Knowledge of water transit times through watersheds is fundamental to understand hydrological and biogeochemical processes. However, its prediction is still elusive, particularly in mountainous terrain where physiography and precipitation change over short distances. In addition, much remains to be studied about the impact of climate change on transit time as it continues to change precipitation form in mountainous terrain. Water isotopic ratios were used to evaluate mean transit time (MTT) and young water fractions ( ) in seven small mountainous watersheds in western Oregon over the 2014–2018 period that included a major regional snow drought in 2015. The MTT was shorter in 2015 across all watersheds compared to any other year while the  was larger in 2015 than in any other year. The short transit times observed in 2015 could be related to low connectivity between surface water and older ground water which resulted in a homogenous hydrologic response across all the investigated watersheds despite their physiographical differences. The 2016–2018 MTT vary widely across all watersheds but especially within the smaller high elevation watersheds indicating that the impact of the 2015 snow drought was stronger for systems that depend heavily on snowmelt inputs. During relatively wet/cold years intrinsic watershed characteristics such as drainage area and terrain roughness explained some of the variability in transit time metrics across all watersheds. Shorter transit times during the drought have implications for water quality and solute concentrations as biogeochemical processes are controlled in part by the time water resides and interacts within the subsurface. Although the impact of the 2015 snow drought appears short lived these results are particularly critical considering the expected regional snowpack decline as the climate warms in the western USA.

How to cite: Segura, C.: Snow drought reduces water transit times in headwater streams, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10842, https://doi.org/10.5194/egusphere-egu22-10842, 2022.

Forests are important to regulate water-climate relationships, providing important ecosystem services locally and elsewhere. Therefore, understanding forest hydrology is crucial to understand the flows of these ecosystem services, and attribute the origins to either transpiration and interception as these can have very different underlying mechanisms. Atmospheric moisture recycling effectively increases the amount of usable water over land as the water can undergo multiple precipitation–evapotranspiration cycles. Forest contribution to atmospheric moisture recycling can come from water pressure deficit driven water transpiration through the stomata, or via evaporation of surface water intercepted in the canopy during precipitation. Disentangling these two pathways is fundamental as the former is dependent on the ability of the deep roots of trees to access groundwater facilitating a constant transpiration flux throughout the dry season, while the latter is fundamentally dependent on precipitation and canopy architecture and leaf morphology. We have demonstrated that forests can buffer precipitation variability elsewhere, for tropical and other types of forests. However, it is not known whether this buffering effect occurs directly through forest transpiration or whether indirect forest interception evaporation has a buffering effect as well. Here we apply a state-of-the-art Lagrangian moisture tracking model (UTrack) to study globally whether forests in the upwind precipitationshed can lead to a reduction in monthly precipitation variability downwind. Indeed, we found that forests are fundamental to reduce precipitation variability downwind in 10 out of 14 global terrestrial biomes, specifically for all forest biomes except Mediterranean forests. On average, if 50% of precipitation originates from forest, there is a strong buffering effect with an average reduction of 60% in the coefficient of variation of monthly precipitation. We also observed that a high fraction of precipitation from non-forest land sources has the opposite effect, that is, no buffering effect. The average variation of monthly precipitation was 69% higher in areas where 50% of precipitation originates from non-forest land sources in the precipitationshed. We also observed that the role of forest interception evaporation is less important than the role of forest transpiration for buffering precipitation variability. The largest buffering effect was found for the tropical forest biomes, mainly Amazon and Congo, while moisture recycling over Southeast Asia was mostly contributed by the surrounding ocean. For temperate biomes, the buffering capacity of forests is lower, related to shallower rooting depths and that large proportions of temperate forests are in areas dominated by precipitation from non-forested land or ocean, such as western Europe. Nevertheless, there is still a significant role in buffering precipitation and potentially this buffering capacity can be increased with large scale reforestation projects to mitigate climate change. Our findings clearly support an important role of forests in buffering precipitation downwind. Forests hereby regulate the climate system, which can become unbalanced if this regulating ecosystem service is removed. Furthermore, the importance of this mechanism is also relevant to maintain other processes, such as food production and highlights the tight connections between forests and other processes and ecosystem services

How to cite: Dekker, S. C., O'connor, J. C., Staal, A., Tuinenburg, O. A., Rebel, K. T., and Santos, M. J.: How forests transpiration and interception evaporation can buffer variations in precipitation downwind, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2794, https://doi.org/10.5194/egusphere-egu22-2794, 2022.

