Gerrit de Rooij

Fingered flow in the unsaturated zone caused by wetting front instability enhances solute leaching to the groundwater. This paper reviews recent progress in fingered flow research, focusing on theoretical results and model development. A variety of stability criteria have been derived to predict wetting front instability, mainly through linear, and sometimes non-linear, stability analysis, but also by theoretically analyzing infiltration

The quasi-linear form of Richards' Equation is used to analyze steady flow from a disc source with uniform flux density in a homogeneous soil with a shallow groundwater table. Additionally, a solution and examples are given for a cylindrical flow region, i.e. of a finite radial extent. Still more general solutions are presented for cylindrical sources, including the limiting cases

<p>Occasionally, there is an interest in groundwater flows over many millennia. The input parameter requirement of numerical groundwater flow models and their calculation times limit their usefulness for such studies.</p><p>Analytical models require considerable simplifications of the properties and geometry of aquifers and of the forcings. On the other hand, they do not appear to have an inherent limitation on the duration of the simulated period. The simplest models have explicit solutions, meaning that the hydraulic head at a given time and location can be calculated directly, without the need to incrementally iterate through the entire preceding time period like their numerical counterparts.</p><p>We developed an analytical solution for a simple aquifer geometry: a strip aquifer between a no flow boundary and a body of surface water with a prescribed water level. This simplicity permitted flexible forcings: The non-uniform initial hydraulic head in the aquifer is arbitrary and the surface water level can vary arbitrarily with time. Aquifer recharge must be uniform in space but can also vary arbitrarily with time.</p><p>We also developed a modification that verifies after prescribed and constant time intervals if the hydraulic head is such that the land surface is covered with water. This excess water then infiltrates in areas where the groundwater level is below the surface and the remainder is discharged into the surface water. The hydraulic head across the aquifer is modified accordingly and used as the initial condition for the next time interval. This modification models the development of a river network during dry periods. The increased flexibility of the model comes at the price of the need to go through the entire simulation period one time step at a time. For very long time records, these intervals will typically be one year.</p><p>Given the uncertainty of the aquifer parameters and the forcings, the models are expected to be used in a stochastic framework. We are therefore working on a shell that accepts multiple values for each parameter as well as multiple scenarios of surface water levels and groundwater recharge rates, along with an estimate of their probabilities. The shell will generate all possible resulting combinations, the number of which can easily exceed 10000, then runs the model for each combination, and computes statistics of the average hydraulic head and the aquifer discharge into the surface water at user-specified times.</p><p>A case study will tell if this endeavor is viable. We will model the aquifer below the mountain range north of Salalah in Oman, which separates the desert of the Arabian Peninsula from the coastal plain at its southern shore. Rainfall estimates from the isotopic composition of stalactites in the area indicate distinct dry and wet periods in the past 300 000 years. In combination with estimated sea level fluctuations over that period, this provides an interesting combination of forcings. We examine the dynamics of the total amount of water stored in the aquifer, and of the outflow of water from the aquifer into the coastal plain.</p>

ABSTRACT More frequent and intense droughts due to global climate change, together with an increasing agricultural water use emphasize the importance of understanding root water uptake under water-stressed conditions. While root water uptake is driven by potential gradients, measurement of soil water potentials was limited by the measurement range of water-filled tensiometers (-0.085 MPa). A recently developed polymer tensiometer (POT) can measure soil water potentials down to -1.6 MPa. Monitoring low soil water potentials in the presence of root water uptake may help gain knowledge of a plant's strategy to cope with water stress, and allows improved determination of local water stress levels in experiments. To investigate plant strategies that cope with water stress, soil water potentials were measured in the vicinity of maize roots in three lysimeters. The lysimeters received different irrigation amounts: an optimal irrigation gift (-0.05 < p < -0.02 MPa) and minimized irrigation to create moderate (minimum p = -0.45 MPa) and severe (minimum p = -0.80 MPa) water stress. Measured soil water potentials showed that the water stressed plants started to take up water from deeper soil layers, and continued to take up water under very dry conditions. This research was funded by the Dutch Technology Foundation (STW).

In densely drained lowland catchments surface water discharge is fed by groundwater flow toward streams, ditches and tile drains, and overland flow. Due to intensive agriculture, high nutrient losses from these catchments cause eutrophication in downstream waters. To determine effective measures to reduce these nutrient loads, we first need to quantify contributions of individual flow routes to the catchment discharge.

The most important parameterizations of the soil water retention curve do not perform very well in either the wet or the dry end. Rossi and Nimmo (WRR 1994) therefore gave the Brooks-Corey (1966) power-law model of the soil water retention curve a non-asymptotic dry range. Ippisch et al. (Adv. Water Resour., 2006) added an air-entry value to the sigmoidal retention model of van Genuchten (SSSAJ 1980). The models of Rossi and Nimmo and Ippisch et al. were Adapted by de Rooij (HESS 2021) to arrive at a sigmoidal, non-asymptotic soil water retention curve with an air-entry value, dubbed RIA. In RIA, the matric potential at oven-dryness, hd, appeared as a derived parameter.Bittelli and Flury (SSSAJ 2009) showed that dry-range soil water retention data points often are unreliable. In order to make RIA robust when this is the case, this presentation explains how hd was made a fitting parameter that can be fixed if needed. This modification was complicated by the peculiar behavior of shape parameter α that made adequate parameter fitting impossible. The presentation elucidates this behavior and explains how this problem was solved by a reformulated model (de Rooij, HESS 2022). It then shows how earlier fits (when the problem had not yet been discovered) corroborate the reformulated model.The work also offers support for a theoretical value for hd proposed by Schneider and Goss (Geoderma 2012), which is very helpful if dry-range data are lacking or of poor quality. The mathematical structure of RIA is such that, for α → ∞, Rossi and Nimmo’s model arises as a special case of RIA, and, by implication, Brooks-Corey as a special case of Ippisch et al.A public-domain code to fit the parameters using shuffled complex evolution (SCE) is available on Zenodo (de Rooij, 2022). It has features that help the user identify issues with local minima and overparameterization, and provides more information than most codes to offer better insight into the fitting process for those familiar with the SCE algorithm. These features may be useful for other parameter identification problems, so they will be discussed as well.