Human impacts on the hydrology of the páramos

Since the 20th century, the páramos have been facing unprecedented anthropogenic pressures (Molina et al., 2015; Roa-García et al., 2011; White, 2013). There is increasing evidence that anthropogenic activities such as intensive and extensive livestock (Molina et al., 2007), cultivation and land management practices affect negatively the páramos’ local biodiversity, their functional capacity (Erwin, 2009) and total water yield (Buytaert et al., 2007a). Scientific and public awareness raised concerning the lack of knowledge about the potential impact of rural development and ecosystem degradation on the hydrology of páramos (Célleri and Feyen, 2009). Research focused in anthropogenic intervention impacts on hydrological services of páramos (Buytaert et al., 2007b), explored the shift from endemic land cover to cultivation lands, livestock and forest plantations, anthropic introduction of fire and degradation (Poulenard et al., 2001). Cultivation tends to reduce the catchment regulation capacity (⁓40% reduction) (Célleri and Feyen, 2009; Ochoa-Tocachi et al., 2016a) increases peak discharge (⁓20%), reduces base flow (⁓50%) (Buytaert et al., 2006a, 2007a) and reduces the soil storage capacity (up to 26% reduction) (Sarmiento, 2000). Cultivation also reduces the soil field capacity (from 100% to 83%) and wilting point (from 83% to 63%) (Díaz and Paz, 2002), and the increase of evapotranspiration rates (up to 66%) (Sarmiento, 2000). Livestock density tends to increase, with often negative impacts on the soil structure. Díaz & Paz, (2002) report an increase of the soil bulk density up to 0.2 g cm3 under extensive and 0.7 g cm3 under intensive conditions compared to undisturbed páramo soils in Popayán Colombia. Water yield reduction as a result of increasing evaporation is typically less than 15% (Crespo et al., 2010), but this tends to be accompanied by an increase in streamflow flashiness and a decrease in hydrological regulation capacity (Ochoa-Tocachi et al., 2016a, 2016a). The introduction of exotic species as a forestation strategy has been a common practice in the Andean páramos. Pine trees in particular have been used as a strategy to improve the inhabitants’ income (Farley et al., 2004) but may affect the hydrological response of páramo ecosystems. Pairwise catchment experiments in southern Ecuador showed that base flow reduced up to 66% (Buytaert et al., 2007b; Ochoa-Tocachi et al., 2016a), water yield decreases between 42% to 50% (Crespo et al., 2010; Ochoa-Tocachi et al., 2016a) as consequence of interception in the canopy and higher evapotranspiration. Burning, another common practice increases the soil erosion, the amount of runoff, reduces the rainfall-runoff time response (Molina et al., 2007), as well as the saturated soil hydraulic conductivity (Poulenard et al., 2001). Drying and hydrophobicity up to 40% have been reported due the direct exposure of dark soils to sunshine (Buytaert et al., 2002). However, a recent study shows that programs aimed at the conservation of hydrological services and soil maintenance do not necessarily need to exclude burning to assure adequate long-term vegetation cover in disturbed páramos (Bremer et al., 2019).

Climate change impacts on páramos

Climate change is very likely to have an impact on the páramos and their ecosystem services. Global warming effects on temperature are generally well quantified with agreement amongst models on the direction of change. This is due to both a higher air moisture content resulting from increased evapotranspiration and the intensification of the Hadley circulation in tropical regions. Both processes are expected to lead to increased temperatures at high altitude (Bradley et al., 2009). However, precipitation and subsequent discharge variations are much more variable with differences up to 50% between various Intergovernmental Panel on Climate Change (IPCC) models from the currently observed values in the Andes (Buytaert and De Bièvre, 2012). For instance, González-Zeas et al. (2019) compared results from a Regional Climate Model (RCM) with observed data in a catchment that provides 30% of the water needs of Quito, the capital of Ecuador. They found considerable disagreement between both data sources with an over prediction of precipitation by the RCM. In addition, major spatial differences exist, in spite of all the uncertainties across climate projections. Whereas part of the Bolivian Altiplano is likely to experience a reduction of 10% precipitation, areas of the Peruvian Andes will potentially be subjected to increases up to 60% (Buytaert and De Bièvre, 2012). Exacerbating the underlying climate is the El Niño Southern Oscillation (ENSO), an imbalance of sea-surface temperatures (SST) and ensuing air pressure in the tropical Pacific (NOAA, 2019). This phenomenon has severe impacts over global and, Andean and Central America weather. El Niño events, where SST anomalies are positive, lead to increased drought risks across the Bolivian Altiplano combined with intense precipitation over northern Peru and Ecuador. Conversely, during a La Niña, where SST anomalies are negative, an intensification of precipitation is observed causing major flooding in certain areas of the tropical Andes (Zolá and Bengtsson, 2006). (Mendoza et al., 2019) proposed intra- and inter-annual scale climate teleconnections with Trans Niño Index (TNI), North Athlantic Oscillation (NAO) and the Caribbean Index (CAR). Additionally, the altitudinal factor plays an important role, when the westerly winds dominate, dry conditions occur especially at high altitude, while the opposite when the easterly winds dominate. In this case all the moisture from the eastern Pacific Ocean is transferred to the mountain range. ENSO occurs naturally and is not a consequence of climate change. However, the intensity and duration of ENSO events has increased substantially in the last decades, indicating a possible link to anthropogenic-induced climatic changes that occurred in the same period. These changes will propagate through the terrestrial water cycle, thus affecting directly its hydrology. Studies have quantified these changes, Wouter Buytaert & De Bièvre (2012) showed that most of the northern Andean region is expected to experience an increase in precipitation. However, the increase in evapotranspiration as a result of warming is likely to compensate the increase in precipitation, resulting in a net reduction in effective precipitation and water availability. Climatic changes will affect ecosystems more broadly through changes in vegetation and soils, which may propagate to the water cycle. In mountain regions, climate change is likely to induce shifts in the distribution of ecosystems and biomes. Logistic regressions coupled with Global climate models (GCM) and Climate Networks (CN) have been used to simulate climate patterns in future scenarios and to estimate the extent of such future changes for the tropical Andes ((Tovar et al., 2013a; Vázquez‐Patiño et al., 2020). Tovar, Arnillas, et al. (2013) estimated that the páramos may lose 30% of their current areas due to a lack of room for upslope migration, necessary in the face of warmer temperatures at higher elevations. Additionally, species distribution is also at risk (Ramirez-Villegas et al., 2014). More than 50% of species considered could experience reductions in quantities up to 45% with 10% potentially extinct. However, despite these advances, estimating changes in mountainous areas remains inherently difficult, making the downscaling of global or regional coarse resolution models highly uncertain as the highly variable topography results in steep and sudden changes in local weather patterns which are difficult to represent in available GCMs (Buytaert et al., 2010). Moreover, simplifications of climate processes and errors in input data exacerbate the problem. As we subsequently discuss, new tools such as citizen science and participatory monitoring, provided agreed-upon data quality and reliability standards, can help bridge some of those hydrometeorological information gaps. In the midst of a highly unpredictable future, water and land decision-makers will have to resort to various methods of incorporating uncertainty in increasing resilience in the face of climate shocks, from scenario analysis in designing infrastructure to implementing measures to control growth on the demand side.