Many shallow landslides and debris flows are precipitation initiated. Therefore, regional landslide hazard assessment is often based on empirically derived precipitation intensity-duration (ID) thresholds and landslide inventories. Generally, two features of precipitation events are plotted and labeled with (shallow) landslide occurrence or nonoccurrence. Hereafter, a separation line or zone is drawn, mostly in logarithmic space. The practical background of ID is that often only meteorological information is available when analyzing (non-)occurrence of shallow landslides and, at the same time, it could be that precipitation information is a good proxy for both meteorological trigger and hydrological cause. Although applied in many case studies, this approach suffers from many false positives as well as limited physical process understanding. Some first steps towards a more hydrologically based approach have been proposed in the past, but these efforts received limited follow-up. Therefore, the objective of our paper is to (a) critically analyze the concept of precipitation ID thresholds for shallow landslides and debris flows from a hydro-meteorological point of view and (b) propose a trigger-cause conceptual framework for lumped regional hydro-meteorological hazard assessment based on published examples and associated discussion. We discuss the ID thresholds in relation to return periods of precipitation, soil physics, and slope and catchment water balance. With this paper, we aim to contribute to the development of a stronger conceptual model for regional landslide hazard assessment based on physical process understanding and empirical data.@en

Rainfall-triggered landslides are among the most widespread hazards in the world. The hydrology in and around a landslide area is key to pore pressure build-up in the soil skeleton which reduces shear strength due to the buoyancy force exerted by water in a saturated soil and to soil suction in an unsaturated soil. Extraordinary precipitation events trigger most of the landslides, but, at the same time, the vast majority of slopes do not fail. The intriguing question is: ‘When and where exactly can a slope become triggered to slide and flow downwards?’ The objective of this article is to present and discuss landslide hydrology at three scales—pore, hillslope, and catchment— which, taken together, give an overview of this interdisciplinary science. In fact, for rainfall-triggered landslides to occur, an unfavourable hydrological interplay should exist between fast and/or prolonged infiltration, and a relatively ‘slow’ drainage. The competition of water storage, pressure build-up and the subsequently induced drainage contains the importance of the timing, which is indisputably one of the more delicate but relevant aspects of landslide modelling, the overlay of hydrological processes with different time scales. As slopes generally remain stable, we can argue that effective drainage mechanisms spontaneously develop, as the best for a slope to stay stable is getting rid of the overload of water (above field capacity), either vertically or laterally. So, landslide hydrology could be framed as ‘Filling-Storing-Draining’. Obviously, ‘Storing’ is added to stress the importance of dynamic pressure build-up for slope stability. ‘Draining’ includes all removal of water from the system (vertical and lateral flow, evaporation and transpiration) and thus pore water pressure release. Furthermore, by addressing landslide hydrology from both earth sciences and soil mechanics perspectives, we aim to manifest the hydrological processes in hillslopes and their influence on behaviour and triggering of landslides and vice versa. The challenge of landslide hydrological research is matching, at hillslope scale, causal hydrological processes, often conceptually described, with detailed physical models of triggering mechanisms. Interdisciplinarity is key in advancing our knowledge on water flows in (un)stable slopes. @en

Rainfall-triggered landslides are among the most widespread hazards in the world. The hydrology in and around a landslide area is key to pore pressure build-up in the soil skeleton which reduces shear strength due to the buoyancy force exerted by water in a saturated soil and to soil suction in an unsaturated soil. Extraordinary precipitation events trigger most of the landslides, but, at the same time, the vast majority of slopes do not fail. The intriguing question is: ‘When and where exactly can a slope become triggered to slide and flow downwards?’ The objective of this article is to present and discuss landslide hydrology at three scales—pore, hillslope, and catchment— which, taken together, give an overview of this interdisciplinary science. In fact, for rainfall-triggered landslides to occur, an unfavourable hydrological interplay should exist between fast and/or prolonged infiltration, and a relatively ‘slow’ drainage. The competition of water storage, pressure build-up and the subsequently induced drainage contains the importance of the timing, which is indisputably one of the more delicate but relevant aspects of landslide modelling, the overlay of hydrological processes with different time scales. As slopes generally remain stable, we can argue that effective drainage mechanisms spontaneously develop, as the best for a slope to stay stable is getting rid of the overload of water (above field capacity), either vertically or laterally. So, landslide hydrology could be framed as ‘Filling-Storing-Draining’. Obviously, ‘Storing’ is added to stress the importance of dynamic pressure build-up for slope stability. ‘Draining’ includes all removal of water from the system (vertical and lateral flow, evaporation and transpiration) and thus pore water pressure release. Furthermore, by addressing landslide hydrology from both earth sciences and soil mechanics perspectives, we aim to manifest the hydrological processes in hillslopes and their influence on behaviour and triggering of landslides and vice versa. The challenge of landslide hydrological research is matching, at hillslope scale, causal hydrological processes, often conceptually described, with detailed physical models of triggering mechanisms. Interdisciplinarity is key in advancing our knowledge on water flows in (un)stable slopes. @en

