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The Paute basin in Ecuador covers some 5900 km2 (according to the count statistic on the basin-masked tree cover map) terminating at the Paute dam which is the largest hydroelectric dam in Ecuador. The basin is outlined here at 1km resolution and here at 1ha resolution. The 1ha resolution simulation cuts small parts of the upper watershed.
WaterWorld v2.91 was applied to examine the hydrological baseline for this area and scenarios for business as usual land use change versus the impact of afforestation and forest protection scenarios that could result from Water Fund investments in the basin.
Baseline
Baseline water balance for the Paute is highly spatially variable, reflecting the spatial variability of rainfall with a mean of 980mm/yr. Fog inputs represent some 12% of the water balance on average but vary from 3% to 34% on a pixel basis. The mean fog contribution is made up of 87 mm/yr of deposition on average and 42 mm/yr of impaction on average.
Business as usual land use change
The following scenario is applied using the EcoEngine land use model and essentially represents a continuation of current rates of deforestation and forest degradation 50 years into the future and allocation of the deforestation along existing and planned roads and according to agricultural suitability. Deforested land is converted to pasture in the scenario. The scenario is described thus:
...and leads to the following change in tree cover:
Figure 1 Tree cover under baseline (left) and scenario conditions (right)
Clearly most of the projected deforestation occurs close to the Paute reservoir. The impacts of this change include reduced evapotranspiration in the deforested area but much larger decreases in fog interception in the deforested area and thus resulting increases in runoff to the reservoir (by 0.85% at the reservoir according to a search for 'Paute Dam' and a map query of the per-cent change in runoff map for the dam reservoir).
We see little impact on net soil erosion at the catchment scale overall. This is because though our scenario led to an increase in catchment mean tree cover from 20% to 24%, tree cover replaces herbaceous cover which decreased from 39 to 37% at the catchment scale (as read from the statistics above the scenario maps). Since herb cover is replaced by tree cover, hillslope net soil erosion changes little since soils remain protected from erosion. The changes in runoff lead to changes in total (hillslopes and channels together) erosion. In some areas we see increases in gross soil erosion , whilst in other areas we see decreases according to the change in runoff and vegetation cover. The lower runoff leads to decreased sediment transportation in the main channels draining into the reservoir and this leads to increased sediment deposition in - and upstream of - the dam. Given the change in the converted areas from mixed forested land to grazing, water quality is reduced in these areas (human footprint increases) which also has implications for the dam.
Afforestation of steep wet slopes
The next scenario applied is one of afforestation of steep (>15 deg.), wet (>1100 mm rainfall) slopes. The following rule-set was applied:
This leads to the change in tree cover shown in Figure 2, below.
Figure 2 Tree cover under baseline (left) and reforestation scenario conditions (right)
Clearly most of the projected afforestation occurs close to the Paute reservoir since this is the area of steep, wet slopes. The impacts of this change include increased evapotranspiration in the afforested area but larger increases in fog interception in the afforested area and thus resulting increases in runoff to the reservoir (by 0.08% at the reservoir according to a search for 'Paute Dam' and a map query of the per-cent change in runoff map for the dam reservoir).
We see little impact on net soil erosion at the catchment scale overall. This is because though our scenario led to a decrease in catchment mean tree cover from 20% to 17%, tree cover was replaced by herbaceous cover which increased from 39 to 43% at the catchment scale (as read from the statistics above the scenario maps). Since tree cover is replaced by a good herbaceous cover, hillslope net soil erosion changes little since soils remain protected from erosion. The changes in runoff lead to changes in total (hillslopes and channels together) erosion. In some areas we see increases in gross soil erosion , whilst in other areas we see decreases according to the change in runoff and vegetation cover. The higher runoff leads to increased sediment transportation in the main channels draining into the reservoir. This leads to increased sediment deposition in the dam. Given the change in land use for the new forested land, water quality is increased in these areas (human footprint increases) which also has implications for the dam.
Optimal afforestation according to RIOS
RIOS was used to generate the optimal afforestation pattern to avoid loss of runoff based on a budget of USD$1.6M for reforestation and using this pattern as a ZOI for an afforestation scenario resulted in the pattern of reforestation shown here. This led to a catchment-wide increase in forest cover from 20% to 21%. This afforestation is targeted to lead to increases in runoff contrary to the decrease in runoff on afforestation to be expected for much of the catchment. It does lead to significant local increases in percentage terms (see here) even generating an increase of 0.09% (0.06 m3/s) of runoff at the reservoir. However this extra runoff means more erosion in the channel and thus soil transportation increases by 220 mm/yr (2200 t/ha) or 26% just upstream of the dam, leading to increases in sediment deposition of around 944 mm/yr (9440 t/ha) or 0.14% at the dam.
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