Klaus Bohne

Numerical experiments to simulate vertical vapor and liquid water transport in unsaturated non-rigid porous media

Abstract We present numerical simulations using a model named VALTRAUDE (VApor and Liquid TRansport in Unsaturated DEformable media). It was developed to simulate thermal induced heat and vapor transport and liquid water flow in unsaturated soil as affected by both hydraulic and thermal gradients. Both of the flow phenomena are coupled using the Philip and de Vries [Philip, J.R., de Vries, D.A., 1957. Moisture movement in porous materials under temperature gradients. Trans. Am. Geophys. Union 38 (2), 222–231.] theory. VALTRAUDE is able to consider the porous medium to be non-rigid. We therefore compare numerical simulations of water flow in rigid and non-rigid soil columns under isothermal and non-isothermal conditions. Simulation results indicate that the isothermal steady-state equilibrium pressure head distribution in a soil column under non-isothermal conditions, when the soil column is sealed at the top and at the bottom, so that water content remains constant, is replaced by a dynamic flow equilibrium, where flow of liquid water is balanced by flow of vapor. Thus, in a state of non-isothermal equilibrium, the hydraulic gradient is not zero, and the matric potential at the top of the soil has lower values than under isothermal conditions. Comparing rigid with non-rigid soils, in the case of dry initial conditions and non-isothermal boundary conditions we found smaller flow rates and therefore less negative values of matric potential at the top of the soil. With wet initial conditions, there is nearly no difference between rigid and non-rigid soil.

Inverse simulation of non-steady-state evaporation using nonequilibrium water retention data: a case study

The aim of this investigation was to check the capability of the Mualem/van Genuchten model to predict unsaturated soil hydraulic conductivity values from water retention data in the absence of spatial variability and effects of macropores. Furthermore, different techniques of parameter estimation were tested. Soil cores of 250 cm3 were filled with a repacked silty loam soil. After measuring the saturated hydraulic conductivity and equilibrium soil water retention, the soil cores were subject to non-steady-state evaporation (bottom sealed). Pressure head and soil mass were recorded every 60 min. Based upon the van Genuchten/Mualem model and the Darcy–Buckingham equation, an inverse simulation was carried out. As a first option, the parameters θr, θs, α, and n were fitted to water retention data. The remaining parameters L (coefficient of tortuosity) and Ks were fitted to pressure head versus time recordings during evaporation. Equilibrium soil water retention data yielded a poor fit. The observed pressure head values during evaporation, h(t), could not be described sufficiently by using hydraulic functions determined from equilibrium retention values. A good fit was obtained in case nonequilibrium water retention measurements from the same experiment were used. Obviously, no equilibrium between pressure head and water content was established during evaporation under atmospheric conditions leading to an evaporation rate of about 3 mm day−1. As a second option, all the parameters were fitted simultaneously to both nonequilibrium soil water content data and tensiometer readings during evaporation. Simultaneous fitting yielded slightly decreased errors of simulated h(t) values and slightly increased errors of calculated soil water content. Another simulation model was used to calculate vapor flow in soil during evaporation. Results indicate that vapor flow may be significant in the range h<−500 cm. Before this range was reached, vapor flow could be neglected. Based on the results, we would like to recommend measuring transient water retention under conditions similar to those which are to be predicted in the field. Not only because of hysteresis but also because of transient effects, water retention characteristics of infiltration and evaporation should be distinguished.

Prediction of long-term groundwater recharge by using hydropedotransfer functions

Abstract The investigations to estimate groundwater recharge were performed. Improved consideration of soil hydrologic processes yielded a convenient method to predict actual evapotranspiration and hence, groundwater recharge from easily available data. For that purpose a comprehensive data base was needed, which was created by the simulation model SWAP comprising 135 different site conditions and 30 simulation years each. Based upon simulated values of actual evapotranspiration, a transfer function was developed employing the parameter b in the Bagrov differential equation dEa/dP = 1- (Ea/Ep)b. Under humid conditions, the Bagrov method predicted long-term averages of actual evapotranspiration and groundwater recharge with a standard error of 15 mm year-1 (R = 0.96). Under dry climatic conditions and groundwater influence, simulated actual evapotranspiration may exceed precipitation. Since the Bagrov equation is not valid under conditions like these, a statistic-based transfer function was developed predicting groundwater recharge including groundwater depletion with a standard error of 26mm(R = 0.975). The software necessary to perform calculations is provided online.