More trees can improve groundwater (Aida Tobella)
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Day 1 (14.00) SLU (http://events.globallandscapesforum.org/agenda/bonn-2017/day-1/discussion-forums-2-parallel-sessions/landscape-lab-griffith-university-climate-change-response-program/)
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More trees can improve groundwater (Aida Tobella)
- 1. AIDA BARGUÉS TOBELLA Global Landscapes Forum Bonn, Dec. 2017 MORE TREES CAN IMPROVE GROUNDWATER RECHARGE IN THE SEASONALLY DRY TROPICS
- 2. Canopy cover Dominant paradigm in forest hydrology: The trade-off theory Groundwaterrecharge Transpiration and interceptionGroundwater recharge
- 3. Groundwaterrecharge Transpiration and interception Surface runoff Soil evaporation Groundwater rechargeInfiltrability Canopy cover Revised paradigm: The optimum tree cover theory
- 4. Groundwaterrecharge Transpiration and interception Surface runoff Soil evaporation Groundwater rechargeInfiltrability Canopy cover ET exportLocal moisture recycling ET exportLocal moisture recycling ET exportLocal moisture recycling Revised paradigm: The optimum tree cover theory
- 5. Groundwaterrecharge Transpiration and interception Surface runoff Soil evaporation Groundwater rechargeInfiltrability Canopy cover ET exportLocal moisture recycling ET exportLocal moisture recycling ET exportLocal moisture recycling ET import ET import ET import Revised paradigm: The optimum tree cover theory
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- So I’m going to talk a bit about the relationship between tree cover and water yields, and I’ll try to put this in a wide perspective, taking into account precipitation recycling which is the main focus of this discussion forum. OK, so, what do we know about tree cover and water yields, either GW recharge, dry season flow or total annual streamflow? There have been many reviews of paired-catchment studies and other catchemtn experiments looking at the impacts of (de)forestation on water yields, and the general conclusion is that increases in tree cover lead to reduced water yields, while decreases in tree cover result in increased water yields. This is generally attributed to the higher evapotranspiration losses from forests compared to shorter vegetation types such as grass or agricultural crops. On the basis of the available scientific evidence, a trade-off theory, in which more trees mean lower water yields, has become the dominant paradigm in forest hydrology. Based on this paradigm, there have been multiple concerns that the establishment of trees on drylands can put into risk already scarce water resources, which in turn has raised criticism against carbon sequestration programmes or restoration activities that involve establishing trees in these areas. However, the scientific evidence for the trade-off theory has several important limitations. First, there is a strong bias of studies towards humid temperate areas, and there are only few studies in the tropics and almost none in the (semi)arid tropics. Second, most studies looking at the effects of deforestation on water yields involve forest cutting, but not conversion to a post-forest land use such as grazing or cropping, and thus soil distrubance is not taken into account. Similalry, the majority of studies looking at the effects of forestation are on non-degraded soils, and as a result, the observed changes in water yeilds only reflect a change in water use by contrasting vegetation types. Third, most studies ahve focused on the impacts of only few tree species, mainly eucalypts and pines. In the tropics, for instance, there are no studies on the impact of forestation with native tree species. And finally, the lack of studies on the effects of intermediate levels of tree cover. This is surprising given the importance of this type of open vegetation in terms of extent, biodiversity and carbon storage. So what most studies do is to compare closed forest vs. open land (non-degraded grassland), which is interesting to see the effects of (de)forestation, but gives no clue about the impacts of establishing an open tree cover, such as in the case of tree-based mosaic restoration activities. Because of these limitations, it is currently not possible to draw any sound conclusion about the impact of inermediate tree cover on gw rechareg and dry season flows in the tropics in general and in the semi(aird) tropics in particular.
- So, given these limitations we developed and tested an alternative theory, namely that under conditions typical of the seaonally dry tropics groundwater recharge is maximized at an intermediate tree cover. This theory is a bit more complex because it takes into account not only ET flows from trees, but also soil infitlrability, surface runoff and soil evaporation, which are also affected by tree cover. It has been shown that trees can improve soil infiltration and thus reduce surface runoff, and that soil evaporation under trees is reduced. According to this optimum tree cover theory, at tree covers below the optimum more trees result in groundwater recharge as the benefits gained from more trees (i.e through enhanced infiltration, preferential flow and resuced soil evaporation) outweigh the additional transpiration and interceptino losses from these trees. In contrast, above the optimum tree cover, increased transpiration and interception losses dominate any benefits trees might have on groundwater recharge, and so more trees lead to less groundwater recharge. So far there is evidence for this from an agroforestry parkland in semiarid Burkina Faso, but we believe that groundwater recharge will be maximized under an intermediate tree cover over widespread areas in the seasonally dry tropics. This has profound implications for livelihoods and the environment. Tree-based landscape restoration and forestation of degraded lands can, if undertaken and managed properly, increase local water availability. This connects very welll SDGs 6 Clean water and sanitation and 15 life on land.
- I’ll try now to put this in a wider context, and add some unknowns that could potentially shift the optimum when larger scale areas are considered. ET, which is typically regarded as ’water consumption’ (in the demand-side) supplies water to the atmosphere and thus can promote precipitation at local, regional and global scales. Overall, we want the ‘rainfall recycling’ idea to be the main storyline, so it would be nice if you can somehow connect the optimum tree cover theory to that – possibly by embellishing the infiltration diagram with a couple of unknowns that could potentially shift the optimum when larger scale areas are considered: 1. On -site ET, 2. Off-site ET (based on groundwater recharged and/or surface runoff generated), 3. Rainfall triggering by on-site tree cover (non-linearly related to tree cover??), 4. Rainfall triggering by vegetation elsewhere supported by the groundwater recharged and/or surface runoff generated. The net effect will depend on various unknown sensitivities of rainfall to time and place of atmospheric vapour recycling plus responses to biological rainfall triggers. This may sound overly complex, but I hope you can find an easy way to explain it and link with thenpreceding talks…
- Then we can also consider the fact that generated surface runoff and groundwater recharge might support vegetation somewhere else and that, this vegetation, in turn, generates ET that can return into our site in form or ET import. These imports, plus local moisture recycling can increase rainfall. In addition tree cover might induce rainfall throug bilogical Ice-Nucleating particles found in plants and released to the atmosphere. Moderate tree cover might be the best to induce bioprecipitation since there are the necessary conditions for uplift of particles plus a moderate amounf of vegetation and thus of particle abundance. However, it is unclear what the net effect will be, as it depends on various factors which change in time and space. Overall, we want the ‘rainfall recycling’ idea to be the main storyline, so it would be nice if you can somehow connect the optimum tree cover theory to that – possibly by embellishing the infiltration diagram with a couple of unknowns that could potentially shift the optimum when larger scale areas are considered: 1. On -site ET, 2. Off-site ET (based on groundwater recharged and/or surface runoff generated), 3. Rainfall triggering by on-site tree cover (non-linearly related to tree cover??), 4. Rainfall triggering by vegetation elsewhere supported by the groundwater recharged and/or surface runoff generated. The net effect will depend on various unknown sensitivities of rainfall to time and place of atmospheric vapour recycling plus responses to biological rainfall triggers. This may sound overly complex, but I hope you can find an easy way to explain it and link with thenpreceding talks…