We used GenRiver to assess the importance of rainfall
variation ('climate change') and land cover change ('deforestation') in the Way
Besai watershed in Sumberjaya, West Lampung (Sumatra,
Indonesia). Sumberjaya is
situated between 4° 56' 6" and 5° 11' 25" South and 104° 17' 52" and 103° 33'
51" East. The elevation ranges between 720 m and 1831 m above sea level. The
area of Sumberjaya is about 415 km2.

Model simulations used rainfall data from August 1976 - May
2007 with annual rainfall of 2500 3500 mm per year. Eight landcover types were
distinguished:

1) Increase
in BD/BDref reflects soil compaction and shifts 'soilquickflow' to 'overland
flow'

2) The
amount of water stored on the canopy that can directly evaporate

3) lower
values imply that the vegetation is less sensitive to drought

The major
soils are inceptisols (Dystropepts, Dystrandepts and Humitropepts) with some
entisols (Troporthent), with differences in soil texture and water holding
capacity.

Result - River flow
model performance

The model simulation could capture most of the observed
pattern across 30-year period. Overall there was a 'moderately good fit' of
model estimates with field measurement data; part of the rainfall and river flow
field data are probably less reliable. Also, spatial variability of rainfall
over the catchment was not fully represented in the field data.

            The
watershed function of the Way Besai catchment can be assessed using criteria
and indicator of water transmission (total water yield per unit rainfall),
buffering capacity (peak flows relative to peak rainfall events) and gradual
release of ground (dry season flow). Simulations and observations for these indicators
match sufficiently to use the model for further scenario studies.

The main source of year-to-year variation in river flow is
the rainfall, with wetter years leading to a higher total discharge fraction.
The main effect of land cover change was an increase in the discharge fraction as
well. Land cover change and associated soil compaction also increased the
overland flow fraction, but this has only a small effect on the daily
hydrograph. Indicators of gradual water release (slow flow and soil quick flow
fractions, and lowest monthly river flow) tended to decrease over the years,
along with more pronounced dry seasons in the 1990's. Further analysis suggests
that this was in

response to El Nino and Indian Ocean Dipole anomaly years.
The buffering capacity (buffering indicator, relative buffering indicator and
buffering of peak events) was relatively low in the 1990's but increased in recent
years.

Overall, the model results show that details of the
rainfall regime dominate the river flow results, but land cover change and soil
compaction do modify the results. The model can be used to explore a wider set
of land use scenarios and impacts of climate change.