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The capture and use of water are critically important in drylands, which collectively constitute Earth's largest biome. Drylands will likely experience lower and more unreliable rainfall as climatic conditions change over the next century. Dryland soils support a rich community of microphytic organisms (biocrusts), which are critically important because they regulate the delivery and retention of water. Yet despite their hydrological significance, a global synthesis of their effects on hydrology is lacking. We synthesized 2,997 observations from 109 publications to explore how biocrusts affected five hydrological processes (times to ponding and runoff, early [sorptivity] and final [infiltration] stages of water flow into soil, and the rate or volume of runoff) and two hydrological outcomes (moisture storage, sediment production). We found that increasing biocrust cover reduced the time for water to pond on the surface (−40%) and commence runoff (−33%), and reduced infiltration (−34%) and sediment production (−68%). Greater biocrust cover had no significant effect on sorptivity or runoff rate/amount, but increased moisture storage (+14%). Infiltration declined most (−56%) at fine scales, and moisture storage was greatest (+36%) at large scales. Effects of biocrust type (cyanobacteria, lichen, moss, mixed), soil texture (sand, loam, clay), and climatic zone (arid, semiarid, dry subhumid) were nuanced. Our synthesis provides novel insights into the magnitude, processes, and contexts of biocrust effects in drylands. This information is critical to improve our capacity to manage dwindling dryland water supplies as Earth becomes hotter and drier. This work was conducted as part of the Powell Working Group “Completing the dryland puzzle: creating a predictive framework for biological soil crust function and response to climate change” supported by the John Wesley Powell Center for Analysis and Synthesis, funded by the US Geological Survey. J.B. and S.R. were funded by USGS Ecosystems and Land Use Change Mission Areas, by the US Department of Energy (DESC-0008168), and by the Strategic Environmental Research and Development Program (RC18-1322). J.D. is supported by grants from the Holsworth Wildlife Research Endowment & The Ecological Society of Australia, and a scholarship from China Scholarship Council (No. 201706040073). B.C. is supported by grants from the National Science Foundation (award DEB-1844531) and DePaul University. M.A.B. is supported by a grant from the National Science Foundation (award DEB-1638966). B.W. was supported by the Max Planck Society and a Paul Crutzen Nobel Laureate Fellowship. E.H.-S. was supported by CONACYT grant 251388 B. F.T.M. was supported by the European Research Council (ERC grant agreement 647038 [BIODESERT]) and Generalitat Valenciana (CIDEGENT/2018/041).

Soil hydraulic properties control the provision of hydrological services. Vegetation and topography influence these properties by altering soil structure and porosity. The underlying mechanisms are not yet fully understood for the high Andean region. In this study, we examined how vegetation and topographic attributes are related to soil hydraulic properties and soil pore structure in young volcanic ash soils, and further correlated them to soil texture, organic carbon, and root characteristics to explain these relationships. In a 0.7 km2 study site located in the Andean páramo of northern Ecuador, we measured soil water retention, saturated hydraulic conductivity, bulk density (BD), and pore size distribution parameters on eight soil profiles with contrasting vegetation types (cushion-forming plants vs. tussock grasses) and topographic positions (summit vs. hillslope). We observed significant differences in soil hydraulic properties and soil pore structure in the uppermost horizons by vegetation type, whereas topography had a minor effect. In the A horizons, we found higher water retention at saturation and field capacity (10%–14%), higher total available water (8%–15%), and higher saturated hydraulic conductivity (4–12 times) under cushion-forming plants compared to tussock grasses. The elevated values under cushion plants were attributed to the presence of larger pores, lower soil BD, and higher soil organic carbon content as a result of coarser root systems. Total available water was generally high (0.34–0.40 cm3 cm−3), and locally not associated with any soil property. The higher water retention in soils under cushion vegetation can enhance soil water storage for plants and the regulation of water flows during prolonged rainfall events. The saturated hydraulic conductivity of the surface horizons is high compared to rainfall intensities resulting in high infiltration capacity; whilst its decline with depth reveals the potential for generation of subsurface stormflow, especially below cushion-forming plants. Our findings highlight that soil hydraulic properties differ among vegetation types, and show the significance of vegetation types for soil hydrology.

