Abstracts

Azeez, J.O., Chikoye, D., et al. 2005, 'Effect of Drought and Weed Management on Maize Genotypes and the Tensiometric Soil Water Content of an Eutric Nitisol in South Western Nigeria', Plant and Soil, vol. 276, pp. 61-68.

In the dry savannas of West and Central Africa, where low soil fertility, unpredictable rainfall, weed competition and recurrent drought are major constraints to maize production, the development of tropical maize genotypes with high and stable yields under drought and low-nitrogen condition is very important since access to these improved genotypes may be the only affordable alternative to many small scale farmers. Field trials were conducted in 2002 and 2003 at Ikenne southwestern Nigeria to investigate the effect of weed pressures and drought stress on 2 maize (Zea mays L.) hybrids (9134-14, 9803-9) and 2 open-pollinated varieties (STREVIWD, IYFDCO1). Irrigation was withdrawn 4 weeks after planting (about four weeks to mid-flowering) in the drought stress while the adjacent watered treatment had irrigation throughout the growing period. The weed pressures were the completely weeded plots (hand weeding every week) and weedy plots (weeded once, 2 weeks after planting).The experiment was a split plot in a randomized complete block design with four replicates. Drought stress reduced the stover weight and grain yield of the maize cultivars by 6% and 34% respectively. Weed-free plots had maize with higher agronomic traits than unweeded treatments. Hybrid 9803-9 was more susceptible to drought and weed stress as indicated in the stover weight and grain yield. STREVIWD an open-pollinated variety (OPV) and Hybrid 9134-14 had superior performances in terms of grain yield and shorter anthesis silking interval. Soil moisture content was higher in the unweeded plots while the uptake of moisture was highest in drought susceptible hybrid 9803-9. Irrespective of the genotypes, maize (hybrid and OPV) was more tolerant to drought in a weed-free environment than in unweeded conditions. There existed a negative but signifcant correlation between weed biomass and chlorophyll content (-0.29, P <0.01), grain yield (-0.45, P <0.05), ear plant^-1 ( -0.27, P <0.05) and kernel-number (-0.366, P <0.01).

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Bradley, C. and Gilvear, D.J. 2000, 'Saturated and Unsaturated Flow Dynamics in a Floodplain Wetland', Hydrological Processes, vol. 14, pp. 2945-2958.

This paper describes initial work modelling unsaturated processes within floodplain wetlands. The limitations of solely considering the dynamics of the wetland water table and hence saturated processes are examined. Water table, precipitation and evapotranspiration data from Narborough Bog, a degraded floodplain wetland of 10 ha in Leichestershire, were used in the development of a one-dimensional saturated-unsaturated flow model using UNSAT1. Hydraulic parameters were derived from a combination of field and laboratory analysis of wetland deposits previously undertaken to develop a saturated flow model of the wetland. Additional parameters describing unsaturated flow were derived by a mathematical approximation based upon the soil moisture characteristic of representative wetland sediments. The resulting model is used to explore the interaction between water table position, bertical soil moisture fluxes and evapotranspiration. Model simulations are completed over a 20-day period, and used to indicate the relationship of moisture content to key hydraulic properties of soil profiles typical of the wetland and to illustrate the wat in which this varies laterally along a transect. The results highlight the need to consider the importance of correct description of the wetland and substrate and, notwithstanding difficulties in describing unsaturated flow parameters, there is a considerable need to improve this description of near-surface wetland water fluxes and their expansion to two dimensions.

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Cavieres, L.A., Badano, E.I., et al. 2006, 'Positive Interactions between Alpine Plant Species and the Nurse Cushion Plant Laretia acaulis do not Increase with Elevation in the Andes of Central Chile', New Phytologist, vol. 169, no. 1, pp. 59-69.

