The International Soil Moisture Network (ISMN) was initiated in 2009 to support calibration and validation of remote sensing products and land surface models, and to facilitate studying the behavior of our climate over space and time. The ISMN does this by collecting and harmonizing soil moisture data sets from a large variety of individually operating networks and making them available through a centralized data portal. Due to the diversity of climatological conditions covered by the stations and differences in measurement devices and setup, the quality of the measurements is highly variable. Therefore, appropriate quality characterization is desirable for a correct use of the data sets. This study presents a new, automated quality control system for soil moisture measurements contained in the ISMN. Two types of quality control procedures are presented. The first category is based on the geophysical dynamic range and consistency of the measurements. It includes flagging values exceeding a certain threshold and checking the validity of soil moisture variations in relation to changes in soil temperature and precipitation. In particular, the usability of global model‐ or remote sensing–based temperature and precipitation data sets were tested for this purpose as an alternative to in situ measurements, which are often not recorded at the soil moisture sites themselves. The second category of procedures analyzes the shape of the soil moisture time series to detect outliers (spikes), positive and negative breaks, saturation of the signal, and unresponsive sensors. All methods were first validated and then applied to all the data sets currently contained in the ISMN. A validation example of an AMSR‐E satellite and a GLDAS‐Noah model product showed a small but positive impact of the flagging. On the basis of the positive results of this study we will add the flags as a standard attribute to all soil moisture measurements contained in the ISMN.
Darcy velocity fields obtained from finite element solutions of heads in groundwater flow exhibit discontinuities at element boundaries, thus giving rise to inaccuracies in path line construction. Several methods have been developed to resolve those inaccuracies including global postprocessing of velocity fields, mixed hybrid finite element formulations, and stream function formulations. All of these techniques either lead to a considerable increase in the computational effort or are not general enough for all purposes. The point of view taken here is that standard finite element flow models yield exact water balances, which in the usual approach to path line computation are not fully used. By starting out from patches with flux‐conserving boundaries, a new postprocessing procedure is derived that allows construction of a continuous flux distribution over the whole model domain. In this procedure only the head gradients of adjacent elements are used, thus avoiding the solution of another global system of equations. Furthermore, application of a stream function interpolation inside each patch allows the fast computation of path lines without any time stepping. The path lines computed with the proposed procedure are compared to path lines computed by usual particle tracking and examples are presented showing that the new method is vastly superior to traditional particle tracking.
Linear Galerkin finite element discretizations of the Laplace operator produce nonpositive stiffness coefficients for internal element edges of two-dimensional Delaunay triangulations. This property, also called the positive transmissibility (PT) condition, is a prerequisite for the existence of an M -matrix and ensures that nonphysical local extrema are not present in the solution.For tetrahedral elements, it has already been shown that the linear Galerkin approach does not in general satisfy the PT condition. We propose a modification of the three-dimensional Galerkin scheme that, if a Delaunay triangulation is used, satisfies the PT condition for internal edges and, if further conditions on the boundary are specified, yields an M -matrix. The proposed approach can also be extended to the general diffusion operator with nonconstant or anisotropic coefficients.
SUMMARYIn standard ÿnite element simulations of groundwater ow the correspondence between hydraulic head gradients and groundwater uxes is represented by the sti ness matrix. In two-dimensional problems the use of linear triangular elements on Delaunay triangulations guarantees a sti ness matrix of type M . This implies that the local numerical uxes are physically consistent with Darcy's law. This condition is fundamental to avoid the occurrence of local maxima or minima, and is of crucial importance when the calculated ow ÿeld is used in contaminant transport simulations or pathline evaluation. In three spatial dimensions, the linear Galerkin approach on tetrahedra does not lead to M -matrices even on Delaunay meshes. By interpretation of the Galerkin approach as a subdomain collocation scheme, we develop a new approach (OSC, orthogonal subdomain collocation) that is shown to produce M -matrices in three-dimensional Delaunay triangulations. In case of heterogeneous and anisotropic coe cients, extra mesh properties required for M -sti ness matrices will also be discussed.
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