scholarly journals Mechanisms of consistently disjunct soil water pools over (pore) space and time

2019 ◽  
Vol 23 (6) ◽  
pp. 2751-2762 ◽  
Author(s):  
Matthias Sprenger ◽  
Pilar Llorens ◽  
Carles Cayuela ◽  
Francesc Gallart ◽  
Jérôme Latron
Keyword(s):  
Stable Isotopes ◽  
Stable Isotope ◽  
Soil Water ◽  
Soil Depth ◽  
Stream Water ◽  
Pore Space ◽  
Bound Water ◽  
Subsurface Water ◽  
Data Set ◽  
Soil Pores

Abstract. The storage and release of water in soils is critical for sustaining plant transpiration and groundwater recharge. However, how much subsurface mixing of water occurs, and how much of the water is available for plants or otherwise percolates to streams and the groundwater is not yet understood. Based on stable isotope (2H and 18O) data, some studies have found that water infiltrating into soils can bypass older pore water. However, the mechanisms leading to the separation of water routed to the streams and water held tightly in smaller pores are still unclear. Here, we address the current limitations of the understanding of subsurface mixing and their consequences regarding the application of stable isotopes in ecohydrological studies. We present an extensive data set, for which we sampled the isotopic composition of mobile and bulk soil water in parallel with groundwater at a fortnightly temporal resolution and stream water and rainfall at a much higher resolution in a Mediterranean long-term research catchment, in Vallcebre, Spain. The data reveal that the mobile and tightly bound water of a silty loam soil in a Scots pine forest do not mix well; however, they constitute two disjunct subsurface water pools with little exchange, despite intense rainfall events leading to high soil wetness. We show that the isotopic compartmentalization results from the rewetting of small soil pores by isotopically depleted winter/spring rain. Thus, stable isotopes, and, in turn, water residence times, do not only vary across soil depth, but also across soil pores. Our findings have important implications for stable isotope applications in ecohydrological studies assessing the water uptake by plants or the process realism of hydrological models, as the observed processes are currently rarely implemented in the simulation of water partitioning into evapotranspiration and recharge in the critical zone.

scholarly journals Mechanisms of consistently disconnected soil water pools over (pore)space and time

2019 ◽  
Author(s):  
Matthias Sprenger ◽  
Pilar Llorens ◽  
Carles Cayuela ◽  
Francesc Gallart ◽  
Jérôme Latron
Keyword(s):  
Soil Water ◽  
Soil Depth ◽  
Stream Water ◽  
Pore Space ◽  
Bound Water ◽  
Subsurface Water ◽  
Data Set ◽  
Silty Loam ◽  
Scots Pine Forest ◽  
Soil Pores

Abstract. Storage and release of water in the soils is critical for sustaining plant transpiration and groundwater recharge. However, the subsurface mixing of water available for plants or quickly flowing to streams and groundwater is not yet understood. Moreover, while water infiltrating into soils was shown to bypass older pore water, the mechanisms leading to a separation between water routed to the streams and water held tightly in smaller pores are unclear. Here we present an extensive data set, for which we sampled fortnightly the isotopic composition (2H and 18O) of mobile and bulk soil water in parallel with groundwater, stream water and rainfall in the Mediterranean long-term research catchment, Vallcebre, in Spain. The data revealed that mobile and tightly bound water of a silty loam soil in a Scots pine forest do not mix, but they constitute two separate subsurface water pools; despite intense rainfall events leading to high soil wetness. We show that the isotopic compartmentation results from rewetting of small soil pores with isotopically depleted winter/spring rain. Thus, stable isotopes, and therefore water residence times too, do not only vary across soil depth, but also across soil pores. Our findings have important implications for stable isotope applications in ecohydrological studies assessing water uptake by plants or process realism of hydrological models, as the observed processes are currently rarely implemented in the simulation of water partitioning into evapotranspiration and recharge in the critical zone.

scholarly journals Water Stable Isotopes in an Alpine Setting of the Northeastern Tibetan Plateau

