Natural Rain Research Articles

Multi‐frequency scatterometer measurements on water surfaces agitated by artificial and natural rain

Images of rain events over the ocean acquired by a multi‐frequency/multi‐polarization Synthetic Aperture Radar (SAR) show different radar contrasts at different frequencies and polarizations. In order to better understand these effects, field and laboratory experiments were performed at different rain rates and wind speeds with scatterometers working at different radar frequencies, polarizations, and incidence angles. Our results show that the dominant scattering mechanism on a rain‐roughened water surface, observed at VV polarization, at all incidence angles is Bragg scattering from ring waves. At HH polarization the radar backscatter is caused by both ring waves and non‐propagating splash products, with the dominating effect depending on incidence angle. The reduction and enhancement of the surface roughness by ring waves and sub‐surface phenomena, respectively, result in a transition wavenumber between reduction of the radar backscattering and its enhancement of about 100 rad m−1. We assume that this transition wavenumber depends on the drop‐size distribution of the rain. Taking into consideration the different dependencies of the radar backscatter at different frequencies and polarizations on rain rate, we suggest a method to estimate rain rates by calculating the ratio of the radar cross‐sections at L band, VV polarization and at C band, HV polarization. Provided an availability of SAR data at the respective frequency–polarization combinations, this method allows for investigating the nature of small‐scale (convective) rain events over the ocean.

Evaluation of forest fire retardant removal from forest fuels by rainfall

Long-term forest fire retardants for fire prevention purposes are currently used, or are under consideration, in many parts of the world. Their use requires, among other things, knowledge about weathering of retardants with time, which may lead to the need for re-application. Rainfall is a factor that can lead to retardant depletion from the fuels. In this study, the rate of depletion was evaluated experimentally using Aleppo pine (Pinus halepensis) needles. The needles were made into small bundles and immersed in retardant, which was FIRE-TROL 936 concentrate, diluted to 20% (v/v) with water. The retardant-treated needles, after drying, were exposed to natural rain in three different rainfall events, and retardant depletion was measured. A regression equation was developed with percentage retardant removal as the dependent variable. The natural logarithm of rainfall quantity, expressed in mm of rain, and the duration of rain, expressed in min, were the two independent variables. A simple equation can be used to support pre-suppression planning and to assess environmental effects.

DEVEX-disdrometer evaluation experiment: Basic results and implications for hydrologic studies

Increasing our understanding of the small scale variability of drop size distributions (DSD), and therefore of several bulk characteristics of rainfall processes, has major implications for our interpretation of the remote sensing based estimates of precipitation and its uncertainty. During the spring and summer of 2002 the authors conducted the DEVEX experiment (disdrometer evaluation experiment) to compare measurements of natural rain made with three different types of disdrometers collocated at the Iowa City Municipal Airport in Iowa City, Iowa in the Midwestern United States. This paper focuses on the evaluation of the instruments rather than analysis of the hydrometeorological aspects of the observed events. The comparison demonstrates discrepancies between instruments. The authors discuss the systematic and random effects in terms of rainfall quantities, drop size distribution properties, and the observed drop size vs. velocity relationships. Since the instruments were collocated, the effects of the natural variability of rain are reduced some with time integration, isolating the instrumental differences. The authors discuss the status of DSD measurement technologies and the implications for a range of hydrologic applications from remote sensing of rainfall to atmospheric deposition to soil erosion and sediment transport in the environment. The data set collected during the DEVEX experiment is made available to the research community.

