When the evaporative demand is greater than the ability of the soil to conduct water in the liquid phase, the soil profile above a watertable exhibits a liquid−vapour discontinuity, known as the evaporation front, that affects the depth of salinisation and the rate of evaporation. We conducted experiments on a sandy loam with shallow saline watertables under high isothermal evaporative demand (24 mm/day), monitoring rates of evaporation from the soil and upward movement of groundwater, and observing profiles of soil water and salinity over periods of up to 78 days. Three zones were distinguished in the soil profile: a zone of liquid flow above the watertable, a zone of vapour flow close to the surface, and an intermediate transition zone in which mixed liquid−vapour flow occurred. The vapour-flow zone above the evaporation front appeared after a few days and progressed downward to depths of 40, 60, and 120 mm, while eventual steady-state rates of evaporation were 1.3, 1.1, and 0.3 mm/day for watertable depths of 300, 450, and 700 mm, respectively. Salts mainly accumulated in the transition zone, suggesting that the depth of the evaporation front should be a criterion to locate and prevent salinisation as a result of capillary flow from a watertable in arid regions.
A field experiment at Cockle Park, Northumberland on a clay loam soil (Dunkeswick series) cropped with winter wheat investigated the effects of drainage and season of application on pesticide movement. Isoproturon, mecoprop, fonofos and trifluralin were applied in two consecutive seasons at normal agricultural rates to three hydrologically isolated plots each of 0.25 ha. Two of the plots were mole-drained and the third was an undrained control. Surfacelayer flow and drainflow from each plot were monitored at 10-min intervals. Samples of flow were analysed for pesticides to evaluate transport of applied chemicals from the site. Despite widely differing properties (Koc 20-8000 ml g-', t,,* 10-60 days), all four pesticides were found in surface-layer flow and mole drainflow from the site. Maximum concentrations of pesticides in flow ranged from 0.1 to 121 pg litre-' (aqueous phase) and cO.2 to 48 pg litre-' (particulate phase). Over two contrasting seasons, total losses of pesticides in flow followed total amounts of flow and were approximately four and five times larger, respectively, in 1990/91 than in 1989/90. The maximum loss occurred from the undrained plot and was 2.8 g isoproturon (0.45% of that applied). Total losses of autumn-applied pesticides from an undrained plot were up to four times greater than losses from a mole-drained plot. Mole drainage decreased movement of pesticides from this slowly permeable soil by reducing the amount of surfacelayer flow. Maximum concentrations of mecoprop and isoproturon in drainflow were 10-20 times larger following spring application than after application in autumn. Bypass flow down soil cracks was an important process by which pesticide was lost from the site, with transport to the drainage system via mole channels (55 cm depth) after less than 0.5 and 6.7 mm net drainage in the two winters.Pesticide applied to the surface, and Pesticide concentra-Silsoe, Bedfordshire, MK45 4DT, UK. tions in soil water will be attenuated through the pro-Pestic. Sci. 0031-613X/95/$09.00 0 1995 SCI. Printed in Great Britain
Summary Root development is described by a simple algebraic model which gives the numbers and lengths of root members of different orders in terms of time and a few properties of each order of root member. The model is tested against experimental results for the early growth of the roots of temperate cereals in pots or in liquid culture. The model is then used to simulate root growth and to explain the observed behaviour of four growth measures (relative multiplication rate, relative extension rate, mean extension rate, average root length) used in the growth analysis of root systems. Three principles governing root development emerge from the model and the simulations.
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