Conceptual Models of Flow and Transport in the Fractured Vadose Zone (2001) / Chapter Skim
Currently Skimming:
Modeling Macropore Flow in Soils: Field Validation and Use for Management Purposes
Modeling Macropore Flow in Soils: Field Validation and Use for Management Purposes
Pages 189-216
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 189... ...
Regulatory applications of macropore flow models should be based either on precalibrated scenarios or on objective automatic parameter estimation routines (pedotransfer functions)
|
From page 190... ...
At the macroscopic scale of measurement, this is reflected in large increases in unsaturated hydraulic conductivity across a small soil water pressure head range close to saturation (Clothier and Smettem, 1990; Jarvis and Messing, 1995~. At the pore scale, macropore flow is generated when the water pressure locally increases to near saturation at some point on the interface with the surrounding soil matrix, such that the waterentry pressure of the pore is exceeded.
|
From page 191... ...
, and a simple instantaneous bypass routine to account for macropore flow. The model was apparently implemented only for nonreactive solute transport, although it was compared to nitrate leaching data during three winter seasons (Barraclough, 1989b)
|
From page 192... ...
Thus, although such approaches may provide some theoretical insights into solute transport processes occurring in structured soils, they are not appropriate to field situations characterized by time-varying soil water content due to evaporation, root water uptake, and intermittent inputs of rainfall. For this reason, their use as management tools is rather limited.
|
From page 193... ...
, applied to a bimodal pore size distribution (Othmer et al., 1991; Durner, 1992~. Richards' equation and the CDE are used to calculate water flow and solute transport, respectively, in both pore domains, while mass exchange of water and solutes is calculated using approximate first-order equations accounting for both convective and diffusive transfer (Gerke and van Genuchten, 1993b)
|
From page 194... ...
_ - Kb C, . _ I o FIGURE 6-1 Schematic representation of soil hydraulic functions in the MACRO model: Ks is the saturated hydraulic conductivity, Os is the saturated water content, sib iS the pressure head defining the boundary between macropores and micropores, fib iS the equivalent water content, and Kb is the hydraulic conductivity at sib.
|
From page 195... ...
A full water balance is considered in MACRO, including precipitation, evapotranspiration, and root water uptake; deep seepage; and lateral fluxes to tile drains in saturated soil. Solute transport and transformation processes in the model include convective-dispersive transport, Freundlich equilibrium sorption with sorption sites partitioned between the two pore domains, microbial degradation according to first-order kinetics, plant uptake, and canopy interception/ washoff.
|
From page 196... ...
described a two-dimensional, dual-porosity, finite element model of water flow and nonreactive solute transport in tile-drained soil. The mass exchange terms between the domains were taken from Gerke and van Genuchten (1993a, 1993b)
|
From page 197... ...
Richards' equation and the CDE may be conceptually valid for predicting water flow and solute transport in homogeneous porous media, but would repeatedly fail operational validation tests for predicting leaching to groundwater in naturally heterogeneous soils, and lose credibility for this purpose. Apart from difficulties with semantics, one reason for the continuing confusion over the meaning and purpose of model validation is that there are no widely accepted, standard, objective performance criteria for testing models (Rykiel, 1996~.
|
From page 198... ...
. The confusion that arises when performance criteria for model validation are not clearly expressed a priori can be demonstrated by the current situation in the European Union with respect to harmonized pesticide registration procedures.
|
From page 199... ...
As an example of the latter, the nonreactive tracer experiment and model application reported by Larsson and Jarvis (1999a) can serve as a useful example to illustrate the range of field data required to properly validate macropore flow models such as MACRO.
|
From page 200... ...
compared model predictions and measurements of drain discharge, concentrating especially on estimating soil macroporosity (0S-0b) and saturated hydraulic conductivity by matching the timing and shape of individual flow hydrographs (i.e., time to peak flows and recessions)
|
From page 201... ...
In the second stage of model calibration, estimates of sensitive parameters regulating the impact of macropore flow on preferential solute transport, particularly the mixing depth, Zd. (Equation 6.1)
|
From page 202... ...
However, it would not be possible to match the pattern of flux concentrations observed in tile-drain outflow without considering macropore flow. This emphasizes that validation of macropore flow models requires measurements of both resident and flux concentrations, and that without flux measurements, there is a clear risk that misleading positive conclusions can be drawn concerning the validity of models that do not account for preferential movement.
|
From page 203... ...
J Jarvis, Evaluation of a dual-porosity model to predict field-scale solute transport in macroporous soil, pp.
|
From page 204... ...
utilized the calibrated Lanna parameter set as input to purely predictive scenario simulations investigating the interactions between pesticide compound properties and macropore flow effects on leaching. It was demonstrated that for mobile leachable compounds, macropore flow at Lanna results in small decreases in leaching, or has no effect at all, while for intermediate leachers, macropore flow may increase leaching by up to three to four orders of
|
From page 205... ...
FIGURE 6-5 A comparison of bentazone contents measured by soil coring at Lanna with those simulated by the MACRO model both with and without considering macropore flow. Redrawn from Journal of Hydrology, 215, Larsson, M
|
From page 206... ...
One practical consequence of this result is that reductions in applied dose would appear to be a relatively more effective means of reducing pesticide leaching in the presence of macropore flow (Larsson, 1999~. 1o2 ~ 101o it, to Q ~0 C' ( 1 on .~ _' ~ 10-1 _ C' C' -20-3 , 10-3 10-2 10-1 10° 1o1 1o2 Percent leached (without macropore flow)
|
From page 207... ...
can also be used to illustrate the importance of year-to-year variability in the rainfall pattern following surface application in determining leaching losses of agrochemicals in the presence of macropore flow. Figure 6-7 shows that for spring applications, annual leaching losses of bentazone predicted during a 10-year period were strongly correlated to the total rainfall in the three- to fourweek period following application (Larsson, unpublished data)
|
From page 208... ...
the simulation without macropore flow could be made to match measured nitrogen contents in both the soil profile and harvested grain, but nitrate concentrations in drain flow were then generally overestimated; (3) in one year, significant increases in nitrate leaching due to macropore flow were observed during a short period following spring fertilizer application; and (4)
|
From page 209... ...
The strength of structural development reflects the presence of cutans and organic linings at macropore/matrix interfaces, which may restrict mass exchange processes between pore domains (e.g., Thoma et al., 1992~. The boundary hydraulic conductivity Kb (Figure 6-1)
|
From page 210... ...
One reason for this may be the difficulty in estimating one critical parameter for reactive solutes, namely, the partitioning of sorption sites between the pore domains. The simple approximation currently used in MACRO_DB is to distribute the sorption sites in proportion to the relative volumes of the pore domains.
|
From page 211... ...
Thus, future regulatory use of macropore flow models is necessary, but requires strict protocols to ensure that simulations are free of subjective user bias. Regulatory applications of macropore flow models should therefore be based either on Recalibrated scenarios or on objective parameter estimation routines (pedotransfer functions)
|
From page 212... ...
Simulation of water movement and isoproturon behaviour in a heavy clay soil using the MACRO model. Hydrology and Earth System Sciences 1: 835-844.
|
From page 213... ...
A multiregion model describing water flow and solute transport in heterogeneous soils. Soil Science Society America Journal 59: 743-751.
|
From page 214... ...
Soil Science Society of America Journal 40: 473-480. van Genuchten, M
|
From page 215... ...
Modeling transient water and solute transport in a biporous soil. Water Resources Research 32: 819-829.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.