The importance of preferential flow in controlling groundwater recharge in tropical Africa and implications for modelling the impact of climate change on groundwater resources

2010 ◽  
Vol 1 (4) ◽  
pp. 234-245 ◽  
Author(s):  
M. O. Cuthbert ◽  
C. Tindimugaya
Keyword(s):  
Soil Moisture ◽  
Preferential Flow ◽  
Unsaturated Flow ◽  
Uniform Flow ◽  
Richards Equation ◽  
Tropical Africa ◽  
Lateritic Soils ◽  
Flow Models ◽  
Spatial Coverage ◽  
The Impact

An improved water table fluctuation technique for estimating recharge is applied to a sustained (10-year) groundwater level monitoring record in Uganda, a rare dataset for tropical Africa, and compared against results from soil moisture balance models (SMBMs) and unsaturated flow models. The results show that recharge is directly proportional to rainfall (long-term average rainfall is around 1200 mm/a), even during times when high soil moisture deficits are anticipated. This indicates that preferential and/or localized flow mechanisms dominate the recharge behaviour. SMBMs and unsaturated flow models, based on uniform flow governed by the Richards equation, are shown to be inappropriate for estimating recharge in this location underlain by lateritic soils. Given the large spatial coverage of lateritic soils both globally and in tropical Africa, and despite the convenience of physically based uniform flow models and, in particular, SMBMs, concern is raised over the use of such models for recharge estimation, and thus for exploring future trends due to climate or land use change, unless backed up by sufficient hydraulic data to enable the recharge processes to be confirmed. More research is needed to assess how widespread preferential flow may be within other major soil groups and climate zones.

scholarly journals Geostatistic Interpolation of Soil Moisture

1997 ◽  
Vol 28 (4-5) ◽  
pp. 307-328 ◽  
Author(s):  
Nils-Otto Kitterød ◽  
E. Langsholt ◽  
W. K. Wong ◽  
L. Gottschalk
Keyword(s):  
Soil Moisture ◽  
Unsaturated Zone ◽  
Preferential Flow ◽  
Unsaturated Flow ◽  
Three Dimensional ◽  
Indicator Kriging ◽  
Flow Paths ◽  
Flow Models ◽  
Semivariogram Analysis ◽  
Preferential Flow Paths

The spatial distribution of soil moisture defines preferential flow paths in the unsaturated zone. Hence, three dimensional (3D) estimates of soil moisture are of great importance to understand transport of contaminants as well as remediation processes in the unsaturated zone. In this study 3D estimates conditioned on spatially frequent observations of soil moisture, have been obtained by kriging. The observations were divided into subdomains consistent with the local stratigraphy and directional semivariogram analysis was applied. It was found difficult to clearly identify a 3D semivariogram function in this case, but from a georadar survey two semivariogram functions were derived, describing two different sedimentological units. By conditioning the estimates of soil moisture on the sedimentological architecture computed by indicator kriging, more accurate estimates were achieved. These improvements were quantified by a ‘jackknife’ cross validation procedure. Besides the practical aspects of finding the most important flow paths estimates of soil moisture are valuable when validating unsaturated flow models.

scholarly journals MEASUREMENT OF PREFERENTIAL FLOW DURING INFILTRATION AND EVAPORATION IN POROUS MEDIA

2017 ◽  
Vol 43 (4) ◽  
pp. 1831
Author(s):  
A. Papafotiou ◽  
C. Schütz ◽  
P. Lehmann ◽  
P. Vontobel ◽  
D. Or ◽  
...  
Keyword(s):  
Porous Media ◽  
Water Distribution ◽  
Preferential Flow ◽  
Unsaturated Flow ◽  
Coupling Effect ◽  
Heterogeneous Systems ◽  
Heterogeneous Porous Media ◽  
Richards Equation ◽  
Hydraulic Coupling ◽  
Experimental Findings

