FLAMMA, 4 (1), 1-4, 2013 ISSN 2171 - 665X CC Attribution-NonCommercial-ShareAlike 3.0 License

Evaluation of two emergency treatments for reducing post-fire runoff and sediment losses M.A.S. Martins (1*), S.A. Prats (1), M. Ben-Hur (2), J.J. Keizer (1) (1) Centre for Environmental and Marine Studies (CESAM), Department of Environment and Planning, University of Aveiro, 3810-193 Aveiro (Portugal) (2) Volcani Center, Department of Soil, Water and the Environment, Israel * Corresponding author: [email protected]

Keywords

Abstract

Erosion Mulching PAM Runoff Wildfires

Portugal suffers every year the effects of wildfires, including an increasing superficial runoff and erosion in affected areas. Post-fire stabilization treatments with mulching have been referred by several authors as one of the best techniques to relieve these impacts. This study evaluates the effectiveness of two treatments in reducing overland flow and erosion in a burned eucalyptus forest in northern central Portugal, at a microplot scale (0.24m2). After the first year, which had an average annual precipitation of 1442 mm, the treatment with chopped bark “mulching”, reduced runoff in 55% and sediment loss in 90%. The treatment with anionic polyacrylamide (PAM), reduced runoff in 20% but was not effective in reducing sediment losses. Plot position on the slope proved to be an important factor in the hydrological and erosive response of burned eucalypt areas.

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INTRODUCTION

OBJECTIVE

This study aims at comparing the effectiveness of two post-fire erosion mitigation treatments - mulching with forest residues versus application of an anionic polyacrylamide (PAM) - in reducing overland flow and erosion in a recently burnt eucalyptus plantation.

Wildfires are a common phenomenon in present-day Portugal and, over the past three decades, have affected an average of 100,000 ha of rural lands per year (Pereira et al., 2005). At the same time, wildfires can produce marked changes in hydrological and erosion processes that can negatively affect both land-use sustainability and downstream “values-at-risk” (Robichaud et al., 2010; Shakesby, 2011). These negative, erosion-related effects of wildfires can be mitigated effectively by the application of mulch before the occurrence of significant post-fire rainfall (Robichaud et al., 2010; Fernández et al., 2011; Prats et al., 2012). The application of synthetic polymers such as the anionic poly-acrylamides (PAM) (Ben-Hur, 2006) has been forwarded as a more cost-effective alternative to mulching but its effectiveness in burnt areas is still not well-established (Ben-Hur et al., 2012).

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METHODOLOGY

3.1 STUDY AREA The study was carried out in a Eucalyptus globulus plantation in the Sever do Vouga municipality, northcentral Portugal, that was burnt by a wildfire during the summer of 2010. This plantation was located on a steep o (25 ) slope with a length of about 40 m facing south-east. Before the occurrence of significant rainfall (< 1 mm), 12 1

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Figure 1. (a) One of the four blocks of three micro-plots; (b) example of one of the micro-plots treated with eucalypt residue mulch, (c) grid used for ground cover descriptions.

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Sediment Loss (Mg ha -1 )

18 16 14 12 10 8 6

PAM

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control

2

Mulching

0 0

10

20 Bare soil (%)

30

40

Figure 2. Relation of total sediment losses with bare soil cover in May 2011.

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100 80

Efectivness (%)

60 40 20 0 -20 0

50

100

150

200

250

300

-40 -60

PAM

-80

Mulching

-100 Total runoff for each sampling period (mm)

Figure 3. Relation of the average effectiveness of the mulching and PAM treatments in reducing sediment losses with the runoff volumes of the individual measurement periods.

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micro-plots of 0.25 m were installed in four blocks situated at regular, 9-m intervals from the base to the stop of the slope (Figure 1a, b). The two treatments were applied randomly to the plots of each block; the mulching with eucalypt residues was done on September 15 2010 and the application of PAM on October 4 2010. Mulching -1 was done at rate of 13.6 Mg ha and produced, on average, a soil cover of 80%, whilst the PAM was applied at -1 a rate of 0.05 Mg ha .

