dc.description.abstract |
The hydrologic effects of forest use and reforestation of degraded lands in the humid tropics has implications
for local and regional hydrologic services but such issues have been relatively less studied when
compared to the impacts of forest conversion. In particular, the ‘‘infiltration-evapotranspiration trade-off’’
hypothesis which predicts a net gain or loss to baseflow and dry-season flow under both, forest degradation
or reforestation depending on conditions has not been tested adequately. In the Western Ghats of
India, we examined the hydrologic responses and groundwater recharge and hydrologic services linked
with three ecosystems, (1) remnant tropical evergreen forest (NF), (2) heavily-used former evergreen forest
which now has been converted to tree savanna, known as degraded forest(DF), and (3) exotic Acacia
plantations (AC, Acacia auriculiformis) on degraded former forest land. Instrumented catchments ranging
from 7 to 23 ha representing these three land-covers (3 NF, 4 AC and 4 DF, in total 11 basins), were established
and maintained between 2003 and 2005 at three sites in two geomorphological zones, Coastal and
Up-Ghat (Malnaad). Four larger (1–2 km2) catchments downstream of the head-water catchments in the
Malnaad with varying proportions of different land-cover and providing irrigation water for areca-nut
and paddy rice were also measured for post-monsoon baseflow. Daily hydrological and climate data
was available at all the sites. In addition, 36 min data was available at the Coastal site for 41 days as part
of the opening phase of the summer monsoon, June–July 2005.
Low potential and actual evapotranspiration rates during the monsoon that are similar across all landcover
ensures that the main control on the extent of groundwater recharge during the south-west monsoon
is the proportion of rainfall that is converted into quick flow rather than differences in evapotranspiration
between the different land cover types. The Flow duration curves demonstrated a higher
frequency and longer duration of low flows under NF when compared to the other more disturbed land
covers in both the Coastal and Malnaad basins. Groundwater recharge estimated using water balance
during the wet-season in the Coastal basins under NF, AC and DF was estimated to be 50%, 46% and
35% respectively and in the Malnaad it was 61%, 55% and 36% respectively. Soil Water Infiltration and
Movement (SWIM) based recharge estimates also support the pattern (46% in NF; 39% in AC and 14%
in DF). Furey–Gupta filter based estimates associated with the Coastal basins also suggest similar groundwater
recharge values and trends across the respective land-covers: 69% in NF, 49% in AC, and 42% in DF.
Soil water potential profiles using zero flux plane methods suggest that during the dry-season, natural
forests depend on deep soil moisture and groundwater. Catchments with higher proportion of forest
cover upstream were observed to sustain flow longer into the dry-season. These hydrologic responses
provide some support towards the ‘‘infiltration-evapotranspiration trade-off’’ hypothesis in which differences
in infiltration between land-cover rather than evapotranspiration determines the differences in
groundwater recharge, low flows and dry-season flow. Groundwater recharge is the most temporally stable
under natural forest, although substantial recharge occurs under all three ecosystems, which helps to sustain dry-season flow downstream in higher order streams that sustain local communities and agroecosystems.
In addition to spatial scale effects, greater attention also needs to be given to the role of
hydrogeology within the context of the above hypothesis and its implications for hydrologic services. |
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