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Areal Heterogeneity

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Yechezkel Mualem – One of the best experts on this subject based on the ideXlab platform.

  • Runoff from heterogeneous small bare catchments during soil surface sealing
    Water Resources Research, 2006
    Co-Authors: Shmuel Assouline, Yechezkel Mualem

    Abstract:

    [1] The combined effects of Areal Heterogeneity of the soil hydraulic properties and the surface seal formation dominate the hydrological response of arid and semiarid water catchments. Here these two phenomena were simulated to study their mutual role in runoff generation in small bare catchments. Seal formation during rainfall was simulated applying the dynamic model of Assouline and Mualem (1997). Areal Heterogeneity of the soil was represented by a lognormal distribution of the saturated hydraulic conductivity of the initially undisturbed soil, Ks, and by related distributions of the other soil parameters. The runoff hydrograph, at the outlet of a hypothetical bare catchment of 0.5 km2, was calculated using the cell model of Diskin et al. (1984). Two water catchments types, homogeneous and heterogeneous, with two different soil surface states, unsealed (mulched) and ongoing dynamic sealing, were considered. Four spatial organizations of the 10 cells in the heterogeneous catchment, all representing the same discrete distribution of Ks, were studied. Rainfall events of two rainfall intensities, 20 and 40 mm h−1, of different durations ranging from 10 to 120 min, were applied uniformly over the catchments to study the effect of rainfall intensity and duration on runoff characteristics under the different soils and catchments conditions. The state of the soil surface seal was found to be a dominant factor with regard to runoff generation. Relative to the runoff produced in the homogeneous unsealed catchment under a (40 mm h−1, 45 min) rainfall, the runoff was augmented by a factor of 10 during soil surface sealing and still more, by a factor of 20, when the soil surface was already sealed. On a relative basis the impact of soil sealing on runoff is much more important than that of soil Heterogeneity. The effect of Areal Heterogeneity on runoff seems to depend on both the soil surface condition and the rainfall intensity and duration. When the soil is unsealed, the total runoff and the discharge peak are higher for the heterogeneous catchment. When the soil surface undergoes a sealing process, the total runoff and the discharge peak are higher for the heterogeneous catchment for the lower rainfall intensity of 20 mm h−1. For the 40 mm h−1 rainfall the catchment relative response was found to depend on the rainfall duration. A higher peak was obtained in the homogeneous catchment for rainfall durations above 60 min, and more runoff was produced for rainfall durations above 90 min. The spatial pattern of the cell organization in the heterogeneous catchment is an additional factor affecting the hydrological response. The hydrographs corresponding to each of four patterns representing the same Areal Heterogeneity displayed differences regarding concentration time (which varied between 7 and 28 min), timing of the peak runoff (varying between 77 and 146 min), and the peak discharge (varying between 0.41 and 0.57 m3 s−1).

  • Infiltration during soil sealing: The effect of Areal Heterogeneity of soil hydraulic properties
    Water Resources Research, 2002
    Co-Authors: Shmuel Assouline, Yechezkel Mualem

    Abstract:

    [1] The combined effects of soil surface sealing and Areal Heterogeneity of the soil hydraulic properties on the mean infiltration curve are studied here for the first time. Seal formation during rainfall is simulated according to the dynamic model of Assouline and Mualem [1997]. Areal Heterogeneity is represented by lognormal distribution of the saturated hydraulic conductivity of the initially undisturbed soil and by related distributions of the other soil parameters. The mean infiltration curve of the heterogeneous field is evaluated presuming that horizontal fluxes are negligible. It is found that when soil surface sealing is taken into account in a heterogeneous field, the ponding time is only slightly affected. However, significantly more runoff is produced compared to nonsealing field (mulched), with the relative area contributing to surface runoff being increased from 65 to 95%. The relative effect of field Heterogeneity on the infiltration curve increases for higher rainfall intensity. The formation of soil surface seal apparently reduces the effect of the field Areal variability on the steady infiltration rate. The infiltration rate after 60 min of rainfall distributes lognormally under sealing conditions, while it distributes normally in the case of the unsealed soil. The final infiltration rate resulting from the assumption of a sealed uniform field is underestimated compared to sealed heterogeneous field, with the relative effect being larger for a loam soil (−51%) than for a sandy loam soil (−26%). Also, the approximation that the infiltration curve is a unique function of cumulative rainfall independent of rainfall intensity is not valid in a sealing heterogeneous field, and its application in such a case would be more erroneous than in uniform fields. On the other hand, one may disregard the variability of all soil parameters, except the hydraulic conductivity, when calculating infiltration.