Wildfires are a growing concern in the Mediterranean area. Prescribed burning (PB) is often used to reduce fire risk, through fine fuel reduction. However, the monitoring of PB effects on ecosystem processes is mandatory before its spread. This study aims to assess hydrological effects of PB on the topsoil by controlled laboratory experiments. The evaporation flux successive to interception of a simulated rain in the litter and the fermentation layers was determined using both a water balance approach and an experimental 2H and 18O isotopes mass balance approach. PB was performed in spring 2014 in three Southern Italy pine plantations, dominated, respectively, by Pinus pinea L. (in Castel Volturno Nature State Reserve), P. halepensis Mill. (in Cilento, Vallo di Diano e Alburni National Park) and P. pinaster Ait. (in Tirone Alto-Vesuvio Nature State Reserve). In each study site, two cores, both including litter and fermentation layers, were sampled, 18 months after PB, in burned and in near unburned (control) areas, respectively, by means of customized collectors allowing to extract “undisturbed” cores. Afterwards, each core was moved into a lysimeter set-up in the laboratory, under controlled conditions (temperature of 22 °C, relative humidity of 50%), to carry out duplicate infiltration and evaporation experiments. To simulate rainfall, 1 L of tap water (=32 mm of rain) was sprinkled uniformly on the litter layer in the lysimeter and intercepted water from the litter and fermentation layer was collected for isotope analysis at two different depths for each layer, two times per day until 2 days after the rain simulation. The results of the water balance and isotope mass balance showed a slightly lower evaporation of intercepted water from the forest floor in burned areas, compared to unburned ones, but in most cases not statistically significant. The isotopic profiles of 2H and 18O also confirmed independently this finding, since they showed more enrichment in the unburned areas compared to the areas treated with PB. This could be due to thinner litter layers in burned areas of the three plantations, at least up to 18 months after treatment.@en

Wildfires are one of the major environmental issue in the Mediterranean area. Prescribed burning (PB) is increasingly used in Europe as a practice to reduce fire risk, through dead fine fuel reduction. Several studies have focused on fire effects on vegetation and soil microbial community, but very few on ecosystem processes involved in water cycle. This study aims to estimate interception by the litter and fermentation layer and the successive evaporation flux in laboratory conditions, using a water balance and 2H and 18O isotopes mass balance calculation, in order to assess PB effects on the hydrology and ecosystem in pine plantations. PB was carried out in spring 2014 in three pine plantations of Southern Italy, dominated by Pinus halepensis (Cilento, Vallo di Diano e Alburni National Park, CVDANP), P. pinaster (Vesuvio National Park, VNP) and P. pinea (Castel Volturno Nature Reserve, CVNR). A dataset concerning the effects of PB on vegetation structure, floristic composition, microbial biomass and activity in the fermentation layer and 5-cm of soil beneath is available for the same stands. In each plantation, two cores of litter and fermentation layer were sampled in a burned area and in a near unburned area (control), respectively, with a collector to extract an “undisturbed” core. Then, each core was transferred in a lysimeter installed in the Water Lab of Delft University of Technology. In total, three lysimeters were set up and each experiment was carried out in duplicate. The laboratory had constant temperature, and both temperature and relative humidity were recorded every 15 minutes. To simulate rainfall, 1 litre of tap water was sprinkled uniformly on the lysimeter with a plant spray (equivalent to 32 mm of rain). The precipitation was sprinkled every 3 days for a period of two months. Soil moisture and temperature were measured during the experiment every 15 minutes in the top and bottom of the litter and fermentation layer. Interception water was collected for isotope analysis from every layer with Rhizon soil moisture samplers by applying a vacuum with 5 ml syringes. Samples were collected two times per day (in the morning and in the evening) and at two different depths for each layer ( 4 cm and 7 cm in litter layer and 10 and 15 cm in fermentation layer) until 2 days after rain simulation. Water samples were analysed with laser spectrometry using the liquid water isotope analyser (LGR-LWIA). The influence of different litter layers and PB on interception and litter layer evaporation was assessed. Then, the evaporation flux measured using the lysimeter was compared with the calculated evaporation flux using the isotopes mass balance. Generally, the preliminary results indicate a slight increase in evaporation flux in burned areas compared to the controls, in P. pinea and P. pinaster stands. By contrast, in P. halepensis stand, a significant decrease in evaporation flux was detected in prescribed burned plot. The isotope mass balance method to measure litter evaporation is promising and could be used in future, in-situ, measurements of evaporation from the litter layer. @en

