mircea sevastel , nicolae petrescu · annals. food science and technology 2011 available on-line at...

Annals. Food Science and Technology 2011

Available on-line at www.afst.valahia.ro

92

INDICATORS FOR GULLY EROSION ASSESMENT ON AGRICULTURAL TORRENTIAL WATERSHEDS

Mircea Sevastel 1, Nicolae Petrescu2

1 University of Agricultural Sciences and Veterinary Medicine-Bucharest, Marasti 59, Romania 2 Valahia University of Târgovişte, Faculty of Environmental Engineering and Biotechnology,

Bd., 18-24, 130082, Târgovişte, Romania E-mail: [email protected]

Abstract It is known that effective erosion triggered by light rain, get local differences depending on the specific impact of these weather factors that are causal and conditional factors, geological structure and soil, vegetation, relief, etc. Therefore these reasons, the analysis of complex phenomena erosion must be made at the scale of basin or sub-basin, depending on the specific conditions of the site studied. In the Sub-Carpatien hills of Romania, the conditions are conducive to intense soil erosion event, the inventary in each river basin scale, considerable areas of degraded by such phenomena. This paper presents some aspects concerning gully erosion process developed on small torrential and agricultural watersheds as well as its impact on environment in Romania, with a case study located in one of the most affected areas by water erosion and landslides, namely Sub-Carpathian Curvature. The case study was carried out in the period 1992-1999 on a number of 11 gullies located in the Slanic River Basin, which is a tributary of the Buzau River. After a short introduction on soil erosion process and its impact on environment, in general, gully erosion - in particular, in Romania, especially in the case study location, the paper presents some aspects regarding the main findings on some state and risk indicators of gully erosion developed in Romania on the small agricultural torrential watersheds. These indicators can be then easily used for soil erosion modeling. Keywords: gully erosion, torrential watersheds, impact on environment, state and risk indicators. Submitted: 18.05.2011 Reviewed: 15.06.2011 Accepted: 25.06.2011 1. SOME DATA CONCERNING GULLY

EROSION AND ITS IMPACT Soil erosion, in general, and gully erosion process, in special, affects an important part of the Romanian territory, both agricultural lands and forests. The most significant types of gullies in the country are the torrents and ephemeral as well as permanent gullies that are present in several locations. Continuously gully erosion development produces important damages to the agricultural lands, human settlements and socio-economic units. Generally speaking, water erosion annually generates about 36 million tons of alluvia, with annual losses of lands estimated at about 2300 ha (Motoc, 1984). The affected areas by water erosion, which include the agricultural lands, forests and the unproductive areas on slopes, are as follows: slight erosion - 46.3%; moderate and high erosion - 41.5% and severe - excessive erosion - 12.2%.

As it is known, the gullies development on the three main directions - in length, wide and depth have an immediate impact on the lands and on environment as entire both on short and long term. These effects could be quantified through specific state and risk indicators, contributing in this way to the general assessment of river basin vulnerability from gully erosion point of view. The consequences of sheet and gully erosion occur both on- and off-site. On-site effects are particularly important on agricultural land where redistribution of soil within a field, the loss of soil from a field, the breakdown of soil structure and the decline in organic matter and nutrients result in a reduction of cultivable soil depth and a decline in a soil fertility. Off-site problems result from sedimentation downstream, which reduces the capacity of rivers and retention ponds, enhances the risk of flooding and muddy floods and shortens the design life of reservoirs. Sediment is also a pollutant in its own right and, together with

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some chemicals carried downstream, can increase the level of nutrients in water bodies, contributing in this way to eutrophication. Compared with on-site impacts, off-site impacts are more easily measured and can be expressed in economic terms, (Valentin et al., 2005). Gullies length’s evolution is becoming more and more important. To better predict the rate of gully headcut movement, there have been developed in time several models, mainly deterministic ones. Most of the models have been in general presented as regressions (simple and multiple correlations), using the following independent variables, (Wathern, 1990): - the catchments area at the gully head-cut; - terraced area, out of the total catchments area at the gully’s head-cut; - annual rainfall measured at the gully head-cut catchments; - soil features (content of clay) at the gully head-cut; - gully’s length at a certain moment; - distance and relief energy between the gully’s head-cut and the highest altitude in the river basin; - valley’s slope upstream of gullies’ head-cut. It has been determined that the gully evolution’s rate in length varies very much from a region to another, as well as within the same area. According to the researches developed in the world so far, it has been resulted that the gully evolution rate covers a very wide range of values. For example, in Tajikistan, in conditions of irrigated crops, the value is of about 674.7 m/year while in natural conditions (soil mainly covered with natural vegetation) the measured value is of about 1.4 – 82.5 m/year in the same region. In Israel, the average rate of gully headcut movement is about 8.5 m/year, and, in India it reaches values between 0.6 – 1.0 m/year (Valentin et al., 2005). In Romania, the annual rate of gully headcut movement varies in some regions as follows: - Colinele Tutovei region, 0.5 – 2.0 m/year (Motoc et al., 1984);

