In order to gain a better understanding of hydrology, it is essential to study the complex interactions and linkages between watershed components, such as drainage network, riparian corridors, headwaters, hillslopes and aquifers and related processes operating at multiple scales (National Research Council, 1999). Hydrological science plays an important and fundamental role only when it provides an integrated knowledge and understanding of the forms and processes that operate in watersheds at multiple space–time scales in the landscape (Marcus et al., 2004). A useful way of understanding the response of catchments to rainfall events is to analyze stream discharge vs. rainfall per unit of time, plotted as a storm flow hydrograph and hyetograph, respectively. In recent decades, hydrologists have carried out numerous studies on catchment and hillslope hydrology in order to define when, how and where runoff is produced and how it progressively increases along the drainage network. Hydrologists generally agree that following rainfall, new-event water components are added to the old, pre-event water components through various hydrological mechanisms that are generally referred to as baseflow components that are derived from deep and shallow aquifers, thus expanding and reducing the runoff-contributing areas (Betson, 1964). The most common general concept that explains the above-mentioned hysteretic behavior is the variable source area (VSA) concept. This concept was originally proposed by Hewlett (1961) and later adopted by other authors (Dunne and Black, 1970; Dunne and Leopold, 1978; Huang and Laften, 1996; Vander Kwaak and Loague, 2001; Zollweg et al., 1995, Pionke et al., 1996). Despite its early formulation, it has provided the hydrological background for more recent research studies (Lyon et al., 2004, Easton et al., 2007, 2008; Buchanan et al., 2012; Moore et al., 1988; Barling et al., 1994; Kwaad, 1991; Easton et al., 2010; White et al., 2011). Contemporarily, the “hydro-geomorphic paradigm” was proposed by Sidle et al. (2000) in order to discriminate the VSA hydrologic sources and pathways, which refers to the connected hydro-geomorphic components of the catchments (hollow, hillslope and riparian corridor). Within a more general program for flood hazard assessment procedures, the hydro-geomorphic paradigm was used to generalize at basin and regional scale in southern Italy by Cuomo (2012), by means of hydro-geomorphology (Okunishi, 1991, 1994; Babar, 2005; Sidle and Onda, 2004; Goerl et al., 2012). Cuomo (2012) introduced and applied a new hydro-geomorphological basic unit, the hydro-geomorphotype, by using the Salerno Geomorphological Mapping System (Dramis et al., 2011; Guida et al., 2012, 2015) as a framework for object-based geomorphological mapping. Based on the up-to-date and shared theoretical geomorphometric background (Baatz and Schäpe, 2000; Dragut and Blaschke, 2006; van Asselen and Seijmonsbergen, 2006; Anders et al., 2011; Dragut et al., 2013, 2014; Eisank et al., 2014), this proposal is currently under experimental calibration as an effective, object-based geomorphometric procedure for spatial individuation, objective delimitation and automatic recognition of the hydro-geomorphotypes from the perspective of an object-based distributed hydrological modeling (Cuomo et al., 2012).

Linking geomorphometry with hydrology towards hydro-geomorphology gives consistency to the suggestion made by Peckham (2009) with the aim of simplifying the issue of the computational cost and time of a fully distributed model.

In the past, many authors made extensive use of chemical and isotopic tracers in order to separate the runoff components recorded in the hydrographs and pinpoint distinctive sources and pathways by using the geochemical and isotopic signature of water at parcel scale or for small catchments (Klaus and McDonnell, 2013). However, applying only the hydro-chemograph and isotopic separation methods to an experimental parcel cannot provide sufficient information on the spatial distribution of runoff sources and paths for basins as a whole, due to their spatial heterogeneity structure and time process variability.