Infiltration and evaporation are governing processes for water exchange between soil and atmosphere. In addition to atmospheric supply or demand, infiltration and evaporation rates are controlled by the material properties of the subsurface and the interplay between capillary, viscous and gravitational forces. This is commonly modeled with semi-empirical approaches using continuum models, such as the Richards equation for unsaturated flow. However, preferential flow phenomena often occur, limiting or even entirely suspending the applicability of continuum-based models. During infiltration, unstable fingers may form in homogeneous or heterogeneous porous media. On the other hand, the evaporation process may be driven by the hydraulic coupling of materials with different hydraulic functions found in heterogeneous systems. To analyze such preferential flow processes, water distribution was monitored in infiltration and evaporation lab experiments using neutron transmission techniques. Measurements were performed in 2D and 3D, using homogeneous and heterogeneous setups. The experimental findings demonstrate the fingering effect in infiltration and how it is influenced by the presence of fine inclusions in coarse background material. During evaporation processes, the hydraulic coupling effect is found to control the evaporation rate, limiting the modeling of water balances between soil and surface based on surface information alone.

Impact of Initial Soil Moisture Anomalies on Subsequent Precipitation over North America in the Coupled Land–Atmosphere Model CAM3–CLM3

2007 ◽  
Vol 8 (3) ◽  
pp. 513-533 ◽  
Author(s):  
Yeonjoo Kim ◽  
Guiling Wang
Keyword(s):  
Soil Moisture ◽  
North America ◽  
Continental Scale ◽  
Model Version ◽  
Initial Soil Moisture ◽  
Spatial Coverage ◽  
Initial Soil ◽  
Atmosphere Model ◽  
Shallow Soils ◽  
The Impact

Abstract To investigate the impact of anomalous soil moisture conditions on subsequent precipitation over North America, a series of numerical experiments is performed using a modified version of the Community Atmosphere Model version 3 and the Community Land Model version 3 (CAM3–CLM3). First, the mechanisms underlying the impact of spring and summer soil moisture on subsequent precipitation are examined based on simulations starting on 1 April and 1 June, respectively. How the response of precipitation to initial soil moisture anomalies depends on the characteristics of such anomalies, including the timing, magnitude, spatial coverage, and vertical depth, is then investigated. There are five main findings. First, the impact of spring soil moisture anomalies is not evident until early summer although their impact on the large-scale circulation results in slight changes in precipitation during spring. Second, precipitation increases with initial soil moisture almost linearly within a certain range of soil moisture. Beyond this range, precipitation is less responsive. Third, during the first month following the onset of summer soil moisture anomalies, the precipitation response to wet anomalies is larger in magnitude than that to dry anomalies. However, the resulting wet anomalies in precipitation quickly dissipate within a month or so, while the resulting dry anomalies in precipitation remain at a considerable magnitude for a longer period. Consistently, wet spring anomalies are likely to be ameliorated before summer, and thus have a smaller impact (in magnitude) on summer precipitation than dry spring anomalies. Fourth, soil moisture anomalies of smaller spatial coverage lead to precipitation anomalies that are smaller and less persistent, compared to anomalies at the continental scale. Finally, anomalies in shallow soil can persist long enough to influence the subsequent precipitation at the seasonal time scale. Dry anomalies in deep soils last much longer than those in shallow soils.

scholarly journals Enhancing Model Skill by Assimilating SMOPS Blended Soil Moisture Product into Noah Land Surface Model

2015 ◽  
Vol 16 (2) ◽  
pp. 917-931 ◽  
Author(s):  
Jifu Yin ◽  
Xiwu Zhan ◽  
Youfei Zheng ◽  
Jicheng Liu ◽  
Li Fang ◽  
...  
Keyword(s):  
Soil Moisture ◽  
Land Surface ◽  
Surface Radiation ◽  
Radiation Budget ◽  
Shortwave Radiation ◽  
Surface Model ◽  
Spatial Coverage ◽  
In Situ Observations ◽  
The Impact

