Landscapes Forest and Global Change - ESA - Escola Superior ...

J.P. Nunes et al. 2010. Impacts of wildfires on catchment hydrology
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3. Research at the hillslope scale
This research was carried out for six hillslopes close to Albergaria, central Portugal: four burnt
in 2005 (Açores 1&2, Jafafe 1&2) and two burnt in 2006 (Soutelo 1&2). All slopes were
covered by eucalypt prior to the wildfires, and represent different management options
including terracing and downslope ploughing. Meteorology, runoff and erosion were monitored
weekly in each slope; soil surface and subsurface conditions were monitored bi-weekly using 5-
point transects. Figure 3 shows some measured results for the slope Açores 1, as reported by
Keizer et al. (2008). They illustrate one of the main findings of this study regarding slope-scale
hydrology in burnt hillslopes. Soils in these slopes are highly water repellent immediately after
fires; the repellency decreases at the start of the wet season, and reappears during the following
dry season (although it can reestablish during dry spells in the wet season, as shown in the
figure for the winter of 2007). Repellency and soil water storage are closely related, and
reasonable amounts of rainfall falling on repellent soils might not lead to an increase of soil
water. Consequently, rainfall events at the start of the wet season (or during other periods with
high soil water repellency) might lead to increased runoff rates when compared with events in
the middle and end of this season; this has the potential for increasing flood risks at the
catchment scale which might be difficult to predict if repellency is not taken into account.
4. Watershed-scale research: micro- and meso-scale catchments
Research at the watershed scale is shown for two micro-catchment sites, Colmeal (central
Portugal) and Esteiro (Galicia); and one meso-catchment site, Águeda (central Portugal).
Colmeal is a micro-catchment (60 ha) which burnt almost completely in 2008; it was
instrumented with a number of slope-scale plots, while runoff and sediment data was also
collected at the catchment outlet in a multi-scale sampling approach. Preliminary results
indicate that runoff and erosion are concentrated in the first months after the wildfire, at the start
of the vegetation disturbance and when soil water repellency is high. They also indicate a nonlinear
relationship between slope-scale and catchment-scale runoff, as the latter also depends on
subsurface flow and saturation-excess runoff generation close to the water line.
The Esteiro fire occurred in 2007; two paired micro-catchments were instrumented, each with
around 450 ha (Figure 5). The Maior catchment was partially burnt while the Arestiño
catchment was not. Preliminary results indicate that runoff was more irregular in the burnt
catchment, with higher values in the winter immediately following the wildfires and lower
values in the subsequent summer (close to zero). These results might indicate an impact of
wildfires on increasing the irregularity of water resources provisioning, although differences in
the underlying geology of both catchments might also play a role in differentiating the
hydrological regimes. The Águeda fire occurred in 1986, and burned c. 25% of the upper
Águeda river basin. Given the scarcity of hydrological data for this wildfire (runoff was only
measured at the outlet), a modeling approach is being used to estimate the impacts on runoff and
water balance on subcatchments with different burnt areas. The approach relies on the SWAT
model (Neitsch et al., 2005), capable of simulating a number of processes inside meso-scale
catchments, including water balance, surface and subsurface runoff, soil erosion, nutrient
exports and vegetation regrowth (including human management operations). The current
modeling approach relies on simulating 5-10 years in both burnt and unburnt conditions,
therefore evaluating the different impacts for dry and wet years as well as assess the relative
importance of wildfires and climate variability for watershed-scale water yield.
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.