Moreover, extensive use of the above-mentioned methods is more expensive and time-consuming than the quantity and quality of the data collected and the knowledge gained. As stated by Ladouche et al. (2001), with these methods alone it is possible to identify the type, timing and volume of the runoff components, but it is impossible to define the spatial origin and related pathways during storm events accurately. In order to overcome these difficulties and by following the general approach used by Latron and Gallart (2007), we used an integrated, hydro-geomorphological approach for studying a Mediterranean research catchment in southern Italy. This approach is based on detailed geomorphological surveys, mapping and 3-year hydro-chemical monitoring. It integrates a new procedure for identifying and separating hydro-chemical runoff components and a geomorphometric application for the objective delimitation of the source areas, where each runoff component is generated (Cuomo and Guida, 2013; Guida and Cuomo, 2014). Starting from these premises, the paper describes the study area as a Mediterranean research catchment and presents the hydro-chemical data set recorded during the monitoring activity carried out in the 2013–2014 calibration period. In the next section an original procedure is described for determining the timing, type and hydro-chemical signature of the runoff components involved during storm events. With the aim of spatially defining these runoff sources, an object-based hydro-geomorphological map was then set by hydrologically oriented segmentation and classification. Finally, the results of combined hydro-chemical and object-based hydro-geomorphometric analysis are discussed in order to determine the variability of the contributing area during a significant storm event (see the storm event hydro-chemical data set in the Supplement data).

The study area is a forested and hilly catchment located in the Bussento River drainage basin, the 3 km2 Ciciriello catchment in the Cilento and Vallo di Diano National Park–UNESCO Global Geopark, southern Italy (Fig. 1).

At the base the terrigenous bedrock is composed of a lower Tertiary, marly-clayey formation passing in unconformity upward to middle Miocene, westward-dipping sandstone strata and pelitic intervals. A lenticular 10 m thick marly layer (“Fogliarina Marl”, as a geosite in the Geopark) outcrops along the right-hand side of the valley. Regosols, regolite and gravelly slope deposits up to 5 m thick cover the above-mentioned bedrock. The mainstream bed, rectilinear and dipping strata subsequent to main faults is incised in alluvial gravelly and smooth deposits and partly in bedrock; the secondary streambed is exclusively in bedrock, subsequent to minor fault systems. From a hydro-geomorphological perspective, the groundwater circulation is controlled by the litho-structural arrangement of the above-mentioned bedrock formations, where the marly-clayey formation constitutes the local aquitard below the sandstone aquifer. The westward dipping of the permeability boundary causes a general westward groundwater flow, convergent toward the lower apex of the wedge-like hydro-structures (“hydro-wedge” in Cascini at al., 2008 and Cuomo and Guida, 2016), where the main permanent springs are located. In the headwaters, colluvial hollows are situated at the bottom of the zero-order basins, and are considered to be the main headwater hydro-geomorphotypes by Cuomo (2012), where dominant saturation excess runoff occurs mainly during the wet season. The streamflow of both permanent springs from the bedrock aquifers and seasonal springs from colluvial headwater increase down valley.

From December 2012, water depth (D), discharge (Q) and specific electrical conductivity (we used either sEC or EC in the following) were measured daily at the main station, hourly during the floods and weekly at the sub-stations during the inter-storm periods (Fig. 1). The Q measurements were obtained with the Swoffer 3000 current meter (Swoffer Inc., USA), and the EC parameter was measured with multi-parametric probe HI9828 (Hanna Instruments Inc., Romania). The monitoring year 2013–2014 (Fig. 2) provided a complete hydro-chemical data set, which enabled us to carry out the analysis at seasonal and event timescales (Cuomo and Guida, 2014).

The contributing area is a dynamic hydrological concept because it may vary seasonally. The extension of the contributing area is strongly influenced by various static factors such as topography and soils, and dynamic factors such as antecedent moisture conditions, rainfall characteristics (Dunne and Black, 1970) and vegetation cover.

In the following sections, an integrated procedure is proposed that uses simple geomorphometric tools to take into account various hydrological and geomorphological factors that cause time–space runoff variability in the catchment case study.

The flowchart in Fig. 3 shows the three integrated approaches used in the application.

