4.2 What are the effects of TMCF conversion to shaded coffee on
streamflow?
At daily scale, shaded coffee had a lower mean streamflow and lower
rainfall to runoff ratio, despite having similar rainfall inputs
compared to PF. In the dry season, SC also showed a slightly lower mean
annual low flow (MALF), and a lower recession constant (k ) (Table
3). A significantly lower RR and MALF was expected in SC, because this
catchment is located at a lower elevation (472 m difference), which
promotes a higher evapotranspiration (Ramirez et al., 2017), while ET in
the PF is likely to be energy-limited, as described in previous
paragraphs. Additionally, the soil evaporation in a shaded coffee farm
may account for around 13 % of the total ET (Holwerda & Meesters,
2019), while the dense canopy in PF can minimize this output. However,
the lower aboveground biomass of SC is also associated with a lower
interception capacity, and an approximately 10% to 15% lower ET
compared to PF (Holwerda et al., 2010; Ponette‐González et al., 2010).
This interception difference may compensate for the increased of soil
evaporation in SC. In this work, SC had a significantly lower RR than
PF, as expected, but surprisingly, SC also exhibited a relatively higher
MALF and k , when compared to the other studied land covers (Table
3) and with previous studies (Muñoz-Villers & McDonnell 2013);
regardless of the expected higher ET described earlier. This suggests
that shade coffee may preserve a large part of its original capacity to
sustain baseflow during the dry season, associated with its higher soil
infiltration capacity when compared to IP (Table 1).
At the event scale, the Dunn’s test indicated that the hydrologic
response of SC in terms of total runoff, quickflow, baseflow and peak
discharge were not statistically different (alphabet grouping Figure
4a-d) compared to the PF. The metric normalized by streamflow
(Qbf/Qt ) also suggested that SC
has larger dominance of baseflow compared to YF and IP. The ANCOVA
analysis largely confirmed these results (Table 5). These findings
indicated that the SC micro-catchment shows a similar capacity to
modulate peak discharge compared to that of PF. However, the average
time to peak and lag time responses of the SC were the lowest (Table 4),
indicating a faster response, even when this micro-catchment exhibit the
largest area of the studied sites (Table 1). We attributed this result
to the fact that SC had an intermediate soil infiltration capacity (48
mm h-1) and the highest soil bulk density (Table 1).
Our results agreed with previous research conducted in the study region
which suggested that shade coffee soils had higher bulk density and
lower soil porosity than forest soils, in addition to intermediate
saturated hydraulic conductivities (Geissert & Ibáñez, 2008;
Marin-Castro et al., 2016). Our study demonstrates that shade coffee
preserved large part of the pre-disturbance capacity to sustain baseflow
and modulate peak discharge, despite the differences in elevation and
slopes that may have affected our results.
At the daily scale, the primary and intermediate forest presented
similar responses characterized by lower variability in their daily
flows, higher water storage capacity and lower flow velocities (high
recession constants), and higher MALF , compared to that of the
young forest (Table 3). Figure 3a demonstrates that, during the dry
season, the intermediate forest had a streamflow closer to that of
primary forest. These results correlated well with the soil infiltration
capacity, where, primary forest and intermediate forest showed a similar
and higher Kfs, compared to the young forest. Thus, our
results suggested that daily scale hydrologic regime of the intermediate
forest more closely resembles that of primary forest than that of a
young forest. However, the YF catchment also had the lowest annual
rainfall inputs and the highest rainfall variability (PVAR and DAYP0),
in addition to a greater land cover heterogeneity (Figure 1), which may
have obscured the response of this land cover. While PF and IF have
dominant land covers in their respective micro-catchments (100% and
77% respectively), the young forest covered only 68% of the area and
the rest of the micro-catchment was covered by more developed land. It
has been shown in other catchments that 20% or less forest removal is
sufficient to cause substantial effects on peak flow and water yield
(Bosch & Hewlett, 1982, Schueler et al., 2009; Evaristo & McDonnell,
2019). Yet, the mean annual low flow and mean annual high flow estimates
(Table 3) for YF were of the same order of magnitude to the results
reported by Muñoz-Villers and McDonnell (2013) for a 20 yr-old
regenerating forest in a neighboring catchment (0.21 and 32 mm
day-1, respectively); this suggests that all the
studied catchments showed high mean annual low flows compared to
previous studies in the region.
In terms of baseflow normalized by total dischargeQbf/Qt , all three catchments
presented a high baseflow percentage (Table 4): primary forest (81%),
intermediate forest (78 %), and young forest (67 %). This metric also
appears to be positively correlated with the percent of forest cover
described in the previous paragraph. Similar results of high baseflow
dominance during storms were previously reported in this region for an
old-growth TMCF by stable isotope-based experiments (Table 3 from
Muñoz-Villers & McDonnell, 2012), where groundwater dominated up to
90% of the storm runoff during the wet season. In terms of quick flow
and total runoff responses to rainfall, YF and IF were less responsive
to storm magnitude compared to PF (Figure 4e and Figure 4g). The high
baseflow contribution to total stormflow in PF appeared to be associated
with increases in the rainfall-runoff response, as observed in Figures
4g. Similar effects were previously reported for an old-growth TMCF, a
20-yr old TMCF, and a pasture catchment (Muñoz-Villers & McDonnell,
2013). Surprisingly, the peak discharge response to rainfall was very
similar for the three forested micro-catchments (Table 5, Figure 4d).
These results indicated that the three forest covers presented a
seemingly similar capacity to attenuate peak flows. Our work suggests
that the secondary forests contributed to restored hydrologic responses
to pre-disturbance conditions. Nonetheless, uncertainty remains to
confirm this hypothesis, since the micro-catchments were not entirely,
nor equally, dominated by the studied forests, as indicated earlier.
We were not able to completely answer the question of whether forest age
impacts hydrology, in terms of baseflow maintenance and peak discharge,
because differences in slope, elevation and percent of forest cover
among the three catchments influenced our results. For instance, the PF
micro-catchment has a steeper mean slope than that of IF and YF, being
6° and 12° greater, respectively. Steep slopes have been associated with
higher quickflow, and lower lag time and time to peak responses (Mu et
al., 2015; Nainar 2018), as observed in our study (Figure 4e-f and Table
4). On the other hand, steeper slopes have also been linked with higher
water storage capacity (Karlsen, 2010; Gabrielli & McDonnell, 2011;
Sayama et al., 2011; Uchida et al., 2008). Thus, the steeper slope of PF
may explain its higher contribution from baseflow and its high rainfall
to runoff ratio (0.67). Additionally, the higher mean elevation of PF
(1756 m a.s.l.), compared to that of IF (1604 m a.s.l.) and YF (1453 m
a.s.l.) seems to correlate with its higher RR.
Our findings showed that 20 and 40 years of natural forest regeneration
were sufficient to restore large part of the hydrology on micro
catchments previously occupied by intensive pastureland and annual crops
(local inhabitants communication). These results clarify that, despite
the commonly accepted notion that these forests have high ET rates, both
regenerating and primary forests provide important hydrological
services, such as dry-season baseflow sustenance and modulation of peak
discharge, due to the higher infiltration rates and water storage
capacities.