Forest canopies redistribute precipitation by processes of interception and stemflow which also change the chemical signature of incoming precipitation. Understanding what controls these transformations and how they evolve across seasons is key to assess forest water cycling and nutrient transport. At the Waldlabor Zurich ecohydrological observatory (Switzerland) we measure the amount and chemical signature of precipitation since April 2020. Our measurement setup focusses on spruce (Picea abies) and beech (Fagus silvatica) trees, as they are the two most common species across Switzerland. In addition to the ion and isotope concentrations in precipitation, throughfall, stemflow as well as in the soil at different depths (10, 20, 40 and 80 cm), we also assess the canopy density in weekly resolution, groundwater depth and streamflow amount at the outlet of our forested catchment, as well as their chemical and isotopic composition

We assessed the seasonal variability of throughfall and stemflow and their relation to canopy density measurements for beech, spruce and young spruce trees. Canopy density had little to no effect on interception and stemflow fractions. We found almost half of the total annual precipitation is intercepted in the canopies of spruce and beech trees, this is because most precipitation events were quite small, resulting in almost no throughfall at all. However, in general, the fraction of interception decreased with increasing event size, on the other hand events below 4 mm did not produce significant amounts of throughfall and stemflow. Water chemistry is showing that major enrichment processes took place in the canopy, subsequently the ion signature was different in throughfall and stemflow compared to open field precipitation. Ion concentrations of sodium, chloride, nitrate, ammonium and potassium were 2 - 10 times higher in throughfall and up to 14 times higher in stemflow compared to concentrations measured in open field precipitation. We hypothesize this is the result of accumulation of wind deposits, especially of anthropogenic contaminants on the tree stem, branches and leaves, as we found concentrations where generally higher during events succeeding long periods without precipitation. In accordance with the much rougher surface of spruce needles and stems compared to beech leaves and stems, we found much higher concentrations in throughfall and stemflow below spruce trees and elevated ion concentrations in the soil waters up to 40 cm depth. Overall, our results highlight the importance of forests, not only in redistributing precipitation, but also in changing the chemical signal of precipitation and thus the forest water and nutrient cycle.

How to cite: Grundmann, M., Floriancic, M. G., and Molnar, P.: Enrichment Processes in Throughfall and Stemflow in a Mixed Temperate Forest, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8574, https://doi.org/10.5194/egusphere-egu22-8574, 2022.

Subalpine forests in the Alps are fragile ecosystems with high importance for human water resources and the local and mesoscale climate. While previous studies have measured different components of the water balance, little is known about the partition of all the water components during the same period and the role of forest age. We were able to measure the water balance components and will show how water distributed in the forest. Some highlights of our study are the evidences that canopies of old forests were able to intercept water in larger extend than the young forest canopies, representing the largest water reservoir, the frequency of fog during the study period, shown for the first-time in this ecosystem, the larger ratio of throughfall related to precipitation during days with mixed precipitation (fog and rainfall) and the role of epiphytes in the water balance.

How to cite: Garcia-Santos, G., Obojes, N., and Montagnani, L.: New insights in the understanding of the water balance in a subalpine forest in the Alps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12591, https://doi.org/10.5194/egusphere-egu22-12591, 2022.