In alpine habitats, positive interactions among plants tend to increase with elevation as a result of altitudinal increase in environmental harshness. However, in mountains located in arid zones, lower elevations are also stressful because of scarce availability of water, suggesting that positive interactions may not necessarily increase with elevation. Here we analysed the spatial association of plant species with the nurse cushion plant Laretia acaulis at two contrasting elevations, and monitored the survival of seedlings of two species experimentally planted within and outside cushions in the semiarid Andes of central Chile. Positive spatial associations with cushions were more frequent at lower elevations. Species growing at the two elevations changed the nature of their association with cushions from neutral or negative at higher elevations to positive at lower elevations. Survival of seedlings was higher within cushions, particularly at lower elevations. The increased facilitation by cushions at lower elevations seems to be related to provision of moisture. This result suggests that cushion plants play a critical role in structuring alpine plant communities at lower elevations, and that climatic changes in rainfall could be very relevant for persistence of plant communities.

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Heschel, M.S. and Hausmann, N.J. 2001, 'Population Differentiation for Abscisic Acid Responsiveness in Impatiens Capenesis (Balsaminaceae)', International Journal of Plant Science, vol. 126, no. 6, pp. 1253-1260.

To test for population differentiation in sensivity to abscisic acid (ABA) in jewelweed (Impatiens capensis), we experimentally manipulated soil water availability and sprayed ABA on inbred lines derived from sites with different water availabilities. Response to these treatments was quantified with both morphological and physiological measurements, including water use efficiency (WUE). Plants from a relatively dry site in Rhode Island responded to drought theatment by significantly increasing survivorship in subsequent drought in subsequent drought compared with plants from a wet site. ABA treatment of the "dry" population plants WUE; however, ABA application did not significantly affect WUE in "wet" population plants. Wet-population plants were also limited by a trade-off between fruit production and drought survivorship. Additionally, regardless of treatment, wet-population plants had lower survivorship than dry-population plants. These environment-specific repsonses to water stress provide strong evidence for adaptive differentiation between populations of I. capensis.

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Huat, B.B.K., Ali, F.H., et al. 2005, 'Response of Suction, Moisture and Temperature of Unsaturated Granitic Residual Soil to Rainfall', Electronic Journal of Geotechnical Engineering, vol. 10, pp.

Soils located above the groundwater table such as residual soils are generally unsaturated and possess negative pore water pressures (matric suction). However, heavy, continuous rainfall can result in the decreased in matric suction and even increased in pore water pressures to a significant depth, resulting in the instability of the slope. This paper describes a study that has been made to determine the response of suction to rainfall of an unsaturated residual soil slope of weathered granite. Suctions are measured using tensiometers, and the rainfall is recorded by using an automatic logging tipping bucket rain gauge. In addition, the soil moisture and temperature response to rainfall are also monitored. The value of suction is found to vary with depth and soil weathering grades. Suction is observed to decrease with increase in depth below the ground surface. Granitic residual soil of weathering grade IV appears to have the lowest value of suction, indicating the more porous nature of soil of this grade. During the dry spell, the suction values at the shallower depth are observed to be generally larger than the suction values at the deeper depths. Upon rain (wetter period), there is a general lost (reduction) of suction in all cases. However, the suction response to antecedent rainfall is less pronounced as the depth increases. The drop in suction at the deeper depth is only significant over a long wet period, indicating longer time required by the water to infiltrate the slope. With regards to soil moisture, there appears to be an increase in the soil moisture with rain. After the rain has stopped, there is a gradual decrease in the moisture content as the soil recovers its suction. Likewise there is a decrease of soil temperature due to the rain.

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McKergow, L.A., Prosser, I.P., et al. 2004, 'Performance of Grass and Rainforest Riparian Buffers in the Wet Tropics, Far North Queensland. 1. Riparian Hydrology', Australian Journal of Soil Research, vol. 42, pp. 473-484.

