Browsing by Author "Snow, Valerie Olga"

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    The pattern of soil water extraction by individual kiwifruit vines : a thesis presented in partial fulfillment of the requirements for the Master of Agricultural Science in Soil Science at Massey University
    (Massey University, 1987) Snow, Valerie Olga
    In order to efficiently design and operate irrigation systems water balance studies are needed. To date few of these studies have been carried out on kiwifruit. Detailed measurements of water extraction were made beneath two 7 year old kiwifruit vines. Under-vine covers were used on these vines to exclude rainfall and irrigation. Measurements of fruit size and leaf water potential were made on the two covered vines and on adjacent irrigated vines. In addition, solar radiation and air temperature were monitored in the orchard block. In concurrent studies, the root distribution of vines in the orchard were determined and heat pulse measurements of sapflow were made. The water extraction pattern showed little variation with depth to the maximum depth of measurement (2.2 m). There was, however, considerable variation in extraction with horizontal distance away from the vine. This variation may be explained in terms of the root distribution. The soil volume may be divided into the zone of occupation, in which the soil is completely occupied by the plant roots, and the zone of exploration, which is the volume of soil in which there are a few roots but the soil is still largely unexplored. Within the zone of occupation, water is uniformly extracted despite variation in root density. Water appears to be extracted from the zone of exploration primarily by flow of water towards the zone of occupation, where the soil water potential is lower. The fruit volume and leaf water potential measurements were used to indicate the onset of water-stress. At this time, soil water potential in the zone of occupation was between -40 and -50 kPa. The size of the reservoir of readily availible water was found to be at least 2.1 m 3 for 7 year old vines, and is projected to rise to a maximum of at least 6.5 m3 in three or so years in this orchard. Whereas the vine canopy may, by management, mature in 3 years, the root system may take 10 years to mature, so irrigation requirements of young vines will be higher than for mature vines. This is contrary to common assumptions made in standard methods for designing horticultural irrigation systems and is due to changes in the size of the reservoir rather than changes in the rate of water use. When there is radial variation in water extraction it is important to take account of the variation when calculating volumes of water extracted from the soil. The rate of water use by the vines, as estimated by the water balance method and the heat pulse technique, was found to be considerably lower than that predicted by the equilibrium evapotranspiration rate. This may be due to experimental error, and further work is required to clarify this matter.
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    Solute transport in a layered field soil : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 1992) Snow, Valerie Olga
    Although concern about the effects of movement of chemicals through soil has brought about a need for greater understanding of solute transport, the question as to where best to focus the research effort remains open. Initially a philosophical fram ework was presented that described in a general sense how research into solute transport has been conducted. It was argued that we must combine modelling with experimentation for effective progress in understanding, and that the efforts in field versus lab experimentation and process- versus nonm echanistic modelling should be balanced. Currently there is a need for more field experimentation, but the preferred direction of the modelling effort is less clear. Both process-based and non-mechanistic models are considered in order to deduce the effect of soil layering on solute transport. Field experiments were carried out on a soil consisting of three layers of distinct texture. This soil was instrumented with porous cup samplers at four depths at twenty sites. There was also a 2 m2 lysimeter within the plot. In the first experiment irrigation was used to supplement rainfall in order to leach a surface application of solid KCl through the soil. Porous cup samples of the soil solution were collected on numerous occasions and soil cores less often . The experiment of the following year was similar in design except that no irrigation was used. Finally, in the third year, the Iysimeter was instrumented with porous cup samplers and the same experimental design repeated on a smaller scale. A convection-dispersion (CDE) model was applied to the lysimeter data. This was successful, provided that the surface soil and assumed Dirac delta solute input were not included in the calibration. Layering within the profile appeared to have little effect on solute transport. The transport porosity was revealed to be two-thirds of the water-filled porosity, thus a substantial part of the water-filled porosity did not transport solute. The CDE modelling of the field data was not particularly successful, probably due to the spatially variable nature of solute transport and water application. The Aggregated Mixing Zone (AMZ) model was also used. This model subdivided the transport porosity i nto convective and dispersive components, and also allowed for non-interacting flow paths. Although the AMZ model was conceptually appealing, parameterisation of the model was found to lack discrimination. Little further understanding of solute transport was gained from this model. Textural differences in the soil seem to be overwhelmed by both small-scale heterogeneity of water application and solute movement in the soil, especially near to the surface. It was apparent that processes occurring in the surface soil require much more attention than they have been afforded in the past. Both process-based modelling and field experimentation will increase our understanding of solute transport. It also seems that an increased effort in improving measurement techniques will be advantageous.