Soil hydrology is a key control for the functioning of the terrestrial environment. Many environmental issues that we need to tackle today are directly linked to soil water dynamics. This includes agricultural production and food security, nutrient cycling and carbon storage, prevention of soil degradation and erosion, and last but not least, clean water resources and flood protection. However, these problems need to be addressed at the scales of fields, regions, and landscapes, while soil water dynamics and soil hydraulic properties are well understood and typically measured at much smaller scales—the comfort zone of soil physics. An obvious problem is how to link these vastly different scales and how to profit from small-scale understanding to improve our capability to predict what is going on at the large scale. In this update, this problem is discussed based on insights gained during the last decades. As a synthesis, a two-step scaling approach is proposed for modeling soil water dynamics from local to landscape scales where the scale of the soil profile is the stepping stone.

We thank Audrey Innes for isotope analysis at University of Aberdeen for Bruntland Burn and Krycklan sites, Johannes Tiwari (SLU) for isotope sampling in Krycklan, Pernilla Lofvenius (SLU) for providing PET data for Krycklan (via SITES), and Jeff McDonnell and Kim Janzen (University of Saskatchewan) for soil water isotope analysis for the Dorset and Wolf Creek sites. The Krycklan part was funded by the KAW Branch-Point project. We acknowledge the funding from the European Research Council (ERC, project GA 335910 VeWa). We thank the Editor and three anonymous reviewers for their critical comments during the peer-review process.

Tremendous progress has been made with respect to ground penetrating radar (GPR) equipment, data acquisition, and processing since the establishment of GPR as a tool for soil water content determination in vadose zone hydrology about 25 yr ago. In this update, we aim to provide a critical overview of recent advances in vadose zone applications of GPR with a particular focus on new possibilities for multi-offset and borehole GPR measurements, the development of quantitative off-ground GPR methods, full-waveform inversion of GPR measurements, the potential of time-lapse GPR measurements for process investigations and hydrological parameter estimation, and recent improvements in GPR instrumentation. We hope that this update encourages the soil hydrology, groundwater, and critical zone community to embrace GPR as a viable tool for soil water content determination and the elucidation of subsurface hydrological processes.

The description of soil water movement at the field scale requires soil hydraulic material properties. These cannot be measured directly and exhibit an intrinsic multiscale heterogeneity. The estimation of soil hydraulic properties through inverse modeling or data assimilation requires measurements of the state of the system. However, these measurements are typically scarce and do not resolve soil heterogeneity leading to effective material properties with limited predictive capabilities. In a synthetic study at the scale of soil profiles, we explore the possibility to estimate an effective reduced one‐dimensional representation based on only four local water content measurements in a vertical profile located in one‐ and two‐dimensional heterogeneous media. We allow the effective material properties to deviate locally to consider the impact of the small‐scale heterogeneity on the water content measurements. In the synthetic experiments, the estimated reduced one‐dimensional representations predict soil water content and fluxes sufficiently well but can fail to predict the accurate timing and magnitude of infiltration fronts. Cumulative fluxes below the water content measurements were predicted correctly even on short time scales, if evaporation was estimated accurately. However, this proved to be challenging if the local evaporation fluxes above the measurement profile deviated strongly from the average evaporation fluxes of the heterogeneous medium. The one‐dimensional estimation leads to soil hydraulic parameters describing effective material properties that differ from the true material properties to compensate the missing representation of heterogeneity and two‐dimensional flow.

The ensemble Kalman filter (EnKF) is a widely used data assimilation method in soil hydrology. However, underestimation of the modeling errors and of the sampling errors may cause systematic reduction of state variances and rejection of the observations. Inflation methods are used to alleviate this phenomenon. Here, we suggest a novel inflation method based on confidence intervals constructed using the collected ensemble of the measurements. The proposed method is illustrated via two synthetic examples of a three‐layer soil with (i) precipitation and evaporation boundary condition and (ii) irrigation boundary condition. We present a comparison of two existing inflation methods and discuss the advantages and limitations of the proposed method. Basically, the suggested method behavior is superior to the behavior of the existing methods.