The long and intense storms of the wet tropics present extreme conditions for testing the effectiveness of riparian buffers. This study presents results of a hydrometric investigation of 4 riparian buffers on 2 commercial banana plantations in Far North Queensland, Australia. It investigates runogg generation and riparian hydrology on hillslopes with differeing slopes, contributing areas and topographic convergence. Both grass and rainforest buffers were examined. Surface and subsurface hydrology were measured for 4 wet seasons (December-April) using paired flumes, piezometers and tensiometers. All bufferes experienced large bolumes of surface runoffm with peak discharges ranging from 30 L/s on planar hillslopes to 350 L/s on a highly convergent site. Event runoff : rainfall ratios ranged between 0.01 and 0.65. Grass buffers with smaller contributing buffers with smaller contributing areas (<0.3 ha) were able to dissipate the energy of surface runoff under all conditions. On a larger (5 ha), highly convergent hillslope surface runoff became chanalised upslpe of the buffer and the vetiver hedges and grass were not able to preven scouring of a channel through the buffer, reducing its performance. Infiltration occurred in all buffers during small events, and at the convergent buffer during large events, most likely due to the presence of deep soil fill. In contrast, exfiltration occurred in the grass buffers on planar and moderately converging slopes during large events. There, the riparian soil apprached saturation and return flow and seepage were measured. Under exfiltration, soil strength may be decreased and riparian buffers are needed to decreas erosion hazard. Localised saturation was observed in the rainforest buffer beneath a planar hillslope during large events, where soils were deeper and dired out more quickly than in the adjacent grass buffer. This study documents the high runoff volumes and peak discharges on cropped slopes in the wet tropics, and evaluates riparian hydrological processes. Infiltration is unlikely to be an important buffer function in this environment, but an affitional role of buffers is to reduce the erosion hazard presented by exfiltration.

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Norton, D.A. 1994, 'Relationships between Pteridophytes and Topography in a Lowland South Westland Podocarp Forest', New Zealand Journal of Botany, vol. 32, pp. 401-408.

The distribution of pteridophytes in a lowland podocarp forest was found to be strongly correlated with environmental variables associated with soil wetness, with poorly drained outwash terraces supporting a different pteridophyte assemblage from better drained moraine ridges. Canopy composition appears to have a secondary influence on pteridophyte distribution, primarily through the restriction of some species to recently disturbed sites. Although individual pteridophyte species showed marked changes in abundance along environmental (soil wetness) gradients, species groups appear better indicators of topological position.

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Rahardjo, H., Lee, T.T., et al. 2005, 'Response of a Residual Slope to Rainfall', Canadian Geotechnical Journal, vol. 42, no. 2, pp. 340-351.

Rainfall induced landslides are a common problem in residual soil slopes of the tropics. It is widely known that rainfall-induced slope failures are mainly caused by infiltration of rainwater; however, the response of a residual soil slope to infiltration is not fully understood. The difficulties lie in the quantification of the flux boundary condition across the slope surface with respect to infiltration and its effect on the pore-water pressure conditions in the slope. Therefore, it is important to understand the response of a slope to different rainfall conditions and the resulting changed in pore water pressures and water contents. A residual soil slope in Singapore was instrumented with pore-water pressure, water content, and rainfall measuring devices, and studies were carried out under natural and simulated rainfalls. Results indicate that significant infiltration may occur in a residual soil slope during a rainfall. Small total rainfalls can contribute to a larger infiltration percentage than large total rainfalls. The percentage of infiltration ususally decreases with increasing total rainfall. The study has indicated the existence of a threshold rainfall of about 10 mm for runoff generation to commence. Infiltration during wet periods may lead to the development of positive pore-water pressures as a consequence of a perched water table condition. Matric suctions are recovered gradually during dry periods due to redistribution. Soil water contents tend to be higher near the toe of the slope than at the crest irrespective of rainfall events, indicating subsurface movement of water in the downslope direction. The study has also indicated a correlation between rainfall amount and relative increase in pore-water pressure. The results can be used to quantify the flux boundary conditions required for the seepage analyses associated with rainfall-induced slope failures.

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Rezaur, R.B., Rahardjo, H., et al. 2002, 'Spatial and Temporal Variability of Pore-water Pressures in Residual Soil Slopes in a Tropical Climate', Earth Surface Processes and Landforms, vol. 27, pp. 317-338.

