Groundwater dynamics
Within an overall context of raising groundwater levels, observations
showed different water table behaviors depending on the temporal scales
considered and the locations of observation points within the whole
catchment and, more locally, regarding landscape features and vegetation
types. Below we introduce long-term trends and their manifestation in a
representative vertical section of the landscape, focusing afterwards on
the more detailed dynamics captured after three years with periodic
observations and, finally, selecting three illustrative cases of
seasonal, daily and hourly level fluctuations.
Over the last four decades the El Morro catchment experienced a dramatic
rise of groundwater levels leading to an expansion of the areas occupied
by wetlands, lagoons and seepage zones. Water table levels raised on
average 3.06 meters per decade across 27 wells where groundwater depth
was measured in the past (most commonly in 1975) and measured or
estimated again over the last decade (Figure 3). Noticeably, while the
majority of those points showed level raises throughout one or two
measuring intervals, those located within or very close (<
500m) to stream incisions (6 sites) showed absolute water table depth
declines that were largest in the intermediate belt of the catchment. In
the lower belt, a site that received the thickest deposit of fluvial
sediments showed a relative decline of its water table level as the
surface raised (Figure 3). The observed groundwater level raises were
highest in the higher belt of the catchment where the most extreme case
approached a 37 m level ascent and levels climbed on average 4.83 meters
per decade (initial mean depth of 15.33 m)(Figure 3). That area was
followed by the intermediate and lowest (terminal plain) belts where,
ignoring the incision cases, decadal raises averaged 3.50 and 3.77
meters per decade, respectively (initial mean depths of 11.55 and 7.85
m, respectively)(Figure 3). The smallest changes were observed in the
lower belt of the catchment, where the average ascent was 0.97 meters
per decade, reflecting the fact that water tables were already shallow
at the beginning of the study period there (initial mean depth of 3.6
m)(Figure 3).
Based on the higher availability of data and fast changes observed in
the contact zone between the higher and intermediate belts of the
catchment we reconstructed a transverse vertical cut depicting major
landscape and groundwater shifts (Figure 4). By the year 2000 the water
table in this section of the catchment (covering the full sub catchments
of Quebrachal and Río Nuevo streams), approached the surface in multiple
low positions that were disconnected from groundwater in the late 70s.
Incisions formed in many of the local low positions in west to east
temporal sequences (Figure 4). A relatively uniform lateral distribution
of incisions or wetlands with permanently running surface wetlands can
be appreciated along the transect with distances of 3.2, 3.2, 2.6, 1.9,
and 2.1 km (2.6 km on average) among them. A similarly regular but
larger spacing is observed in the intermediate and lower catchment,
averaging 12.5 km between the main collectors of each sub catchment.
Water table depth observations in the network of monitoring wells
reflected the dry conditions of the 3-year study period with an overall
deepening trend (Figure 5). However, important contrasts were found
between vegetation types and landscape positions. At site A, the
wetland, which appeared in 2000 and has the crystalline basement close
to the surface (see Figures 3 and S1), showed the most stable levels,
always less than 15 cm below the surface, likely as a result of
sustained seepage at that site (Figure 5a). The neighboring cropland
instead, showed seasonal oscillations (summer declines, winter raises)
over imposed to the general declining trend, suggesting that water
consumption during the period of high vegetation activity and
evapotranspirative demand, which is also the rainy season, outpaced
recharge fluxes during the study years (Figure 5a, d, e). Wells at site
B showed a more extreme, steady and parallel decline during the first
year (Figure 5b) (they had to be deepened to continue measurements
afterwards), revealing the likely effects of the recently carved or
deepened incisions flanking the site (Upper Rio Nuevo and Uke
tributaries), drying a wetland that emerged in the late 90s and
interrupting its stream (site 7 in Figures 2b and 6). A particular
aspect of this level decline is its constant rate in the wet and dry
season of the first year, which suggests drainage from below rather than
consumption from above as the likely cause of this behavior. Located by
the deepest incision of the catchment (enlarged from 9 to 20 m of depth
and 28 to 60 m of width in 2015), site C represented the deepest water
table situation showing a strong contrast for its three pairs of
forest-cropland wells (Figure 5c). There, the forest, similarly to the
cropland and wetland sites with shallower water tables, displayed
seasonal oscillations suggesting a pulsed consumption of groundwater by
the dominant winter-deciduous trees whose active roots were found
growing around sensors at 9 m of depth. Croplands, only 600 m away at
this site, showed a steady decline with very subtle seasonal rates of
change. Differences along the transect at this site suggest that wells
at position 1, ~100 m away from the deepened incision
are closer to rich there a new equilibrium while those at positions 2
and 3 may still be yielding water towards the stream (Figure 5c). Two
additional monitoring wells not shown in the figure reflected similar
patterns with Site F hosting a cropland with deep water tables close to
the Quebrachal incision. That site followed croplands in site C until no
saturated conditions could be detected above the crystalline bedrock at
14 m of depth. Site G, in a suburban area of the low catchment affected
by periodic waterlogging followed the same pattern of the wetland at
site E.
