Trees reduce soil erosion, thus reducing run-off. At the same time, trees incur huge water losses through evapo-transpiration. Evapotranspiration losses through forest are nearly 16 times more than barren land. Are tree plantation / afforestation good strategies for water conservation and drought mitigation?
Well,
Certainly, it conserves water and reduces the effect of droughts in drainage basins, provinces and continents, and has a greater impact on the water balance of the Earth throughout the movement of water naturally within the hydrological cycle.
Sincerely
There is a substantial body of research on water yield from experimental catchments and watersheds. On study in NC, USA mountains, moderately high rainfall, estimates of ET from pine, hardwood and grass cover was approximately 30, 20 and 10 inches annually on average. Forests have higher ET rates, but also tend to have high infiltration rates, no to limited surface runoff, and ability to provide a normal extent and duration of baseflow. However, many converted forests to grasslands do have greater water yield, but also may have been overgrazed, severely eroded, infiltration impaired, rapid runoff losses and unable to maintain baseflow. Some of this erosion has led to desertification, with sparse cover and most soil productivity and organics lost. The resultant streams are often ephemeral gullies, having lost most of their capacity to maintain extended to perennial flow. About 20+ years ago at a watershed conference, researcher Moshe Cohen discussed how they took some of this dry desert land (historically forest, cedars I think) and scooped holes in landscape and planted tree in each scooped hole. The dry ephemeral high erosion channels eventually returned back to perennial streams with lowered sediment loading. If water yield and quality are of great concern, consider native cover with deep roots, soil maintaining or building capacity, and with ability to weather the normal disturbances and variation in rainfall and climate, insects and disease common to area. When native cover types are too difficult to restore heavily effected and damaged, eroded areas, avoid using species that are invasive. Special measures are sometimes necessary (ripping, fertilization, mulch, terracing), as for example some of the eroded, gullied and barren lands in SC, USA that we treated. Restoration of barren lands in arid conditions is perhaps the greatest challenge. The body of literature is very suggestive of the benefits of forest landscapes, but also that some forest types such as some pine species do use more water in transpiration than other forest species. Planting a forest or other species with high water demand, or inability to effectively regulate water loss through stomates is likely to reduce water yield. Consultation with specialists in soils, botany, forestry and hydrology can help develop recovery plans or strategies to fit the circumstance.
You can see this article
https://www.researchgate.net/publication/310458060_IMPORTANCE_OF_FOREST_AND_TREES_IN_SUSTAINING_WATER_SUPPLY_AND_RAINFALL
As mentioned in the answers above, overall, planting trees is a good but complex strategy for water conservation and drought mitigation. As all strategies, it must be carefully planned and executed. For example, choosing an inappropriate species might cause a great loss of time, money, and efforts if these species are unable to adapt to current site condtions. In addition, it can cause collateral problems such as invasions that can alter original ecosystems, or agravatte the problem if the planting is executed without care (e.g. more erosion). Continous monitoring is essential for assessment of beneficial or negative effects.
Regards
To add to the detailed answers above, it is important to consider what was the native vegetation in the area. If it was a natural grassland, any tree planting can lead to higher ET along with lower water tables and springs drying up. If it was a forest, or a forest savannah, planting fast-growing exotic species can lead to similar situations. Only by restoring the native vegetation can streamflow approach the pre-disturbance flow regimes. Add to that wetland restoration, if any wetlands were present.
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