This paper provides a simple way to convert Brooks‐Corey (BC) parameters to van Genuchten (vG) parameters and vice versa, for use primarily in situations where saturated conditions are likely to be encountered. Essential in this conversion is the preservation of the maximum value of a physical characteristic, the “effective capillary drive” HcM [Morel‐Seytoux and Khanji, 1974], defined with a good approximation for a soil water and air system as HcM = ∫0∞ krw dhc, where krw is relative permeability (or conductivity) to water and hc is capillary pressure (head), a positive quantity. With this conversion, infiltration calculations are essentially insensitive to the model used to represent the soil hydraulic properties. It is strictly a matter of convenience for the user which expression is used. On the other hand, the paper shows that other equivalences may lead to great variations in predictions of infiltration capacity. Consequently, the choice of the proper equivalence to use in calculations for rainfall‐runoff modeling or for low‐level radioactive waste disposal design is a serious matter.

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Since the publication of the article of Green and Ampt (191!) the two constants appearing in their 'equation of infiltration' have been considered to be empirical constants. In fact, they can be deduced simply from the soil characteristics, and they have a very precise physical meaning. However, the assumption of an abrupt front separating a saturated zone from the zone at the initial water content can lead to prediction errors of the order of 10-70%. Derived formulas display explicitly the functional dependence of the effective (or flowing) capillary head and of the viscous correction factor as a function of the initial water content. These formulas are both simple and accurate and of practical value to the hydrologist. AN UNRESOLVED PROBLEMSince the publication of the article of Green and Amoet [1911] the two constants that appear in their 'equation of infiltration' have been considered as empirical parameters to be determined by experiments. One form of the Green and Ampt equation [Childs, 1969, p. 276] is I = K(H + z/ + H/)/z/ = A + (B/z/) (1) where ! is the infiltration rate (L T-•), K is the fully saturated hydraulic conductivity (that is, the conductivity at a water content equal to porosity) (LT-X), H is the depth of ponded water above the soil surface, zr is the vertical extent of the saturated zone, and H/is the capillary pressure at the front expressed as a water height (L). The demonstrated applicability of the formula [Green and Ampt, 1911] and the surprisingly good agreement with data [Swartzendruber and Hubert),, 1958] make one wonder whether the formula is really empirical. The key to this question is the relation of the parameters A and B, or K and HI, to the soil characteristics. It has been recognized already for a long time that the hydraulic conductivity to appear in (1) should not be the conductivity at full saturation but the wetting conductivity at residual air saturation ,• [Bouwer, 1966, p. 738], which has also been called the 'resaturated hydraulic conductivity' [Whisler and Bouwer, 1970, pp. 11-12] and can be estimated as 0.5K [Bouwer, 1966, p. 732]. Thus the curtain of mystery regarding A has been lift/d at least for the case of a homogeneous soil with an initially uniform water content. There is of course no such thing as an equation of infiltration in general. In this article (following Childs [1969]) the expression equation of infiltration has a very restricted meaning, namely, the relation between time and the water flux across the surface of a column of homogeneous soil of unlimited depth with an initially uniform water content 0• and a maintained ponding depth H over the surface from time zero on. Under these conditions, Bouwer [!964, p. 142] suggested an approximate determination of Hr by substituting for it what he called the 'critical pressure head,' defined by the relation Hb = k•,• dh• (2) where kr,,, is the relative water permeability (that is, relative to wetting permeability at residual air saturation) and hc is the capillary pressure (expressed as a water height). Intui...

A combined analytical-numerical approach is presented to characterize properly the exchange flow between a stream and a hydraulically connected aquifer. It eliminates the need to use a three-dimensional fine grid under and in the vicinity of the river cross section in order to obtain accurate results. Basically the approach matches an analytical solution in a vertical two-dimensional (2D) plane with the numerical description of the aquifer behavior in a 2D horizontal plane. The approach is compared with a finite-difference formulation such as used in MODFLOW.

Abstract. This paper provides an alternative method to describe the water retention curve over a range of water contents from saturation to oven dryness. It makes two modifications to the standard Brooks and Corey [1964] (B-C) description, one at each end of the suction range. One expression proposed by Rossi and Nimmo [1994] is used in the high-suction range to a zero residual water content. (This Rossi-Nimmo modification to the Brooks-Corey model provides a more realistic description of the retention curve at low water contents.) Near zero suction the second modification eliminates the region where there is a change in suction with no change in water content. Tests on seven soil data sets, using three distinct analytical expressions for the high-, medium-, and low-suction ranges, show that the experimental water retention curves are well fitted by this composite procedure. The high-suction range of saturation contributes little to the maximum capillary drive, defined with a good approximation for a soil water and air system as HcM = f• krw dhc, where krw is relative permeability (or conductivity) to water and h c is capillary suction, a positive quantity in unsaturated soils. As a result, the modification suggested to describe the high-suction range does not significantly affect the equivalence between Brooks-Corey (B-C) and van Genuchten [1980] parameters presented earlier. However, the shape of the retention curve near "natural saturation" has a significant impact on the value of the capillary drive. The estimate using the Brooks-Corey power law, extended to zero suction, will exceed that obtained with the new procedure by 25 to 30%. It is not possible to tell which procedure is appropriate. Tests on another data set, for which relative conductivity data are available, support the view of the authors that measurements of a retention curve coupled with a speculative curve of relative permeability as from a capillary model are not sufficient to accurately determine the (maximum) capillary drive. The capillary drive is a dynamic scalar, whereas the retention curve is of a static character. Only measurements of infiltration rates with time can determine the capillary drive with precision for a given soil.