Wildfires are one of the major environmental issue in the Mediterranean area. Prescribed burning (PB) is increasingly used in Europe as a practice to reduce fire risk, through dead fine fuel reduction. Several studies have focused on fire effects on vegetation and soil microbial community, but very few on ecosystem processes involved in water cycle. This study aims to estimate interception by the litter and fermentation layer and the successive evaporation flux in laboratory conditions, using a water balance and 2H and 18O isotopes mass balance calculation, in order to assess PB effects on the hydrology and ecosystem in pine plantations. PB was carried out in spring 2014 in three pine plantations of Southern Italy, dominated by Pinus halepensis (Cilento, Vallo di Diano e Alburni National Park, CVDANP), P. pinaster (Vesuvio National Park, VNP) and P. pinea (Castel Volturno Nature Reserve, CVNR). A dataset concerning the effects of PB on vegetation structure, floristic composition, microbial biomass and activity in the fermentation layer and 5-cm of soil beneath is available for the same stands. In each plantation, two cores of litter and fermentation layer were sampled in a burned area and in a near unburned area (control), respectively, with a collector to extract an “undisturbed” core. Then, each core was transferred in a lysimeter installed in the Water Lab of Delft University of Technology. In total, three lysimeters were set up and each experiment was carried out in duplicate. The laboratory had constant temperature, and both temperature and relative humidity were recorded every 15 minutes. To simulate rainfall, 1 litre of tap water was sprinkled uniformly on the lysimeter with a plant spray (equivalent to 32 mm of rain). The precipitation was sprinkled every 3 days for a period of two months. Soil moisture and temperature were measured during the experiment every 15 minutes in the top and bottom of the litter and fermentation layer. Interception water was collected for isotope analysis from every layer with Rhizon soil moisture samplers by applying a vacuum with 5 ml syringes. Samples were collected two times per day (in the morning and in the evening) and at two different depths for each layer ( 4 cm and 7 cm in litter layer and 10 and 15 cm in fermentation layer) until 2 days after rain simulation. Water samples were analysed with laser spectrometry using the liquid water isotope analyser (LGR-LWIA). The influence of different litter layers and PB on interception and litter layer evaporation was assessed. Then, the evaporation flux measured using the lysimeter was compared with the calculated evaporation flux using the isotopes mass balance. Generally, the preliminary results indicate a slight increase in evaporation flux in burned areas compared to the controls, in P. pinea and P. pinaster stands. By contrast, in P. halepensis stand, a significant decrease in evaporation flux was detected in prescribed burned plot. The isotope mass balance method to measure litter evaporation is promising and could be used in future, in-situ, measurements of evaporation from the litter layer. @en

