Heterogeneity Of Soil Hydraulic Properties Research Articles

Hydraulic Issues Concerning Injection of Harvested Rainwater to the Subsurface Through Drywells: Insight From Numerical Simulations of Flow in a Realistic Combined Vadose Zone‐Groundwater Flow System

AbstractThis study focuses on the analyses of field‐scale water flow originating from a drywell, using the 3‐D Richards equation for the local description of the flow in a spatially heterogeneous, combined vadose zone‐groundwater flow system. Realistic features of the flow system, that is, height of the groundwater table, records of the time‐dependent rainfall data, spatial heterogeneity of soil hydraulic properties and flow‐controlled attributes, as well as man‐controlled characteristics of the drywell, were considered in the analyses. The effect of (a) the depth‐dependency of the soil type, (b) the vertical position of the drywell's screen, and (c) the mode of water injection to the drywell (constant or time‐dependent), on the performance of the drywell, were examined. Results, and in particular those related to the importance of the soil type depth‐dependency with regard to the drywell performance, can be explained by the physics of flow in unsaturated soils of differing soil texture. Of specific significance is the transition zone between the two different soil types in the case in which the spatially‐heterogeneous coarse‐textured soil overlies the spatially heterogeneous fine‐textured soil. In this case, when most of the drywell screen is located within this transition zone and when the flow is transient and non‐monotonous, the downward flow from the drywell is maximized. Although this study is site‐specific, we believe that its results are novel and relevant for sandy to sandy‐clay soils associated with managed aquifer recharge technology based on a device whose vertical axis is perpendicular to the soil layering.

Experimental evaluation of different subsurface water movement models in an agricultural field with soil heterogeneity

Spatial heterogeneity in soil hydraulic properties poses severe challenges to the modeling of soil moisture dynamics in agricultural fields. For simplicity, 1-D water flow models that ignore soil heterogeneity are commonly used to simulate water movement in agricultural fields. As soil hydraulic properties are one of the most crucial input data, it is particularly desirable to investigate how the simulations of these water flow models compare with experimental observations. This study aims to examine the performances of streamtube, 1-D, and 3-D Richards equation based water flow models with experimental observations. Experiments were conducted at an agricultural plot of 20 m × 30 m located in IIT Kanpur, India, for 1 year under wheat (Triticum aestivum L.) and rice (Oryza sativa L.) crop covers. The site was heavily instrumented to monitor soil moisture variation, weather conditions, root depth, and LAI. Further, 54 undisturbed soil samples were analyzed to determine the heterogeneity in soil hydraulic parameters. The results indicate that the performance of the models varies with land cover, atmospheric boundary condition, and crop growth stages. Besides, the difference between model estimates varies with soil moisture wetness and a maximum relative error of 13% was observed under wetting conditions. The streamtube model showed the best performance under wheat crop cover for all boundary conditions and growth stages, as it considers spatial variability in soil hydraulic properties and irrigation application at each subplot. The numerical-1D model was the best predictor under rice crop cover. Further, the results suggest that an increase in model complexity does not lead to an improvement in water simulation, and accurate representation of soil hydraulic properties plays a more important role.

Retrieving Heterogeneous Surface Soil Moisture at 100 m Across the Globe via Fusion of Remote Sensing and Land Surface Parameters

Successful monitoring of soil moisture dynamics at high spatio-temporal resolutions globally is hampered by the heterogeneity of soil hydraulic properties in space and complex interactions between water and the environmental variables that control it. Current soil moisture monitoring schemes via in situ station networks are sparsely distributed while remote sensing satellite soil moisture maps have a very coarse spatial resolution. In this study, an empirical surface soil moisture (SSM) model was established via fusion of in situ continental and regional scale soil moisture networks, remote sensing data (SMAP and Sentinel-1) and high-resolution land surface parameters (e.g. soil texture, terrain) using a quantile random forest (QRF) algorithm. The model had a spatial resolution of 100 m and performed well under cultivated, herbaceous, forest, and shrub soils (overall R2 = 0.524, RMSE = 0.07 m3 m-3). It has a relatively good transferability at the regional scale among different soil moisture networks (mean RMSE = 0.08–0.10 m3 m-3). The global model was applied to map SSM dynamics at 30–100 m across a field-scale soil moisture network (TERENO-Wüstebach) and an 80-ha cultivated cropland in Wisconsin, USA. Without the use of local training data, the model was able to delineate the variations in SSM at the field scale but contained large bias. With the addition of 10% local training datasets (“spiking”), the bias of the model was significantly reduced. The QRF model was relatively insensitive to the resolution of Sentinel-1 data but was affected by the resolution and accuracy of soil maps. It was concluded that the empirical model has the potential to be applied elsewhere across the globe to map SSM at the regional to field scales for research and applications. Future research is required to improve the performance of the model by incorporating more field-scale soil moisture sensor networks and assimilation with process-based models.

