Martyn N. Futter

On the forest cover–water yield debate: from demand‐ to supply‐side thinking

Several major articles from the past decade and beyond conclude the impact of reforestation or afforestation on water yield is negative: additional forest cover will reduce and removing forests will raise downstream water availability. A second group of authors argue the opposite: planting additional forests should raise downstream water availability and intensify the hydrologic cycle. Obtaining supporting evidence for this second group of authors has been more difficult due to the larger scales at which the positive effects of forests on the water cycle may be seen. We argue that forest cover is inextricably linked to precipitation. Forest-driven evapotranspiration removed from a particular catchment contributes to the availability of atmospheric moisture vapor and its cross-continental transport, raising the likelihood of precipitation events and increasing water yield, in particular in continental interiors more distant from oceans. Seasonal relationships heighten the importance of this phenomenon. We review the arguments from different scales and perspectives. This clarifies the generally beneficial relationship between forest cover and the intensity of the hydrologic cycle. While evidence supports both sides of the argument – trees can reduce runoff at the small catchment scale – at larger scales, trees are more clearly linked to increased precipitation and water availability. Progressive deforestation, land conversion from forest to agriculture and urbanization have potentially negative consequences for global precipitation, prompting us to think of forest ecosystems as global public goods. Policy-making attempts to measure product water footprints, estimate the value of ecosystem services, promote afforestation, develop drought mitigation strategies and otherwise manage land use must consider the linkage of forests to the supply of precipitation.

THE EFFECT OF EL NIÑO-RELATED DROUGHT ON THE RECOVERY OF ACIDIFIED LAKES

AbstractAlthough SO2 emissions and deposition rates havedeclined substantially since the implementation of sulphuremission control programmes in North America [1], recovery(measured as decreases in

Riparian soil temperature modification of the relationship between flow and dissolved organic carbon concentration in a boreal stream

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Patterns and trends in Southern Ontario lake ice phenology.

concentrations) of affected lakes in central Ontario has been much less substantial thananticipated based on the decrease in deposition. The slowrecovery is attributed to the reoxidation and release of storedsulphur in catchments. Reduced sulphur retained in previousyears when sulphur deposition was higher is exposed to air andoxidized during severe droughts, then exported duringsubsequent wet periods. Elevated stream

Total Phosphorus Budgets and Nitrogen Loads: Lake Simcoe, Ontario (1990 to 1998)

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Long-term trends in water chemistry of acid-sensitive Swedish lakes show slow recovery from historic acidification

concentrations and export rates occur in the autumns of yearswith prolonged severe droughts, particularly in catchments withextensive wetlands. Drought in our study catchments occurred inyears following strong El Niño events. When the SouthernOscillation Index (SOI) was strongly negative (1976–77, 1982–83,1986–87, 1991–92, 1993–94) the frequency of occurrence ofdrought the following summer in small catchments with shallowoverburden was extremely high. A lake‘s rate of recovery fromacidification depends upon the amount of excess reduced Sthat has been stored in anoxic zones in the catchment (largely afunction of the extent of wetlands) during years of elevated Sdeposition rates, and the frequency and severity of droughts. Iflong-term changes in global or regional climate alter thefrequency or magnitude of El Niño-related droughts, therecovery of acidified lakes will be affected.