Water ◽  
2019 ◽  
Vol 11 (4) ◽  
pp. 770 ◽  
Author(s):  
Xue Qiu ◽  
Mingjun Zhang ◽  
Shengjie Wang ◽  
Athanassios A. Argiriou ◽  
Rong Chen ◽  
...  
Keyword(s):  
Stable Isotopes ◽  
Soil Water ◽  
Water Cycle ◽  
Soil Depth ◽  
Soil Layer ◽  
Water Source ◽  
Hydrological Processes ◽  
Deep Soil ◽  
Monthly Basis ◽  
Mountainous Regions

Hydrological processes produce effects on water resources in inland mountainous regions. To perform a comprehensive investigation of the important segments of the water cycle, using the Qilian Mountains as a case study, precipitation, soil, plant, river, and groundwater were collected during the plant growing season of 2016. All samples were collected on a monthly basis, except precipitation, which was collected on a per event basis. The results showed that: the “temperature effect” was apparent, which suggested a drier climate background; there were differences in the slope and intercept of the local meteoric water line, using different regression methods; and the δ18O of soil water varied greatly in the topsoil, tended to be similar in the deep soil, and became increasingly depleted as the soil depth increased. The responses of the soil water isotopes to precipitation pulses had different boundaries. The major water source for Caragana Fabr. in no-precipitation month was located in the 0–30 cm soil layer, but was different in months when precipitation occurred. Overall, the findings from the stable isotopes provide insights into hydrological processes and offer a platform to understand mountainous water cycle in arid areas.

scholarly journals Continual in-situ monitoring of pore water stable isotopes in the subsurface

2013 ◽  
Vol 10 (11) ◽  
pp. 13293-13331 ◽  
Author(s):  
T. H. M. Volkmann ◽  
M. Weiler
Keyword(s):  
Stable Isotopes ◽  
Water Vapor ◽  
Stable Isotope ◽  
Monitoring System ◽  
Pore Water ◽  
Sampling Method ◽  
Transport Processes ◽  
In Situ Monitoring ◽  
Subsurface Water

Abstract. The stable isotope signature of pore water provides an integral fingerprint of water origin, flow path, transport processes, and residence times and can thus serve as a powerful tracer of hydrological processes in the unsaturated and saturated zone. However, the full potential of stable isotopes to quantitatively characterize subsurface water dynamics is yet unfolded due to the difficulty in obtaining extensive detailed and continual measurements of spatiotemporally variable pore water signatures. With the development of field-deployable laser-based isotope analyzers, such measurements are now becoming feasible. This study presents the development and application of a functional, automatable, and cost-efficient system for non-destructive continual in-situ monitoring of pore water stable isotope signatures with high resolution. The monitoring system uses automatic-controllable valve arrays to continuously extract diluted soil air water vapor via a branching network of multiple small microporous probes into a commercial isotope analyzer. Soil temperature observations are used to convert obtained vapor phase into liquid phase water isotope signatures, but these can also be obtained based on vapor concentration measurements. In-situ sampling was conducted at six depths for each of three plots planted with varying vegetation on an experimental site in SW Germany. Two different methods based on advective and diffusive soil water vapor probing were employed suitable under unsaturated and all (including saturated) moisture conditions, respectively. The advective sampling method was applied using multiple permanently installed probes (continual mode) and using a single probe subsequently inserted to sample the various locations (push-in mode), while the diffusive sampling method was applied in push-in mode only. Using a specific identical treatment onsite calibration approach along with basic corrections for instrument bias and temperature dependent free water-vapor isotopic equilibrium fractionation, the monitoring system facilitated inference of normalized liquid pore water isotopic composition with sufficiently high accuracy and precision at sampling intervals of less than four minutes and resolved the isotopic variability along natural depth profiles. Comparison indicated that the presented in-situ approaches may be used interchangeably with each other and with concurrent laboratory-based direct equilibration measurements of destructively collected samples, such that the choice of method will depend upon the task and anticipated conditions of sampling. The introduced sampling techniques provide powerful tools towards a detailed quantitative understanding of dynamic and heterogeneous shallow subsurface and vadose zone processes.

scholarly journals Technical note: Unresolved aspects of the direct vapor equilibration method for stable isotope analysis (<i>δ</i><sup>18</sup>O, <i>δ</i><sup>2</sup>H) of matrix-bound water: unifying protocols through empirical and mathematical scrutiny