Conditions of Hydraulic Heterogeneity under Which Bayesian Estimation is More Reliable

Natural heterogeneity of soil hydraulic properties is significant for the design and construction of geotechnical structures, and should be adequately characterized. Accurate measurements of hydraulic properties remain a difficult job and do not always work well for further design and analysis. Field hydraulic monitoring data reflects the overall slope performance and provide a more representative estimation of in-situ soil hydraulic properties for back analysis. The objective of this study is to explore the conditions under which monitoring data can provide reliable estimates of hydraulic parameters. Different distributions of soil heterogeneity generate a total number of 500 sets of synesthetic monitoring data. Bayesian inversion with the integration of Karhunen-Loève (K-L) and polynomial chaos expansion (PCE) is chosen to estimate the spatially varied saturated coefficient of permeability ks. The results show that the method is accurate and reliable, with less than 3% percentage error and 0.08 coefficient of variation (COV) around the monitoring points. There are two characteristics of the best-estimated fields. First, the ranges of ks for best-estimated fields are much narrower than the worst estimated fields. Second, when the larger ks values are distributed in the unsaturated zone of slope crest, it will lead to the best estimation. It is suggested that monitoring data can provide a reliable estimation of heterogeneous ks when the ratio of ground surface flux to ks in the unsaturated zone of slope crest is less than 1/150. Small values of ks in the slope crest result in the response of pressure head far from the responses of homogenous ks in the unsaturated zone. This complex response of the pressure head further causes the ill identification of ks by Bayesian estimation.

Soil moisture dynamics modelling of a reclaimed upland in the early post-construction period

Mine reclamation landscapes typically comprise layers of mine waste materials such as tailings sands, capped with a cover soil. In addition to the arrangement and placement of these materials, their hydraulic properties govern the performance of the built system. Soil evolution due to freeze–thaw cycling can result in dramatically altered soil hydraulic properties compared to the as-built material. Therefore, prediction of present and future hydrologic behaviour relies on understanding the nature and magnitude of this change and the elapsed time associated with stabilization. This research quantifies the transient hydraulic properties of mine reclamation materials at a constructed upland within a reclaimed watershed, and models the effect of this evolution on the partitioning of soil moisture between evaporation and groundwater recharge. Soil moisture dynamics were simulated using HYDRUS-1D for the ice-free period two, three, and five years after construction. A capillary barrier between the fine-grained cover soil and coarse-grained tailings sand regulated percolation past the interface. Soil evolution of the cover soil was responsible for an increase in saturated hydraulic conductivity by an order of magnitude, decrease in air-entry pressure by a factor of 4, and decrease in the van Genuchten n parameter by a factor of 2. The altered soil hydraulic properties associated with the weathered cover soil ultimately resulted in a 64% increase in groundwater recharge as a consequence of the capillary barrier weakening. The cover soil exhibited minor spatial heterogeneity in soil hydraulic properties, and did not contribute substantial uncertainty to the estimates of groundwater recharge and evaporation. Cover soil thickness exerted a strong influence on the partitioning of soil moisture. Reclaimed uplands will provide the most recharge to downgradient ecosystems in the period following the completion of soil evolution (~4 years) but preceding substantial vegetation development.

Representation of Plot‐Scale Soil Heterogeneity in Dual‐Domain Effective Flow and Transport Models with Mass Exchange

