. Basic properties of the correlation-distance function such as cumulating period, exclusion versus inclusion of dry periods, and data volume are analysed. Analysis of the correlation patterns for each month shows that two periods (February–May, and September–November) are delimited by a period of great instability (December–January) and by the dry period (June–August). Three main rainfall regions can be distinguished, delimited and governed mainly by the topography and the coastal influence.

The Spatial Coherence of African Rainfall Anomalies: Interhemispheric Teleconnections

Abstract The spatial patterns of rainfall variability over the African continent are examined using a regionally averaged data set comprising the records of 1087 stations. Typical rainfall departure patterns, or anomaly types, are derived using a linear correlation method to assess the similarity of annual departure maps for the years 1901 to 1973. For the region north of the equator, most years fall into one of six types, four of which show a strong opposition between equatorial and subtropical latitudes and two of which show departures of the same sign over the whole region. For the continent as a whole, six types are also derived. These reflect the patterns derived using northern Africa alone, with conditions in the southern subtropics resembling those in subtropical latitudes north of the equator. Continentally, two principal spatial patterns of rainfall variation are evident: anomalies of the same sign over most of Africa and anomalies of the opposite sign in equatorial and subtropical latitudes. Thu...

The nature of rainfall variability in Africa South of the Equator

AbstractThe spatial patterns of rainfall variability over southern and equatorial Africa are examined using a regionally averaged data set comprising approximately 700 station records. Seasonal persistence of anomalies is also considered. Typical rainfall departure patterns, or anomaly types, are derived using a linear correlation method to assess the similarity of annual departure maps for the years 1901 to 1973. Six basic types describe most of the variability in the area 0 to 35°S. These indicate a remarkable spatial coherence of anomalies, but with a strong tendency for opposition between the equatorial and subtropical latitudes. The results are given an interpretation in terms of circulation features and synoptic situations affecting southern Africa.

Analytical results for prediction of variable rainfall infiltration

The basic equations that govern water movement in unsaturated soils are presented for a boundary condition of variable rainfall rate at the soil surface. In particular, a differential equation for the water content at the soil surface is derived. Solutions are then obtained for the time evolution of water content at the soil surface assuming a power law form for the relative permeability to water as a function of normalized water content. Ponding time formulae are obtained and compared with other previously published relations. Water content profiles are also obtained and their shapes are displayed graphically for the case of an exponent of 2 in the power law form of the relative permeability to water for a constant rainfall rate. Formulae are also derived for the situation of a variable rainfall pattern. A methodology for the coupling of the analytical procedures with an implicit numerical solution of the water content at the soil surface is suggested as a cost-effective alternative to the strict numerical solution of the partial differential equation that governs the water content profile evolution. Postponding infiltration formulae and water content profile equations are also provided.

A study of rainfall pattern in relation to crop planning over the Haryana State

Haryana State receives a range of 200 to 1200 mm f annual rainfall in 7 agroclimatic zones. The rainfall patterns of these zones have been discussed with special reference to the amount, variation and probabilities based on the rainfall data of the rain recording stations. The existing cropping patterns have been analysed at block level in relation to rainfall pattern variability etc.

Water Infiltration and Runoff under Rain Applications

AbstractExperiments to verify the validity and to evaluate the performance of rain infiltration models based on extending the Green and Ampt approach are lacking in the literature. There are severe gaps between theory, validity, and practicality of these models. In this study, a physically‐based model for describing infiltration into a homogeneous deep stable soil profile with uniform initial water content distribution is presented. The derivation of the model is based on the concept of mapping the actual wetted soil zone into an equivalent rectangular saturated zone with constant effective hydraulic conductivity and capillary pressure head at the abrupt wetting front. The model takes into account the intial water mobility in the soil profile and predicts the system response under any rainfall pattern. The proposed model is further simplified, for practical purposes, by grouping three basic input parameters into a single parameter, the Surface Saturation Index.The validity of the model was tested and strongly supported by the results of laboratory experiments conducted on samples of three soils, two silt loams and a silty clay loam, under five different (variable and constant) water application rate patterns. It was found that the amount of runoff from a nearly symmetrical convex variable rainfall pattern was very close to that which resulted from its equivalent average constant application rate pattern, regardless of the significant differences between the times at which runoff begins and the characteristics of infiltration rates thereafter.

Effects of management on soil fertility under pasture. 3. Changes in total soil nitrogen, carbon, phosphorus and exchangeable cations

Soil samples from pasture plots which received differential superphosphate, nitrogen and grazing treatments for five years were analysed chemically. Changes in total soil nitrogen were studied annually on different sections of each plot. Changes in carbon, phosphorus and cations were studied in the final samples. High grazing pressure by sheep tended to reduce the accumulation of organic nitrogen and carbon, but the effects were variable on different sections of the plots and it was uncertain (P > 0.05) that a real difference existed when averaged over the whole experimental area. The grazing treatments caused a concentration of soil nitrogen towards one end of each plot, particularly where high numbers of sheep were used to produce a high grazing pressure. Substantial losses of incoming nitrogen from fertilizer and clover occurred at both grazing pressures. Nitrogen accumulations during the drier years of the experiment increased at the higher rates of superphosphate but this effect was lost during the final wetter year. The accumulation of nitrogen on each plot appeared to depend on the initial soil nitrogen content at least as much as on the experimental treatment. Annual nitrogen changes were not constant and the overall accumulations appeared to be the net effect of gains and losses in good and bad clover years respectively. Organic phosphorus and exchangeable potassium increased to a similar extent under all treatments. Inorganic phosphorus was affected only by fertilizer additions. These results suggest that, in regions of variable rainfall patterns, the rate of nitrogen and organic matter accumulation under dryland pastures is difficult to control, and that seasonal rainfall and initial soil nitrogen content are dominant factors.