2021 ◽  
Vol 25 (9) ◽  
pp. 5219-5235
Author(s):  
Benjamin Gralher ◽  
Barbara Herbstritt ◽  
Markus Weiler
Keyword(s):  
Stable Isotope ◽  
Data Quality ◽  
Stable Isotope Analysis ◽  
Isotope Analysis ◽  
Bound Water ◽  
Transport Processes ◽  
Subsurface Water ◽  
Water Volume ◽  
Equilibration Time ◽  
Matrix Bound

Abstract. The direct vapor equilibration laser spectrometry (DVE-LS) method has been developed for obtaining matrix-bound water stable isotope data in soils, the critical zone, and bedrock, deriving therefrom subsurface water flow and transport processes and, ultimately, characterizing, for example, groundwater recharge and vulnerability. Recently, DVE-LS has been increasingly adopted due to its possible high sample throughput, relative simplicity, and cost-efficiency. However, this has come at the cost of a non-unified standard operation protocol (SOP), and several contradictory suggestions regarding protocol details do exist which have not been resolved to date. Particularly, sample container material and equilibration times have not yet been agreed upon. Beside practical constraints, this often limits DVE-LS applicability to interpreting relative isotope dynamics instead of absolute values. It also prevents data comparability among studies or laboratories, and several previous comparisons of DVE-LS with other, more traditional approaches of water extraction and subsequent stable isotope analysis yielded significant discrepancies for various sample matrices and physical states. In a series of empirical tests, we scrutinized the controversial DVE-LS protocol details. Specifically, we tested 10 different easily available and cost-efficient inflatable bags previously employed or potentially suitable for DVE-LS sample collection and equilibration. In storage tests similar to the DVE-LS equilibration process but lasting several weeks, we quickly found heat-sealed bags made of laminated aluminum (Al) sheets to be superior by several orders of magnitude over more frequently used freezer bags in terms of evaporation safety and accompanying adverse isotope effects. For the first time, Al-laminated bags allow the applied equilibration time to be adapted exclusively to sample requirements instead of accepting reduced data quality in a trade-off with material shortcomings. Based on detailed physical considerations, we further describe how to calculate the minimum available container headspace and sample-contained liquid water volume and how their ratio affects analytical precision and accuracy. We are confident that these guidelines will expand DVE-LS applicability and improve data quality and comparability among studies and laboratories by contributing to a more unified, physically well-founded SOP based on more appropriate components.

scholarly journals Isotopic discrimination factors and nitrogen turnover rates in reared Atlantic bluefin tuna larvae (Thunnus thynnus): effects of maternal transmission

2016 ◽  
Vol 80 (4) ◽  
pp. 447 ◽  
Author(s):  
Amaya Uriarte ◽  
Alberto García ◽  
Aurelio Ortega ◽  
Fernando De la Gándara ◽  
José Quintanilla ◽  
...  
Keyword(s):  
Stable Isotopes ◽  
Stable Isotope ◽  
Bluefin Tuna ◽  
Gilthead Seabream ◽  
Half Time ◽  
Turnover Rates ◽  
Thunnus Thynnus ◽  
Data Set ◽  
Nitrogen Turnover ◽  
Discrimination Factors

The use of stable isotope analysis to study animal diets requires estimates of isotopic turnover rates (half time, t50) and discrimination factors (Δ) for an accurate interpretation of trophic patterns. The stable isotopes of carbon and nitrogen were analysed for eggs and reared larvae of Thunnus thynnus, as well as for the different diets supplied during the experiment. The results showed high values of δ15N in eggs and larvae (n=646) until 4 DAH. After this time lapse, the stable isotope values declined progressively until 12 DAH, when notochord flexion began. The δ13C showed an inverse trend, suggesting that maternal inheritance of the stable isotopes is evident until pre-flexion stages. This study proposes a model for estimating maternal isotopic signatures of bluefin broodstock. After notochord flexion, larvae were fed with aquaculture-bred gilthead seabream, which resulted in a rapid increase of bluefin larvae δ15N values together with a rapid decrease in δ13C values. The estimated nitrogen half-time to reach the steady state from the diet was 2.5±0.3 days and the discrimination factor was 0.4±0.3(‰). These results represent the first data set that has allowed isotopic nitrogen turnover rates and discrimination factors of the larval stages of bluefin tuna to be estimated.

scholarly journals Macropore flow of old water revisited: where does the mixing occur at the hillslope scale?