Summer rain pulse size and rainwater uptake by three dominant desert plants in a desertified grassland ecosystem in northwestern China

To examine the different effects of rain pulse size on uptake of summer rains by three dominant desert plants in field conditions of desertified grasslands on the Ordos Plateau of northwestern China, we studied relationships between precipitation event size and rainwater uptake using stable isotopes of hydrogen in plant and soil water. Four natural precipitation events that represented precipitation sizes of 5.3, 8.3, 13.3, and 65.3 mm in the summer were chosen for the experiment. The perennial grass Stipa bungeana, the shrub Artemisia ordosia, and the herb Cynanchum komarovii – the dominant species in the communities – were compared for their use of summer rains with different pulse sizes based on the changes in the hydrogen isotope ratios (dD) of their stem water 7 days following each natural rain event. We found that S. bungeana and C. komarovii took advantage of shallow water sources derived from small ( 65 mm) rain events, and C. komarovii relied primarily on rain events of intermediate (10 –20 mm) size. These different responses to rain pulses among species suggested that more frequent small rain events will promote the dominance of S. bungeana and C. komarovii, medium-sized events will facilitate development of C. komarovii, and large events will advance A. ordosia in this community. The rainwater utilization patterns of the three species would allow the coexistence of S. bungeana and A. ordosia or the coexistence of A. ordosia and C. komorovii in various successional serals following the disturbances. With an increase in variability of summer rain pulse size as predicted by climate change models, we expect that the structure of this community will undergo significant change in the future. Altered precipitation regimes, especially in combination with anthropogenic-related disturbances such as over-grazing, are likely to accelerate rates of degradation in northwestern China.

Fractionation of total water soluble aluminium in extracts from peat and sand soil samples by ion‐exchange method followed by GF AAS

This study attempted the use ion‐exchange resins in order to fractionate total water soluble aluminium in extracts collected over acidic peat and alkaline sand soils. Extraction was performed in neutral and acidic conditions in order to imitate the behaviour of natural or acid rains. The total aluminium content in soil extracts and fractions was determined by graphite furnace atomic absorption spectrometry (GF AAS). In order to retain aqua Al3+ and hydroxycomplexes of aluminium Chelex 100 was used. It was found that fractionation of the aluminium in extracts depends strictly on the type of soil. When neutral spring water was used for the extraction the content of water soluble aluminium was found to be over two times higher in case of peat soil when compared with sand soil extract. Acidification of water by addition of HNO3 and H2SO4 (2:1) pronounced in much extend the extraction efficiency of aluminium only form sand soils (over 10 times) but did not change proportion in fractionation of the particular aluminium species. Admittedly in the case of peat soils no significant increase in total aluminium content was observed but the concentration of cationic forms in acidic extract increased twofold when compared with the neutral extract.

Effects of nickel and copper on growth and mycorrhiza of Scots pine seedlings inoculated with Gremmeniella abietina

SummaryScots pine (Pinus sylvestris) seedlings were planted in soil originating from two localities with different background levels of nickel and copper. In addition, some of the seedlings were exposed to additional nickel (20.5 mg Ni/l of soil) or copper (63.5 mg Cu/l of soil), or a combination of both Ni and Cu, via soil without direct shoot exposure during their second growing period. The seedlings were either irrigated with spring water (pH 6) or got only natural rain during the whole field experiment. All seedlings were inoculated with conidia of a shoot‐pathogen Gremmeniella abietina during their third growing season, and harvested the following spring. Lengths of shoots of different year‐classes were used as growth estimates. In roots, the proportion of fungal (assumedly mycorrhizal) biomass was estimated by measuring ergosterol concentration. Guajacol peroxidase activity was measured. Short roots were classified into two groups according to their condition and the composition of the mycorrhizal community was expressed as a proportion of morphotypes in the roots. The seedlings exposed to additional Ni had higher shoot growth than the seedlings in the other treatments. The mean Ni concentration in the roots of seedlings exposed to additional Ni was 79 p.p.m. and in other seedlings 16 p.p.m. Additional Ni also decreased the frequency of clearly senescent short roots and the proportion of the mycorrhizal morphotype with the thinnest mantle. These results indicate that the Ni exposure levels used in this experiment had some positive effects on the seedlings. The relative fungal biomass was about 6% lower (p = 0.0981) in the fine roots of seedlings treated with additional Cu. The mean Cu concentration in the roots of seedlings exposed to additional Cu was 256 p.p.m. and in other seedlings 29 p.p.m. Peroxidase activity, which was used as a general stress indicator in this study, was not affected by any of the treatments. The shoot growth and the relative biomass of fungi in the fine roots were positively correlated in all seedlings, and this correlation was stronger in seedlings exposed to additional Ni that were not irrigated compared with seedlings not exposed to additional Ni that were irrigated. The frequency of asymptomatic infections of G. abietina was positively correlated with the proportion of senescent short roots in the irrigated seedlings but not in not‐irrigated seedlings. The general condition of seedlings may be an important factor for infection by G. abietina when moisture is high enough for the fungi to infect seedlings by conidia.