Core Ideas The heterogeneity of soil hydraulic properties can be described with effective parameters. Increasing model complexity can be used to represent plot‐scale soil heterogeneity. One‐dimensional dual‐domain flow models are used to reproduce 2D preferential transport. Local subscale variability effects are included as mass transfer in an effective model. Agricultural soils are characterized by a structure that is strongly dependent on farming practices like tillage and trafficking. These practices can create compacted zones in the soil, thus initiating preferential flow. Two‐ or three‐dimensional models can be used to account for the spatial variability of the soil hydraulic and transport properties. Since it is challenging to obtain such data, it is logical to find simpler approaches. Our objective was to design a one‐dimensional (1D) modeling approach that effectively accounts for plot‐scale soil structure variability. A 1D dual‐permeability model was tested in which compacted soil was represented by a matrix domain and uncompacted soil by a fracture domain and eventually by assuming an additional immobile water region (MIM) in the fracture domain representing compacted clods embedded within the uncompacted soil. Models (1D) were compared with two‐dimensional single‐porosity (2D_SP) modeling results for water flow and Br− transport based on a previously performed field tracer experiment. Results indicated good agreement between 1D dual‐domain approaches (1D_DPERM and 1D_DPERM_MIM) and the 2D_SP representative model simulation results with high model efficiency and with respect to the field observations. This implied that a 1D vertical model description was sufficient to represent plot‐scale variability if smaller scale soil structure heterogeneities could be accounted for as effective parameters in dual‐domain models. Variation in the mass transfer term had a large effect on the vertical Br− profile distribution. The parameters describing the sizes and shapes of the domains were most relevant for estimating mass transfer between soil structural features in heterogeneous agricultural fields. Still, the calibration of the upscaling approach of two‐domain interactions in larger scale models remains challenging.

Modelling the impacts of spatial heterogeneity in soil hydraulic properties on hydrological process in the upper reach of the Heihe River in the Qilian Mountains, Northwest China

AbstractSpatial heterogeneity of soil has great impacts on dynamic processes of the hydrological systems. However, it is challenging and expensive to obtain spatial distribution of soil hydraulic properties, which often requires extensive soil sampling and observations and intensive laboratory analyses, especially in high elevation, hard to access mountainous areas. This study evaluates the impacts of soil heterogeneity on hydrological process in a high elevation, topographically complex watershed in Northwest China. Two approaches were used to derive the spatial heterogeneity of soil properties in the study watershed: (1) the spatial clustering method, Full‐Order‐CLK was used to determine five soil heterogeneous clusters (configurations 97, 80, 60, 40 and 20) through large number of soil sampling and in situ observations, and (2) the average values of soil hydraulic properties for each soil type were derived from the coarse provincial soil data sets (Gansu Soil Handbook at 1 : 1 000 000 scale). Subsequently, Soil and Water Assessment Tool model was used to quantify the impact of the spatial heterogeneity of soil hydraulic properties on hydrological process in the study watershed. Results show the simulations by Soil and Water Assessment Tool with the spatially clustered soil hydraulic information from the field sampling data had much better representation of the soil heterogeneity and had more accurate performance than the model using the average soil property values for each soil type derived from the coarse soil data sets. Thus, incorporating detailed field sampling, soil heterogeneity data greatly improve performance in hydrological modelling. Copyright © 2015 John Wiley & Sons, Ltd.

Regionalization of subsurface stormflow parameters of hydrologic models: Up-scaling from physically based numerical simulations at hillslope scale

Summary Subsurface stormflow is an important component of the rainfall-runoff response, especially in steep forested regions. However; its contribution is poorly represented in current generation of land surface hydrological models (LSMs) and catchment-scale rainfall-runoff models. The lack of physical basis of common parameterizations precludes a priori estimation (i.e. without calibration), which is a major drawback for prediction in ungauged basins, or for use in global models. This paper is aimed at deriving physically based parameterizations of the storage-discharge relationship relating to subsurface flow. These parameterizations are derived through a two-step up-scaling procedure: firstly, through simulations with a physically based (Darcian) subsurface flow model for idealized three dimensional rectangular hillslopes, accounting for within-hillslope random heterogeneity of soil hydraulic properties, and secondly, through subsequent up-scaling to the catchment scale by accounting for between-hillslope and within-catchment heterogeneity of topographic features (e.g., slope). These theoretical simulation results produced parameterizations of the storage-discharge relationship in terms of soil hydraulic properties, topographic slope and their heterogeneities, which were consistent with results of previous studies. Yet, regionalization of the resulting storage-discharge relations across 50 actual catchments in eastern United States, and a comparison of the regionalized results with equivalent empirical results obtained on the basis of analysis of observed streamflow recession curves, revealed a systematic inconsistency. It was found that the difference between the theoretical and empirically derived results could be explained, to first order, by climate in the form of climatic aridity index. This suggests a possible co-dependence of climate, soils, vegetation and topographic properties, and suggests that subsurface flow parameterization needed for ungauged locations must account for both the physics of flow in heterogeneous landscapes, and the co-dependence of soil and topographic properties with climate, including possibly the mediating role of vegetation.