- Buzau, in the Curvature Carpathians region, 1.4 – 5.4 m/year (Ichim and Mihaiu, 1988), and, respectively 1.75 – 6.70 m/year (Mircea, 1999); - Barlad Plateau region, 12.5 m/year (Mircea, 1999), etc. 2. LOCALIZATION OF THE RESEARCH AND SHORT CHARACTERIZATION OF THE STUDIED GULLIES The study was carried out in one of the most affected area by surface and gully erosion and landslides in Romania, Buzau County, being located in the South-Eastern part of the country. This is the main part of the Subcarpanthians Curvature region, where the soil losses reach up to 30-45 tons/ha/year, coming mainly from sheet and gully erosion. The agricultural lands located on the slopes cover about 35% from the total surface of the County. The gullies represent about 1000 km, which actually cover a surface of about 1000 hectares. In fact it can be said that in this region all the valleys are affected in different degrees by gully erosion, mainly located on the valley’s thalwegs. More specifically, the study was carried out on 11 gullies located in the most representative subcatchment of the region from gully erosion point of view, the Slanic River Basin.

Fig. 1. Sketch of the studied area, Sub-Carpathians Curvature region, Buzau County, Romania



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This subcathment is a tributary of the Buzau River and the gullies taken into study have been located in the medium and lower sectors of Slanic River catchment (Figure 1). From the climatic point of view the area taken into study is characterized by an average multi-annual amount of rainfalls during the vegetation period (April – September) of about 350-400 mm and the maximum 15-minutes rainfall intensity (I15) of about 1.2-1.5 mm/min.

Regarding the pedology, there are different types of soil in the area, but mainly there are cernozioms and pseudorendzines. As regard to the land use and vegetation from the gullies catchments, there are mainly agricultural lands, covered by arable crops, grasslands, vineyards and orchards. The forests are mainly present along the gullies riverbanks, contributing to their stabilization. The main morphological characteristics of the studied gullies are presented in the Table no 1.

Table 1. Some morphological characteristics of the studied gullies in Slanic River basin (Mircea, 1999)

No crt.

Gully

Total gully watershed

(ha)

Gully headcut watershed

(ha)

Gully length

(m)

Gully headcut depth (m)

Valley slope (%)

Average slope of thalwegs

(%) 0 1 2 3 4 5 6 7 1 Baiasca 377.50 4.50 2748 2,35 12,3 9.1 2 Oarzei 90.00 20.25 1372 1,50 9,5 8.3 3 Irimesti 86.25 2.56 1702 1,00 10,6 4.6 4 Caldaresti 198.73 9.37 2950 2,45 21,6 8.0 5 Vladului 98.73 23.45 1375 2,90 15,3 9.8 6 Plutesului 120.50 23.25 1552 3,10 14,7 8.6 7 Galbeaza 101.70 7.54 1674 0,90 20,1 11.7 8 Balaurul 288.12 14.06 3786 1,30 15,8 5.4 9 Mereului 86.25 35.00 1423 1,70 12,5 8.7 10 Tatarului 51.25 15.75 900 3,30 10,3 11.4 11 Funduri 92.85 28.85 892 1,20 14,7 10.5

3. STATE INDICATORS FOR GULLY EROSION ASSESMENT The issue of seting up of such an indicators presents a relevance related to the risc assesment of producing floods in the torrential watersheds, as well as producing damages in the localities, reservoirs and infrsatructure network. Continuously development of the gulies on the 3 directions – in lenght, wide and depth, has effects, both on-site and off-site, on short and long term. These effects can be roughly estimated using a series of state and risk indicators concerning the vulnerability of the watersheds to the gully erosion, as they are presented below. a) Status of affectability of hydrographical network, (%)

- it determines as the ratio 100⋅L

l, (%)

where:

l is the length of the hydrographical network having a permanent or ephemeral flow adn presents an active erosion of the riverbed and gully banks, in km; L - is the total length of the hydrographical network from the same categorie, in km. b) Status of fragmentation of watershed, (km/km2) - it determines as the ratio L’/S , (km/km2); where: L’ is the length of the main gully as well as of its entire network, in km; S - total surface of the watershed taken into account, in km2. 4. RISK INDICATORS FOR GULLY EROSION DEVELOPMENT

Apart from the state indicators category, the risk indicators category is a very important and



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complex one for the gully development. Generally speaking, natural risk (climate, geology etc.) means that a certain natural event that produces damages to the people or affects their activities may occur. In the specialized literature concerning water erosion and landslides the notion of risk rather than hazard is frequently used. For gully erosion the following indicators are mainly used: - gullies’ advance in length, (m/year); - gullies’ development in width, (ha/year); - gullies’ development in volume, (m3/year). In Romania, (Motoc and Vatau, 1992) has proposed a series of risk indicators concerning gully erosion, as follows: a) Rate of fragmentation of watersheds and jeopardizing of some social-economic objectives located upstream of the gully headcut, such as: civil constructions, infrastructure and facilities etc., in m/year/ha: - it is represented by the ratio between the annual rate of gully headcut advance, in m/year, and the gully headcut watershed, in ha;

b) Risk of gully development in width, in ha/year: - it does refer to the yearly rate of land lost due to the development of the gullies; c) Risk by downstream inundation and/or siltation of the lands or some social-economic objectives: - it does refer to the annual damages, financially evaluated, that are produced downstream of gullies on lands or some social-economic objectives. For the above mentioned 11 small watersheds from Slanic/Buzau River basin a set of state and risk indicators concerning the vulnerability of the watersheds to the gulling have been estimated, at the level of the year 1999, as they are presented below in Table no 2. Based on the values of these indicators a general evaluation of the impact of gully erosion can be done for the studied region.