Corrosion of 85-5-5-5 bronze in natural and synthetic acid rain

In order to investigate the decay of bronzes exposed to acid wet depositions, a comparative study has been performed by following the corrosion behaviour of different sets of bronze specimens exposed either to natural rain or to a similar solution, without organic compounds, artificially reproduced in laboratory. The as cast G85 bronze specimens were exposed to aggressive solutions for different periods through a wet-dry technique. The pH trend of the solutions and the amount of metals transferred into the solutions were periodically monitored. OM, SEM, XRD, RAMAN analyses and ac electrochemical measurements were performed on the artificially weathered specimens. Preliminary results, showing the difference between the ageing in natural and synthetic rain, suggest the influence of the organic components on the corrosion process. In particular, the growth of a more uniform protective layer of corrosion products on the metal surface exposed to the natural rain could be attributed to these components.

Effects of hygroscopic seeding on raindrop formation as seen from simulations using a 2000-bin spectral cloud parcel model

A 2000-bin cloud spectral parcel model is used to investigate the effect of hygroscopic seeding on warm rain formation under different thermodynamic conditions. Simulations show that utilization of commercial hygroscopic flares (“French”, “South African”, New AI and D383) increases raindrop production in those cloud parcels where the natural warm rain process is inefficient. The most effective flare was found to have a maximum fraction of large seeding cloud condensational nuclei (SCCN). An optimum seeding particle radius, which provides the maximum raindrop production under a given mass of the seeding reagent varies from 1.5 to 2.5 μm and slightly depends on the reagent mass, as well as on the dynamic properties of cloud parcels. The existence of the optimum size of seeding particles is important from both a business perspective and an environmental perspective. In the presence of natural, large CCN, the seeding effect decreases due to the efficient collision process initiated by them. The decrease depends on the concentration of large, natural CCN. Thus, to evaluate the effects of hygroscopic seeding one needs to know the properties of large, natural CCN in the region of seeding activity. It is shown that, when a reagent consisting of CCN of the optimum radius is used, a significant increase in the raindrop production can be achieved even when large, natural CCN are present as well. Owing to the limitations of warm rain cloud parcel models in representing the microphysical and dynamical properties of real clouds, the results presented in this study should be verified using more complicated multidimensional models with spectral microphysics.

Hydrochemistry of runoff and subsurface flow within Sahelian microdunes

SummaryThe sandy microdune systems of the Sahel are important for biomass production, in that they trap and store water. We have studied the movement of water over and in a dune and the chemistry of the water to understand this aspect of the systems. We experimented with simulated rain using a field sprinkling infiltrometer. We applied demineralized water with a chemical composition similar to that of the natural rain on a 1‐m2 plot. The plot was delimited by a metallic two‐level setting: the first enabled us to collect surface runoff, while the second measured subsurface flow. Water samples were taken at 5‐ to 10‐minute intervals throughout each simulation for chemical analysis (alkalinity, SO42–, F–, NO3–, Ca2+, Mg2+, Na+, K+ and Si). Mass balances, combined with a simple mixture model involving one tracer (chloride) and two reservoirs (old and new waters), were calculated. The equilibrating pressures of the CO2 (pCO2) and the saturation index with respect to specified minerals (e.g. calcite, fluorite, silicates) were also calculated by the AQUA ion‐pair model. The solute concentrations decrease in surface runoff as well as in subsurface water, except for F– and Si in the subsurface. The pCO2 decreased to a pressure less than the atmospheric pressure. The difference between measured concentrations and concentrations computed with the mixing model highlighted interactions between the soil and water. The dissolution of calcite which consumes CO2, and the cation exchanges, dominated, whereas the dissolutions of fluorite, silicates and gypsum appear secondary. Reactive mineral stocks were quickly exhausted, especially in the surface flow.