The first approach on the left-hand side highlights the expert-based activities by geomorphological surveys and direct monitoring carried out at basin scale before and during the application event and the derivation of traditional, hand-drawn, expert-based geomorphological maps. The field-oriented flow accumulation scenarios were obtained from data collected at the control points (Fig. 1) for each event time step (five time steps) and each hydro-geomorphotype and by using the flow accumulation map derived from the second step described below. The expert-based activities are illustrated in Sect. 3.1. The second approach (see the flowchart in the center) shows the geomorphometric routine activities carried out during the application, as illustrated in Sect. 3.2. Starting from the topographic data source, a hydrologically corrected DEM was obtained and the log of the flow accumulation map was derived, which was reclassified in the first approach in order to obtain the best agreement with the field evidence highlighted during the storm event at each hydro-geomorphotype. The field-oriented flow accumulation maps were obtained as a proxy for the contributing area scenarios. As better explained in Sect. 3.2, after five elaboration steps, the geomorphometric analysis provided us with the object-based hydro-geomorphological map of the catchment, quantitatively defining the spatial extension of the basic hydro-geomorphotypes. The hydro-geomorphotype map was calibrated with the hydro-chemical analysis illustrated in Sect. 3.3 and was then overlaid with the five contributing area scenarios, thus obtaining the final hydro-geomorphological scenarios maps.

For the storm study, the variability of the contributing area was obtained by combining the hydro-chemical procedure and the object-based hydro-geomorphotype map. As a result of this analysis, contributing area space–time variability was obtained for the selected storm event by combining hydro-chemical procedure outcomes, the hydro-geomorphotype map and the contributing area scenarios.

On the right-hand side of Figs. 9–13, hydro-chemograph evolution at the five time steps discussed in Fig. 6a is illustrated, while on the left-hand side of Figs. 9–13, we can see the progressive expanding contributing areas shown on the hydro-geomorphotype map. Specific observations are provided in the figure captions and the corresponding values for the increasing contributing area are listed in Table 4.

Figure 9 shows pre-event conditions, when only the baseflow and the decreasing groundwater ridging from previous events were activated.

By plotting the S vs. Q data from Table 3 on a normal plot, we can follow the pattern of the progressive involvement of the runoff components as specific contributing areas in streamflow (Fig. 14).

In our case, a positive exponential function was obtained for each hydro-geomorphotype curve as shown in Fig. 14. This approach is similar to the calculations proposed by Latron and Gallart (2007), but in this case the contributing area is calculated according to the baseflow component as well as the other components related to hydro-geomorphotypes. All the curves have a general exponential pattern (Eq. 1): S(t)=S0eaQ(t), where S(t) is the total contribution area at instant t, S0 the initial contribution area, ea is a constant for a specific component considered, and Q(t) is the discharge at the time of S(t).

Equation (1) can be re-written as logS(t)=aQ(t)+logS0. The riparian contribution trend is higher than the hollow and hillslope trends for a discharge from 50 to 1000 L s-1, but the specific hollow and hillslope contributing areas progressively reach the same values as the riparian corridor in the event of high discharge. In fact, a slight increase in the discharge from the riparian corridor was observed during the event (a= 0.0012). On comparing the behavior of the hollow and the hillslope, it seems that the hollow has a higher contributing area for lower discharge (from 50 to 600 L s-1) than the hillslope contributing area (Fig. 14). However, after the discharge increased, the two hydro-geomorphotypes reached the same percentages as the contributing areas (A2 in Table 4). A lower contribution originated from the nose, whose contributing area is not influenced by the discharge until it reaches 1000 L s-1, after which it increases rapidly (a= 0.0041).

Since 1970 authors have studied the relationships between the contributing area and the baseflow discharge (Fig. 15a). In fact, Ambroise (1986), Myrabo (1986) and Latron (1990) found good relationships for some catchments in which the increasing rate of the relative saturated area decreases with the increase in a specific discharge.

Dunne et al. (1975) observed that an increase in the saturated area leads to an increase in the discharge. More recently the same relationship was observed by Martinez-Fernandez (2005). Latron and Gallarat (2007) found a linear relationship between the specific discharge and the extent of the contributing area. The authors believe that, unlike the other catchments, the linear trend could be reasonable since the saturation of the catchment under study is not conditioned by its topography.

For the Ciciriello catchment we examined the relationships between the percentage of the contributing area (A1 in Table 4) and the specific discharge for each hydro-geomorphotype considered (Fig. 15b), and we believe that this trend is similar to that observed by Dunne et al. (1975).

When a low discharge occurs, the riparian corridor slowly contributes to the increasing discharge, and only for q= 100 L s-1 km-2 does this hydro-geomorphotype widen its contributing areas. Fig. 15 shows the increase in faster contributing areas for hollow, hillslope and nose at specific discharges q= 300, 200 and 100 L s-1 km-2, respectively. In this case these q values are considered as the q threshold values for activating runoff mechanisms.