Abstract Many studies that have assimilated remotely sensed soil moisture into land surface models have generally focused on retrievals from a single satellite sensor. However, few studies have evaluated the merits of assimilating ensemble products that are merged soil moisture retrievals from several different sensors. In this study, the assimilation of the Soil Moisture Operational Products System (SMOPS) blended soil moisture (SBSM) product, which is a combination of soil moisture products from WindSat, Advanced Scatterometer (ASCAT), and Soil Moisture and Ocean Salinity (SMOS) satellite sensors is examined. Using the ensemble Kalman filter (EnKF), a synthetic experiment is performed on the global domain at 25-km resolution to assess the impact of assimilating the SBSM product. The benefit of assimilating SBSM is assessed by comparing it with in situ observations from U.S. Department of Agriculture Soil Climate Analysis Network (SCAN) and the Surface Radiation Budget Network (SURFRAD). Time-averaged surface-layer soil moisture fields from SBSM have a higher spatial coverage and generally agree with model simulations in the global patterns of wet and dry regions. The impacts of assimilating SMOPS blended data on model soil moisture and soil temperature are evident in both sparsely and densely vegetated areas. Temporal correlations between in situ observations and net shortwave radiation and net longwave radiation are higher with assimilating SMOPS blended product than without the data assimilation.

scholarly journals Assessment of the Temperature Effects in SMAP Satellite Soil Moisture Products in Oklahoma

2021 ◽  
Vol 13 (20) ◽  
pp. 4104
Author(s):  
Kim Oanh Hoang ◽  
Minjiao Lu
Keyword(s):  
Dielectric Constant ◽  
Soil Moisture ◽  
Brightness Temperature ◽  
Temperature Effects ◽  
The United States ◽  
Satellite Observations ◽  
Soil Moisture Data ◽  
Spatial Coverage ◽  
The Impact

Soil moisture is a notably important component in various studies in water sciences, including hydrology, agriculture, and water management. To achieve extensive or global spatial coverage, satellites focusing on soil moisture observation have been launched, and many satellite products, such as SMAP and SMOS soil moisture products, have been provided. Most of these satellite observations are based on the dielectric properties of wet soil, and most soil moisture retrieval algorithms are calibrated or evaluated using in situ soil moisture. While the in situ data observed by dielectric sensors, which are the most widely used, are reported to include errors caused by the so-called “temperature effects” of these sensors, the temperature dependency of bulk soil dielectric constant has rarely been discussed on satellite data. Since both in situ dielectric measurements and satellite observations are based on the same physical variable, the dielectric constant and the dielectrically measured in situ soil moisture data are also used as ground truth, it is necessary to assess the impact of temperature effects on satellite products. In this work, we attempted to identify the existence of the temperature effects and evaluate the consequences of removing these effects on in situ and satellite soil moisture and the relationships between the brightness temperature at the soil surface and the brightness temperature provided by satellite observation. To achieve the goals of this study, we analyzed the temperature effects on surface soil moisture data provided by a SMAP mission in Oklahoma, the United States. The results show that temperature effects exist in SMAP soil moisture products in Oklahoma, and the removal of these effects will potentially improve the accuracy of these products.

scholarly journals Model predicts the impact of root system architecture on soil water infiltration.

2021 ◽  
Author(s):  
Andrew Mair ◽  
Lionel Xavier Dupuy ◽  
Mariya Ptashnyk
Keyword(s):  
Soil Moisture ◽  
Hydraulic Conductivity ◽  
Root System ◽  
Moisture Transport ◽  
Water Pressure ◽  
Root Systems ◽  
Saturated Hydraulic Conductivity ◽  
Water Infiltration ◽  
Richards Equation ◽  
The Impact

There is strong experimental evidence that root systems substantially change the saturated hydraulic conductivity of soil. However, the mechanisms by which roots affect soil hydraulic properties remain largely unknown. In this work, we made the hypothesis that preferential soil moisture transport occurs along the axes of roots, and that this is what changes a soil's saturated hydraulic conductivity. We modified Richards' equation to incorporate the preferential flow of soil moisture along the axes of roots. Using the finite element method and Bayesian optimisation, we developed a pipeline to calibrate our model with respect to a given root system. When applied to simulated root systems, the pipeline successfully predicted the pore-water pressure profiles corresponding to saturated hydraulic conductivity values, observed by Leung et al. (2018), for soils vegetated by willow and grass. Prediction accuracy improved for root systems with more realistic architectures, therefore suggesting that changes in saturated hydraulic conductivity are a result of roots enabling preferential soil moisture transport along their axes. The model proposed in this work improves our ability to predict moisture transport through vegetated soil and could help optimise irrigation, forecast flood events and plan landslide prevention strategies.

