Clifem – Climate Forcing and Erosion Modelling in the Sele River Basin (Southern Italy)
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Nat. Hazards Earth Syst. Sci., 9, 1693–1702, 2009 www.nat-hazards-earth-syst-sci.net/9/1693/2009/ Natural Hazards © Author(s) 2009. This work is distributed under and Earth the Creative Commons Attribution 3.0 License. System Sciences CliFEM – Climate Forcing and Erosion Modelling in the Sele River Basin (Southern Italy) N. Diodato1, M. Fagnano2, and I. Alberico3 1TEDASS – Technologies Interdepartmental Center for the Environmental Diagnostic and Sustainable Development, University of Sannio, Via Bartolomeo Camerario 35, Benevento, Italy 2DIAAT – Dipartimento Ingegneria Agraria e Agronomia del Territorio, University of Naples Federico II, Via Universita` 100, Portici, Italy 3CIRAM – Centro Interdipartimentale di Ricerca Ambiente, University of Naples Federico II, Via Mezzocannone 16, Napoli, Italy Received: 15 January 2009 – Revised: 23 July 2009 – Accepted: 25 Aug 2009 – Published: 14 October 2009 Abstract. This study presents a revised and scale-adapted several Mediterranean Europe river basins (Rickson, 2006), Foster-Meyer-Onstad model (Foster et al., 1977) for the where there aren’t satisfactory geo-data series because of transport of soil erosion sediments under scarce input data, mutability, discontinuity and sparsity of the available data with the acronym CliFEM (Climate Forcing and Erosion (Poesen and Hooke, 1997a). In these basins, stream and Modelling). This new idea was addressed to develop a tillage erosion probably are the dominant sediment sources monthly time scale invariant Net Erosion model (NER), (Poesen and Hooke, 1997b) and are considered an important with the aim to consider the different erosion processes limiting factor of farm soil fertility (Lal, 2001). Prolonged operating at different time scales in the Sele River Basin or accelerated erosion events cause irreversible soil losses, (South Italy), during 1973–2007 period. The sediment de- thus reducing soil ecological functions such as biomass pro- livery ratio approach was applied to obtain an indirect es- duction and filtering capacity (Gobin et al., 2004). Changes timate of the gross erosion too. The examined period was in the climate time patterns may have important effects on affected by a changeable weather regime, where extreme the interaction among erosive rainfalls, vegetation covers and events may have contributed to exacerbate soil losses, al- runoff, which can affect soil degradation. In Mediterranean though only the 19% of eroded sediment was delivered at areas, erosion is particularly pronounced both in the semi- outlet of the basin. The long-term average soil erosion was arid regions (200–300 mm year−1), and in the sub-humid very high (73 Mg ha−1 per year ± 58 Mg ha−1). The estimate ones (900–1500 mm year−1) and the strong variability in an- of monthly erosion showed catastrophic soil losses during the nual precipitation with frequent events of extreme rainfalls soil tillage season (August–October), with consequent land may result in time-transgressive susceptibilities of regional degradation of the hilly areas of the Sele River Basin. erosion (after Boardman and Favis-Mortlock, 2001). Many studies were made on erosion simulations and on ecosys- tem responses to future climate changes (e.g., Newson and Lewin, 1991; Easterling et al., 2000; Yang et al., 2003; Near- 1 Introduction ing et al., 2005; Michael et al., 2005; Phillips et al., 2006; Quilbe et al., 2007), but few researches allowed to quantify The full knowledge of climate drivers of soil erosion the effects of past climate variability on the dynamics of ge- needs extended meteorological, hydrological and land-cover omorphological processes (Rumsby, 2001; Diodato, 2006; records and the knowledge of the processes linking weather Foster et al., 2008), while they study episodic impacts of ex- and geomorphology at different time and spatial scales. The treme rainfalls (e.g., Gomez et al., 1997; Hooke and Mant, nature of such linkages remains still poorly understood for 2000; Coppus and Imeson, 2002; Mart´ınez-Casasnovas et al., 2002; Mul et al., 2008). Other researches are mainly Correspondence to: I. Alberico based on plot covers and soil types and they cannot be ([email protected]) scaled up to catchment scales (Dickinson and Collins, 2007). Published by Copernicus Publications on behalf of the European Geosciences Union. 