Influência de lâminas de água e adubação mineral na nutrição e produtividade de arroz de terras altas

Para recomendações de adubação mais racionais, é fundamental o conhecimento das exigências nutricionais da cultura do arroz, nos diversos sistemas de cultivo. Objetivando estudar a influência de lâminas de água na nutrição e exportação de nutrientes pelo arroz de terras altas, cultivar IAC 201, sob dois níveis de adubação, foram instalados experimentos em um Latossolo Vermelho distrófico, em Selvíria (MS), nos anos agrícolas de 1994/95 e 1995/96. O delineamento foi de blocos casualizados, com quatro repetições. Os tratamentos constituíram-se da precipitação natural e de quatro lâminas de água fornecidas por aspersão. A lâmina L2 foi baseada no coeficiente de cultura (Kc) do arroz de terras altas. As lâminas L1 e L3 foram definidas como 0,5 e 1,5 vez os Kcs utilizados em L2, respectivamente, e na lâmina L4 foi adotado Kc = 1,95 durante todo o ciclo da cultura. Em 1995/96, foram utilizados os mesmos tratamentos em parcelas subdivididas, sendo as subparcelas constituídas por duas doses de adubação (AD1 - 12 kg ha-1 de N, 90 de P2O5 e 30 de K2O, e AD2 - 24 kg ha-1 de N, 180 de P2O5 e 60 de K2O). A menor disponibilidade de água durante a fase vegetativa e reprodutiva proporcionou redução na produção de matéria seca, nos teores e quantidades de nutrientes acumuladas na parte aérea. O sistema irrigado por aspersão, independentemente da lâmina utilizada, proporcionou maior produtividade de grãos e exportação de nutrientes. Em solos com teores adequados de nutrientes para o sistema de sequeiro, não há resposta ao aumento da adubação mineral pelo arroz no sistema irrigado por aspersão, apesar da maior extração de nutrientes.

The Shape–Slope Relation in Observed Gamma Raindrop Size Distributions: Statistical Error or Useful Information?

Abstract The three-parameter gamma distribution n(D) = N0Dµ exp(–ΛD) is often used to characterize a raindrop size distribution (DSD). The parameters µ and Λ correspond to the shape and slope of the DSD. If µ and Λ are related to one another, as recent disdrometer measurements suggest, the gamma DSD model is simplified, which facilitates retrieval of rain parameters from remote measurements. It is important to determine whether the µ–Λ relation arises from errors in estimated DSD moments, or from natural rain processes, or from a combination of both statistical error and rain physics. In this paper, the error propagation from moment estimators to rain DSD parameter estimators is studied. The standard errors and correlation coefficient are derived through systematic error analysis. Using numerical simulations, errors in estimated DSD parameters are quantified. The analysis shows that errors in moment estimators do cause correlations among the estimated DSD parameters and cause a linear relation between est...

Temperature and Wetness-Duration Requirements for Grape Leaf and Cane Infection by Phomopsis viticola.