2012 ◽  
Vol 9 (4) ◽  
pp. 4333-4380 ◽  
Author(s):  
J. Klaus ◽  
E. Zehe ◽  
M. Elsner ◽  
C. Külls ◽  
J. J. McDonnell
Keyword(s):  
Stable Isotope ◽  
Isotopic Composition ◽  
Soil Water ◽  
Irrigation Water ◽  
Initial Conditions ◽  
Soil Depth ◽  
Lower Boundary ◽  
Base Flow ◽  
Macropore Flow ◽  
Hillslope Scale

Abstract. The mechanisms allowing the rapid release of stored water to streams are poorly understood. Here we use a tile drained field site to combine naturally structured soils at the hillslope scale with the advantage of at least partly controlled lower boundary conditions. We performed a series of three irrigation experiments combining hydrometric measurements with stable isotope and bromide tracers to better understand macropore-matrix interactions and stored water release processes at the hillslope scale. Stable isotope concentrations were monitored in the irrigation water, the tile drain discharge and the soil water before and after the experiment. Bromide was measured at mainly every 5–15 min in the tile drain hydrograph. Different initial conditions for each experiment were used to examine how pre-event soil moisture conditions influenced flow and transport. Different amounts of irrigation water were necessary to increase tile drain discharge above the base flow level. Hydrograph separation based on bromide data revealed that irrigation water contributions to peak tile drain discharge were on the order of 20%. Oxygen-18 and deuterium data were consistent with the bromide data and showed that pre-event soil water contributed significantly to the tile drain event flow. However, the isotopic composition of soil water converged towards the isotopic composition of irrigation water through the course of the experiment. Mixing calculations revealed that by the end of the irrigation experiments 20% of the soil water in the entire profile was irrigation water. The isotopic data showed that the pre-event water in the tile drain was mobilized in 20–40 cm soil depth were the macropore-matrix interaction leads to an initiation of macropore flow after a moisture threshold is exceeded.

Subsurface runoff and recharge dynamics in a Mediterranean catchment based on StorAge Selection functions and end-member splitting analysis

2020 ◽  
Author(s):  
Matthias Sprenger ◽  
Pilar Llorens ◽  
Francesc Gallart ◽  
Jérôme Latron
Keyword(s):  
Stream Water ◽  
Pore Space ◽  
Data Driven ◽  
High Temporal Resolution ◽  
Rainfall Runoff ◽  
Catchment Scale ◽  
Data Set ◽  
Isotope Data ◽  
Subsurface Runoff