Binayak P. Mohanty – One of the best experts on this subject based on the ideXlab platform.

  • Spatial Averaging of van Genuchten Hydraulic Parameters for Steady‐State Flow in Heterogeneous Soils: A Numerical Study
    Vadose Zone Journal, 2002
    Co-Authors: Jianting Zhu, Binayak P. Mohanty

    Abstract:

    For meso- or regional-scale Soil–Vegetation–Atmosphere Transfer (SVAT) schemes in hydroclimatic models, pixel dimensions may range from several hundred square meters to several hundred square kilometers. Pixel-scale soil hydraulic parameters and their accuracy are critical for the success of hydroclimatic and soil hydrologic models. This study tries to answer a major question: What will be the effective and average hydraulic properties for the entire pixel (or footprint of a remote sensor) consisting of several textures if the soil hydraulic properties can be estimated for each individual texture? In this study, we examined the impact of Areal Heterogeneity in soil hydraulic parameters on soil ensemble behavior for steady-state evaporation and infiltration. Using the widely used van Genuchten model and hydraulic parameter statistics obtained from neural network–based pedotransfer functions (PTFs) for various soil textural classes, we address the impact of Areal hydraulic property Heterogeneity on ensemble behavior and uncertainty in steady-state vertical flow in large-scale heterogeneous fields. The various averaging schemes of van Genuchten parameters are compared with “effective parameters” calculated by conceptualizing the Areally heterogeneous soil formation as an equivalent homogeneous medium that will discharge approximately the same amount of ensemble flux of the heterogeneous soil. The impact of boundary conditions and parameter correlation on the effective parameters, as well as the accuracy and uncertainty of the averaging schemes for the hydraulic parameters, are investigated and discussed. In light of our results, we suggest the following guidelines for van Genuchten hydraulic parameter averaging: arithmetic means for Ks and n , a value between arithmetic and geometric means for α when Ks and α are highly correlated, and a value between geometric and harmonic means for α when Ks and α are poorly correlated.

  • Upscaling of soil hydraulic properties for steady state evaporation and infiltration
    Water Resources Research, 2002
    Co-Authors: Jianting Zhu, Binayak P. Mohanty

    Abstract:

    [1] Estimation of effective/average soil hydraulic properties for large land areas is an outstanding issue in hydrologic modeling. The goal of this study is to provide flow-specific rules and guidelines for upscaling soil hydraulic properties in an Areally heterogeneous field. In this study, we examined the impact of Areal Heterogeneity of soil hydraulic parameters on soil ensemble behavior for steady state evaporation and infiltration. The specific objectives of this study are (1) to address the impact of averaging methods of shape parameters and parameter correlation on ensemble behavior of steady state flow in an Areally heterogeneous field and (2) to investigate the effectiveness of the “average parameters” in terms of the degree of correlation between hydraulic property parameters for the steady state evaporation and infiltration in unsaturated soil. Using an analytical solution of Richards’ equation, the ensemble characteristics and flow dynamics based on average hydraulic property parameters are studied for evaporation and infiltration. Using various flow and average scenarios, we illustrated the resulting differences among the various averaging schemes. For vertical evaporation and infiltration the use of a geometric mean value for the shape parameter α of Gardner-Russo model and Brooks-Corey model and arithmetic mean value for the saturated hydraulic conductivity Ks simulates the ensemble flow behavior the best. The efficacy of the “average parameters” depends on the flow condition and the degree of correlation between the hydraulic property parameters. With the α and Ks parameters perfectly correlated, the “average parameters” were found to be generally most effective. The correlation between the hydraulic conductivity Ks and the parameter α results in an ensemble soil behavior more like a sand.

Shmuel Assouline – One of the best experts on this subject based on the ideXlab platform.