1694 N. Diodato et al.: CliFEM in the Sele River Basin Although various models are reported in literature, it is dif- Data exploration, development and model performance ficult to relate the occurrence of historical soil erosion rates were supported by XLStatistics – Excel add-in Software for to climate variability because there aren’t long-term research Statistical Data Analysis© Rodney Carr (1997–2007). projects in river basins, especially as regards the sediment data. Some authors (Larson et al., 1997; Boardman, 2006) 2.1 Process-based monthly Net Erosion (NER) model stressed the need to study the driving variables of soil loss with an high frequency, for the importance of the extreme In this study we expanded and adapted a sub-routine intro- rain time-clusters on climate aggressiveness and thus on soil duced by Foster et al. (1977) to take into account the runoff losses. shear stress effect on soil detachment for single storms. The According to the aforementioned Authors, our effort was new Time Scale-Invariant process-based monthly Net ERo- to produce monthly sediment budget, for develop a process- sion (NERTSI) model was used for predicting monthly net 2 based time-scale invariant Net Erosion model (NER). After- erosion over medium basins (around 3000 km ). The hydro- wards, monthly, seasonal and annual gross erosion rates were logical ecosystem module, considers the interrelationships of carried out within a down-scaling approach by using Sed- rainfall erosivity, runoff, erodibility and vegetation cover as iment Delivery Ratio (SDR), on the contrary of up-scaling follows: procedure commonly used by RUSLE (Renard et al., 1997). ψ NER =K· α·EI m+β·Q · As remarked by Larson et al. (1997), the USLE-RUSLE ap- TSI m proach is aimed to simulate the average long-term soil ero- exp −γ · NDVIm·100 (2) sion, but it neglects land vulnerability during severe rain- where the first term in bracket is the modified erosivity fac- storms because it was not designed to accurately simulate tor as adapted by Foster et al. (1977), while the second term soil losses of single events. is the modified vegetation erosion exponential function, as CliFEM approach, derived from Foster et al. (1977), was adapted by Thornes (1990); K is the RUSLE erodibility fac- developed to include the major conceptual advantages of tor (Mg h MJ−1 mm−1) changing with basin soils, that was some erosion models. However, while NER model was set equal to 0.0362 (an approximate range of K values can be largely determined by the required experimental data, the founded in van der Knijff et al., 2000); α, β and ψ are em- SDR model was constrained by weakness of the available pirical parameters equal to 0.40, 0.60 and 2.05, respectively; data in Sele River Basin. Another important characteristic γ is the parameter of the vegetation exponential function, set of the NER model was to consider the different erosion pro- equal to 0.04 (early placed equal to 0.07 by Thornes, 1990). cesses at different time scales (from monthly to annual). The Note that the terms are averaged values upon an area (in our major weakness of the approach here proposed may be in case of the basin-area). the sediment data quality, and in the consequent uncertainty Monthly rainfall erosivity at gauged station EIm in Sediment Delivery Ratio, that is a critical point in many (MJ mm ha−1 h−1 month−1) was derived from rainfall models (L. Borselli, personal communication, 2007). measurements in the Italian area, according to RUSLE scheme (Diodato, 2005a): 2 Model description √ 0.53 1.18 EIm = 0.1174 · p · d · h (3) The process-based monthly time scale invariant Net Erosion where p is the monthly precipitation amount (mm), d is model (NER) was aimed to consider the erosion processes the monthly maximum daily rainfall (mm), and h is the at different time scales. The amount of sediments from a monthly maximum hourly rainfall (mm), respectively. In this basin (net erosion) is in fact generally much smaller than the approach, d and h are descriptors of the extreme rainfalls amount indicated by soil loss rate (gross erosion). The ratio (storms and heavy showers respectively) (Diodato, 2005a). between net and gross erosion is the sediment delivery ratio Since long series of meteorological variables do not include (SDR). Considering then that only a fraction of the eroded hourly rainfall intensities (i.e. the term h), the Eq. (3) was soils is transported to the basin outlet, as indicated by SDR, simplified as in the Eq. (4) (Diodato and Bellocchi, 2009a): we can use this link to evaluate the soil amount removed by water within the watershed slope. A well known relationship 0.35 0.60 0.70 EIm = 0.0400 · p · d · (d · αm) (4) to convert net erosion to gross soil loss is the following: NER where αm is a seasonal scale-factor in function of the month, GER= (1) m=1, . , 12: SDR 3 where GER is the monthly gross soil loss, NER is the net m αm = 1 − 0.30 · cos 2π (5) erosion, and SDR is the sediment delivery ratio. 26 − m Since with Eq. (3) it was not possible to have continuity in erosivity modelling between the first and the last time of the Nat. Hazards Earth Syst. Sci., 9, 1693–1702, 2009 www.nat-hazards-earth-syst-sci.net/9/1693/2009/ N. Diodato et al.: CliFEM in the Sele River Basin 1695 series, the whole erosivity dataset was subjected to a rigorous set constant for the whole simulation period, since MODIS control of internal consistency (e.g., inspection and cross- NDVI data were not available before 2000 year.