&lt;p&gt;Investigations at the long-term experimental catchment Vallcebre in the Pyrenees revealed that rainfall-runoff dynamics are highly variable due to the Mediterranean climatic conditions affecting the storage and release of water in the subsurface&lt;sup&gt;1&lt;/sup&gt;. In a changing climate, to the consequences of which could lead to more variations in catchment wetness due to an increase in both droughts and high intensity rainfalls, there is a strong need to better understand subsurface storage and runoff processes.&lt;/p&gt;&lt;p&gt;While our previous isotope studies (using &lt;sup&gt;2&lt;/sup&gt;H and &lt;sup&gt;18&lt;/sup&gt;O) demonstrated a pronounced heterogeneity of water flow in the unsaturated zone at the plot scale&lt;sup&gt;2&lt;/sup&gt;, we also observed that the contributions of young waters to catchment runoff are highly dependent on the catchments wetness&lt;sup&gt;3&lt;/sup&gt;. These analyses provided a basis from which we present new insights into the relationship between subsurface runoff and storage dynamics applying StorAge Selection functions&lt;sup&gt;4&lt;/sup&gt; and end-member splitting analysis&lt;sup&gt;5&lt;/sup&gt;. Thus, we combined modeling and data-driven approaches to disentangle the partitioning of subsurface waters into storage and runoff based on water age dynamics.&lt;/p&gt;&lt;p&gt;We gathered an extensive isotope data set with &gt;550 rainfall samples and &gt;980 stream samples taken at high temporal resolution (30 minutes to one week), with highest frequencies during high discharge to improve the coverage of rainfall-runoff events. Using this high-frequency isotope data set, we calibrated the StorAge Selection functions and put special emphasis on the representation of the isotopic response during high flow rainfall-runoff periods. We further tested if time-variant representations of StorAge Selection functions dependent on varying wetness improves the stream water isotope simulations and the ways in which isotope data from different compartments (groundwater and tree water) can assist in constraining the parameter space. Furthermore, end-member splitting analysis provided an independent view into the flow dynamics based on these long-term isotope data sets. As such, the analysis allowed us to derive estimates of the dynamics of rainfall partitioning into runoff and evapotranspiration. Therefore, the combination of the modeling and data-driven approaches enabled an assessment of the dynamics of subsurface runoff at the catchment scale underlining the relevance of heterogeneous flow pattern that were observed on the plot scale.&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;ol&gt;&lt;li&gt;Llorens, P. et al. What have we learnt about mediterranean catchment hydrology? 30 years observing hydrological processes in the Vallcebre research catchments. Geogr. Res. Lett. &lt;strong&gt;44, &lt;/strong&gt;475&amp;#8211;502; 10.18172/cig.3432 (2018).&lt;/li&gt; &lt;li&gt;Sprenger, M., Llorens, P., Cayuela, C., Gallart, F. &amp; Latron, J. Mechanisms of consistently disjunct soil water pools over (pore) space and time. Hydrol. Earth Syst. Sci. &lt;strong&gt;23, &lt;/strong&gt;2751&amp;#8211;2762; 10.5194/hess-23-2751-2019 (2019).&lt;/li&gt; &lt;li&gt;Gallart, F. et al. Investigating young water fractions in a small Mediterranean mountain catchment: both precipitation forcing and sampling frequency matter. Hydrol. Process. (in review).&lt;/li&gt; &lt;li&gt;Benettin, P. &amp; Bertuzzo, E. tran-SAS v1.0: a numerical model to compute catchment-scale hydrologic transport using StorAge Selection functions. Geosci. Model Dev. &lt;strong&gt;11, &lt;/strong&gt;1627&amp;#8211;1639; 10.5194/gmd-11-1627-2018 (2018).&lt;/li&gt; &lt;li&gt;Kirchner, J. W. &amp; Allen, S. T. Seasonal partitioning of precipitation between streamflow and evapotranspiration, inferred from end-member splitting analysis. Hydrology and Earth System Sciences, &lt;strong&gt;24&lt;/strong&gt;, 17&amp;#8211;39; 10.5194/hess-24-17-2020 (2020).&lt;/li&gt; &lt;/ol&gt;

scholarly journals Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

2017 ◽  
Vol 21 (7) ◽  
pp. 3839-3858 ◽  
Author(s):  
Matthias Sprenger ◽  
Doerthe Tetzlaff ◽  
Chris Soulsby
Keyword(s):  
Stable Isotopes ◽  
Isotopic Composition ◽  
Soil Water ◽  
Scots Pine ◽  
Soil Depth ◽  
Critical Zone ◽  
Soil Evaporation ◽  
Pore Waters ◽  
Catchment Scale ◽  
Scottish Highlands

Abstract. Understanding the influence of vegetation on water storage and flux in the upper soil is crucial in assessing the consequences of climate and land use change. We sampled the upper 20 cm of podzolic soils at 5 cm intervals in four sites differing in their vegetation (Scots Pine (Pinus sylvestris) and heather (Calluna sp. and Erica Sp)) and aspect. The sites were located within the Bruntland Burn long-term experimental catchment in the Scottish Highlands, a low energy, wet environment. Sampling took place on 11 occasions between September 2015 and September 2016 to capture seasonal variability in isotope dynamics. The pore waters of soil samples were analyzed for their isotopic composition (δ2H and δ18O) with the direct-equilibration method. Our results show that the soil waters in the top soil are, despite the low potential evaporation rates in such northern latitudes, kinetically fractionated compared to the precipitation input throughout the year. This fractionation signal decreases within the upper 15 cm resulting in the top 5 cm being isotopically differentiated to the soil at 15–20 cm soil depth. There are significant differences in the fractionation signal between soils beneath heather and soils beneath Scots pine, with the latter being more pronounced. But again, this difference diminishes within the upper 15 cm of soil. The enrichment in heavy isotopes in the topsoil follows a seasonal hysteresis pattern, indicating a lag time between the fractionation signal in the soil and the increase/decrease of soil evaporation in spring/autumn. Based on the kinetic enrichment of the soil water isotopes, we estimated the soil evaporation losses to be about 5 and 10 % of the infiltrating water for soils beneath heather and Scots pine, respectively. The high sampling frequency in time (monthly) and depth (5 cm intervals) revealed high temporal and spatial variability of the isotopic composition of soil waters, which can be critical, when using stable isotopes as tracers to assess plant water uptake patterns within the critical zone or applying them to calibrate tracer-aided hydrological models either at the plot to the catchment scale.

