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Impacts of land use change and mining on water quality and quantity to the communities of the Alto Putumayo

WaterWorld v2.90 was used to examine the baseline hydrology and impacts of change for the Alto Putumayo, bordering Colombia and Ecuador.

Baseline hydrology
Alto Putumayo is a hydrologically rich area with a mean water balance of 2600 mm, largely produced in the Amazon lowlands and Andean foothills (Figure 1, left).  The region zero human footprint on water quality over 19% of the area and a mean footprint of only 4.8% over the remaining 81%, largely from agriculture in the Andes, mining and oil and gas (Figure 1, right).  Baseline water resource stress is 0 over 94% of the catchment and only 8.7% on average over the remaining 6% of area (largely in Ecuador).  The vast majority of the population have access to high quality drinking water (<50% human footprint).

 
 
Figure 1 Baseline water balance (left) and human footprint on water quality (right)

Impacts of land use change associated with continued agricultural development and the IIRSA roads project
A scenario was applied in which recent rates of deforestation from terra-i were applied for a further 50 years with the existing cover converted to a cropland use with a dominantly herbaceous cover (10% tree, 80% herb, 10% bare).  The allocation of deforestation was relative to the existing roads network but also planned roads as part of the IIRSA project.  The resulting change in forest cover was -14% at the national scale (icnluding all planned roads and the recent rates of deforestation) and -16% within the basin of the upper Putumayo changing forest cover from 60% to 44% as shown in Figure 2.
 
 

Figure 2 Deforestation scenario for the Alto Putumayo.  The baseline (2010) forest cover in on the left, the scenario (2060) cover is on the right.  The blue areas on the right are rhose newly deforested.

The deforestation leads to increases in water balance (figure 3, left) over 19% of the area, decreases over 11% and no change over 70% of the area.  The decreases are largely at higher elevations where loss of cloud forests leads to lower water inputs and the increases are at lower elevations where tree cover loss reduced evapotranspiration.

As a result this deforestation leads to no change in water resource stress (% of demand not supplied) over 99% of the area (which has hydrologically insignificant water resource demands  on the basis of low population) but a decrease of 8% in water resource stress  in the remaining 1% of land (largely in the more densely populated oil areas of N Ecuador, see here).

An overall increase in the human footprint on water quality index of 22% is observed (see figure 3, right), leading to 20,000 more people in the basin with poor quality water (human footprint >50%) , especially near the border with Narino.

 
 
Figure 3 Change in water balance (left) and human footprint on water quality (right)

Impacts of increased mining

A mining scenario was developed assuming that 10% of the current mining concessions would be developed and converted to opencast mines.  The resulting scenario led to an increase in area under mines of 5270 from 830 to 6100 km2 at the national scale and 0 to 62 km2 within the alto Putumayo basin.  The resulting distribution of mines is shown in Figure 4 (left).  The resulting removal of vegetation leads to an increase in water balance of 0.31 mm/yr over the catchment, combining increases in water balance of 130 mm/yr over 0.3% of the basin and decreases of -79 mm/yr over 0.06%.  Tree loss associated with the mines leads to decreases in water balance in the cloud forest belt and increases in the lowlands but at the catchment scale the effects are low because the .areas mined are relatively small and thus the impact on water resource stress is miniscule at the catchment scale.  The human footprint on water quality at the national scale does not change over 98% of the area but over the remaining 2% at or downstream of new mines decreases by 14% on average.  At the national scale the average increase in human footprint is thus 0.28%.  At the alto Putomayo scale 97% of the area is unaffected but the human footprint increases by 9.3% in the remaining 3% of areas leading to a catchment wide deterioration of water quality of 0.27%, though the effects is focused on the areas immediately downstream of mines (Figure 4, right).  This leads to an increase in the number of people with poor quality water  over 0.2% of the area, affecting <100 people.