Modeling the effect of soil structure on water flow and isoproturon dynamics in an agricultural field receiving repeated urban waste compost application

Transport processes in soils are strongly affected by heterogeneity of soil hydraulic properties. Tillage practices and compost amendments can modify soil structure and create heterogeneity at the local scale within agricultural fields. The long-term field experiment QualiAgro (INRA–Veolia partnership 1998–2013) explores the impact of heterogeneity in soil structure created by tillage practices and compost application on transport processes. A modeling study was performed to evaluate how the presence of heterogeneity due to soil tillage and compost application affects water flow and pesticide dynamics in soil during a long-term period. The study was done on a plot receiving a co-compost of green wastes and sewage sludge (SGW) applied once every 2years since 1998. The plot was cultivated with a biannual rotation of winter wheat–maize (except 1year of barley) and a four-furrow moldboard plow was used for tillage. In each plot, wick lysimeter outflow and TDR probe data were collected at different depths from 2004, while tensiometer measurements were also conducted during 2007/2008. Isoproturon concentration was measured in lysimeter outflow since 2004. Detailed profile description was used to locate different soil structures in the profile, which was then implemented in the HYDRUS-2D model. Four zones were identified in the plowed layer: compacted clods with no visible macropores (Δ), non-compacted soil with visible macroporosity (Γ), interfurrows created by moldboard plowing containing crop residues and applied compost (IF), and the plow pan (PP) created by plowing repeatedly to the same depth. Isoproturon retention and degradation parameters were estimated from laboratory batch sorption and incubation experiments, respectively, for each structure independently. Water retention parameters were estimated from pressure plate laboratory measurements and hydraulic conductivity parameters were obtained from field tension infiltrometer experiments. Soil hydraulic properties were optimized on one calibration year (2007/08) using pressure head, water content and lysimeter outflow data, and then tested on the whole 2004/2010 period. Lysimeter outflow and water content dynamics in the soil profile were correctly described for the whole period (model efficiency coefficient: 0.99) after some correction of LAI estimates for wheat (2005/06) and barley (2006/07). Using laboratory-measured degradation rates and assuming degradation only in the liquid phase caused large overestimation of simulated isoproturon losses in lysimeter outflow. A proper order of magnitude of isoproturon losses was obtained after considering that degradation occurred in solid (sorbed) phase at a rate 75% of that in liquid phase. Isoproturon concentrations were found to be highly sensitive to degradation rates. Neither the laboratory-measured isoproturon fate parameters nor the independently-derived soil hydraulic parameters could describe the actual multiannual field dynamics of water and isoproturon without calibration. However, once calibrated on a limited period of time (9months), HYDRUS-2D was able to simulate the whole 6-year time series with good accuracy.

Preferential Flow Effects on Infiltration and Runoff in Grassland and Forest Soils

Due to the heterogeneity of soil hydraulic properties, there are many ways in which natural soils respond to rainfall, making upscaling of flow processes from plot to catchment scale difficult. The objectives of this study were to qualitatively characterize the flow pathways on forest and grassland hillslopes and to quantitatively define the relevant parameters controlling surface runoff generation by using infiltration and dye tracer experiments supplemented with measurements of the saturated hydraulic conductivity Ksat, the structural porosity nSP, and the pore‐size distribution PSD. While infiltration excess overland flow dominates in grassland, forest soil structure characterized by relatively high values of Ksat and nSP enhances the infiltrability of the soil and consequently prevents or at least reduces surface runoff. The dye patterns suggest that macropores are more efficient in forest than in grassland soil. The low efficiency of grassland soil macropores in transporting all water vertically downward can be explained by (i) the fine and dense few topsoil layers caused by the land use that limit water flux into the underlying macropores and (ii) their restricted number, their tortuosity, and the restricted interaction between macropores and the matrix below the topsoil layer. The larger root water uptake of forest soil as compared to grassland soil can be viewed as an additional factor enhancing its storage capacity and, consequently, may reduce the generation of surface runoff. It remains unclear, however, what effect the low interaction between macropores and soil matrix in the upper part of the subsoil has on surface runoff in grassland soil; this should be investigated in future studies.