Soil water repellency can impact soil hydrology, overland flow generation and associated soil losses. However, current hydrological models do not take it into account, which creates a challenge in repellency-prone regions. This work focused on the adaptation for soil water repellency of a daily water balance model. Repellency is estimated from soil moisture content using site-specific empirical relations and used to limit maximum soil moisture. This model was developed and tested using approximately 2 years of data from one long-unburned and two recently burned eucalypt plantations in northern Portugal, all of which showed strong seasonal soil water repellency cycles. Results indicated important improvements for the burned plantations, with the Nash–Sutcliffe efficiency increasing from −0.55 and −0.49 to 0.55 and 0.65. For the unburned site, model performance was already good without the modification and efficiency only improved slightly from 0.71 to 0.74, mostly due to the better simulation of delayed soil wetting after dry periods. Results suggested that even a simple approach to simulate soil water repellency can markedly improve the performance of hydrological models in eucalypt forests, especially after fire. Copyright © 2015 John Wiley & Sons, Ltd.

Although fire-induced soil water repellency (SWR) and its effects on soil hydrology and geomorphology have been studied in detail, very few studies have considered the effect of rock fragments resting on the soil surface or partly embedded in soil. In this research, we have studied the effect of rock fragments on the strength and spatial distribution of fire-induced SWR at different fire severities. A fire-affected area was selected for this experiment and classified into different zones according to fire severity (unburned, low, moderate and high) and rock fragment cover (low, 60%). During 7 days after fire, SWR and infiltration rates were assessed in the soil surface covered by individual rock fragments and in the midpoint between two adjacent rock fragments (with maximum spacing of 20 cm). SWR increased with fire severity. Rock fragments resting on the soil surface increased the heterogeneity of the spatial distribution of fire-induced SWR. SWR increased significantly with rock fragment cover in bare areas under moderate and high fire severity, but quantitatively important changes were only observed under high fire severity. In areas with a low rock fragment cover, water repellency from soil surfaces covered by rock fragments increased relative to bare soil surfaces, with increasing SWR. In areas with a high rock fragment cover, SWR increased significantly from non-covered to covered soil surfaces only after low-severity burning. Rock fragment cover did not affect infiltration rates, although it decreased significantly in soil surfaces after high-severity burning in areas under low and high rock fragment cover. Copyright © 2013 John Wiley & Sons, Ltd.

Many peatlands have been subjected to wildfire or prescribed burning, but it is not known how these fires influence near-surface hydrological processes. Macropores are important flowpaths in the upper layers of blanket peat and were investigated through the use of tension disc infiltrometers, which also provide data on saturated hydraulic conductivity. Measurements were performed on unburnt peat (U), where prescribed burning had taken place 2years (B2), 4years (B4) and >15 (B15+) years prior to sampling, and where a wildfire (W) had taken place 4months prior to sampling. Where there had been recent burning (B2, B4 and W), saturated hydraulic conductivity was approximately three times lower than where there was no burning (U) or where burning was last conducted >15years ago (B15+). Similarly, the contribution of macropore flow to overall infiltration was significantly lower (between 12% and 25% less) in the recently burnt treatments compared to B15+ and U. There were no significant differences in saturated hydraulic conductivity or macropore flow between peat that had been subject to recent wildfire (W) and those that had undergone recent prescribed burning (B2 and B4). The results suggest that fire influences the near-surface hydrological functioning of peatlands but that recovery in terms of saturated hydraulic conductivity and macropore flow may be possible within two decades if there are no further fires.

