4. DISCUSSION
Our results illustrate how physical water vapor tracers, such as stable
isotopes in precipitation, allow tracking water sources in atmospheric
circulation as they connect evaporation and precipitation, and their
concentration indicates the meteorological conditions and the proximity
to the main atmospheric moisture sources that originate precipitation,
as expected. Although the use of stable isotopes in precipitation does
not provide quantitative information about the amount of atmospheric
moisture from the sources, it complements the information generated by
models, such as in our FLEXPART experiment. More generally, this
information has the potential to be integrated into atmospheric models
to diagnose the percentage of moisture originated from the different
sources (Wang et al., 2004; Fuka et al., 2014; Arias et al., 2015; Hoyos
et al., 2018; Molina et al., 2019; Liu et al., 2020). Further, the
combined use of physical tracers and models are particularly useful in
regions with complex topographical and meteorological setups where model
results can be highly uncertain.
The consideration of the optimal transport day (when the moisture
transference is maximum), instead of the canonical 10-day mean lifetime
of water vapor in the atmosphere, allowed us to improve the
quantification of contributions from atmospheric moisture sources,
highlighting the predominance of terrestrial sources all through the
year. The natural scales of distance and time are closely linked to the
regional structure of moisture source composition. Therefore,
assumptions about uniformity or similarity to a global mean need to be
revisited for each region, after consideration of the particular
climatic and meteorological features. Conversely, the proper transport
scales are footprints for each particular target area and include
regional features like the mechanisms involved in the atmospheric
moisture transport, orographic features, and distance between the target
and source regions. The lack of knowledge of these scales could lead to
over (or under) estimations of the degree of contribution from different
sources and reduce the forecasting power of teleconnections.
Physical water vapor tracers help identify signals of the origin of
incoming atmospheric moisture when the source is not influenced by
mixing effects associated with different oceanic and terrestrial sources
present in the majority of the seasonal analysis. Our results allowed us
to identify oceanic sources connected with the seasonality migration of
ITCZ, as well as terrestrial sources such as Amazon and Orinoco basins,
that agree with meteorological criteria. Both of these observations are
evident in the reconstruction of the D-excess, even when the spatial and
temporal coverage of isotopic data is very low for Colombia. Although we
have reconstructed a baseline for the isotopic structure of
precipitation, it is also evident that a better sampling network is
necessary to improve the monitoring network of stable isotopes to
produce a more detailed analysis of the moisture transport processes, as
they have done in more instrumented areas (Friedman, Smith, Gleason,
Warden, & Harris, 1992; Kendall & Coplen, 2001; Bowen, Ehleringer,
Chesson, Stange, & Cerling, 2007; Bowen, Kennedy, Liu, & Stalker,
2011).
Our results showed a multiannual moisture convergence average from
terrestrial sources of 59% for the Andean region and for the Caribbean
region of 56% (Figure 6), which is greater than previous estimations by
our group (38% in Hoyos et al., 2018). By using a combination of
physical tracers and modeling, we confirm previous modeling results
(Hoyos et al., 2018) that indicate how, in the study area, the
contribution of terrestrial moisture sources to local precipitation is
significant (always greater than 44%), such that most ecosystems and
water security for society and the economy may depend on the stability
of major regional ecosystems such as the Orinoco plains (8% - 28% per
month) and the Northern Amazon (17% contribution). More importantly,
our results highlight that most terrestrial moisture originates within
the same region (NOSA), with contributions larger than 23% per month in
some seasons and up to 40% per month in other seasons.
The fact that a significant proportion of rainfall comes from recycling
(Fig. 6), highlights how precipitation and, more generally, water
availability in the Andean and Caribbean regions of Colombia could
potentially be altered by changes in vegetation and land cover, directly
affecting transpiration and atmospheric circulation. This is a clear
indication that the region is particularly vulnerable to ongoing
widespread ecosystem transformation in the region and the surrounding
basins. According to Ruiz‑Vásquez et al. (2020), under scenarios that
consider deforested areas of approximately 28% and 38% of the Amazon
basin, terrestrial sources reduce their annual contributions to northern
South America by an incredible average of 40 and 43%. Likewise, Badger
and Dirmeyer (2016), confirm that the rise of air masses over northern
South America is inhibited with Amazon deforestation, which could also
induce inhibition of precipitation over the region. Similarly, the
teleconnections with the Orinoco basin reveal that the regional regime
of precipitation is highly dependent on a zone in which raising cattle
is one of the main economic sources since this is the predominant
activity in the area, occupying more than 50% of the productive
territory. The expansion of the areas dedicated to this activity is the
main source of deforestation (González-González, Villegas, Clerici, &
Salazar, 2021). Overall, our results indicate the importance of the
hydrological coupling of terrestrial ecosystems in Northern South
America. Particularly in Colombia, rainfed agriculture and hydropower
generation are an important proportion of the nation’s economy.
Overall, our results highlight an advantage of considering stable
isotopes of precipitation over using only numerical modeling, given that
many models can either underestimate or overestimate the amount of
atmospheric moisture (Hoyos, 2017). Additionally, many atmospheric
models have difficulties to represent the topography of the area (Inse,
Poulsen, & Ehlers, 2010). Here, this difficulty is faced by analyzing
moisture transport phenomena with stable isotopes in precipitation,
precisely due to the inverse correlation between the amount of isotopic
composition and altitude (Dansgaard, 1964; Rozanski et al., 1993; Mook,
2002) The simultaneous application of both techniques (physical tracer
and atmospheric modeling) results in a better interpretation of the
transport of atmospheric moisture. The integration of stable isotopes of
precipitation and the spatial-temporal modeling can be an accurate tool
that reduces the uncertainty associated with the understanding of the
climate system and provides information for foreseeing possible changes
in hydroclimatic patterns provenience from sources region (Risi, Bony,
Vimeux, & Jouzel, 2010; Sánchez-Murillo et al., 2013; Hu & Dominguez,
2015).
In summary, most studies have focused on atmospheric moisture from
oceanic sources (Rueda & Poveda, 2006; Sakamoto et al., 2011; Arias et
al., 2015; Yepes, Poveda, Mejía, Moreno, & Rueda, 2019) and although a
few show the importance of terrestrial sources (Kumar et al., 2016;
Hoyos, 2017; Hoyos et al 2018; Molina et al., 2019; Ruiz-Vásquez, Arias,
Martínez, & Espinoza, 2020). Our results illustrate the influence of
moisture recycling in two important areas of Colombia, with the largest
population in the country, and whose economy and ecosystems vitally
depend on water availability for ensuring moisture security. Regional
atmospheric moisture composition largely controls ecosystem structure,
increasing the vulnerability to climate and environmental change in the
study area. Further, these natural systems are directly threatened by
human activities such as deforestation and intensive agriculture,
altering the exchange of water in the land-atmospheric interactions, the
energy balance, evapotranspiration, and therefore, the atmospheric
moisture content and transport.