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A framework for modelling emergent sediment loss in the Ombrone River Basin, central Italy [1]
['Nazzareno Diodato', 'Met European Research Observatory International Affiliates Program Of The University Corporation For Atmospheric Research', 'Benevento', 'Fredrik Charpentier Ljungqvist', 'Department Of History', 'Stockholm University', 'Stockholm', 'Bolin Centre For Climate Research', 'Swedish Collegium For Advanced Study', 'Linneanum']
Date: 2023-02
Water can represent a hazard causing soil erosion and it is essential to anticipate the potential environmental impacts of sustained rainwater energy to achieve sustainability. Here, we present the modelling of the erosive force of water for the production of soil sediment in a Mediterranean basin of central Italy (Ombrone River Basin, ORB). A point of departure is the historical recognition of the environmental factors causing sediments loss (SL) by water. A semi-empirical framework was then proposed for the upscaling of SL based on the Foster-Thornes approach (EUSEM: Environmental Upscaling Sediment Erosion Model) in order to give an insight into annual sediment losses (SL) over the period 1949–1977 (calibration) and over a longer time-frame (1942–2020: reconstruction). Two change-points were detected: 1967 and 1986. During this period, SL was affected by a sharp decrease from 625 Mg km -2 yr -1 , before the first change-point (when SL was only occasionally below the tolerable soil loss threshold of 150 Mg km -2 yr -1 ), to 233 Mg km -2 yr -1 , during the transition phase 1967–1985 (mostly above the warning treshold of 140 Mg km -2 yr -1 ). This decrease coincided with an enhancing of vegetation throughout the basin due to an ongoing afforestation process. After this period, a resurgence of climatic forcing led to a further, but more contained, increase in SL, from 1996 onwards. This case-study illustrates the application and results that can be obtained with the framework for the outcome of environmental change due to sediment losses in a Mediterranean fluvial basin. Limitations and perspectives of this approach are given as conclusion.
Funding: FCL was supported by the Swedish Research Council (Vetenskapsrådet, grant no. 2018-01272) and conducted the work with this article as a Pro Futura Scientia XIII Fellow funded by the Swedish Collegium for Advanced Study through Riksbankens Jubileumsfond. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
1. Introduction
Despite considerable advances in model-based work on the resposse of sediment erosion to climatic and other environmental changes, research is still confronted with the need to produce experimental evidence on the erosional dynamics associated with long-term responses [1]. The strength and temporal progression of erosion-sediment can have a major impact on landscape layout through long-term environmental pressures [2], as erosion of agricultural land in many regions is rapidly degrading soil at a faster rate than its natural renewal [3]. Studies show that soil erosion by running water depends mainly on rainfall intensity and duration, soil properties, vegetation, land use and antecedent soil wetting conditions [4, 5]. Although the effects of wind on soil erosion can be severe in arid and semi-arid areas [6], wind erosion is less than water erosion on a global scale [7]. Water represents perhaps the most challenging aspect compared to the other elements [8], due to its unpredictability [9], the power of rainfall and its aggressiveness related to runoff and flooding [10].
Hydrological extremes have widespread, often harmful, impacts on ecosystems resulting in soil losses and economic damage [11], especially in the Mediterranean region [12]. The Mediterranean region, in fact, experienced signifcant climatic variations during the Holocene period (last ~11,700 years). The Little Ice Age (LIA, ~1250–1850 CE) featured in the Mediterranean region prolonged wet intervals [13], which had important consequences for terrestrial ecosystems [14]. In fact, during 16th century the process of extension of ploughing on the hilly soils, which had already started in the 12th century, developed further, involving new areas further away from inhabited centers, and expanding more and more often also to uplands [15, 16]. Fig 1A gives us an image of the erosion of the shallow slopes and, at the same time, of the deforested areas upstream of the Ombrone River Basin (ORB), which is the focus of this study.