The long-term measurement of soil redox potential (EH) by permanently installed Pt electrodes may be restricted by electrode breakdown (electrode rupture and resin leakage) and contamination, especially under wet and strongly reducing soil conditions. The EH of a slightly alkaline (pH 7.1 to 7.3) Calcaric Gleysol developed from marine sediment in the dyked marsh of Schleswig-Holstein, Northern Germany, was monitored weekly during a 4-year period using permanently installed Pt electrodes. Measurements were performed in fivefold at 10, 30, 60, 100, and 150 cm. Furthermore, water table level was recorded. Sulfide occurred in 150 cm as a heritage of the previous marine environment. Mean water table level was 84 cm below the soil surface but was characterized by both short-term and seasonally strong fluctuations. Levels of water table ranged from 33 to >200 cm below soil surface. In consistence with water table level, the EH continually decreased with soil depth. Mean redox conditions were oxidizing at 10 (550 mV) and 30 cm (430 mV), weakly reducing at 60 cm (230 mV), and moderately reducing at 100 (120 mV) and 150 cm depth (–80 mV). Soil hydrology differed markedly during the study as expressed by periods of water saturation for each depth. This was reflected by Pt electrodes response, since period of water saturation and EH were significantly negatively correlated as calculated for each year and depth (rs = –0.971; n = 20; P < 0.01). The 60-cm depth was most frequently influenced by water table fluctuations, showed the largest EH range (920 mV) and the most distinct seasonal pattern in EH. Good function of the electrodes in this depth can be concluded even after long time of installation in soil. Although established in a sulfide-bearing environment, three of five electrodes at 150 cm showed a substantial increase (+500 mV) in EH during summer of the third and fourth years of investigation, which had low water tables. It is not clear whether the non-response of two electrodes was due to electrode contamination or spatial variation in EH. When operating in reducing systems, this problem can be circumvented by installing a large number of electrodes or by a regular replacement of electrodes. Using properly constructed and permanently installed Pt electrodes, soil EH can be monitored for extended periods under wet and reducing soil conditions. In situ-Langzeitmessung des Redoxpotenzials in einem eingedeichten Marschboden Die Langzeitmessung des Redoxpotentials (EH) mittels dauerhaft im Boden installierter Pt-Elektroden kann durch Bruch der Elektroden, Auflosen des Harzes und Kontamination der Elektroden eingeschrankt werden, insbesondere unter nassen und stark reduzierenden Bodenbedingungen. In dieser Arbeit wurde das EH einer schwach alkalischen (pH 7,1 bis 7,3) Norm-Kalkmarsch aus marinen Sedimenten in der eingedeichten Marsch von Schleswig-Holstein, Norddeutschland, wochentlich mittels permanent im Boden installierter Pt-Elektroden gemessen. Die Messungen erfolgten mit funffacher Wiederholung in 10, 30, 60, 100 und 150 cm Bodentiefe. Weiterhin wurde der Grundwasserstand gemessen. In 150 cm trat Sulfid infolge vorheriger mariner Bedingungen auf. Der mittlere Grundwasserstand lag 84 cm unterhalb der Bodenoberflache. Er war durch kurzzeitige als auch saisonale starke Schwankungen gekennzeichnet und reichte von 33 bis >200 cm unter Bodenoberflache. In Ubereinstimmung mit dem Grundwasserstand nahmen die EH mit der Bodentiefe kontinuierlich ab. Die mittleren Redoxbedingungen waren oxidierend in 10 (550 mV) und 30 cm (430 mV), schwach reduzierend in 60 cm (230 mV) und masig reduzierend sowohl in 100 (120 mV) als auch in 150 cm (–80 mV). Die bodenhydrologischen Verhaltnisse unterschieden sich wahrend der Messkampagne deutlich, was in der Dauer der Wassersattigung fur jede Bodentiefe zum Ausdruck kam. Dies wurde durch die Messwerte der Elektroden widergespiegelt, denn Berechnungen fur jedes Jahr und jede Tiefe ergaben signifikante negative Korrelationen zwischen EH und Dauer der Wassersattigung (rs = –0,971; n = 20; P < 0,01). Die Tiefe 60 cm war am haufigsten durch Grundwasserschwankungen beeinflusst, zeigte die groste Spannweite (920 mV) und das ausgepragteste saisonale Muster im EH wahrend der gesamten Messkampagne. Daraus wird eine gute Funktion der Elektroden gefolgert, die auch nach langer Dauer der Installation im Boden anhalt. Obwohl in einer sulfidhaltigen Umgebung eingesetzt, zeigten drei von funf Elektroden in 150 cm einen wesentlichen EH-Anstieg (+500 mV) in den Sommern des dritten und vierten Untersuchungsjahres, die niedrige Grundwasserstande aufwiesen. Es ist nicht klar, ob die Nichtreaktion der beiden Elektroden auf Elektrodenkontamination oder raumlicher Variabilitat des EH beruht. Wenn in reduzierenden Systemen gearbeitet wird, kann dieses Problem umgangen werden, indem eine grosere Anzahl von Elektroden installiert wird oder indem die Elektroden regelmasig ersetzt werden. Falls Pt-Elektroden sorgfaltig angefertigt sind, wird eine Langzeitaufnahme des Redoxpotenzials von Boden mit dauerhaft installierten Elektroden empfohlen, auch unter standig nassen und reduzierenden Bodenbedingungen.