Non-equilibrium in soil hydraulic modelling

A notorious problem in modelling water dynamics within the unsaturated zone is preferential flow and temporal non-equilibrium between water content and water potential. These phenomena originate from the ubiquitous spatial heterogeneity of soil hydraulic properties and substantially hampers the prediction of solute transport from the soil surface towards groundwater. The characteristic scale of these processes is the soil profile or ‘pedon scale’ where modelling of water dynamics is typically based on the well established Richards equation which has been augmented by various conceptual approaches to represent preferential flow phenomena. In this paper we investigate through numerical case studies hydraulic non-equilibrium during infiltration of water into relatively dry, heterogeneous soil profiles, which is the most prominent scenario where non-equilibrium conditions affect the overall dynamics of water and solutes. We start from well-defined local heterogeneities in a two-dimensional domain and we analyze the dynamics of water content and water potential at the models full spatial resolution. Based on our numerical results we approach the problem of how to represent the heterogeneous setting by an effective one-dimensional description. For practical purposes of large scale applications, such one-dimensional schemes are highly required. As a prerequisite, soil types need to be represented by a reduced set of effective hydraulic properties. The main challenge is to preserve preferential flow phenomena in such simplified one-dimensional models. We demonstrate that classical approaches of volume averaging do not reflect non-equilibrium effects. From the different concepts for representing hydraulic non-equilibrium we focus on an approach proposed by Ross and Smettem [Ross, P., Smettem, K.R.J., 2000. A simple treatment of physical non-equilibrium water flow in soil. Soil Sci. Soc. Am. J. 64, 1926–1930] to decouple water content and water potential. This concept is corroborated by our numerical results and we discuss the possibility to link non-equilibrium dynamics and the related model parameters to observable structural properties of the material.

Three‐Dimensional Modeling of the Scale‐ and Flow Rate‐Dependency of Dispersion in a Heterogeneous Unsaturated Sandy Monolith

Spatial heterogeneity in soil hydraulic properties strongly affects solute transport. However, the level of precision needed in the description of spatial variability to properly reproduce the solute mixing regime is still an open question. We investigated this problem by analyzing observed inert solute transport using three‐dimensional simulations with different levels of complexity in the material description. The scale‐ and flow‐dependency of the dispersivity was first characterized from a series of leaching experiments during unsaturated steady‐state flow in a heterogeneous sandy monolith. The structure and hydraulic property variability within the monolith was investigated as well by means of an exhaustive survey of the monolith, and by intensive soil core sampling allowing for hydraulic characterization. In this study, three three‐dimensional models are constructed, involving several levels of complexity. In Case I, only the macrostructure variability is represented. In Case II, scaling factors encoding the spatial variability in the hydraulic properties of the sandy matrix are implemented. In Case III, an anisotropy factor for hydraulic conductivity is added to the macrostructure and the microheterogeneity of the sand matrix. Results show that microheterogeneity is needed to reproduce qualitatively the scale‐ and flow rate‐dependency of the transport parameters. Despite the elaborate effort devoted to the structure characterization, no model was fully capable of reproducing observed solute transport in the monolith and at the outlet.

Land-use induced spatial heterogeneity of soil hydraulic properties on the Loess Plateau in China

On the Loess Plateau in China, soil erosion amounts to between 10 000 and 25 000 tons/km 2/year. In 1998, the EROCHINA project was started, with the major objective of developing alternative land-use and soil and water conservation strategies for the Loess Plateau, using the LISEM soil erosion model. In order to provide the model with accurate input on soil hydraulic properties of the catchment, this study tried to quantify these properties for major land-use units and to examine the effects of the statistically identified in-field heterogeneity on model outcome. The study area (Danangou catchment) is located in the middle part of the Loess Plateau in the northern part of Shaanxi Province. The catchment is about 3.5 km 2 in size. To determine the hydraulic properties of the soil, a sampling scheme was implemented to measure unsaturated conductivity and water retention characteristics. The saturated conductivity measurements were performed on land-use clusters, based on treatment and plant and soil differences. A 100×100 m sampling grid was set out on 12 fields, with 1×1 m grid squares. On each field, 10 sampling spots were randomly selected, using Simple Random Sampling. The sensitivity of the LISEM model to the measured heterogeneity of saturated conductivity was analysed by using the geometric mean and standard deviation (S.D.) of the K s-optimized of the various land-use units to calculate discharge and soil loss during a single rain event. This proved that, using the standard deviation of the saturated conductivity, which was 30–50%, the calculated discharge and total sediment losses varied by a factor 2. Using the standard deviation had a minor effect on the calculated sediment concentration. As regards the on-site effects, the use of the geometric mean of K s minus the S.D. resulted in an increase in the level of erosion, while the use of geometric mean of K s plus the S.D. value resulted in a significant decrease in erosion level relative to that obtained using the geometric mean of K s itself. Using randomly selected sampling spots and a calibration procedure as was done in the present study make detailed information on K s available, which can be used to compare alternative land-use options for their effectiveness in reducing discharge and sediment losses.