In 1998 and 1999, controlled-environment studies were conducted in growth chambers to determine the temperature and wetness-duration parameters required for leaf and cane infection of grape by Phomopsis viticola. Greenhouse-grown 'Catawba' (Vitis labrusca) and 'Seyval' (French hybrid) grapes were inoculated with P. viticola and incubated at constant temperatures of 5, 10, 15, 20, 25, 30, and 35°C and at wetness durations of 5, 10, 15, and 20 h for each temperature. Data from each cultivar were analyzed by nonlinear regression analysis to determine the relationship between disease severity and temperature and wetness duration. A generalized form of the Analytis Beta model was found to provide the best fit to the data. Disease severity on leaves and canes increased with increasing wetness duration at most temperatures. Minimum and maximum temperatures for infection were around 5 and 35.5°C, respectively. Optimum temperatures for leaf and cane infection were between 16 and 20°C. In the 2000 and 2001 growing seasons, the generalized Beta model was validated in 'Catawba' and 'Seyval' vineyards by inoculating vines during natural rain events. Average temperature and hours of wetness for each event and inoculation were recorded and used in the model equation to predict disease severity on leaves and internodes. Correlation coefficients between observed disease severities following field inoculations and predicted disease severities for both cultivars were between 0.71 and 0.81 and always significant (P < 0.01). These results indicate that the model reliably predicted leaf and cane infection on both cultivars over a wide range of wetness durations and temperatures. The model may be useful in developing disease-forecasting systems for Phomopsis cane and leaf spot on grapes.

Defoliation alters water uptake by deep and shallow roots of Prosopis velutina (Velvet Mesquite)

Summary Prosopis velutina Woot. (Velvet Mesquite) at a site with limited groundwater availability derived a greater percentage of water from shallow soil at the onset of the summer rainy season than did trees at a site with greater availability of groundwater. Predawn leaf water potentials (Ψpd) were not a strong indicator of shallow water use for this species with roots in multiple soil layers. We experimentally defoliated P. velutina plants to determine if reduced‐canopy photosynthesis would alter vertical patterns of root activity. After natural rain events, hydrogen isotope ratios of xylem sap indicated that defoliated P. velutina took up a greater percentage of its water from shallow soils than did undefoliated plants. Irrigation with deuterium‐labelled water further demonstrated that undefoliated plants were able to use shallow soil water. Defoliation appeared to affect the ability of trees to use deep‐water sources. Reduced carbon assimilation limited water uptake from deep soil layers. These data highlight that there are internal physiological controls on carbon allocation that may limit water uptake from different soil layers. During periods of high vapour pressure deficit or soil drought, when leaf gas exchange and carbon assimilation decline, this may create positive feedbacks where plants are unable to forage for deep water due to carbon limitations.

Infiltration on mountain slopes: a comparison of three environments

Water is well established as a major driver of the geomorphic change that eventually reduces mountains to lower relief landscapes. Nonetheless, within the altitudinal limits of continuous vegetation in humid climates, water is also an essential factor in slope stability. In this paper, we present results from field experiments to determine infiltration rates at forested sites in the Andes Mountains (Ecuador), the southern Appalachian Mountains (USA), and the Luquillo Mountains (Puerto Rico). Using a portable rainfall simulator–infiltrometer (all three areas), and a single ring infiltrometer (Andes), we determined infiltration rates, even on steep slopes. Based on these results, we examine the spatial variability of infiltration, the relationship of rainfall runoff and infiltration to landscape position, the influence of vegetation on infiltration rates on slopes, and the implications of this research for better understanding erosional processes and landscape change. Infiltration rates ranged from 6 to 206 mm/h on lower slopes of the Andes, 16 to 117 mm/h in the southern Appalachians, and 0 to 106 mm/h in the Luquillo Mountains. These rates exceed those of most natural rain events, confirming that surface runoff is rare in montane forests with deep soil/regolith mantles. On well-drained forested slopes and ridges, apparent steady-state infiltration may be controlled by the near-surface downslope movement of infiltrated water rather than by characteristics of the full vertical soil profile. With only two exceptions, the local variability of infiltration rates at the scale of 10° m overpowered other expected spatial relationships between infiltration, vegetation type, slope position, and soil factors. One exception was the significant difference between infiltration rates on alluvial versus upland soils in the Andean study area. The other exception was the significant difference between infiltration rates in topographic coves compared to other slope positions in the tabonuco forest of one watershed in the Luquillo Mountains. Our research provides additional evidence of the ability of forests and forest soils to preserve geomorphic features from denudation by surface erosion, documents the importance of subsurface flow in mountain forests, and supports the need for caution in extrapolating infiltration rates.

