Quantify Piston and Preferential Water Flow in Deep Soil Using Cl− and Soil Water Profiles in Deforested Apple Orchards on the Loess Plateau, China

Piston and preferential water flow are viewed as the two dominant water transport mechanisms regulating terrestrial water and solute cycles. However, it is difficult to accurately separate the two water flow patterns because preferential flow is not easy to capture directly in field environments. In this study, we take advantage of the afforestation induced desiccated deep soil, and directly quantify piston and preferential water flow using chloride ions (Cl−) and soil water profiles, in four deforested apple orchards on the Loess Plateau. The deforestation time ranged from 3 to 15 years. In each of the four selected orchards, there was a standing orchard that was planted at the same time as the deforested one, and therefore the standing orchard was used to benchmark the initial Cl− and soil water profiles of the deforested orchard. In the deforested orchards, piston flow was detected using the migration of the Cl− front, and preferential flow was measured via soil water increase below the Cl− front. Results showed that in the desiccated zone, Cl− migrated to deeper soil after deforestation, indicating that the desiccated soil layer formed by the water absorption of deep-rooted apple trees did not completely inhibit the movement of water. Moreover, there was an evident increase in soil water below the downward Cl− front, directly demonstrating the existence of preferential flow in deep soil under field conditions. Although pore water velocity was small in the deep loess, preferential water flow still accounted for 34–65% of total infiltrated water. This study presented the mechanisms that regulate movement of soil water following deforestation through field observations and advanced our understanding of the soil hydrologic process in deep soil.

Spatial Variability of Soil Water and Evapotranspiration

Spatial variability in evapotranspiration from a crop covered field exposed to homogeneous climatic conditions is partly caused by field variability in soil physical properties and partly by field variability in pertinent crop properties. The present paper presents an analysis of the spatial variability in soil water content and evapotranspiration for two 0.5 ha grass fields of different soil texture, viz. a coarse sand and a sandy loam. Soil physical properties and soil water profiles were determined 1 m apart at 16 points systematically located in each field. The analysis of soil water profiles in relation to soil physical properties indicates an appreciable variability within a range of 1 m. For this reason a relatively simple model was used to simulate the variability pattern of the evapotranspiration, taking into account the variability in plant available water content. The results of the simulations are compared with the evapotranspiration patterns determined on the basis of a short term water balance.

Water relations of Pinusradiata in competition with weeds

Soil water profiles up to 2 m in depth and diurnal patterns of needle water potential and stomatal resistance were measured over a dry summer period in 5-, 16-, and 28-month-old radiata pine (Pinusradiata D. Don) plantations, growing with and without weed competition. Severe water stress with consequent productivity loss occurred in trees with weed competition but the severity of stress decreased progressively with increasing tree age. Transplanted seedlings in their first growing season had shallow root systems which could not efficiently exploit water at depth. By contrast, seedlings in their second and third growing seasons extracted water from at least a depth of 2 m. Trees without weed competition were not water stressed over this period even when planted at three times their normal stocking rate. Thus there is considerable capacity to increase early growth rates of radiata pine without water being the limit to growth if weeds are adequately controlled.

Towards more efficient soil-water modelling: moisture profiles for constant application or removal of water at the surface

SUMMARYSoil-water profiles are often calculated using models based on the application to successive soil layers of an approximate formulation of Darcy's Law. An arbitrary decision is usually made concerning the magnitude of the layer thickness, Δz, and time increment Δt, a decision often based on the intuitive criterion that both should be as small as possible. An unsuitable choice can either give rise to oscillations in the solutions that can be eliminated only by recalculation using different increments, or can culminate in an accuracy unjustified by the precision of the data, both cases resulting in unnecessarily lengthy and wasteful computing. However, mathematicians have shown, for problems which are analogous to particular soil-water models, that oscillations will not occur if τ = D(Δt)/(Δz)2 ≥ 0·50 (where D is the soil-water diffusion coefficient, assumed constant).This paper considers the development of moisture profiles in soils with a constant rate of loss (e.g. evaporation) or entry (e.g. irrigation) of water imposed at the surface, and demonstrates that the criterion τ ≥ 0·50 can be extended to apply to waterdependent diffusion coefficients by re-interpreting D as the maximum value occurring in the profile. Examples for 10-layer models suggest that it is sufficient for practical purposes to start with Δt values that give τ-values close to 0·50, and to proceed to smaller Δt-values only when necessary. The total gain or loss of water given by integrating the calculated profiles is compared with the known amount entering or leaving at the surface.The profiles for both evaporation and irrigation are found to advance too rapidly.

Effects of Planting Patterns and Population on Water Relations of Maize

SUMMARYSeven plant populations, ranging from 24,000 to 73,000 plants/ha of the maize variety White Composite, were established in four growing seasons between 1976 and 1978 by varying row width and number of plants per hill. Soil water profiles, leaf and soil water potentials, and leaf resistances were monitored. The results indicated a close link between these parameters, number of plants per hill and overall plant populations. Grain yield increased with increasing plant populations during the early growing season but dropped beyond 54,000 plant/ha (spaced two plants per hill at 60x60 cm) during the late season, due to adverse plant water status.