Infiltration during soil sealing: The effect of areal heterogeneity of soil hydraulic properties

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.

Influence of spatially variable soil hydraulic properties and rainfall intensity on the water budget

Monte Carlo simulation is employed to investigate the separate and simultaneous effect of horizontal heterogeneity of soil hydraulic properties and rainfall intensity on various statistical properties of the components of the one‐dimensional water budget. One year of micrometeorological time series consisting of half‐hourly values, generated with a one‐dimensional climate model, constitutes the upper boundary conditions. Both a loamy and a sandy soil type are used. Spatial heterogeneity of rainfall is generated by disaggregation of the spatial mean rainfall. Compared with the annual water balance obtained using the scaled average soil hydraulic properties and spatial mean rainfall intensities, spatial heterogeneity of these induces significant changes in the mean values of the annual balance terms. Spatial variation of saturated water content has a negligible effect on the annual water balance and the dynamics of water storage. It appears that a homogeneous “equivalent” soil exists for the sandy soil but not for the loamy soil.

Simulating the effect of spatial variability of soil hydraulic properties on microwave brightness temperature over a corn field

Abstract Spatial variability of L-band (21 cm wavelength) microwave brightness temperature over a corn field, caused by spatial heterogeneity of soil hydraulic properties, is simulated by combining physically based models for microwave emission and for dynamics of soil water. The scaling theory is used for the spatial variability of soil hydraulic parameters, the scaling parameter being represented by a histogram corresponding to a log-normal frequency distribution. The mean and the standard deviation of brightness temperatures over a corn field are calculated as a saturated soil dries progressively under clear-sky conditions. Results are presented for two values for the coefficient of variation (CV)of the scaling parameter, namely 0·45 and 0·65, which encompass the range of a few available field observations. For CV=0·45, the mean brightness temperatures are higher and the standard deviations are lower by about 2 deg K compared with those for CV = 0·65. Results of the present simulation suggest that spat...

Estimating spatial variability in soil moisture with a simplified dynamic model

Observed spatial variability of soil moisture in the field may be due to several factors. Topographic effects, soil heterogeneity, and spatial variability in precipitation and vegetation may induce variability in measured soil moisture. Scaling theory has been suggested as a promising tool for describing soil heterogeneity and, consequently, a portion of spatial variability of measured soil moisture. We investigate the appropriateness of ascribing soil moisture variability to heterogeneity of soil properties through the use of a model of soil moisture dynamics and scaling theory. The model, termed a wetting front model, is a simplified representation of water movement in soils. The model conceptually depicts the moisture profile as discrete blocks of soil moisture separated by sharp fronts. Probabilistic simulations were run with input data appropriate to watersheds R‐5 and R‐7 at Chickasha, Oklahoma. Results from the simulations indicate that variability due to soil heterogeneity remains relatively constant through time with only slight increases during redistribution following infiltration. A comparison of simulated and observed soil moisture integrated from the surface to the 15‐cm depth shows that heterogeneity in soil hydraulic properties may account for approximately 75% of observed standard deviation.

Spatial Variability of Soil Hydraulics and Remotely Sensed Soil Parameters

AbstractThe development of methods to correctly interpret remotely sensed information about soil moisture and soil temperature requires an understanding of water and energy flow in soil, because the signals originate from the surface, or from a shallow surface layer, but reflect processes in the entire profile. One formidable difficulty in this application of soil physics is the spatial heterogeneity of natural soils. Earlier work has suggested that the heterogeneity of soil hydraulic properties may be described by the frequency distribution of a single scale factor. The sensitivity of hydraulic and energetic processes to the variation of this scale factor is explored with a suitable numerical model. We believe that such an analysis can help in deciding how accurately and extensively basic physical properties of field soils need to be known in order to interpret thermal or radar waveband signals. It appears that the saturated hydraulic conductivity needs to be known only to its order of magnitude, and that the required accuracy of the soil water retention function is about 0.02 volume fraction. Furthermore, the results may be helpful in deciding how the total scene or view field, as perceived through a sensor, is composed from the actual mosaic of transient soil properties, such as surface temperature or surface soil moisture. However, the latter proposition presupposes a random distribution of permanent properties, a condition that may not be met in many instances, and no solution of the problem is apparent.