  • Runoff from heterogeneous small bare catchments during soil surface sealing
    Water Resources Research, 2006
    Co-Authors: Shmuel Assouline, Yechezkel Mualem

    Abstract:

    [1] The combined effects of Areal Heterogeneity of the soil hydraulic properties and the surface seal formation dominate the hydrological response of arid and semiarid water catchments. Here these two phenomena were simulated to study their mutual role in runoff generation in small bare catchments. Seal formation during rainfall was simulated applying the dynamic model of Assouline and Mualem (1997). Areal Heterogeneity of the soil was represented by a lognormal distribution of the saturated hydraulic conductivity of the initially undisturbed soil, Ks, and by related distributions of the other soil parameters. The runoff hydrograph, at the outlet of a hypothetical bare catchment of 0.5 km2, was calculated using the cell model of Diskin et al. (1984). Two water catchments types, homogeneous and heterogeneous, with two different soil surface states, unsealed (mulched) and ongoing dynamic sealing, were considered. Four spatial organizations of the 10 cells in the heterogeneous catchment, all representing the same discrete distribution of Ks, were studied. Rainfall events of two rainfall intensities, 20 and 40 mm h−1, of different durations ranging from 10 to 120 min, were applied uniformly over the catchments to study the effect of rainfall intensity and duration on runoff characteristics under the different soils and catchments conditions. The state of the soil surface seal was found to be a dominant factor with regard to runoff generation. Relative to the runoff produced in the homogeneous unsealed catchment under a (40 mm h−1, 45 min) rainfall, the runoff was augmented by a factor of 10 during soil surface sealing and still more, by a factor of 20, when the soil surface was already sealed. On a relative basis the impact of soil sealing on runoff is much more important than that of soil Heterogeneity. The effect of Areal Heterogeneity on runoff seems to depend on both the soil surface condition and the rainfall intensity and duration. When the soil is unsealed, the total runoff and the discharge peak are higher for the heterogeneous catchment. When the soil surface undergoes a sealing process, the total runoff and the discharge peak are higher for the heterogeneous catchment for the lower rainfall intensity of 20 mm h−1. For the 40 mm h−1 rainfall the catchment relative response was found to depend on the rainfall duration. A higher peak was obtained in the homogeneous catchment for rainfall durations above 60 min, and more runoff was produced for rainfall durations above 90 min. The spatial pattern of the cell organization in the heterogeneous catchment is an additional factor affecting the hydrological response. The hydrographs corresponding to each of four patterns representing the same Areal Heterogeneity displayed differences regarding concentration time (which varied between 7 and 28 min), timing of the peak runoff (varying between 77 and 146 min), and the peak discharge (varying between 0.41 and 0.57 m3 s−1).

  • Infiltration during soil sealing: The effect of Areal Heterogeneity of soil hydraulic properties
    Water Resources Research, 2002
    Co-Authors: Shmuel Assouline, Yechezkel Mualem

    Abstract:

    [1] The combined effects of soil surface sealing and Areal Heterogeneity of the soil hydraulic properties on the mean infiltration curve are studied here for the first time. Seal formation during rainfall is simulated according to the dynamic model of Assouline and Mualem [1997]. Areal Heterogeneity is represented by lognormal distribution of the saturated hydraulic conductivity of the initially undisturbed soil and by related distributions of the other soil parameters. The mean infiltration curve of the heterogeneous field is evaluated presuming that horizontal fluxes are negligible. It is found that when soil surface sealing is taken into account in a heterogeneous field, the ponding time is only slightly affected. However, significantly more runoff is produced compared to nonsealing field (mulched), with the relative area contributing to surface runoff being increased from 65 to 95%. The relative effect of field Heterogeneity on the infiltration curve increases for higher rainfall intensity. The formation of soil surface seal apparently reduces the effect of the field Areal variability on the steady infiltration rate. The infiltration rate after 60 min of rainfall distributes lognormally under sealing conditions, while it distributes normally in the case of the unsealed soil. The final infiltration rate resulting from the assumption of a sealed uniform field is underestimated compared to sealed heterogeneous field, with the relative effect being larger for a loam soil (−51%) than for a sandy loam soil (−26%). Also, the approximation that the infiltration curve is a unique function of cumulative rainfall independent of rainfall intensity is not valid in a sealing heterogeneous field, and its application in such a case would be more erroneous than in uniform fields. On the other hand, one may disregard the variability of all soil parameters, except the hydraulic conductivity, when calculating infiltration.