Wildfires are one of the major environmental issue in the Mediterranean area. Prescribed burning (PB) is increasingly used in Europe as a practice to reduce fire risk, through dead fine fuel reduction. Several studies have focused on fire effects on vegetation and soil microbial community, but very few on ecosystem processes involved in water cycle. This study aims to estimate interception by the litter and fermentation layer and the successive evaporation flux in laboratory conditions, using a water balance and 2H and 18O isotopes mass balance calculation, in order to assess PB effects on the hydrology and ecosystem in pine plantations. PB was carried out in spring 2014 in three pine plantations of Southern Italy, dominated by Pinus halepensis (Cilento, Vallo di Diano e Alburni National Park, CVDANP), P. pinaster (Vesuvio National Park, VNP) and P. pinea (Castel Volturno Nature Reserve, CVNR). A dataset concerning the effects of PB on vegetation structure, floristic composition, microbial biomass and activity in the fermentation layer and 5-cm of soil beneath is available for the same stands. In each plantation, two cores of litter and fermentation layer were sampled in a burned area and in a near unburned area (control), respectively, with a collector to extract an “undisturbed” core. Then, each core was transferred in a lysimeter installed in the Water Lab of Delft University of Technology. In total, three lysimeters were set up and each experiment was carried out in duplicate. The laboratory had constant temperature, and both temperature and relative humidity were recorded every 15 minutes. To simulate rainfall, 1 litre of tap water was sprinkled uniformly on the lysimeter with a plant spray (equivalent to 32 mm of rain). The precipitation was sprinkled every 3 days for a period of two months. Soil moisture and temperature were measured during the experiment every 15 minutes in the top and bottom of the litter and fermentation layer. Interception water was collected for isotope analysis from every layer with Rhizon soil moisture samplers by applying a vacuum with 5 ml syringes. Samples were collected two times per day (in the morning and in the evening) and at two different depths for each layer ( 4 cm and 7 cm in litter layer and 10 and 15 cm in fermentation layer) until 2 days after rain simulation. Water samples were analysed with laser spectrometry using the liquid water isotope analyser (LGR-LWIA). The influence of different litter layers and PB on interception and litter layer evaporation was assessed. Then, the evaporation flux measured using the lysimeter was compared with the calculated evaporation flux using the isotopes mass balance. Generally, the preliminary results indicate a slight increase in evaporation flux in burned areas compared to the controls, in P. pinea and P. pinaster stands. By contrast, in P. halepensis stand, a significant decrease in evaporation flux was detected in prescribed burned plot. The isotope mass balance method to measure litter evaporation is promising and could be used in future, in-situ, measurements of evaporation from the litter layer. @en

Rainfall induced landslides are a hydrological phenomenon. It deals with all hydrological processes from rainfall to discharge focussing on the role of the variably saturated hillslope soil. However, where much of the hillslope and catchment hydrology traditionally focus on the lumped fluxes of the entire slope, the landslide community is more interested in the distributed storage of the water in the hillslope and concentrated seepage points. In recent years, water storage got increasing research attention as it became clear that state-variables needed to be taken into account to improve our hydrological understanding of the behaviour of hillslopes. Interestingly, slope deformation is the direct result of water storage at that specific point! Furthermore, the role of preferential flow paths and perched water bodies is very important in stable slopes, but even more in slowly deforming slopes or active landslides. However, the role of a dual permeability system is not unambiguous; it can increase not only infiltration, but also drainage. Not mentioning that (vertical) infiltration takes place at the soil surface whereas lateral drainage can be concentrated somewhere within the soil profile. Infiltration, drainage and temporarily storage of water within the different parts of the landslide are in delicate balance. Lastly, many landslides take place in fine textured soils, like clay-shales or in very steep environments like pyroclastic deposits. These lithologies are less abundant in hillslope and catchment observatories and research projects and as such can help expanding our hydrological knowledge. We will highlight and discuss recent insights in landslide hydrology and how we think this can add to our knowledge on hillslope hydrological behaviour. Interdisciplinarity is key in advancing our knowledge on water flows in (un)stable slopes. @en

Rainfall induced landslides are a hydrological phenomenon. It deals with all hydrological processes from rainfall to discharge focussing on the role of the variably saturated hillslope soil. However, where much of the hillslope and catchment hydrology traditionally focus on the lumped fluxes of the entire slope, the landslide community is more interested in the distributed storage of the water in the hillslope and concentrated seepage points. In recent years, water storage got increasing research attention as it became clear that state-variables needed to be taken into account to improve our hydrological understanding of the behaviour of hillslopes. Interestingly, slope deformation is the direct result of water storage at that specific point! Furthermore, the role of preferential flow paths and perched water bodies is very important in stable slopes, but even more in slowly deforming slopes or active landslides. However, the role of a dual permeability system is not unambiguous; it can increase not only infiltration, but also drainage. Not mentioning that (vertical) infiltration takes place at the soil surface whereas lateral drainage can be concentrated somewhere within the soil profile. Infiltration, drainage and temporarily storage of water within the different parts of the landslide are in delicate balance. Lastly, many landslides take place in fine textured soils, like clay-shales or in very steep environments like pyroclastic deposits. These lithologies are less abundant in hillslope and catchment observatories and research projects and as such can help expanding our hydrological knowledge. We will highlight and discuss recent insights in landslide hydrology and how we think this can add to our knowledge on hillslope hydrological behaviour. Interdisciplinarity is key in advancing our knowledge on water flows in (un)stable slopes. @en