Fourteen popular, representative infiltration models, some physically based, some semi-empirical and some empirical, were selected for a comparative evaluation. Using the Nash and Sutcliffe efficiency criterion, the models were evaluated and compared for 243 sets of infiltration data collected from field and laboratory tests conducted in India and the USA on soils ranging from coarse sand to fine clay. Based on a relative grading scale, the semi-empirical Singh–Yu general model, Holtan model and Horton model were graded respectively as 6·52, 5·57 and 5·48 out of 10. The empirical Huggins and Monke model, modified Kostiakov and Kostiakov model were graded as 5·57, 5·30 and 5·22, respectively. The physically based non-linear and linear models of Smith–Parlange were graded as 5·48 and 5·22, respectively. Other models were ranked lower than these models. All the models generally performed poorly in field tests on Georgia's sandy soils, except the Robertsdale loamy sand. Copyright © 2003 John Wiley & Sons, Ltd.

The articles in this issue are a selection of the presentations made at the 2nd International Conference of Biohydrology. This special issue ‘Biohydrology—coupling biology and soil hydrology from pores to landscapes’ contains a range of articles on biological and hydrological interactions in soil, including large-scale systems research on the influence of forests on catchments and small-scale reductionist research on processes operating at the scale of soil pores. Copyright  2010 John Wiley & Sons, Ltd.

Pedohydrological properties were investigated on a piped slope in the Bergisches Land in the Rhenish Slate Mountains (Nordrhein-Westfalen, Germany). The study confirmed that genesis and development of pipes decisively depend on hydrological conditions in the soils. Vertical water permeability of saturated samples was very high. This promoted fast seepage. Many macropores produced by earthworms also caused high transport capacity for soil water. Even more efficient were the burrows of moles and mice, enabling immediate infiltration and direct vertical and lateral water movement. On the contrary, the horizontal saturated permeability was low indicating no correlation with piping. Porosity of piped soils was not different to that of other soils of the region without pipes. Hydrologische Parametrisierung von Tunnelerosion in lossreichen Boden des Bergischen Landes, Nordrhein-Westfalen Im Bergischen Land (Rheinisches Schiefergebirge, Nordrhein-Westfalen, Deutschland) wurden bodenhydrologische Eigenschaften auf einem von Tunnelerosion betroffenen Hang untersucht. Die Studie zeigte, dass fur Entstehung und Entwicklung der unterirdischen Rohren die bodenhydrologischen Bedingungen von entscheidender Bedeutung waren. In den Boden war die vertikale gesattigte Wasserleitfahigkeit sehr hoch, was schnelle Wasserversickerung ermoglichte. Auch die zahlreichen, von Regenwurmern geschaffenen Makroporen sorgten fur eine hohe Transportkapazitat fur Bodenwasser. Die Grabgange von Maulwurfen und Mausen waren sogar noch effektiver fur die sofortige Infiltration und eine direkte vertikale sowie horizontale Ableitung des Niederschlagswassers. Demgegenuber war die horizontale gesattigte Wasserleitfahigkeit gering und zeigte keinen Zusammenhang mit der Tunnelerosion. Die Porositat der unterirdisch erodierten Boden unterschied sich nicht von der anderer, nicht betroffener Boden in der Region.

Laboratory and field experiments demonstrated that solid state pressure transducers are accurate and reliable devices for frequent measurements of soil suction. However, each transducer had to be individually calibrated before use and a hanging column procedure designed for this purpose is described. Analysis showed that each transducer had a linear response and that environmental conditions such as temperature had minimal influence. Twenty four tensiometers with pressure transducers were intalled in a forest soil to test their operation and their output was monitored by a data logger. An example of soil suction results measured during four storms is given to demonstrate their stability and their rapid response. The transducers were found to perform accurately and were only affected by temperatures below 0°C.