There is an evident anomaly regarding the riparian corridor, as it shows a percentage of contributing area over 100 %. In our opinion, this result is due to a DEM resolution and the riparian corridor must be carefully defined due to the possible overlap with other hydro-geomorphotypes, especially the hollows. In Fig. 15 it is important to note the intersection between all the curves at high q values. In our opinion, it shows the interaction between all the runoff mechanisms occurring in the catchment during high-magnitude events before reaching the stream, as assumed by Cuomo and Guida (2016).

One of the most interesting results of this study is the experimental confirmation of the pre-event water contributions to streamflow by the rapid mobilization of the capillary fringe inducing groundwater-ridging mechanisms. This mechanism is still poorly understood despite the number of processes proposed and widespread acceptance (Cloke et al., 2006). Therefore, this case study can be considered the preliminary identification, recognition and quantification of the mechanisms at catchment scale.

According to the premises, the case study confirms the close link between geomorphometry and hydrology, since geomorphometry describes land surface quantitatively and land surface is the spatial expression of the geomorphic processes acting in time and resulting in landforms that are generated by hydrological mechanisms mainly in temperate and Mediterranean eco-regions. This demonstrates how geomorphometry can support hydrological analysis, by improving an interdisciplinary approach for future research developments in connecting hydrology and geomorphology in data acquisition, mapping, analysis, modeling and general-purpose applications. This is the purpose of object-based hydro-geomorphology, based on the methods for recognizing and classifying distinctive hydro-objects within catchments, involving ontology and semantics of landforms and processes in significant catchment areas with distinctive hydrological behavior and response in order to allow for their objective description, holistic analysis and inter-catchment comparison.

From this perspective, firstly by means of a recursive training-target approach (Guida et al., 2015), good agreement was observed between expert-based geomorphological mapping and an object-based geomorphometric map.

Therefore, by combining hydro-chemical analysis and an object-based hydro-geomorphotype map, the variability of the contributing area during a significant storm event was spatially modeled using the log values of the flow accumulation. In spite of its simplicity, a good agreement was observed between the spatial distribution of these parameters with the observed contributing areas detected during the event by carrying out direct surveys and taking surface and groundwater discharge measurements. The runoff components were determined for the storm event under study and specific runoff discharge from each contributing hydro-geomorphotype was calculated for each time step on the hydro-chemograph.

This study is the experimental confirmation of the role and entity of pre-event water contributions to streamflow by the rapid mobilization of the capillary fringe inducing the groundwater-ridging mechanism in steep sloping terrains. This mechanism is still poorly understood despite the number of processes proposed and widespread acceptance (Cloke et al., 2006); therefore, this case study can be considered as being a preliminary identification, recognition and quantification of this particular mechanism at catchment scale. According to Marcus et al. (2004), this study emphasizes the fact that field-based process studies must “continue to form the underpinning of hydrologic application in GIS's” and “GIScience should not come at the expense of sacrificing field-based studies of hydrologic processes and responses”.

This is an approach that can fill the gap between simple lumped hydrological models and sophisticated hydrological distributed models based on numerous quantitative parameters and expensive data collection. This kind of interdisciplinary and integrated approach can be applied to similar, rainfall-dominated, forested no-karst catchments in the Mediterranean eco-region by using an inexpensive, parsimonious and effective methodology for water resource assessment and management as suggested by the Biosphere2 program. In fact, in UNESCO International Designation Areas (such as the Cilento Global Geopark), the Global Geopark Network mission must guarantee hydro-geodiversity in compliance with the regulations laid down by the World Heritage Cultural Landscape Management, and natural and managed ecosystems (A1) must be safeguarded as established by the MAN AND BIOSPHERE program.

From this perspective, geomorphometry plays a fundamental role in quantifying and objectively mapping hydro-geomorphological entities with hydrological relevance that require monitoring and modeling in production, transferring and routing the flows between the various units in the catchments, as the base knowledge of progressive ecological planning for the sustainable use of water resources and best practices in land use improvements.

The underlying research data can be publicly accessed and are available from the Supplement.