Continuous monitoring of groundwater depth with pressure transducers
offered a more precise perspective on the recharge and discharge
dynamics of the study sites (Figures 6 and 7). At Site A we highlight
contrasting periods of net water gains (summer-fall 2018) and losses
(spring-summer 2019-2020), showing the contrasting behavior of the
wetland and the cropland. In the first of these periods, the wetland
displayed high daily level fluctuations that amplified progressively
following rainfall events (Figure 6, left panels), suggesting an
increasing reliance on groundwater as the rainfall pulse was consumed.
No fluctuations were observed in the cropland, that at that time was in
fallow stage prior to the establishment of a pasture (see NDVI in figure
6, left panel). A single and intense rainfall event in this period (74
mm on April 1 measured at the precise site) switched the wetland to a
flooded condition reducing but not eliminating fluctuations, likely as a
result of the shifting groundwater yield-depth relationship under
flooding. The same event and the following two triggered direct recharge
episodes under the fallow with level rises of 2.7 – 3.8 mm per mm of
rainfall (or a specific yield of 0.26-0.37)(Figure 6, left panels). In
the second period, which followed three consecutive dry growing seasons,
the wetland displayed wider diurnal fluctuations with the same pulsed
reduction and gradual amplification in response to rainfall events
(Figure 6, right panels). These fluctuations averaged 15.2 cm
d-1 (daily maximum – minimum level) and, based on
Loheide, Butler and Gorelick (2005) methodology, generated an average
groundwater level depletion of 16.6 cm d-1.
Noticeably, in spite of its apparent reliance on groundwater, the
wetland greenness increased after rainfall events (see NDVI in figure 6,
right panel). The cropland in this period was covered by an active (and
typically deep-rooted) alfalfa pasture which was likely responsible for
sustained water table fluctuations averaging 1.6 cm
d-1 with an estimated mean depletion of 4.9 cm
d-1. The largest rainfall pulse of this period (125 mm
in November 17-20), triggered a slow 36 cm level raise of 10 days in the
cropland which had a much lower velocity than the recharge pulses of the
previous study period (Figure 6, right vs. left panel), suggesting that
the level recovery resulted from the interruption of groundwater
consumption.
Deep water table levels at site C, likely favored by its neighboring
incision, provided an opportunity to explore vegetation-groundwater
interactions under conditions that could have been more common for the
catchment in the past (Figure 7). Sensor data provided here a more
detailed description of the seasonal departure of water tables at forest
stands with regard to their neighboring croplands (Figure 7 vs. Figure
5), showing that forest trees produce similar diurnal fluctuation,
seasonal depression and slow level recoveries after rainfall events than
those described for the alfalfa pasture above, yet they do so with water
tables that are 8-10 m deep (Figure 7). Groundwater uptake, as indicated
by daily water table fluctuations, started a few days after the
deciduous tree species leafed out in mid-October (see NDVI cycles in
Figure 7) and levels initiated their sustained and asymptotic recovery
when the trees gradually lost their leaves in late May, approaching
equilibrium with the surrounding cropland matrix throughout the dry (by
ecologically inactive) season (Figure 7). Diurnal water table level
fluctuations during the active period of the forest (October 15 - April
15) averaged 1.7 cm d-1 with an estimated mean
depletion of 3.1 cm d-1. In the three consecutive
years of observations, levels dropped/recovered -89/47, -71/55 and
-79/57 cm in the forest site. The same net drop of -80 cm over the
three-year measurement period was observed in the cropland but without
seasonal fluctuations. No direct recharge episodes were documented at
any of these wells (Figure 7), yet slow and partial level recoveries in
response to rainfall pulses were observed in the forest, as depicted in
detail for a series of events in March 2019 (Figure 7, bottom left
panel) and recorded in November 2019 and January 2020 when of 15
cm-recoveries took place along 8 and 17 days following rainfall pulses
of 90 and 77 mm, respectively (Figure 7).