It is critical to understand and quantify the temporal and spatial variability in hillslope hydrological data in order to advance hillslope hydrological studies, evaluate distributed parameter hydrological models, analyse variability in hydrological response of slopes and design efficient field data sampling networks. The spatial and temporal variability of field-measured pore-water pressures in three residual soil slopes in Singapore was investigated using geostatistical methods. Parameters of the semivariograms, namely the range, sill and nugget effect, revealed interesting insights into the spatial structure of the temporal situation of pore-water pressures in the slopes. While informative, mean estimates have been shown to be inadequate for modelling purposes; indicator semivariograms together with mean prediction by kriging provide a better form of model input. Results also indicate that significant temporal and spatial variability in pore-water pressures exists in the slope profile and thereby induces variability in hydrological response of the slope. Spatial and temporal variability in pore-water pressure decreases with increasing soil depth. The variability decreases during wet conditions as the slope approaches near saturation and the variability increases with high matric suction development following rainfall periods. Variability in pore-water pressures is greatest at shallow depths and near the slope crest and is strongly influenced by the combined action of microclimate, vegetation and soil properties.

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Wanjura, D.F., Mahan, J.R., et al. 1996, 'Irrigation Starting Time Effects on Cotton under High-Frequency Irrigation', Agronomy Journal, vol. 88, pp. 561-566.

Deciding when to begin irrigating cotton (Gossypium hirsutum L.) is difficult because the positive effect of eliminating water stress may be offset by the negative effect of lowered soil temperature. Different starting times for high-frequency drip irrigation of cotton were investigated in a field having Olton clay loam soil (fine, mixed, thermic Aridic Paleustolls). Early irrigation (EI), started when seedlings had 3 mainstem nodes, was compared with delayed irrigation (DI-L), started at squaring when there were 7 mainstem nodes. Another delayed irrigation treatment (DI-H) received an initial large irrigation that equaled the cumulative amount of water applied by that time to the EI treatment. During the early irrigation period, 130 mm of water was applied in 1993 and 120 mm in 1994. Average soil temperature in the top 1000 mm during the early irrigation period was 25°C in the EI treatment, compared with 27°C in the DI treatment. During the late irrigation period, the average soil temperature was 24.5°C for EI and 25.5°C for DI. At first bloom, plants were 30 mm taller in 1993 and 70 mm taller in 1994 in the EI irrigation treatment, compared with the DI-L treatment. Lint production was greatest in the DI-L treatment in both years (1548 and 1630 kg ha^-1), compared with the DI-H treatment (1467 and 1460 kg ha^-1) and the EI treatment (1447 and 1481 kg ha^-1). Water use efficiency, based on the total of irrigation and rainfall, averaged 3.33, 2.42, and 2.36 kg lint ha-mm^-1 for the DI-L, DI-H, and EI irrigation treatments, respectively. Initiating automated irrigation scheduling before squaring in cotton increased seedling height but decreased lint yield and the efficiency of use of total applied water.

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Yang, H., Rahardjo, H., et al. 2004, 'A Study of Infiltration on three Sand Capillary Barriers', Canadian Geotechnical Journal, vol. 41, no. 4, pp. 629-643.

The capillary barrier effect was investigated by conducting infiltration tests on three soil columns of fine sand over medium sand, medium sand over gravelly sand, and fine sand over gravelly sand. The barrier effect was verified in the underlying layer of coarser material, and the water-entry values of the coarser laters were confirmed to be nearly equal to the residual matric suctions of the soils. The coarser layer of gravelly sand, which had a lower water-entry value, was more effective in forming a barrier than the coarser layer of medium sand, which had a higher water-entry value/ When the capillary barrier was comprised of a coarser layer of gravelly sand, there ware more water stored in the finer layer at the end of the drying stage than when the capillary barrier was comprised of a coarser layer of medium sand. Non-equilibrium static conditions of pore-water pressure profiles were observed in the rhtree soil columns, and a generalized ultimate pore-water pressure porfile of a caplillary barrier system was proposed. In addition, the final volumetric water contents versus matric suctions of the soils as measured from the soil columns were reasonably consistent with the soil-water characteristic curves (SWCCs) of the soils, suggesting that the drying SWCC of a sloil could also be obtained from the drying process in a soil column (or a capillary open tube). The drying SWCC could be establised from measurements in the soil column uo to a height corresponding to two times the residual matric suction head of the soil.

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