Water availability – both now and in the future – is of the utmost importance.

However, the role of forests, their impact on precipitation, water yield and the hydrologic cycle more generally remain hotly contested. Afforestation strategies to ameliorate dry season flows have come under increasing scrutiny and attack.

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.

Tree type can also impact the local water balance. Eucalyptus trees in particular have been criticized in the more arid regions of Africa for their heavy drain on available water supply. Deciduous tree species typically generate less ET than evergreen, potentially mediating forest impact on the local water balance. Overall, however, regardless of the tree species, the effect on the water balance is typically assumed to be negative.

Forests also have the ability to passively ‘attract’ or ‘catch’ atmospheric moisture, in particular, humidity in fog and clouds. Condensation on plant surfaces provides additional moisture for tree growth. Estimates suggest that anywhere from 200 mm year−1 to 425 mm year−1 additional moisture can be obtained through fog condensation. Some also find a positive correlation between the surface roughness of forests (varied age class) and drag coefficients, a precursor for potential precipitation. Equally important, a share of the moisture captured from the atmosphere in this way is again returned to the atmosphere as ET and remains available for vegetation growth in other areas.

Changing land-use patterns, both in the developing and in the developed world, should be subject to increased scrutiny. Progressive deforestation and wetland destruction have direct implications for the global hydrologic cycle. In this regard, maintaining and/or significantly increasing current levels of forest cover seems advisable. Deforestation, due to its impact on the ET regime, on soil degradation and loss, and reduced soil water retention represents a significant threat to planetary survival. Increasing forest and wetland cover is likely to have beneficial feedbacks on regional water budgets. Thus, along with accepted forest ecosystem functions (such as carbon sequestration, climate mitigation, biodiversity preservation and fossil fuel substitution through bioenergy), forests play an important role in helping manage the world's water regime.

Finally, Forests … play a major role in the atmospheric circulation and the water cycle on land and may have a role in mitigating regional climate, desertification and water security problems.

Water availability – both now and in the future – is of the utmost importance.

However, the role of forests, their impact on precipitation, water yield and the hydrologic cycle more generally remain hotly contested. Afforestation strategies to ameliorate dry season flows have come under increasing scrutiny and attack.

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.

Tree type can also impact the local water balance. Eucalyptus trees in particular have been criticized in the more arid regions of Africa for their heavy drain on available water supply. Deciduous tree species typically generate less ET than evergreen, potentially mediating forest impact on the local water balance. Overall, however, regardless of the tree species, the effect on the water balance is typically assumed to be negative.

Forests also have the ability to passively ‘attract’ or ‘catch’ atmospheric moisture, in particular, humidity in fog and clouds. Condensation on plant surfaces provides additional moisture for tree growth. Estimates suggest that anywhere from 200 mm year−1 to 425 mm year−1 additional moisture can be obtained through fog condensation. Some also find a positive correlation between the surface roughness of forests (varied age class) and drag coefficients, a precursor for potential precipitation. Equally important, a share of the moisture captured from the atmosphere in this way is again returned to the atmosphere as ET and remains available for vegetation growth in other areas.

Changing land-use patterns, both in the developing and in the developed world, should be subject to increased scrutiny. Progressive deforestation and wetland destruction have direct implications for the global hydrologic cycle. In this regard, maintaining and/or significantly increasing current levels of forest cover seems advisable. Deforestation, due to its impact on the ET regime, on soil degradation and loss, and reduced soil water retention represents a significant threat to planetary survival. Increasing forest and wetland cover is likely to have beneficial feedbacks on regional water budgets. Thus, along with accepted forest ecosystem functions (such as carbon sequestration, climate mitigation, biodiversity preservation and fossil fuel substitution through bioenergy), forests play an important role in helping manage the world's water regime.

Finally, Forests … play a major role in the atmospheric circulation and the water cycle on land and may have a role in mitigating regional climate, desertification and water security problems.

Dear Anisa,

The forest canopy controls the moisture rate returned to the atmosphere via evapotranspiration. So more moisture is exchanged more water will be available. This fact can effect water retention and the forest ecosystems structures.

The effect of a forest on the water cycle change temporarily and spatially depending on the existing ecological units and its density.

Kindly find attached an interesting research in the link below: https://eos.org/research-spotlights/how-forest-structure-influences-the-water-cycle

Greeting.

Ghada.

I am not going to repeat the excellent points already made. It is true, there is some variability of opinion on the effects of forests. Generally speaking, forests do have higher ET than other land types or covers. But forests improve soil organics, macropores, ability to take up and store water, augment groundwater supplies and as a result flood waters and sediment loads are often reduced. Water quality is generally substantially better from forest ecosystems. The availability of water however may also depend on not only ground cover, but climate, rainfall, geology, soils, land uses, past hydrologic modifications, water uses including instream and water withdrawals from wells. Areas with history of extensive rilling and gullying tend to hasten runoff and deplete groundwater levels, increasing also sediment loading, but down stream, areas with lower gradient experience aggravation, braided streams, increased water tables and flooding frequency. So there can be spatial differences.

In a few instances, I became aware of anomolies of flow disappearance in the Suwannee River (Ga, Fl) and Stevens Creek (SC, USA) in the USGS flow records between stations. Geologic materials such as limestone and faults such as shear lineament were the likely explanations.

Some geoclimactic zones support primarily loosing streams, as the water table is seldom elevated enough to maintain flow. Under this condition, attempting afforestation might be difficult. Native grasses may be another approach to consider to gain land cover, they are typically used to the native environmental conditions and once established (which may be difficult), offer quality soil cover, organic building with lower ET demand. Some native plant and tree types are probably going to be too difficult to establish, and sometimes easier to grow non-invasive annual grasses might be provide soil cover until perennial grasses or trees can be established. In some instances, efforts to provide or specifically select soil depressions or terracing has been used to capture and collect rainfall or get closer to or increase duration of roots in moist soil.

à l'échelle du bassin versant, la forêt a un effet sur le bilan hydrique ou disponibilité de l'eau important (ça dépend de plusieurs paramètres étendu, taille, espèce.. il y aura plus de précipitations et moins de perte d'eau de ruissellement) mais aussi un effet sur la répartition de la ressource (l'eau sera retenue en partie sur l'amont du bassin).

Des outils de simulation peuvent vous apporter des éléments de réponse

Through their impact on precipitation

Although the Ganges is largely snow fed, forest cover helps precipitation and reduces/controls erosion.