Peter J. Shouse

SIMULTANEOUS INVERSE ESTIMATION OF SOIL HYDRAULIC AND SOLUTE TRANSPORT PARAMETERS FROM TRANSIENT FIELD EXPERIMENTS: HOMOGENEOUS SOIL

Inverse estimation of unsaturated soil hydraulic and solute transport properties has thus far been limited mostly to analyses of one–dimensional experiments in the laboratory, often assuming steady–state conditions. This is partly because of the high cost and difficulties in accurately measuring and collecting adequate field–scale data sets, and partly because of difficulties in describing spatial and temporal variabilities in the soil hydraulic properties. In this study, we estimated soil hydraulic and solute transport parameters from several two–dimensional furrow irrigation experiments under transient conditions. Three blocked–end furrow irrigation experiments were carried out, each of the same duration but with different amounts of infiltrating water and solutes resulting from water depths of 6, 10, and 14 cm in the furrows. Two more experiments were carried out with the same amounts of applied water and solute, and hence for different durations, on furrows with water depths of 6 and 10 cm. The saturated hydraulic conductivity (Ks) and solute transport parameters in the physical equilibrium convection–dispersion (CDE) and physical nonequilibrium mobile immobile (MIM) transport models were inversely estimated using the Levenberg–Marquardt optimization algorithm in combination with the HYDRUS–2D numerical code. Soil water content readings, cumulative infiltration data, and solute concentrations were used in the objective function during the optimization process. Estimated Ks values ranged from 0.0389 to 0.0996 cm min–1, with a coefficient of variation of 48%. Estimated immobile water contents (.im) were more or less constant at a relatively low average value of 0.025 cm3 cm–3, whereas the first–order exchange coefficient (.) varied between 0.10 and 19.52 min–1. The longitudinal dispersivity (DL) ranged from 2.6 to 32.8 cm, and the transverse dispersivity (DT) ranged from 0.03 to 2.20 cm. DL showed some dependency on water level and irrigation/solute application time in the furrows, but no obvious effect was found on Ks and other transport parameters, most likely because of spatial variability in the soil hydraulic properties. Agreement between measured and predicted infiltration rates was satisfactory, whereas soil water contents were somewhat overestimated, and solute concentrations were underestimated. Differences between predicted solute distributions obtained with the CDE and MIM transport models were relatively small. This and the value of optimized parameters indicate that observed data were sufficiently well described using the simpler CDE model, and that immobile water did not play a major role in the transport process.

SPATIAL VARIABILITY OF SOIL WATER RETENTION FUNCTIONS IN A SILT LOAM SOIL

Soil water characteristic curves are a prerequisite for quantifying the field soil water balance and predicting water flow in unsaturated soils. The spatial variation of water retention in the root zone influences water availability for plants, evaporation, and fluxes of water and solutes through soils. The purpose of this study was to determine the ability of a popular model for the soil water retention function to describe the spatial variability of measured retention data and to investigate the application of a water content scaling theory to reduce the apparent spatial variation of soil water retention. Using a combination of Tempe cells and 1.5-MPa pressure plate extractors, we measured soil water retention at six pressure heads. In total, 281 undisturbed soil core samples were taken from the Ap horizon (0 to 17-cm depth increments) along an 80-m transect on a bare silt loam soil at 0.30-cm intervals. Sample statistics were calculated to identify outliers and erroneous data. A four-parameter retention model (&thetas;s, &thetas;r, α, n) was fitted to the data, and water content scale factors were also calculated. The soil water retention model was found to be extremely flexible in fitting the measured data. The parameters in the retention model showed a structured variance with a range of influence between 12 and 30. The number of parameters needed to characterize the field variability was 912 for the retention model. Scaling theory applied to the water retention data significantly reduced the apparent spatial variability. One scale factor also showed a structured variance, indicating a spatial correlation distance of greater than 30 m. Using the Akaike information criterion, we found that scaling theory could adequately represent the spatial variation in water retention with only 460 parameters. Sampling, calibration and/or experimental errors were thought to account for more than 50% of the total variability.

Scaling of near-saturated hydraulic conductivity measured using disc infiltrometers