PPT PowerPoint slide
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TIFF original image Download: Fig 1. Historical evolution of the landscapes in the Ombrone River Basin. a) The degradation of the hilly and mountainous landscape of Gavinana (43° 56’ N, 10° 55’ E) during the 16th century in the painting Presa di Gavinana (1556–1562), a fresco from the school of Giorgio Vasari (1511–1574), available free of charge from the Fondazione Federico Zeri, Università di Bologna (
http://catalogo.fondazionezeri.unibo.it/scheda/opera/39379/Stradano%20Giovanni%2C%20Battaglia%20tra%20le%20truppe%20fiorentine%20e%20imperiali), b) wooded landscape with Mount Amiata reforestation area (photo of the Archivio Italia Nostra, from I paesaggi rurali storici della Toscana, from Regione Toscana, 2014b, p. 9 [89], freely available at
https://www.regione.toscana.it/-/piano-di-indirizzo-territoriale-con-valenza-di-piano-paesaggistico), c) stable agricultural and forest landscape of high naturalistic value in the upper valley if Albegna River, near Murci (42° 44’ N, 11° 24′ E), in the province of Grosseto (photo taken by A. Chiti-Batelli, archive NEMO, from Regione Toscana, 2014b, p. 29 [89], freely available at
https://www.regione.toscana.it/-/piano-di-indirizzo-territoriale-con-valenza-di-piano-paesaggistico).
https://doi.org/10.1371/journal.pwat.0000072.g001
The timing and extent of the transition from the mid-19th centrury and the onset of the current warmer and, in the study region, drier climatic conditions have led to a stabilisation of the landscape, to which human reforestation has contributed (Fig 1B and 1C). However, erosional soil degradation remains a subject of much debate due to the timing and erraticity of extreme hydrological events accompanying this phase of climate change. For instance, erosion studies have focused on long-term mean areal estimates [17–19], on short time-series [20, 21], or even on scattered years [22], leaving unexamined the historical evolution of erosional sediment transport in the landscapes of many Tuscan river basins.
Fig 2 illustrates the spatial pattern of soil erosion in 2012. Over the 21st century, the global mean amount of potential soil erosion has increased (+2.5% from 35.0 Pg yr−1 in 2001 to 35.9 Pg yr−1 2012), mainly driven by land-use changes, such as the decline of forests and the expansion of semi-natural vegetation and cropland [23]. In addition, hydrological systems in different river basins worldwide have come under increasing pressure on land resources due to urbanisation and poor land management practices [24], as well as changes in precipitation patterns [25].
Tuscany has become one of the most vulnerable regions in Italy [26], with most of its territory classified as being at high risk of erosion [27]. In fact, in recent decades, extreme weather-climatic events have highlighted the effects of ongoing climate change indicating a trend towards an increase in very intense precipitation events that may have important repercussions on the territory from a hydrogeological point of view [28]. For instance, the ORB has been affected by extreme events in the past decade, such as during the Grosseto provinceflood of November 12, 2012, which altered the soil properties that regulate plant and animal life, causing heavy economic losses [29]. Intense rainfall also affected the Mount Amiata area, including the Senese slope, with daily pluviometric values advancing between c. 100 and 400 mm in the 2010–2020 period from the north (near Siena) to the south (near the coast) of the Ombrone basin [30]. Between October 18 and 21, 2014, another major flood was recorded, leading to the closure of the Grosseto-Siena railway line [31]. However, already in the past, many ORB lands were affected by remarkable erosive processes, especially under the pressure of deforestation of the Apennines, as reconstructed by historians of the time (ARAAT, 1904, p. 268 [32], our translation):
The deforestation that is often carried out indiscriminately in the Apennines […], while very harmful in various respects, has instead increased the richness of the solid material […]. I remember, for example, the Ombrone which, after increased deforestation, saw the amount of sediment increased [by 23%].
