Mapping the water erosion’s threat on the Madeira’s island (Portugal)

May 2015

The catastrophe of 20 February 2010 which hit Madeira’s island, with intense water erosion and runoff, triggering huge debris flows, was responsible for large devastation and with more than 50 losses of lives. Shortly after the event it was decided to study the occurrence and to assess of the water erosion threat on the island. The evaluation of soil erosion’ threat, at regional level, is an important tool for the planning of land use and territory occupation and for the definition of environmental policies and measures to combat land degradation. The Madeira island is an high mountainous island, with an area of about 742,0 km2, a maximum altitude of 1858 m, an average surface slope of about 65 %, which is exceeded in 304 km2 (about 41 % of the area). In the island, the average annual precipitation ranges between 420 and 3 125 mm, with a mean value of about 1380 mm. First, the RUSLE methodology was adopted to assess the water erosion rates and to provide erosion maps; the major issue was the estimation of the average erosivity (R) values for the entire island, since, besides the Funchal-Observatório, there was only a few more weather stations with precipitation records of 10 minute discrimination, and most with less than 10 years durations. Due to the data’s scarcity, correlations were established to estimate erosivity; the analyzed relations showed most appropriate correlations with the Modified Fournier Index (MFI), and then with MFI and R. The adopted methodology allowed the mapping of RUSLE’s average R - erosivity parameter for Madeira’s island. The other RUSLE parameters where obtained using standard procedures. The obtained values of Modified Fournier Index (MFI) vary from about 81,0 to 465,0 mm. The equation adopted to estimate the rainfall erosivity from the MFI is given by the relation R = 1.363·MFI1.408. This equation was adjusted from some data obtained for Funchal Observatory, regional data processed in Southern Portugal and from equations reported in the literature. The estimated values for the erosivity parameter R, have an average value of 2 964 MJ.mm.h-1.ha-1.yr-1, ranging from about 660 to 8 515 MJ.mm.h-1.ha-1.yr-1. Second, the preliminary evaluation of “in situ” erosion’s threat was obtained by dividing the volume of the soil cover’s surface horizons by the value of the annual erosion. The resulting value, expressed as the number of years required for the total loss of soil cover, is then obtained. From the preliminary findings one can state that the average erosion obtained for Madeira Island is about 28 t ha-1 year-1, and the value of 100 t ha-1 year-1 is exceeded in more than 4.9 % of the area. It is also observed that the central zone of Madeira Island presents high values of soil loss, such as more steep slope areas located along the coast. As for the threat of “in situ” water erosion, despite the estimated average value for soil resource depletion exceeds 20 000 years, the area that corresponds to high and very high threat levels is very significant; the areas were the is threat is high and very high (depletion in less than 100 yrs) correspond to 263 km2 (35.6 %) and for very high threat (depletion in less than 10 yrs) the area is approximately 40 km2 (5.4 %). Areas which have relatively high slopes are associated to a high (or very high) erosion threat level. Studies are still underway, in particular those associated with extreme events and the effects of concentrated erosion and sediment transport to be carried to the drainage network (off site effects).

Authors in the community:

• 

  M

  Miguel Maria Jonet de Azevedo Coutinho

  ist10759

  Affiliations and sources

  IST > CEHIDRO

  Centro de Estudos de Hidrossistemas

• 

  R

  Rodrigo De Almada Cardoso Proença de Oliveira

  ist126343

  Affiliations and sources

  IST > CEHIDRO

  Centro de Estudos de Hidrossistemas

VII Congress of the European Society for Soil Conservation ESSC

Moscow, Russia

civil-engineering - Civil engineering

eng - English