Afforestation is recognized as one of the commonly used nature-based solutions for flood risk mitigation as well as for reduction of anthropogenic impacts on sediment and nutrient flushing. Processes of nutrient cycles (e.g., nitrogen cycle) are directly dependent on the amount of precipitation and its temporal and spatial distribution as water is the main transport medium and the driving force for many ecosystem processes. To understand the natural background of biogeochemical processes and transport of their products, it is therefore necessary to improve understanding of the hydrological control mechanisms, especially the formation of rainfall-runoff. Forest ecosystems without any or with negligible anthropogenic influences represent such kind of a reference, i.e. natural state in the field of nutrient flushing research. For this reason, we established an experimental monitoring system in the small, almost completely afforested Kuzlovec river catchment, Slovenia. Most of the hydrometeorological variables were measured continuously at a 20-min time step accompanied with measurements of concentrations of nitrate-nitrogen in the stream and leaf area index, which was used as an indicator of seasonal growth periods. Based on a two-year data set of various variables, we identified rainfall events for which we investigated: i) the influences of rainfall characteristics on the nitrate-nitrogen flushing, ii) the rainfall-runoff formation processes taking into account nitrate-nitrogen concentration changes during rainfall events, and iii) the role of the forest on the dynamics of the nitrate-nitrogen flushing (i.e., are there differences between the seasons). The focus of this contribution will be on the latter. We will present the most important findings obtained by using statistical analyses, such as hierarchical clustering, k-means, and principal component analysis. We believe that information obtained from such research is extremely important for improving the understanding of the processes’ controlling factors in areas with anthropogenic activities that affect the circulation and amount of nutrients exported to water bodies. 

How to cite: Lebar, K. and Rusjan, S.: The role of forest vegetation seasonality on the dynamics of nitrate-nitrogen export during rainfall events, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3352, https://doi.org/10.5194/egusphere-egu22-3352, 2022.

Land cover change and its effect on water resources has been a continuous concern in south-central Chile. In this regard, we use two methodologies to study the effect of different land covers on the hydrological processes in several experimental catchments (<100km²) located between 36°S to 39°S. These experimental catchments include native forest and commercial plantations (Monterrey Pine and Eucalyptus) land covers. The first methodology is the use of hydrological modelling to analyze the land cover influence on the water yield of a catchment. [1] compared hydrological and evapotranspiration models and selected the best which represented daily stream flow. The best results were obtained through the more complex model (i.e GR6J and Oudin PET model) which can account for groundwater interaction. The second methodology is the use of the recession coefficient (a) as a metric for baseflow recession analysis, which is a good index of the risk of flow decreasing below a threshold. This coefficient represents the rate of decrease in streamflow after a rainfall event. [2] investigated through mathematical modelling how (a) differs between land cover within the same study sites as [1] study mentioned above. This coefficient did not differ in winter, indicating a similar soil saturation, but some differences were found in summer.  It was determined that (a) was similar between land cover types. Considering both methodological approaches we can conclude that at the experimental catchment scale geology and the fractured rock system play a crucial role in surface-groundwater interactions in these ecosystems. Therefore, future investigations should be focused on subsoil processes and surface-groundwater interaction. Due to its Mediterranean climate, rainfall in Chile is concentrated during winter months, when trees are dormant, meaning that they do not significantly transpire, and a larger proportion of rainfall can infiltrate into the soil. Therefore, metrics such as (a) can help develop landscape planning strategies to increase water availability in conjunction with hydrological modelling. 

References:

[1] Flores, N., Rodríguez, R., Yépez, S., Osores, V., Rau, P., Rivera, D., & Balocchi, F. (2021). Comparison of Three Daily Rainfall-Runoff Hydrological Models Using Four Evapotranspiration Models in Four Small Forested Watersheds with Different Land Cover in South-Central Chile. Water, 13(22), 3191.

[2] Balocchi, F., Flores, N., Arumí, J. L., Iroumé, A., White, D. A., Silberstein, R. P., & Ramírez de Arellano, P. (2021a). Comparison of streamflow recession between plantations and native forests in small catchments in Central‐Southern Chile. Hydrological Processes, 35(6), e14182.

How to cite: Balocchi, F., Arumi, J. L., Rivera, D., and Iroumé, A.: Understanding land cover influence on water resources in south-central Chile: A hydrological modelling with baseflow recession analysis approach , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1842, https://doi.org/10.5194/egusphere-egu22-1842, 2022.