Qualidade industrial e teores de nutrientes dos grãos do arroz de terras altas sob diferentes lâminas de água e níveis de adubação mineral

With the objective of evaluate the effects of water levels and mineral fertilizers on the mill revenue and nutrient content ot the grains of upland rice cv. IAC 201, two experiments were conducted in Selviria , state of Mato Grosso do Sul , Brazil. The experimental design was randomized blocks and treatments consisted of five water levels: natural rain and four water levels supplied by sprinkler irrigation. Irrigation levels were based on crop coefficient (Kc) for dryland rice that resulted in water level 2 (L2). Water levels L1 and L3 were then defined as 0.5 and 1.5 Kcs used in L2, respectively, and on water level 4 (L4) was used Kc=1,95, during all plant cicle. In 1995/96 the same treatments were applied in a split-plot scheme, where subplots consisted of two NPK-fertilization levels (300 e 600kg ha -1 of formula 04-30-10). Sprinkler irrigation incresed hulling yield, and undamaged grains; low water availability incresed P, Ca, Mg, and Fe and, decreased K and S content of polished grains; the mineral fertilizer levels did not interfere in the nutrients and protein content of polished grains and industrial quality of grains, in soil with appropriete nutrients levels.

Absorção, exportação e eficiência de utilização de nutrientes pela cultura do arroz de terras altas em função de lâminas de água aplicadas por aspersão

A field experiment was carried out on a Typical Haplustox, in Selvíria, MS, Brazil, to evaluate the effects of water levels applied through sprinkler irrigation on plant dry matter, macronutrients uptake at flowering, exportation and nutrient use efficiency of dry land rice, cv. Caiapó. The water levels used were: natural rain and four water levels supplied by sprinkler irrigation. Irrigation levels were based on crop coefficient (Kc) for dry land rice that resulted in water level 2 (L2). Water levels L1,/sub> and L3 were defined as 0.5 and 1.5 Kcs used in L2, respectively, and Kc=1.95 was used on water level 4 (L4) during all plant cycle. The water level 4 (L4) promoted greater nutrient uptake, however the same was not verified in relation to the exportation, because there was not translocation for the grains in the same proportion. The greatest grain yield and nutrient use efficiency was obtained under water level 2 (L2P>Mg>Ca>K>N.

Produtividade e qualidade industrial de grãos de arroz de terras altas em função de lâminas de água no sistema irrigado por aspersão

The objective of this paper was to evaluate the effects of water level applied by sprinkler irrigation on the phenology, grain yield and quality of the dry land rice, cv. Caiapo, Oryza sativa L. on a Typical Haplustox, grown in Selviria, MS, Brazil. The water levels used were: natural rain and four water levels supplied by sprinkler irrigation. Irrigation levels were based on crop coefficient (Kc) for dry land rice that resulted in water level 2 (L 2 ). Water levels L 1 and L 3 were defined as 0.5 and 1.5 Kcs used in L 2 , respectively, and Kc=1.95 was used on water level 4 (L 4 ) during all plant cycle. Low water availability, during vegetative phase, increased plant cycle and reduced plant height. The greatest grain yield was obtained under water level 2 (L 2 ). The sprinkle irrigation increased hulled yield and percentage of undamaged grains.

Manejo de irrigação por aspersão com base no "Kc" e adubação mineral na cultura de arroz de terras altas

