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Case Study 2 - Impact of forests on the catchment response for extreme rainfall events
BackgroundLa Reina is a 0.35 km2 basin in Region 10, 41°S, in Chile (Figure 4.1). A picture of the lower part of the basin and the outlet flume in the mid 1990’s can be seen above. Until January 2000 there was a commercial radiata pine forest (with some deciduous areas). The entire basin was then logged in that month and in January 2001 it was replanted. Soils are mainly well drained and around 2.5m deep. Annual precipitation is around 2500 mm/year and this is mostly from frontal rainfall that falls in the winter (June – August). Annual potential evaporation is around 1000 mm/year.
SHETRAN ModelThe SHETRAN mesh for La Reina basin uses 141 50m by 50m grid squares and 53 river links (3m wide) that run along the edge of the grid squares (see below). The elevations can also be seen. Calibration of the model was carried out for two periods. Firstly from 1997-1999 when the basin was forested and secondly from 2000-2001 when the basin had been logged and then replanted. These two periods used exactly the same soil parameters but different vegetation parameters. The simulations were then run again for both the forested and logged cases using a 1000 years rainfall record based on the seven years of hourly data from La Reina and 45 years of daily data from a near-by site.
ResultsThe mean annual runoff increased by 110% after logging in La Reina basin in January. This is well simulated by SHETRAN as can be seen below for the forested and logged cases. The mass balance shows this decrease in annual runoff was due to a major reduction in the annual evaporation
The results of the 1000 year simulation can be seen below. This compares the maximum daily discharges for each corresponding day of 1000 years for the forested and logged cases and show that there can be a range of “logged” responses for a given “forested” discharge, depending on antecedent soil moisture conditions. In some cases there is no difference between the two discharges, in others there is: if the soil under the forest is very wet, the response is similar to the logged case. The difference between the two cases is least in the winter and greatest in the summer (when trees have the least and greatest effects respectively on soil moisture).
More details can be found on the EPIC FORCE website, where all the deliverables from the project are available.
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