Advancing the selection of soil hydraulic property models for soil-crop modelling

<p>From hypothesis testing to impact assessment, soil-crop models serve a multitude of purposes in researching climate change impact on the water cycle in cropping systems. As such, model complexity undeniably differs amongst approaches and intended purpose but must include the full range of soil-water states. Climate change is predicted to increase prolonged periods of dry conditions, however, soil-crop models, often misrepresented drying of soils, inducing misquantifications in root water uptake, evapotranspiration, and concomitantly, water use efficiency and ground water recharge rates. While in soil hydrology, this has been recognised and effective solutions exist, soil-crop models typically neglect these more comprehensive approaches.</p><p>Our study serves two aims: For one, we shed light into the chaotic multitude of soil hydraulic property models, by representing the models in a systematic framework, of sub-models and processes. This model is called the Brunswick Soil Hydraulic Property Model Framework. With this we hope to make soil hydrology more tangible, for non-experts. Secondly, we i) demonstrate that representing soil hydraulic properties over the entire moisture range, i.e. from fully saturated to completely dry soils, changes simulated root water uptake under dry soil conditions, which sensitively affects crop yields. We quantify these effects by simulating root water uptake using the agro-ecosystem models Expert-N and Daisy, comparing the standard as well as oversimplified approach to a comprehensive model (of the Brunswick model framework). This was possible by using new pedotransfer functions to obtain all required model parameters. The framework and pedotransfer functions are available in the R package spsh on CRAN.</p>

Post-fire soil water repellency, sorptivity, and the measurement of infiltration

<p>Wildfires are frequently associated with the increased potential for runoff, flooding and debris flows during and after subsequent rainfall events. Specifically, wildfires can cause soils to become water repellent, which is believed to slow, if not halt, the infiltration of water into the soil. However, there exists no mechanistic way to determine the effect of post-wildfire water repellency on infiltration and runoff—until now. We have recently developed a simple physically-based model to account for the effect of water repellency on the soil hydraulic property of sorptivity. Further, since sorptivity is crucial to understanding the relationship between water repellency and post-fire infiltration, there is a need for a robust and accurate method to measure sorptivity in the field. All research was conducted in the laboratory using a fine silica sand, some of which was treated with Scotchgard<sup>TM</sup> to induce water repellency. Treated and untreated sand was mixed proportionally by weight to create various degrees of water repellent sand. Upward infiltration tests (wicking experiments) were used to validate the sorptivity model. Then, two common field methods (a 1D infiltrometer ring and a 3D tension infiltrometer) were used to measure downward infiltration and extract sorptivity data. Results showed that we able to predict sorptivity given the degree of water repellency and basic soil properties, that sorptivity measured using common field methods reflected the degree of water repellency, and that both downward infiltration measurement methods yielded similar sorptivity values.</p>

A Simulation and Validation of CLM during Freeze-Thaw on the Tibetan Plateau

The applicability of a new soil hydraulic property of frozen soil scheme applied in Community Land Model 4.5 (CLM4.5), in conjunction with an impedance factor for the presence of soil ice, was validated through two offline numerical simulations conducted at Madoi (GS) and Zoige (ZS) on the Tibetan Plateau (TP). Sensitivity analysis was conducted via replacing the new soil hydraulic property scheme in CLM4.5 by the old one, using default CLM4.5 runs as reference. Results indicated that the new parameterization scheme ameliorated the surface dry biases at ZS but enlarged the wet biases which existed at GS site due to ignoring the gravel effect. The wetter surface condition in CLM4.5 also leads to a warmer surface soil temperature because of the greater heat capacity of liquid water. In addition, the combined impact of new soil hydraulic property schemes and the ice impedance function on the simulated soil moisture lead to the more reasonable simulation of the starting dates of freeze-thaw cycle, especially at the thawing stage. The improvements also lead to the more reasonable turbulent fluxes simulations. Meanwhile, the decreased snow cover fraction in CLM4.5 resulted in a lower albedo, which tended to increase net surface radiation compared to previous versions. Further optimizing is needed to take the gravel into account in the numerical description of thermal-hydrological interactions.