A function relating unsaturated soil hydraulic conductivity K and soil water pressure head h is most important for understanding water flow and chemical transport in the vadose zone. Furthermore, the K(h) function near saturation is critical for describing flow in macropores and other structural voids. The usefulness of similar media scaling and functional normalization to describe the near-saturated hydraulic conductivity function K(h) measured in situ at 296 spatial locations across a heterogeneous agricultural field was tested. Disc (ponded and tension) infiltrometers were used to measure K(h) at different field positions (corn row, no traffic interrow, and traffic interrow) cutting across different soil types (Nicollet and Clarion loam derived from glacial till material). The K(h) data ranged several orders of magnitude for different field positions and soil types and were found to be statistically different between different field positions. Using a Gardner type K(h) function, relative hydraulic conductivity values, and a hybrid of similar media scaling and functional normalization concepts, all disc infiltrometer data sets were coalesced to a single reference curve. Poor to moderately correlated K and h scale factors did not show any significant spatial structure across the field. A novel finding is that saturated hydraulic conductivities (Ksat) could be successfully used as the scale factor for the near-saturated K(h) functions (e.g., 0-15 cm soil water tension) under all field positions and soil types at the experimental field. Among others, Warrick et al. (1977) and Jarvis and Messing (1995) suggested that further research should be carried out with respect to both experimental technology and scaling con- cepts for an optimum coevolution of techniques addressing soil heterogeneity. More recently, in situ measurement of near- saturated hydraulic conductivlty (K(h)) using disc (ponded and tension) infiltrometers has opened up new avenues to assess spatial variability of hydraulic properties of field soils. These in situ K(h) measurements are better suited to repre- sent (near-saturated) flow and transport scenarios in the field than K(h) measurements obtained using detached soil cores in the laboratory (Mohanty et al., 1994a). Near-saturated K(h) measurements are important for understanding the influence of macropores and other structural voids in the soil water regime of field soils and useful for multidomain models for soil hydraulic properties. The effects of soil structure and macro- pores might be more reliably predicted, as shown by Mohanty et al. (1997). Spatial variability of these K(h) measurements using different geostatistical and/or scaling concepts need to be studied further for different soils, crops, tillage practices, traf- fic conditions, and other extrinsic/intrinsic field variables. Mo- hanty et al. (1994b, 1996) used geostatistical techniques to an- alyze disc infiltrometer K(h) data under different soil and traffic conditions. To date, only Jarvis and Messing (1995) used a similar media scaling technique to analyze disc infiltrometer K(h) data obtained by means of four to six disc infiltrometer experiments at each of six different soil types in Sweden. The objective of our study was to test the appropriateness of

Lateral Water Diffusion in an Artificial Macroporous System: Modeling and Experimental Evidence

In two-domain schematizations of macroporous soils or fractured rock systems, lateral mass exchange between macropores and the soil matrix is generally modeled as an apparent first-order process. With respect to lateral diffusion, the system is thus characterized by a single parameter, the transfer rate coefficient, which is difficult to estimate a priori. We conducted water infiltration experiments in a laboratory column with an artificial macropore. The novel design of the experimental setup allowed us to discriminate between matrix flow and macropore flow, from which we could estimate the water exchange flux between the two domains. Most of the parameters in a dual-permeability model could be determined independently of the experimental data. In particular, a theoretical expression for the transfer rate coefficient was derived by assuming lateral water and solute diffusion to be similar processes. Numerical analysis of the water exchange process revealed that the transfer coefficient depended also on the macropore conductivity. When this dependency was taken into account, the model reproduced the experimental data reasonably well.

Modification of soil structural and hydraulic properties after 50 years of imposed chaparral and pine vegetation

Although biotic communities have long been recognized as important factors in soil development, especially of A horizons, few studies have addressed their influence on soil physical properties in nonagricultural settings. A biosequence of 50-year-old soils supporting near monocultures of Coulter pine (Pinus coulteri), scrub oak (Quercus dumosa), and chamise (Adenostoma fasciculatum) was used to determine the relative influence of vegetation type and associated soil organisms on the development of soil structural characteristics and water flow. Total porosity ranged from a high of 51% in the heavily worm-worked A horizon under oak to a low of 39% within the 35- to 50-cm depth under pine, where earthworms were absent. Macroporosity (pores with diameters >300 Am) was highest in the A horizon under oak (15.6%) and lowest under pine (9.5%). Saturated hydraulic conductivity of surface soils ranged from 10.8 cm h � 1 under oak to 3.2 cm h � 1 under pine. Soil under chamise, which had fewer earthworms than that under oak, had Ksat and bulk density values intermediate between oak and pine. Root and macrofauna distributions suggest that roots are the dominant factor in the development of macroporosity under pine, while earthworms have had the greatest effect under oak. Porosity has increased at an average rate of 0.22% per year in the 0- to 7-cm depth under oak (from 41% to 56%), but has not been significantly altered within the same depth under pine. Below the 7-cm depth, porosity values are similar for each vegetation type and the original parent material. Available water capacity (AWC) within the first 0to 7-cm depth has increased from the original values (about 0.11 m 3 m � 3 ) to 0.17 m 3 m � 3 under oak, 0.16 m 3 m � 3 under chamise, and 0.13 m 3 m � 3 under pine. The data show that the presence of

Measuring Particle Size Distribution Using Laser Diffraction: Implications for Predicting Soil Hydraulic Properties

Methods to predict soil hydraulic properties frequently require information on the particle size distribution (PSD). The objectives of this study were to investigate various protocols for rapidly measuring PSD using the laser diffraction technique, compare the obtained PSD with those determined using the traditional hydrometer-and-sieves method (HSM), and assess the accuracy of soil hydraulic properties predicted from the measured PSD. Ten soil samples encompassing a wide textural range were analyzed using the HSM and 3 different laser diffraction methods (LDM1, LDM2, and LDM3). In LDM1, the soil sample was thoroughly mixed before analysis. In LDM2, the sand fraction was sieved out and analyzed separately from the silt-clay fraction. LDM3 was similar to LDM2 except that the silt-clay fraction was diluted so that a large sample volume could be used while maintaining an acceptable level of obscuration. LDM2 and LDM3 improved the agreement between the PSD with the HSM in comparison to LDM1, without the need of altering the Mie theory parameters or the use of scaling factors. Moreover, a reasonable prediction of measured saturated hydraulic conductivity and water retention curve was achieved when using the PSD from LDM2 and LDM3, in conjunction with bulk density information.