Today, the use of satellite remote sensing for soil erosion mapping and modelling has received considerable attention [33], but these records tend to cover single events up to, at most, a few decades [34]. Thus, the short periods of these records mean that we cannot assess whether the spatial patterns associated with erosion initiation events persist over the time-scales that are notable for landscape change and evolution [35]. To operate over extended time periods, models engaged in soil erosion require, however, a variety of environmental inputs as well as comprehensive parameterisation. In this context, the application of dynamic and physical models is often illusory due to a lack of detailed data required for these types of models when going back in time. A challenge is to find out how climate variability affects soil erosion over interdecadal and longer time scales. In fact, sediment discharge data on such long time scales are scarcely available for most river basins [36] and this motivates the development of hydrological models to reconstruct the long-term variability of sediment data at the scale of basins with relatively low data availability. Semi-empirical models developed to assess sediment loss at the basin scale use environmental factors to characterise drainage basins in terms of sensitivity to rainfall erosivity and sediment transport [37]. They consider erosion processes to some extent with a limited amount of data, which makes them suitable for estimating in-basin effects on soil erosion [38]. In particular, parsimonious mathematical models can simulate the combined effects of hydroclimatic forcing agents, such as the estimation of sediment budgets using–in the absence of spatially and temporally distributed data–the drainage basin as a homogeneous landscape unit in which sediment fluxes and land surface changes can be calculated [39].
The classical soil erosion determination procedure of Wischmeier and Smith [40], the Universal Soil Loss Equation and its revised forms–(R)USLE [41]–or its WaTEM/SEDEM extension to continental scale [42] are applicable to the calculation of long-term mean annual erosion, but their use at basin level for estimating annual soil loss values from single storms has led to a reinterpretation of the original formulation [43]. This has allowed the identification of concepts for the development of parsimonious modelling solutions for sediment load assessment of river systems [44–47]. Given the issue of assessing erosive sediment by complex models in recognition of the detailed input for the historical period, we arranged a parsimonious erosion model adapted to the annual scale from the original algorithms of Foster et al. [48] and Thornes [49] because they provide an interpretation of empirically determined factors shaping active erosional landscapes in basin areas based on the parsimonious balance between driving and resisting forces in the sediment budget [50, 51]. The driving forces are essentially simplified to proxies or indicators or rainfall and runoff erosivity associated with splash and transport erosion, respectively. This simplification seems to be adequate for the generation of basin-wide annual outputs, while it may not be fully applicable for the generation of monthly outputs, as seasonal effects become prominent due to the concurrence of rainfall erosivity with different seasonal conditions of soil erodibility [50, 51]. Resisting forces are expressed as an exponential function of the fraction of vegetation cover in the basin, as erosion rates show a steep decay compared to that of a bare soil as plant cover increases [52]. This relatively simple approach is desirable to reconstruct interannual and interdecadal variability in the evolution of landscape responses over periods of several decades in historical time, for which only monthly and daily rainfall amounts can be used. Empirical models to be applied–even if based on simple physical concepts–are much more attractive from an interdisciplinary point of view as they are able to capture the attention of a wide range of environmental scientists, where engineers, physicists, agronomists, historians, geographers, ecologists and geomorphologists can interact more easily than physically-based models. This is also so due to the interaction between variables which, in empirical models, are more explicit, in contrast to physical models where often the interaction mechanisms are locked in complex algorithms that are difficult to understand for a group of people with different and not-specialised knowledge. In this way, simple models can also integrate the work of several research groups into a single working product that summarises understanding [53].
Our objective was to reconstruct the dynamics of sediment loss in the ORB from a parsimonious interpretation of the relationship between input data and a basin-responsive erosion variable. The adoption of a parsimonious model is motivated by its ability to capture important environmental changes (including temperature, land cover and erosive resistance properties) using readily available data. The ability to reproduce (at basin scale) the combined effects of hydro-climatological processes, including sediment transport, in the absence of distributed spatial and temporal data, relies on the possibility of considering a drainage basin as a homogeneous ecosystem unit, counting only on two stations with continuous and homogeneous data. The sediment loss time-series, with a total of 28 sediment samples from Sasso d’Ombrone station (derived from the former SIMN dataset [54]), offered a unique opportunity to explore erosion processes in the basin. Thus, for the first time, it was possible to apply this approach to reconstruct the sediment loss from the Ombrone River Basin (ORB), over the 1942–2020 period.
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