Modelling non-equillibrium unsaturated flow in soils during sudden pressure head changes by solving Richards' equation with a Method of lines approach

2021 ◽  
Author(s):  
Robert Mietrach ◽  
Thomas Wöhling ◽  
Niels Schütze
Keyword(s):  
Water Content ◽  
Preferential Flow ◽  
Unsaturated Flow ◽  
Full Range ◽  
Method Of Lines ◽  
Pressure Head ◽  
Richards Equation ◽  
Extended Model ◽  
Solution Methods ◽  
Numerical Robustness

<p>The classical formulation of Richards' equation is relying on a unique functional relationship between water content, conductivity and pressure head. Some phenomena like hystersis effects in the water content during wetting and drying cycles and hydraulic non-equillibrium cannot be accounted for with this formulation. Therefor it has been extended in different ways in the past to be able to include these effects in the simulation. Each modification comes with its own challenges regarding implementation and numerical stability.<br>The Method Of Lines approach to solving the Richards' equation has already be shown to be an efficient and stable alternative to established solution methods, such as low-order finite difference and finite element methods applied to the mixed form of Richards' equation.<br>In this work a slightly modified Method Of Lines approach is used to solve the pressure based 1D Richards' equation. A finite differencing scheme is applied to the spatial derivative and the resulting system of ordinary differential equations is reformulated as differential-algebraic system of equations. The open-source code IDAS from the Sundials suite is used to solve the DAE system. Different extensions to Richards' equation have been incorporated into the model to address the shortcomings mentioned above. These extensions are a model able to simulate preferential flow using a coupled two domain approach, a simple hysteretic model to account for hysteresis in the water retention curve and also two models to either fully or partially calculate hydraulic non-equillibrium effects. To verify the numerical robustness of the extended model, stochastic parameterizations were generated that represent the full range of all soil types. Simulations were carried out using these parameter sets and real-world meteorological boundary conditions at 10 minutes time intervals, that exhibit drastic flux changes and poses numerical challenges for classical solution methods.</p><p>The results show that not only does the extended model converge for all parameterizations, but that numerical robustness and performance is maintained. Where applicable the results have been verified against solutions from the software Hydrus and show good agreement with those.</p>

Capillary effect on flow in the drainage layer of highway pavement

2012 ◽  
Vol 39 (6) ◽  
pp. 654-666 ◽  
Author(s):  
Han-Cheng Dan ◽  
Pei Xin ◽  
Ling Li ◽  
Liang Li ◽  
David Lockington
Keyword(s):  
Unsaturated Flow ◽  
Groundwater Table ◽  
Richards Equation ◽  
Capillary Effect ◽  
Saturated Flow ◽  
Drainage Layer ◽  
Flow Models ◽  
Model Based ◽  
D Model ◽  
Highway Pavement

This paper aims to examine capillarity effect on flows in the drainage layer of highway pavement. A two-dimensional (2-D) model based on the Richards equation was used to simulate saturated and unsaturated flows in the drainage layer. For comparison, flows were also simulated using a 1-D Boussinesq equation based model and a 2-D model based on the Laplace equation, both assuming saturated flow only. The drainage layer was modeled with sand and gravel, which possess similar hydraulic properties to those of commonly used filling materials in practice. The results showed that the two saturated flow models agreed well with each other, indicating the dominance of horizontal flow in the drainage layer. However, their predictions differed significantly from those of the variably saturated flow models. The latter model predicted significant flow activities in a relatively large unsaturated zone, especially for a sandy drainage layer. Such unsaturated flow contributes to and enhances the capacity of the drainage layer. With the unsaturated flow neglected, the saturated flow models over-predicted the extent of the saturated zone and hence the groundwater table elevation. As the current engineering design of the drainage layer is typically based on the groundwater table elevation predicted by the saturated flow models, the finding of this study suggests that the design criterion is likely to lead to over-design of the drainage system. Further work is also required to prove the practical significance of the capillary effect and account for other factors.

