In this update, we review some of the more significant advances that have been made in the last decade in the study of preferential flow through the vadose zone as well as suggest some research needs in the coming years. We focus mostly on work that aims to improve understanding of the processes themselves and less on more applied aspects concerning the various consequences of preferential flow (e.g., for surface water and groundwater quality). In recent years, the research emphasis has shifted somewhat toward the two extremes of the scale continuum, the pore scale and the scale of management (field, catchments, and landscapes). This trend has been facilitated by significant advances in both measurement technologies (e.g., noninvasive imaging techniques and high frequency–high spatial resolution monitoring of soil moisture at field and catchment scales) and application of novel methods of analysis to large datasets (e.g., machine learning). This work has led to a better understanding of how pore network properties control preferential flow at the pore to core scales as well as some new insights into the influence of site attributes (climate, land uses, soil types) at field to landscape scales. We conclude that models do not at present fully reflect the current state of process understanding and empirical knowledge of preferential flow. However, we expect that significant advances in computational techniques, computer hardware, and measurement technologies will lead to increasingly reliable model predictions of the impacts of preferential flow, even at the larger scales relevant for management.

Understanding Preferential Flow in the Vadose Zone: Recent Advances and Future Prospects

Underground pore fissure and bedrock bareness are the main factors that lead to complicated mechanisms of soil erosion in karst regions. The objectives of this study were as follows: a) to measure runoff and sediment generated from the surface and underground in the karst bare slope; and b) to analyse the impacts of underground pore fissures and bedrock bareness rates on soil erosion. To achieve these objectives, we designed an experiment in which underground pore fissure degrees and bedrock bareness rates were varied, and their impacts on soil erosion were measured. The results showed that і) runoff and sediment yields from the surface and underground increased with increasing rainfall intensity, and the surface runoff was much more easily generated when the rainfall intensity was greater; ii) surface runoff yield first increased and then decreased with increasing bedrock bareness rates, while the runoff yield in underground pore fissures increased with increasing bedrock bareness rates; thus, sediment production on the surface and in underground pore fissures showed the same pattern, i.e., first decreased and then increased; iii) the surface runoff and sediment generally decreased with increasing degrees of underground pore fissures, while that of underground pore fissures presented a different pattern; and iv) rainfall intensity, bedrock bareness rates and underground pore fissure degrees were the important factors affecting soil erosion processes in karst bare slopes because the bedrock bareness rates and underground pore fissure degrees were the main factors causing the uneven distribution of runoff and sediment on soil surfaces and underground. Therefore, soil and water loss in underground pore fissures was the special pattern and main difference in soil erosion processes in the karst region compared with other regions. These results have theoretical and practical significance for understanding soil erosion mechanisms and controlling soil and water loss in karst regions.

Runoff and erosion processes on bare slopes in the Karst Rocky Desertification Area.

In this review, we explore the significance of rock fragments in studies of soil hydrological processes, because according to research, the effects of rock fragments on soil hydrological processes are inconsistent (positive/negative). Rock fragments play a critical role in the biosphere as the primary filter for water and solutions containing contaminants and heavy metals. Rock fragments have a complex influence on soil hydrological processes (e.g. soil erosion, runoff generation, water infiltration, solute transport and water flow) and are highly relevant in a typical system in the soil–plant–atmosphere continuum. Cracks related to rock fragments provide preferential flow paths through which pollutants are transported into groundwater. In this paper, the published literature is reviewed concentrating on rock fragments and their effects on soil hydrological processes. Systematic studies and examples illustrate the effects of rock fragments architecture (i.e. coverage, content, size, position, spatial heterogeneity, morphology, weathering and topography) on soil hydrological processes. Methods and models applied to evaluate the effects of rock fragments on soil hydrological processes are examined in detail. We conclude that the relationship between rock fragments and soil hydrological processes is complicated and requires more international research efforts. This review concludes with a discussion of perspectives on areas of research that can improve understanding of the effects of rock fragments on soil hydrological processes, with insights and suggestions also being provided regarding potential research trends, requirements and solutions.

