How does climate forcing impact flow regulation services?


In order to determine the relative impact of climate forcing on water flow regulation in two watersheds (2 and 6) at HBEF, we used logistic regression to determine the impact of antecedent precipitation (48 hrs) on the likelihood that the flood regulation service will be provided (in other words, that a flood WILL NOT occur). Similarly, we analyzed the effects of moisture availability on the likelihood that the drought mitigation service will occur. We calculated moisture availability as the difference of potential evapotranspiration (PET; Thornthwaite 1948) and precipitation during the antecedent month. In order to calculate Thornthwaite PET, we used weather data from the HBEF archive. In both cases, the dataset was constrained to the spring months (April-October) and to the 10 years after harvest in watershed 2 (1965-1975).

In both watersheds, as precipitation in the antecedent 48 hours increases the likelihood that floods will be avoided decreases. At low levels of precipitation (less than 50mm), the likelihood of the service being provided is close to 100%. As precipitation approaches 200 mm, the likelihood that the service will be provided is negligible. At levels of precipitation between these extremes, the odds that floods will be regulated are higher in watershed 6 than in watershed 2. This is likely due to the loss of storage and ET associated with vegetation, and is an indicator of the service provided by the standing forest. On the other hand, the de-vegetated watershed is more likely to effectively mitigate droughts than the reference watershed under conditions of moisture deficit (antecedent moisture availability less than 0 mm) for the same reason – reduced storage and ET. At a moisture availability of -100 mm, for example, watershed 6 has a 35.0% chance that the drought mitigation service will occur whereas watershed 2 has a 65.4% chance. At available moisture levels above ~60 mm, both watersheds are almost 100% likely to mitigate droughts.


In order to determine the relative impact of climate forcing on water flow regulation in two watersheds (5 and 6) at HBEF, we used logistic regression to determine the impact of antecedent precipitation (48 hrs) on the likelihood that the flood regulation service will be provided (in other words, that a flood WILL NOT occur). Similarly, we analyzed the effects of moisture availability on the likelihood that drought mitigation services will be intact. We calculated moisture availability as the difference of potential evapotranspiration (PET; Thornthwaite 1948) and precipitation during the antecedent month. In order to calculate Thornthwaite PET, we used weather data from the HBEF archive. In both cases, the dataset was constrained to the spring months (April-October) and to the 10 years after harvest in watershed 5 (1983-1993).

In both watersheds, as precipitation in the antecedent 48 hours increased the likelihood of sufficient flood regulation decreased. At low levels of precipitation (less than 50 mm), the likelihood that the service will be provided is close to 100%. As precipitation exceeds 147 mm, the odds that flood regulation will occur become negligible. At levels of precipitation between these extremes, the odds of a flood in both watersheds are very similar. One would imagine, given the reduction in standing biomass in watershed 5, that ET would be lower and, consequently, likelihood of service provision would be lower in this watershed than in the reference watershed. This is not true; the odds that flood regulation will occur are slightly lower in the reference watershed at these intermediary levels of climate forcing. A more intuitive picture emerges, however, when looking at the graph of DM likelihood. At a moisture availability of -100 mm, for example, watershed 5 has a 40.0% chance of that the service will occur whereas watershed 6 has only a 19.7% chance. At available moisture levels above ~60 mm, both watersheds are almost certain to be free from droughts.


In order to determine the relative impact of climate forcing on water flow regulation in the catchments at TLW, we used logistic regression to determine the impact of antecedent precipitation (48 hrs) on the likelihood that flood regulation would occur (in other words, that floods would NOT OCCUR). Similarly, we analyzed the effects of moisture availability on the likelihood of the provision of the drought mitigation service. We calculated moisture availability as the difference of potential evapotranspiration (PET; Thornthwaite 1948) and precipitation during the antecedent month. In order to calculate Thornthwaite PET, we used weather data from the HBEF archive. In both cases, the dataset was constrained to the spring months (April-October) and to the 10 years after harvest (1997-2007).

In all catchments, as precipitation in the antecedent 48 hours increases the likelihood of flood regulation decreases. Al low levels of precipitation (less than ~40mm), the likelihood of service provision is close to 100%. As precipitation approaches 200 mm, the odds of service provision drop to less than 16.0%. At intermediate levels of precipitation, the odds of service provision are lowest in the clearcut catchment (31), followed by the catchments cut using the selection harvest (33) and shelterwood harvest (34) and finally by the reference catchment (32). This relationship can be explained by the relative rate of biomass removal; as more biomass is removed during harvest, ET is further reduced, runoff increases, and the odds that flood regulation services will be provided decrease. The reverse relationship holds true with droughts. At low levels of available moisture (-100 mm), the odds that the drought mitigation service will be provided are 59.8% in the reference catchment and 88.3% in catchment 31, the clearcut catchment. The catchments that underwent intermediate levels of harvest diverged significantly, however. Catchment 33, cut using the selection system (29% reduction in basal area), was very similar to the reference catchment in its propensity for droughts. In fact, the odds of that DM services would be provided in this catchment were slightly lower than in the reference catchment at all levels of available moisture. On the other hand, the odds of service provision in catchment 34 (42% reduction in basal area), cut using a shelterwood system, were very similar to those in the clearcut catchment. This suggests that somewhere above a 30% reduction in basal area, a threshold may exist above which harvesting has a disproportionately greater effect on the odds that this service will be provided to beneficiaries. At levels of available moisture above 200 mm, the odds of the service were nearly 100% in all catchments.