Table 2. State and risk indicators set up for some watersheds from Slanic River Basin (Mircea, 1999)

Indicators No crt.

Gully Status of hydrographical network affectability (%)

Status of watershed fragmentation

(km/km2)

Rate of fragmentation and damage of watershed

(m/year/ha) 1 Baiasca 60,2 1,32 1,70 2 Oarzei 64,3 1,52 0,21 3 Irimesti 68,4 1,97 2,61 4 Caldaresti 73,5 2,03 0,26 5 Vladului 62,6 2,91 0,09 6 Plutesului 64,3 2,28 0,10 7 Galbeaza 55,8 1,96 0,81 8 Balaurului 48,7 1,35 0,41 9 Mereului 43,6 1,65 0,06 10 Tatarului 52,8 1,76 0,13 11 Funduri 56,4 0,97 0,06

Function of the values of these state and risk indicators, established for some sub-watersheds from a given area, it can be achieved a general appreciation of the gully erosion impact on environment, by using the indexation method. To do that, there is prior necesarely a qualitative clasification of the studied watersheds in 4 groups, function of the maximum and minimum values of the

state and risk indicators. Afterwards, the evaluation matrice is build up, using the established indicators and the marks given to the each watershed, as proportion. In the end, a general mark of gully erosion impact assesment is given, in accordance with the current methodologies of environment impact assesment (Table 3).



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Table 3. Watersheds’ classification function of state and risk indicators (Mircea, 1999)

No Indicator, on groups Impact Mark crt. Status of

hydrographical network affectability

(%)

Status of watershed fragmentation

(km/km2)

Rate of fragmentation and damage of

watershed (m/year/ha)

assesment

1 0 – 25 0 – 1 0 – 1 Slight 0 – 25 2 25 – 50 1 – 2 1 – 2 Moderate 25 – 50 3 50 – 75 2 – 3 2 – 3 Strong 50 – 75 4 75 – 100 > 3 > 3 Very strong 75 – 100

5. CONCLUSIONS A such impact assesment for the Slanic/Buzau watersheds, having a predominant agricultural land cover, shows a moderate to strong impact of the gully erosion on environment. Such an analysis can be used in the end to the fundamentation of the antierosion strategy, both for the new works or for the rehabilitation of the already existing works. In adition to that, the results of applying the conceptual model DPSIR for soil erosion can be used fot the decision-makers. Having in view the great importance of soil conservation all over the world, and especially at the EU level, as soil has been one of thematic strategies under the 6 Environmental Action Programme, the European Environmental Bureau (EEB) is currently preparing Soil Framework Directive (SFD). (Gobin et al., 2004). The Directive, which is intended to be something similarly to WFD, will seek to address key threats to Europe’s soils, such as erosion, contamination, sealing, organic matter decline, salinisation, compactation, biodiversity loss and floods.

6. ACKNOWLEDGMENTS This work was supported by CNCSIS –UEFISCDI, project number 1084/2009, PNII – IDEI, code 1385/2008.

7. REFERENCES [1] Motoc M., Participarea proceselor de eroziune si a folosintelor terenului la diferentierea transportului de aluviuni in suspensie pe raurile din Romania, Buletinul informativ ASAS, Bucuresti, 1984, nr 13: 20-28. [2] Ichim, I., Mihaiu Ghe.: Aspects concerning geomorphology of the gully erosion formations and provenience of the alluvia. In Second Symposium Alluvia provenience and runoff effluence, Piatra Neamt, 1988. [3] Motoc M., Vatau A., Indicatori privind eroziunea solului, Revista Mediul Inconjurator, Bucuresti, 1992, vol III, nr.3: 20-26. [4] Mircea S., Study concerning gully erosion evolution in natural and managed watersheds in Buzau region. PhD Thesis, University of Agricultural Sciences and Veterinary Medicine, Bucharest, 1999. [5] Valentin C., Poesen J., Zong Li., Gully erosion – impacts, factors and control. Elsevier Science Publishers B.V.Catena 2005, 63: 132-153. [6] Wathern P., (Editor), Environmental Impact Assessment - Theory and practice, Routledge, London and New York. 1990. [7] Gobin A. et. al., Indicators for pan-European assessment and monitoring of soil erosion by water, ELSIVIER,.Environmental Science & Policy, 2004, 7, 25–38.

mircea sevastel , nicolae petrescu · annals. food science and technology 2011 available on-line at...

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