Mikkel Mollerup

A MATLAB program for estimation of unsaturated hydraulic soil parameters using an infiltrometer technique

We combined an inverse routine for assessing the hydraulic soil parameters of the Campbell/Mualem model with the power series solution developed by Philip for describing one-dimensional vertical infiltration into a homogenous soil. We based the estimation routine on a proposed measurement procedure especially suitable for early-time infiltrometer experiments where the flow can be considered as one-dimensional. The routine requires input of the initial soil water content and cumulative infiltration in two experiments with different pressures at the upper boundary and/or initial conditions. An independent measurement of the soil water content at saturation may reduce the uncertainty of estimated parameters. Response surfaces of the objective function were analysed. Scenarios for various soils and conditions, using numerically generated synthetic cumulative infiltration data with normally distributed errors, show promising results for finding the true values of the optimized parameters. We also investigated the effects of measurement frequency for the cumulative infiltration and errors in water content determinations.

Comparison of simulated water, nitrate, and bromide transport using a Hooghoudt‐based and a dynamic drainage model

For large-scale hydrological modeling, the accuracy of the models used is a trade-off with the computational requirements. The models that perform well on the daily/meter scale may not perform well when applied at the yearly/kilometer scale. We compare two models of water flow and nitrate and bromide transport in a tile drained soil. The first model is based on a 2-D grid with an explicit drain node, here called the Dynamic Drainage Model (DDM). The second and less computationally expensive model is based on an 1-D vertical discretization where the horizontal flow is included as a sink term based on the Hooghoudt theory, here called the Hooghoudt Drainage Model (HDM). Both are based on Finite Volume Method solutions to Richards equation and to the advection-dispersion equation (ADE), and embedded within the Daisy agroecological model, which includes the nitrogen cycle. The two models are run with 10 years of weather data and three different lower-boundary conditions. Losses of water, nitrogen, and bromide to both drain pipes and deep percolation/leaching are compared between the models, at daily and yearly time scales. In no case do we find the discrepancy large enough to warrant a rejection of the use of the faster HDM instead of DDM. For the daily time scale, we find in general a higher Nash-Sutcliffe efficiency coefficient for water (0.98–1.00) than for nitrate (0.97–1.00), and the lowest for bromide (0.95–1.00). The results are explained with a low concentration gradient along the water flow pathway toward the drain.