scholarly journals Macropore flow of old water revisited: experimental insights from a tile-drained hillslope

2013 ◽  
Vol 17 (1) ◽  
pp. 103-118 ◽  
Author(s):  
J. Klaus ◽  
E. Zehe ◽  
M. Elsner ◽  
C. Külls ◽  
J. J. McDonnell
Keyword(s):  
Stable Isotope ◽  
Isotopic Composition ◽  
Soil Water ◽  
Irrigation Water ◽  
Initial Conditions ◽  
Soil Depth ◽  
Lower Boundary ◽  
Hydrograph Separation ◽  
Macropore Flow ◽  
Hillslope Scale

Abstract. The mechanisms allowing the rapid release of stored water to streams are poorly understood. Here we use a tile-drained field site to combine macroporous soils at the hillslope scale with the advantage of at least partly controlled lower boundary conditions. We performed a series of three irrigation experiments combining hydrometric measurements with stable isotope and bromide tracers to better understand macropore–matrix interactions and stored water release processes at the hillslope scale. Stable isotope concentrations were monitored in the irrigation water, the tile-drain discharge and the soil water before and after the experiment. Bromide was measured every 5–15 min in the tile-drain hydrograph. Different initial conditions for each experiment were used to examine how these influenced flow and transport. Different amounts of irrigation water were necessary to increase tile-drain discharge above the baseflow level. Hydrograph separation based on bromide data revealed that irrigation water contributions to peak tile-drain discharge were on the order of 20%. Oxygen-18 and deuterium data were consistent with the bromide data and showed that pre-event soil water contributed significantly to the tile-drain event flow. However, the isotopic composition of soil water converged towards the isotopic composition of irrigation water through the course of the experiment. Mixing calculations revealed that by the end of the irrigation experiments 20% of the soil water in the entire profile was irrigation water. The isotopic data showed that the pre-event water in the tile drain was mobilized in 20–40 cm soil depth where the macropore–matrix interaction leads to an initiation of macropore flow after a moisture threshold is exceeded.

scholarly journals Using Soil Water Stable Isotopes to Investigate Soil Water Movement in a Water Conservation Forest in Hani Terrace

Water ◽  
2020 ◽  
Vol 12 (12) ◽  
pp. 3520
Author(s):  
Huimei Pu ◽  
Weifeng Song ◽  
Jinkui Wu
Keyword(s):  
Stable Isotopes ◽  
Soil Water ◽  
Water Conservation ◽  
Rainy Season ◽  
Preferential Flow ◽  
Water Movement ◽  
Soil Depth ◽  
Dry Season ◽  
Water Infiltration ◽  
Soil Water Movement

Water conservation forests significantly contribute to the stability of mountain agricultural ecosystems in Hani Terrace. In this study, we analyzed the relationship between the stable isotopic composition of soil water and precipitation to determine the mechanisms of soil water movement in the small watershed of Quanfuzhuang. We observed significant seasonal variations in soil water sources: antecedent precipitation was the dominant supply during the dry season, and current precipitation dominated during the rainy season. The recharge ratio of precipitation to soil water in the grassland was significantly higher than that in the arbor land and shrubland. The influence of water infiltration, old and new soil water mixing, and soil evaporation on the soil water stable isotopes gradually decreased from the surface (0–20 cm) to the deep (60–80 cm) soil. We observed significant seasonal variability in average soil water δ18O in the upper 0–60 cm and lower variability at 60–100 cm. The average soil water δ18O was generally higher in the dry season than in the rainy season. The mixing of old and new water is a continuous and cumulative process that is impacted by soil structure, soil texture, and precipitation events. We therefore identified a significant time delay in soil water supply with increasing soil depth. Moreover, the piston flow of soil water co-occurred with preferential flow, and the latter was the dominant supply during the rainy season.

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