Stream flow and yield
Periodic base flow measurements obtained during the monitoring program
together with previous data revealed contrasting seasonal and
multiannual trends across different streams in El Morro catchment
(Figure 8). During the three years of frequent measurements, a clear
seasonal pattern with higher flow in the dry (and cold) season and lower
flow in the rainy (and warm) season was evident at all sites (-31 to
-50% in summer compared to the rest of the year, Table 1) except Site
2. Matching the seasonal patterns of water table depths (Figure 5),
these fluctuations illustrate the dominant effect of groundwater uptake
rather than precipitation inputs controlling stream base flow.
Noticeably, the only site with no seasonal fluctuation corresponded to a
segment of the Río Nuevo with deep incisions down to the gaging point
(riparian consumption less likely), whereas most intense fluctuation
were seen at site 3 and 5 (running dry in February 2020), which had
shallow incisions and a large fractions of their trajectory surrounded
by wetlands (Table 1). Older base flow data obtained before and shortly
after the last erosion episode show how sites 1, 2, and 3 gained flow
while site 7, which was flanked by the first two, got completely dried
out after being a high-yielding stream before (Figure 8, Table 1).
Matching water table observation at site B, this switch in stream
dominance suggests competition of neighboring incisions capturing
groundwater as they deepen. While showing no seasonal trends, the deeply
incised segment of Río Nuevo gaged at site 2 showed a slight but
significant decline in its base flow with time between (-23 l
s-1 per year between November 2016 and August 2020,
r2=0.60, p<0.01).
Base flow water yield estimates were obtained considering a topographic
(larger) and geometric (smaller) definition of the feeding catchment
area for each site (Table 1). These two criteria suggested specific
yields ranging from 4 to 56 and 8 to 159 mm y-1,
respectively, with the highest values corresponding to the upper Río
Nuevo sub catchment (Site 2) and the lowest to the Quebrachal sub
catchment (Site 1). Along the Río Nuevo catchment, water yields appear
to be highest in the higher belt (Sites 2 and 3), declining but
maintaining a positive contribution in the intermediate belt (site 4 and
“site 4 - (sites 2 + 3)” in Table 1). In the final segment where the
merged La Guardia and Río Nuevo streams enter the terminal plain,
negative yields were consistently observed (site 6 and “site 6 -
(sites 4 + 5)” in Table 1). At this closure point, the total specific
yield of the catchment was 19/29 mm y-1(topographic/geometric area definition)(Table 1). While not accurately
measured, a high-end estimate of peak flow contributions achieved at
this site suggested that it would add less than 11% to the total yield
of the catchment (Supplementary material).