From the five ground cover categories, that of bare soil (as recorder in May 2011) was most closely related to the marked variation in total sediment losses of the individual plots from August 2010 to August 2011 (Figure 2). There was no suggestion of apparent differences in sediment losses between the untreated and the PAM-treated plots. The overall difference in effectiveness of the two treatments was also evident for the individual measurement period (Figure 3). The effectiveness of mulching in reducing sediment losses never dropped below 60%, neither for the extreme events with more than 100 mm of runoff at the control plots. The effectiveness of the PAM application, however, was highly variable, ranging from -100 to +80%, and revealed no apparent relationship with runoff amounts.

3.2 LABORATORY ANALYSIS The runoff samples were filtered in the laboratory using filters with a 12-14 µm pore size. The filters were then o dried in an oven for 24 hours at 105 C to determine their sediment concentrations and afterwards placed in a muffle o for 4 hours at 550 C to determine their organic matter content.

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CONCLUSIONS

Thus, the main conclusions of this study on post-fire runoff and erosion in a recently burnt eucalypt stand in northcentral Portugal were the following: (1) the runoff and erosion rates of the untreated plots were rather high, exceeding substantially the 1 Mg ha-1 yr-1 threshold for tolerable hillslope soil erosion proposed by (Verheijen et al., 2009); (2) the application of eucalypt residue mulch greatly reduced overland flow generation but especially sediment losses; (3) the application of PAM reduced somewhat overland flow generation but even increased

RESULS AND DISCUSSION

The total precipitation during the study period amounted to 1442 mm, producing an average runoff coefficient of 45% at the control plots and an average sediment loss of -1 7.8 Mg ha . On average, the mulching treatment reduced runoff by 55% and sediment losses by 90%. The PAM treatment was markedly less effective, reducing the average runoff by 20% but even increasing the average sediment losses by 24%. 3

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sediment losses. The latter is contradictory to other experimental evidence, from both laboratory and field studies with burnt soil from Spain and Israel (Ben-Hur et al., 2012), possibly due to the specificities of the PAM applied in this study and/or to the role of the ash layer in the effectiveness of poly-acrylamides.

REFERENCES Ben-Hur M. 2006. Using synthetic polymers as soil conditioners to control runoff and soil loss in arid regions – A review. Australian Journal of Soil Research 44: 191-204. Ben-Hur M, Inbar A, Lado M. 2012. Using synthetic polymers to prevent soil erosion after fire in Mediterranean forests. Conf. Proc. Climate Change & Forest Fires in the Mediterranean Basin: management & risk reduction (Nir Etzion, Mt. Karmel, Israel, 24-26/01/2012). Fernández C, Vega JA, Jiménez E, Fonturbel MT. 2011. Effectiveness of three post-fire treatments at reducing soil erosion in Galicia (NW Spain). International Journal of Wildland Fire 20: 104– 114. Pereira J, Carreira J, Silva J, Vasconcelos M. 2005. Alguns conceitos basicos sobre os fogos rurais em Portugal. In: Pereira JS, Pereira JMC. (Eds.), Incêndios florestais em Portugal - caracterização, impactes e prevenção. ISAPress, Lisbon, pp. 133. Prats SA, Macdonald LH, Monteiro MSV, Ferreira AJD, Coelho COA, Keizer JJ. 2012. Effectiveness of forest residue “mulching” in reducing post-fire runoff and erosion in a pine and a eucalypt plantation in north-central Portugal. Geoderma 191:115-124. Robichaud PR, Ashmun LE, Sims BD. 2010. Post-Fire Treatment Effectiveness for Hillslope Stalibization. USDA Forest Service Gen. Tech. Rep. RMRS-GTR240. Shakesby RA. 2011. Post-Wildfire soil erosion in the Mediterranean: Review and future research directions. Earth-Science Reviews 105:71-100. Verheijen FGA, Jones RJA, Smith CJ, Rickson RJ. 2009. Tolerable versus actual soil erosion rates in Europe. Earth-Science Reviews 94: 23-38

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