Regolith Water in Zero-Order Chaparral and Perennial Grass Watersheds Four Decades after Vegetation Conversion

In 1960, areas of chaparral were converted to perennial grass after a fire burned most of the San Dimas Experimental Forest in southern California. This conversion provided an opportunity to compare regolith moisture patterns of zero-order watersheds under native chaparral with those under nonnative veldt grass ( Ehrharta calycina Sm.). We collected data as a function of vegetation type and watershed element to test the hypothesis that conversion from chaparral to grass altered water distribution in the vadose zone as a result of changes in the physical environment, including rooting depth and soil horizonation. Patterns in vadose zone water distribution during the dry season, including soil water potential and residual flux, were significantly different in converted areas, reflecting the different rooting habits of the two vegetation types. In chaparral areas, there was no significant change in soil water potential between the surface and the 150-cm depth; soil water potential was consistently below −1.5 MPa, reflecting the extensive root system. In grass areas, soil water potential was most negative close to the surface, where grass roots were most abundant. Plant available water was present below the 100-cm depth, suggesting that recharge to groundwater may occur under grass in average or wetter years. Under both vegetation types, the largest differences in residual water fluxes were near the soil–weathered rock contact. However, there was a significant relation between minor differences in fluxes and soil horizon boundaries, confirming the effects of vegetation conversion on soil properties and vadose zone soil water.

Smog Nitrogen and the Rapid Acidification of Forest Soil, San Bernardino Mountains, Southern California

We report the rapid acidification of forest soils in the San Bernardino Mountains of southern California. After 30 years, soil to a depth of 25 cm has decreased from a pH (measured in 0.01 M CaCl2) of 4.8 to 3.1. At the 50-cm depth, it has changed from a pH of 4.8 to 4.2. We attribute this rapid change in soil reactivity to very high rates of anthropogenic atmospheric nitrogen (N) added to the soil surface (72 kg ha–1 year–1) from wet, dry, and fog deposition under a Mediterranean climate. Our research suggests that a soil textural discontinuity, related to a buried ancient landsurface, contributes to this rapid acidification by controlling the spatial and temporal movement of precipitation into the landsurface. As a result, the depth to which dissolved anthropogenic N as nitrate (NO3) is leached early in the winter wet season is limited to within the top ~130 cm of soil where it accumulates and increases soil acidity.

Solute Transport in Heterogeneous Field Soils

The purpose of this paper is to briefly review current approaches to quantifying (modeling) solute transport in the unsaturated (vadose) zone of field soils. Much progress has been attained in the analytical and numerical description of vadose zone transfer processes. A variety of mathematical models are now available to describe and predict water flow and solute transport between the land surface and the groundwater table. The most popular models remain the classical Richards’ equations for unsaturated flow and the Fickian-based convection-dispersion equation for solute transport. While deterministic solutions of these equations remain useful tools in both fundamental and applied research, their practical utility for predicting actual field-scale water and solute distributions is increasingly being questioned. Problems caused by preferential flow through soil macro-pores, spatial and temporal variability in the soil hydraulic properties, various nonequilibrium processes affecting chemical transport, and a lack of progress in improving our field measurement technology, have contributed to some disillusionment with the classical models. A number of alternative deterministic and stochastic approaches have been proposed to better deal with field-scale heterogeneities. These models have greatly increased our quantitative understanding of field-scale flow and transport processes, and in some cases also resulted in better practical tools for management purposes.

Use of Geostatistics in the Description of Salt-Affected Lands

Geostatistical methods are increasingly popular tools in the anaylysis of a variety of agricultural problems. The methods are typically used to determine various spatially related quantities which, in turn, characterize the variability of one or more parameters in space and/or time. Simple, ordinary and universal kriging methods produce linear estimators which are useful for obtaining estimates of a spatially distributed property over a region, especially at locations for which no data are available. For the most part, the final result of an analysis is a map showing the spatial distribution of the property of interest. Many examples appear in the literature (Burgess and Webster 1980a, b; Webster and Burgess 1980; Vieira et al. 1981; Vauclin et al. 1983; Warrick et al. 1986; Yates et al.1986a; Yates and Warrick 1987; ASCE 1990a, b). At other times, descriptors such as variograms and correlation scales are the ultimate goal of a geostatistical investigation.