Rock fragments and soil hydrological processes: Significance and progress

This study constitutes a review of spring hydrograph recession analysis, and it is focused on karst aquifers. The different literature models have been separated into empirical and physically-based models; in the last ones, only analytical models have been considered, as they provide the discharge equation during recession. Under constant geometrical and hydraulic aquifer characteristics, it has been found that the “exponential form” appears to be the most recurrent theoretical type, at least during the long-term flow recession. During this stage, any deviation from the exponential form, may suggest hydraulic anisotropy of actual aquifers, as well as aquifer geometry has a fundamental role in controlling the shape of spring hydrographs. The hydrodynamics of karst aquifer under recession has been described, associating any segment of the hydrograph to a specific hydrologic condition of the aquifer, and also to a specific physical law which control the water flow.

The Recession of Spring Hydrographs, Focused on Karst Aquifers

Karst aquifers provide large parts of the water supply for Mediterranean countries, though climate change is expected to have a significant negative impact on water availability. Recharge is therefore a key variable that has to be known for sustainable groundwater use. In this study, we present a new approach that combines two independent methods for karst recharge estimation. The first method derives spatially distributed information of mean annual recharge patterns through GIS analysis. The second is a process-based karst model that provides spatially lumped but temporally distributed information about recharge. By combining both methods, we add a spatial reference to the lumped simulations of the process-based model. In this way, we are able to provide spatiotemporal information of recharge and subsurface flow dynamics also during varying hydroclimatic conditions. We find that there is a nonlinear relationship between precipitation and recharge rates resulting in strong decreases of recharge following even moderate decreases of precipitation. This is primarily due to almost constant actual evapotranspiration amounts despite varying hydroclimatic conditions. During the driest year in the record, almost the entire precipitation was consumed as actual evapotranspiration and only little diffuse recharge took place at the high altitudes of our study site. During wettest year, recharge constituted a much larger fraction of precipitation and occurred at the entire study site. Our new method and our findings are significant for decision makers in similar regions that want to prepare for possible changes of hydroclimatic conditions in the future.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014WR015685

Modeling spatiotemporal impacts of hydroclimatic extremes on groundwater recharge at a Mediterranean karst aquifer

Knowledge of soil moisture dynamics in the unsaturated soil zone provides valuable information on the temporal and spatial variability of groundwater recharge This is especially true for the Mediterranean region, where a substantial fraction of long-term groundwater recharge is expected to occur during high magnitude precipitation events of above-average wet winters To elucidate process understanding of infiltration processes during these extreme events, a monitoring network of precipitation gauges, meteorological stations, and soil moisture plots was installed in an area with a steep climatic gradient in the Jordan Valley region In three soil moisture plots, Hydrus-1D was used to simulate water movement in the unsaturated soil zone with soil hydraulic parameters estimated by the Shuffled Complex Evolution Metropolis algorithm To generalize our results, we modified soil depth and rainfall input to simulate the effect of the pronounced climatic gradient and soil depth variability on percolation fluxes and applied the calibrated model to a time series with 62 years of meteorological data Soil moisture measurements showed a pronounced seasonality and suggested rapid infiltration during heavy rainstorms Hydrus-1D successfully simulated short and long-term soil moisture patterns, with the majority of simulated deep percolation occurring during a few intensive rainfall events Temperature drops in a nearby groundwater well were observed synchronously with simulated percolation pulses, indicating rapid groundwater recharge mechanisms The 62-year model run yielded annual percolation fluxes of up to 66% of precipitation depths during wet years and of 0% during dry years Furthermore, a dependence of recharge on the temporal rainfall distribution could be shown Strong correlations between depth of recharge and soil depth were also observed

/pdf/recharge-estimation-and-soil-moisture-dynamics-in-a-321q7dv3l8.pdf

Recharge estimation and soil moisture dynamics in a Mediterranean, semi-arid karst region