A consequence of climate change, less investigated from the hydrological point of view, is the alteration of the frequency and intensity of forest disturbances that can reduce forest productivity, change the distribution of tree species, and shift their range and density. Based on this, this study evaluates the ability of remnants of native forests to resist or adapt to a changing climate. The case study is the island of Sardinia, located within the Mediterranean basin. Sardinia covers a latitudinal gradient of 300 km; and it is one of the least populated and the most forested regions in Italy. It is considered one of the most important biodiversity reservoirs inside the Mediterranean hotspot. From one hand Sardinia is experiencing a decreasing pressure on forests due to human factors, from the other, some studies demonstrate that winter precipitation and runoff are decreasing alarmingly, and this can have an impact on forests. Based on the above, this study aims, through the analysis of 20 years of satellite images (MOD44B), to evaluate forest cover changes and to detect any possible relationship with some of the most important climate variables such as precipitation, air temperature and vapor pressure deficit. Results indicate that in the last 40 years Sardinia has experienced a simultaneous increase in air temperatures (0.026 °C*y^-1) and VPD (0.001 kPa*y^-1) combined with reductions in both total precipitation (14.03 mm*y^-1) and winter precipitation (12.09 mm*y^-1), and that the areas with a mean annual precipitation lower than 700 mm went from the 26% in the period 1922-1979, to 63% during 1980-2018, and in effect 1980 was detect as changing point for annual precipitation. These climatic variations have led to an important reduction of the tree cover in some historical forests of Sardinia, and in the broad-leaved ones particularly. The reduction in TC shows a positive correlation with mean annual precipitation (ρ= 0.66) and altitude (0.72), while negative correlations were detected with temperature (ρ= -0.57) and VPD (ρ= -0.48). Results highlight that forests are adapting to climate change, and this may have an impact on local water resources.

How to cite: Cipolla, S. S. and Montaldo, N.: Spatiotemporal evolution of forest cover and of historical climate data: a case study in the Mediterranean basin , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5265, https://doi.org/10.5194/egusphere-egu22-5265, 2022.

According to the Forest European Process, the recent Climate Change Conference (COP26), and EU policies, conservation of forest ecosystems is a critical step in mitigating climate change and combating deforestation; accordingly plantation forests will be critical in achieving these goals. While limitations in monoculture plantation are well established in silvicultural practices and documented in research studies, in the face of intensifying climate change and resources scarcity, the need for knowledge on mixed-species plantations has grown. There has been also a recent develop in innovative and sustainable forest management practices, including irrigation and fertilization (fertigation) that aim to improve the productivity of forestry plantations, and therefore their carbon sequestration capacity, as well as the ecosystem services associated with healthy forests. However, the exact effects of fertigation on forests plantation have yet to be established. This study examines the growth patterns, productivity, and carbon storage capacity of four–year-old mixed-species and monoculture plantations in response to an intensive fertigation management. Particularly, our findings highlight differences in tree growth patterns and their performance in paired experimental plots based on different soil types. Such differences are associated varying soil moisture conditions due the interaction of fertigation and the soil water holding capacity. In general, trees growth is higher in sandy soil, due to the positive relation between high soil water diffusivity and fertigation. On the other hand, no such tree growth has been observed in clay soil, most likely due to the high susceptibility to waterlogging, which requires careful fertigation management to avoid limiting conditions for the healthy development of young forest plantation. Our research provides important insights into how young plantation trees respond to different soil moisture conditions, which can aid to in the design and fertigation management of mixed-species plantations resulting in more productive, biodiverse, economically viable, and healthier forests than monocultures.

How to cite: Rabbai, A., Krause, S., Kettridge, N., Ullah, S., Curioni, G., Rob Mackenzie, R., and Hart, K.: Fertigation management of mixed-species plantation versus monoculture in plantation forestry: key aspects and future perspective., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6224, https://doi.org/10.5194/egusphere-egu22-6224, 2022.