scholarly journals Soil Moisture Dynamics in Response to Precipitation and Thinning in a Semi-Dry Forest in Northern Mexico

Water ◽  
2021 ◽  
Vol 13 (1) ◽  
pp. 105
Author(s):  
Argelia E. Rascón-Ramos ◽  
Martín Martínez-Salvador ◽  
Gabriel Sosa-Pérez ◽  
Federico Villarreal-Guerrero ◽  
Alfredo Pinedo-Alvarez ◽  
...  
Keyword(s):  
Soil Moisture ◽  
Forest Management ◽  
Water Availability ◽  
Dry Forest ◽  
Rainfall Events ◽  
Rainfall Characteristics ◽  
Northern Mexico ◽  
The Difference ◽  
The Impact ◽  
Moisture Dynamics

Understanding soil moisture behavior in semi-dry forests is essential for evaluating the impact of forest management on water availability. The objective of the study was to analyze soil moisture based in storm observations in three micro-catchments (0.19, 0.20, and 0.27 ha) with similar tree densities, and subject to different thinning intensities in a semi-dry forest in Chihuahua, Mexico. Vegetation, soil characteristics, precipitation, and volumetric water content were measured before thinning (2018), and after 0%, 40%, and 80% thinning for each micro-catchment (2019). Soil moisture was low and relatively similar among the three micro-catchments in 2018 (mean = 8.5%), and only large rainfall events (>30 mm) increased soil moisture significantly (29–52%). After thinning, soil moisture was higher and significantly different among the micro-catchments only during small rainfall events (<10 mm), while a difference was not noted during large events. The difference before–after during small rainfall events was not significant for the control (0% thinning); whereas 40% and 80% thinning increased soil moisture significantly by 40% and 53%, respectively. Knowledge of the response of soil moisture as a result of thinning and rainfall characteristics has important implications, especially for evaluating the impact of forest management on water availability.

scholarly journals Towards Including Dynamic Vegetation Parameters in the EUMETSAT H SAF ASCAT Soil Moisture Products

2021 ◽  
Vol 13 (8) ◽  
pp. 1463
Author(s):  
Susan C. Steele-Dunne ◽  
Sebastian Hahn ◽  
Wolfgang Wagner ◽  
Mariette Vreugdenhil
Keyword(s):  
Soil Moisture ◽  
Interannual Variability ◽  
Incidence Angle ◽  
Pearson Correlation ◽  
The United States ◽  
Research Network ◽  
Window Width ◽  
High Frequency Variability ◽  
Vegetation Parameters ◽  
The Impact

The TU Wien Soil Moisture Retrieval (TUW SMR) approach is used to produce several operational soil moisture products from the Advanced Scatterometer (ASCAT) on the Metop series of satellites as part of the EUMETSAT Satellite Application Facility on Support to Operational Hydrology and Water Management (H SAF). The incidence angle dependence of backscatter is described by a second-order Taylor polynomial, the coefficients of which are used to normalize ASCAT observations to the reference incidence angle of 40∘ and for correcting vegetation effects. Recently, a kernel smoother was developed to estimate the coefficients dynamically, in order to account for interannual variability. In this study, we used the kernel smoother for estimating these coefficients, where we distinguished for the first time between their two uses, meaning that we used a short and fixed window width for the backscatter normalisation while we tested different window widths for optimizing the vegetation correction. In particular, we investigated the impact of using the dynamic vegetation parameters on soil moisture retrieval. We compared soil moisture retrievals based on the dynamic vegetation parameters to those estimated using the current operational approach by examining their agreement, in terms of the Pearson correlation coefficient, unbiased RMSE and bias with respect to in situ soil moisture. Data from the United States Climate Research Network were used to study the influence of climate class and land cover type on performance. The sensitivity to the kernel smoother half-width was also investigated. Results show that estimating the vegetation parameters with the kernel smoother can yield an improvement when there is interannual variability in vegetation due to a trend or a change in the amplitude or timing of the seasonal cycle. However, using the kernel smoother introduces high-frequency variability in the dynamic vegetation parameters, particularly for shorter kernel half-widths.

Sign in / Sign up

Export Citation Format

Share Document