In many semi-arid environments, the surface materials are frequently characterized by thin soils and large proportions of bare rock, both of which are expected to considerably influence infiltration and groundwater recharge processes. In this study, small plot scale irrigation experiments in combination with dye tracer application, as well as real-time and subsequent soil moisture measurements and soil particle size analysis, were conducted to investigate the significance of preferential flow at the rock soil interface. Large scale assessments of soil depth and surface type distribution were used to upscale the findings of the irrigation experiments to the scale of a complete hillslope and, further, to estimate percolation properties of a hypothetical rainfall event. The two experimental plots were each located in the West Bank Mountains close to Ramallah. Each irrigation experiment was designed with 50 mm of precipitation, applied on an area of 1 m 2 during the course of ca. 2.5 h. Each irrigation plot had a surface share of approximately 50% rock outcrop and 50% soil surface. Percolation properties were investigated by subsequent soil moisture measurements at high spatial resolution on vertical soil profiles on the irrigated plot. Dye tracer application on the rock outcrop during the sprinkling allowed for tracking of resulting outcrop runoff in the subsurface. The soil depth survey included 2100 measurement points on 7 transects, which allowed the estimation of local soil depth distribution. Results of the irrigation experiments showed that precipitation-induced runoff from rock outcrops continued below ground as preferential flow along the rock–soil interface, while water from the soil surface percolated mainly vertically and much more homogeneously. This was evident from the high density soil moisture measurements, as well as from the dye tracer patterns. Outcrop runoff percolated faster and to greater depths than water infiltrating directly from the soil surface, thus possessing a greater potential for groundwater recharge.

Significance of preferential flow at the rock soil interface in a semi-arid karst environment

Characterisation and modelling of conduit restricted karst aquifers – Example of the Auja spring, Jordan Valley

Controls on runoff generation along a steep climatic gradient in the Eastern Mediterranean

 Pluvial or flash floods generated by heavy precipitation
events cause large economic damage and loss of life worldwide As discharge
observations from such extreme occurrences are rare, especially on the scale
of small catchments or even hillslopes, data from artificial sprinkling
experiments offer valuable information on runoff generation processes,
overland and subsurface flow rates, and response times We conducted 132
large-scale sprinkling experiments on natural hillslopes at 23 sites with
different soil types and geology on pastures and arable land within the
federal state of Baden-Wurttemberg in south-western Germany The
experiments were realized between 2016 and 2017 Simulated rainfall events
of varying durations were based on (a) the site-specific 100-year return
periods of rainfall with different durations and (b) the maximum rainfall
intensity observed locally The 100 m 2 experimental area was
divided into three individual plots, and overland and subsurface flow, soil
moisture, and water level dynamics in the temporarily saturated soil zone
were measured at 1 min resolution Furthermore, soil characteristics were
described in detail for each site The data were carefully processed and
corrected for measurement errors and combined into a consistent and easy-to-use database The experiments revealed large variability in possible
runoff responses to similar rainfall characteristics In general,
agricultural fields produced more overland flow than grassland The latter
generated hardly any runoff during the first simulated 100-year event on
initially dry soils The data set provides valuable information on runoff
generation variability from natural hillslopes and may be used for the
development and evaluation of hydrological models, especially those
considering physical processes governing runoff generation during extreme
precipitation events The data set presented in this paper is freely
available from the FreiDok plus data repository at
 https://doiorg/106094/UNIFR/151460 (Ries et al, 2019)

/pdf/runoff-reaction-from-extreme-rainfall-events-on-natural-3rqwe96nlp.pdf

Fabian Ries

Papers

Recharge estimation and soil moisture dynamics in a Mediterranean, semi-arid karst region

Significance of preferential flow at the rock soil interface in a semi-arid karst environment

Characterisation and modelling of conduit restricted karst aquifers – Example of the Auja spring, Jordan Valley

Controls on runoff generation along a steep climatic gradient in the Eastern Mediterranean

Runoff reaction from extreme rainfall events on natural hillslopes: a data set from 132 large-scale sprinkling experiments in south-western Germany