A irrigação por aspersão diminui bastante o risco de perda da lavoura por deficiência hídrica e aumenta a produtividade de grãos, incentivando maior uso de tecnologias como adubação mineral. Com o objetivo de avaliar o efeito de diferentes manejos da água da irrigação por aspersão com base no coeficiente de cultura (Kc) e da adubação mineral sobre a cultura do arroz cv. IAC 201, foram instalados dois experimentos em Latossolo Vermelho Distrófico, em Selvíria (MS). O delineamento foi em blocos casualizados, com quatro repetições. Os tratamentos constituíram-se de precipitação pluvial natural e três manejos de água fornecidos por aspersão. O manejo (M2) foi realizado com base no Kc do arroz de terras altas. Os manejos M1 e M3 foram definidos como 0,5 e 1,5 vezes os Kcs utilizados em M2 respectivamente. Em 1995/96, utilizou-se o esquema de parcelas subdivididas, sendo as subparcelas constituídas por dois níveis de adubação: AD1 - 12 kg de N, 90 kg de P2O5 e 30 kg de K2O ha-1, e AD2 - 24 kg de N, 180 kg de P2O5 e 60 kg de K2O ha-1. A deficiência hídrica da emergência da plântula até a diferenciação do primórdio da panícula provocou aumento do ciclo e redução do porte da planta. A deficiência hídrica entre os estádios de diferenciação do primórdio da panícula e os de emborrachamento reduziu o número de espiguetas por panícula. A utilização de 1,5 vezes os valores de Kc recomendados, no manejo da irrigação por aspersão proporcionou maior produtividade de grãos. Os níveis de adubação utilizados não influenciaram a resposta da cultura ao manejo da irrigação por aspersão.

PROBLEMS IN ESTIMATING IMPACTS OF FUTURE CLIMATE CHANGE ON MIDWESTERN CORN YIELDS

Recent studies of trends in Midwestern precipitation show marked increases over the last 50 years of the 20th Century, and most climate models project that future rainfall in the Corn Belt will be increased further. During five years, 1988–1991 and 1994, field tests were conducted on agricultural test plots in central Illinois, an area typical of the Corn Belt, to discern how corn yields reacted to varying levels of added rainfall (+10%, +25%, and +40%) during the growing season. The best treatment over the five years was a 40% rain increase, with an average yield increase of 9%. Its yield increase was up to 34% in a hot-dry year, but below that of natural rainfall in a wet year as were the yields of the other lesser increases. The average yield changes from the three treatments were not statistically significantly different. Major interannual yield differences were found in the yields for each rain treatment, reflecting how rain timing and temperatures also have major effects on yields. A 40% summer rain increase has little influence if natural rains do not occur in the high stress period of mid summer. The plots results show that only small average increases in corn yields occur from growing season rain additions in the 10% to 40% range, except in dry years. Weather-crop yield regression models incorporating the same rain increases predict greater yield increases than found in these field tests. This suggests that future yields projected for a wetter climate using yield-weather models may be over-estimated. The plot sample size is small but conditions sampled in the five years represented 43% of all past 97 growing seasons in central Illinois and extremely good and bad weather years, which resulted in large between-year yield differences. Hence, the experimental results provide useful information about how increased rainfall may affect future corn yields, especially since the sample included three of the five types of dry growing seasons found in the area's climate since 1900.

Aggregates stability and water erosion in Andosols of the Canary Islands

AbstractThe aim of this paper is to assess the mechanisms of water erosion in andic soils using two tests, which in a certain way simulate the two principal mechanisms of aggregate destruction in the process of water erosion—water dispersion and raindrops impact—and compare them with the aggregation observed in material dettached by inter‐rill erosion (sediments) in experimental plots with natural rain. In accordance with the obtained results, the erosive process in these soils seems to come about through a picking off of surface material of larger aggregates, due to the impact of raindrops. The intensity of pull off and fragment size from larger aggregates depends on the kinetic energy of the drops (rain intensity), but the size generally ranges between 0ċ2 and 0ċ5 mm. Therefore inter‐rill erosion initially proceeds by a washing down of smaller aggregates (&lt;0ċ5 mm) (of less bulk density than larger aggregates 0ċ4 Mg m−3 against 0ċ9 Mg m−3), enriching the soil in larger sized aggregates which, on being fragmented by picking off of raindrops, supply new material for washing down by inter‐rill erosion. Copyright © 2002 John Wiley &amp; Sons, Ltd.