Storm-water infiltration and focused recharge modeling with finite-volume two-dimensional Richards equation: application to an experimental rain garden

Rain gardens are infiltration systems that provide volume and water quality control, recharge enhancement, as well as landscape, ecological, and economic benefits. A model for application to rain gardens based on Richards equation coupled to a surface water balance was developed, using a two-dimensional finite-volume code. It allows for alternating upper boundary conditions, including ponding and overflow, and can simulate heterogeneous soil-layering or more complex geometries to estimate infiltration and recharge. The algorithm is conservative, and exhibits good performance compared to standard models for several test cases (less than 0.1% absolute mass balance error); simulations were also performed for an experimental rain garden and comparisons to collected data are presented. The model accurately simulated the matrix flow, soil water distribution, as well as deep percolation (potential recharge) for a natural rainfall event in the controlled experimental setup. Read More: http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29HY.1943-7900.0000111?prevSearch=authors%3A%28Dussaillant%2C%29&searchHistoryKey

More green and less blue water in the Alps during warmer summers

Climate change can reduce surface-water supply by enhancing evapotranspiration in forested mountains, especially during heatwaves. We investigate this ‘drought paradox’ for the European Alps using a 1,212-station database and hyper-resolution ecohydrological simulations to quantify blue (runoff) and green (evapotranspiration) water fluxes. During the 2003 heatwave, evapotranspiration in large areas over the Alps was above average despite low precipitation, amplifying the runoff deficit by 32% in the most runoff-productive areas (1,300–3,000 m above sea level). A 3 °C air temperature increase could enhance annual evapotranspiration by up to 100 mm (45 mm on average), which would reduce annual runoff at a rate similar to a 3% precipitation decrease. This suggests that green-water feedbacks—which are often poorly represented in large-scale model simulations—pose an additional threat to water resources, especially in dry summers. Despite uncertainty in the validation of the hyper-resolution ecohydrological modelling with observations, this approach permits more realistic predictions of mountain region water availability

An urban ecohydrological model to quantify the effect of vegetation on urban climate and hydrology (UT&C v1.0)

Increasing urbanization is likely to intensify the urban heat island effect, decrease outdoor thermal comfort and enhance runoff generation in cities. Urban green spaces are often proposed as a mitigation strategy to counteract these adverse effects and many recent developments of urban climate models focus on the inclusion of green and blue infrastructure to inform urban planning. However, many models still lack the ability to account for different plant types and oversimplify the interactions between the built environment, vegetation, and hydrology. In this study, we present an urban ecohydrological model, Urban Tethys-Chloris (UT&C), that combines principles of ecosystem modelling with an urban canopy scheme accounting for the biophysical and ecophysiological characteristics of roof vegetation, ground vegetation and urban trees. UT&C is a fully coupled energy and water balance model that calculates 2 m air temperature, 2 m humidity, and surface temperatures based on the infinite urban canyon approach. It further calculates all urban hydrological fluxes, including transpiration as a function of plant photosynthesis. Hence, UT&C accounts for the effects of different plant types on the urban climate and hydrology, as well as the effects of the urban environment on plant well-being and performance. UT&C performs well when compared against energy flux measurements of eddy covariance towers located in three cities in different climates (Singapore, Melbourne, Phoenix). A sensitivity analysis, performed as a proof of concept for the city of Singapore, shows a mean decrease in 2 m air temperature of 1.1 °C for fully grass covered ground, 0.2 °C for high values of leaf area index (LAI), and 0.3 °C for high values of Vc,max (an expression of photosynthetic activity). These reductions in temperature were combined with a simultaneous increase in relative humidity by 6.5 %, 2.1 %, and 1.6 %, for fully grass covered ground, high values of LAI, and high values of Vc,max, respectively. Furthermore, the increase of pervious vegetated ground is able to significantly reduce surface runoff. These results show that urban greening can lead to a decrease in urban air temperature and surface runoff, but this effect is limited in cities characterized by a hot, humid climate.ISSN:1991-962XISSN:1991-961

Numerical simulation of ground surface subsidence due to coal-bed methane extraction

Coal bed methane (CBM) has gained significant attention as a source of natural gas. CBM recovery is achieved through either primary production or enhanced CBM production, the later of which remains at an infant stage. Primary CBM extraction involves production of CBM reservoir fluids using production wells to facilitate pressure drawdown within the targeted formation. De-pressurization is required to release adsorbed methane within the interior surface of the coal matrix. However, de-pressurization can cause compaction within the CBM reservoir, especially in the vicinity of production wells. This, in turn, can lead to ground surface subsidence. The objective of this project is to develop a semi-analytical solution to explore ground surface subsidence above CBM extraction wells. To achieve this, an existing analytical solution, for ground surface subsidence above a cylindrical uniform pressure change, is extended to allow for a non-uniform pressure distribution using the principle of superposition. The non-uniform effective pressure to drive the semi-analytical solution for ground surface subsidence is derived from a numerical fluid flow model describing water and methane production from a CBM formation, also developed as part of this project. The numerical fluid flow model describes two-phase fluid flow (gas and water) in porous media in conjunction with non-equilibrium gas adsorption and stress dependent porosity and permeability. The resulting set of partial differential equations is solved using the method of lines by discretising in space using finite difference and then solving the resulting set of coupled non-linear ordinary differential equations (ODE) using MATLAB's ODE solver, ODE15s. The numerical fluid flow model was verified by comparison with published modeling results from the literature. As a further verification, the model's ability to simulate field production and pressure data was demonstrated using field data from a CBM case study in the US. The potential role of initial water saturation on ground surface subsidence was investigated by studying the associated spatial distributions of fluid pressure. It was found that, for a given time, the mean fluid pressure within the reservoir reduces with increasing initial water saturation. However, the spatial distribution of fluid pressure, for a given volume of produced gas, was found to be insensitive to initial water saturation. This can be attributed to the fact that the volume of water stored in the cleats of the coal-bed is very small as compared to the volume of gas stored within the coal matrix. Consequently, the presence of water in the cleats was found to have no influence on ground surface subsidence for a given gas production volume. It was also found that ground surface subsidence for a given gas production volume is insensitive to initial coal permeability and cleat volume compressibility. A simplified analytical solution for ground surface subsidence was derived assuming that the pressure distribution within the reservoir is uniform. Sensitivity analysis showed that the simplified analytical solution is effective at predicting ground surface subsidence for a given gas production volume, predicted by the numerical model, for all of the scenarios studied. This suggests that pressure distribution within a CBM reservoir is not important for determining ground surface subsidence in this context