[1] The use of inverse one-dimensional (1-D) analytical methods for estimating vertical stream-aquifer exchange flux is now commonplace. However, the application of such simple models can lead to significant errors in estimates of vertical exchange flux where the model assumptions are violated in real systems. An idea that is gaining acceptance in the literature is that the presence of nonvertical flow is such a violation. However, it is shown here that nonvertical flow by itself will not necessarily lead to errors in vertical flux estimation but rather that significant errors can stem from nonuniform (convergent/ divergent) flow fields and/or hydrodynamic dispersion even within uniform flow fields. Nonuniform flow may also be expected, in some cases, to create discrepancies between flux estimates made on the basis of vertical head gradient measurements and those made using 1-D analytical heat tracer methods. Significant differences are observed in the estimates of heat-derived fluxes obtained by the amplitude ratio and phase-shift time-series methods when convergent and divergent flows are apparent. Such differences may potentially be used to infer that convergent or divergent flow is occurring and that a 1-D analysis is inappropriate.Citation: Cuthbert, M. O., and R. Mackay (2013), Impacts of nonuniform flow on estimates of vertical streambed flux, Water Resour.
Participatory Impact Pathways Analysis (PIPA) is a practical planning, and monitoring and evaluation approach developed for use with complex projects in the water and food sectors . PIPA begins with a participatory workshop where stakeholders make explicit their assumptions about how their project will achieve an impact. Participants construct problem trees, carry out a visioning exercise and draw network maps to help them clarify their 'impact pathways'. These are then articulated in two logic models. The outcomes logic model describes the project's medium term objectives in the form of hypotheses: which actors need to change, what are those changes and which strategies are needed to realise these changes. The impact logic model describes how, by helping to achieve the expected outcomes, the project will impact on people's livelihoods. Participants derive outcome targets and milestones which are regularly revisited and revised as part of project monitoring and evaluation (M&E). PIPA goes beyond the traditional use of logic models and logframes by engaging stakeholders in a structured participatory process, promoting learning and providing a framework for 'action research' on processes of change.
Abstract. Fast upscaling of hydraulic conductivity is a recurrent problem in modeling flow through heterogeneous porous media. We propose a new renormalization technique. It is based on the iterative application of the Cardwell and Parsons [1945] bounds on elementary groups of cells. The combination of the bounds with a heuristic formula allows anisotropy to be taken into account. The new technique is tested and compared with other fast techniques. Among the tested techniques the two most reliable ones are the tensorial renormalization and the new simplified renormalization. The numerical efficiency of the simplified renormalization leads us to recommend it when a diagonal tensor of equivalent conductivity is sufficient.
IntroductionModeling groundwater flow and transport requires a realistic description of the spatial distribution of hydraulic conductivity to capture flow paths and to make realistic forecasts of groundwater behavior. As discussed by de Marsily et al. [1998] and Koltermann and Gorelick [1996], statistical and genetic tools can be used to describe the underground distribution of hydraulic conductivity. Geological models generated by these tools often produce a very high spatial resolution which cannot be used directly in groundwater flow and transport models given currently available computer resources. It is necessary to adopt a much coarser description. This upscaling comprises the calculation of spatial averages of the hydraulic conductivity over blocks of the geological model that can be used directly in the groundwater model. This problem is different from the problem of finding a unique effective conductivity for the whole aquifer. Renard and de Marsily [1997] and Wen and G6mez-Herngndez [1996] present extensive reviews of upscaling theories, analytical results, and numerical techniques.In this paper, we develop a fast real-space renormalization algorithm to calculate block conductivities. The essence of renormalization is to apply composition rules to groups of cells at the local scale to produce "composite" cells and then to successively reapply the same rules to the resulting composite cells until the desired coarse-scale resolution is achieved. The basic assumption is that the upscaling rules are scale invariant. When the probability distribution of the local-scale conductivity is known, it is possible to follow the evolution of the prob-
Abstract.Results are presented of a detailed study into the vadose zone and shallow water table hydrodynamics of a field site in Shropshire, UK. A conceptual model is presented and tested using a range of numerical models, including a modified soil moisture balance model (SMBM) for estimating groundwater recharge in the presence of both diffuse and preferential flow components. Tensiometry reveals that the loamy sand topsoil wets up via preferential flow and subsequent redistribution of moisture into the soil matrix. Recharge does not occur until near-positive pressures are achieved at the top of the sandy glaciofluvial outwash material that underlies the topsoil, about 1 m above the water table. Once this occurs, very rapid water table rises follow. This threshold behaviour is attributed to the vertical discontinuity in preferential flow pathways due to seasonal ploughing of the topsoil and to a lower permeability plough/iron pan restricting matrix flow between the topsoil and the lower outwash deposits. Although the wetting process in the topsoil is complex, a SMBM is shown to be effective in predicting the initiation of preferential flow from the base of the topsoil into the lower outwash horizon. The rapidity of the response at the water table and a water table rise during the summer period while flow gradients in the unsaturated profile were upward suggest that preferential flow is also occurring within the outwash deposits below the topsoil. A variation of the source-responsive model proposed by Nimmo (2010) is shown to reproduce the observed water table dynamics well in the lower outwash horizon when linked to a SMBM that quantifies the potential recharge from the topsoil. The results reveal new insights into preferential flow processes in cultivated soils and provide a useful and practical approach to accounting for preferential flow in studies of groundwater recharge estimation.
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