Hoe snelle stroming door preferente banen het grondwater kan verontreinigen

Pesticiden en andere chemicaliën blijken vaak sneller in het grondwater terecht te komen dan modellen voorspellen. De meeste van deze modellen zijn namelijk gebaseerd op de veronderstelling dat de bodem homogeen is, het water in de onverzadigde zone verticaal infiltreert en het vochtfront evenwijdig is aan het bodemoppervlak. In werkelijkheid stroomt het water vaak via preferente stroombanen door de bodem. Deze stroombanen kunnen in nagenoeg alle gronden optreden. Water en daarin opgeloste stoffen bereiken hierdoor veel sneller het grondwater dan op grond van de gemiddelde verblijftijd verwacht zou worden. Vooral ondiep grondwater en oppervlaktewater hebben door dit proces een vergrote kans op verontreiniging

Analysis of stomatal and convective resistances to transpirational flow

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47839/1/484_2005_Article_BF01554062.pd

Three-dimensional analysis of infiltration from the disc infiltrometer: 1. A capillary-based theory

The hydraulic properties of an unsaturated homogenous and isotropic soil can be obtained from the unconfined flux out of a disc infiltrometer into the soil over the depth of wetting. The disc infiltrometer is becoming increasingly popular, but methods of analysis have generally relied on the restrictive assumptions of one-dimensional flow at early times or quasi-steady state flow at large times. We provide an approximate analytical expression for three-dimensional unsteady, unconfined flow out of a disc infiltrometer, and this includes the geometric effect of the circular source but ignores gravity. This physically based solution is tested against data obtained from laboratory experiments on repacked material. The results illustrate that the difference between three-dimensional and one-dimensional flow is linear with time

Three-dimensional analysis of infiltration from the disc infiltrometer: 2. Physically based infiltration equation

In situ measurement of soil hydraulic properties may be achieved by analyzing the unconfined efflux from disc tension infiltrometers, once consistent infiltration equations can be derived. In this paper an analytical, three‐dimensional infiltration equation is developed, based on the use of parameters with sound physical meaning and adjustable for varying initial and boundary conditions. The equation is valid over the entire time range. For practical purposes, a simplified solution is also derived. The full and simplified equations give excellent agreement with published experimental results and are particularly useful for determining soil hydraulic properties through application of inverse procedures

Application of a twin disc infiltrometer

A twin disc infiltrometer is developed to simultaneously measure one-dimensional cumulative infiltration. The assumption that flow at early times is essentially one-dimensional is shown to introduce error when estimating sorptivity, resulting in estimates that are greater than the true one-dimensional sorptivity. In contrast, sorptivity calculations using our recently developed three-dimensional infiltration equation give quite precise estimates of sorptivity. The difference between three-dimensional and one-dimensional infiltration is used to calculate the value of the infiltration parameter γ from the twin disc experiment. This parameter, together with the correct one-dimensional sorptivity, provide the information necessary to calculate the contribution of gravitational flow during three-dimensional infiltration. It is shown that when three-dimensional infiltration is dominated by capillary effects, as is often the case, estimation of hydraulic conductivity using quasi-steady-state solutions may not be possible

Three-dimensional analysis of infiltration from the disk infiltrometer: 3. Parameter estimation using a double-disk tension infiltrometer

A double-disk tension infiltrometer is developed to measure simultaneously one-dimensional (1-D) and three-dimensional (3-D) infiltration subject to identical initial and boundary conditions. The hypothesis that cumulative three-dimensional infiltration is proportional to the square root of time is incorrect after a few seconds, and in consequence, estimates of sorptivity based on this hypothesis are in error. In contrast, sorptivity calculations using a recently developed 3-D infiltration equation that includes the edge effects of the disk give accurate estimates of sorptivity. The difference between 3-D and 1-D cumulative infiltration is used to calculate the value of an additional infiltration parameter from the double-disk experiment. This parameter, together with the sorptivity, provides the information required to calculate the contribution of gravitational flow during three-dimensional infiltration. It is shown that estimation of hydraulic conductivity using a quasi-steady-state solution may not be justified when 3-D infiltration is dominated by capillary flow effects. Finally, an analysis of the different timescales governing disk infiltrometer experiments under most practical circumstances is provided. A simple expression is given for the time at which the infiltration rate is within a specified amount of the final steady rate