Introduction 

Acid rain directly affects the chemical and pH balances of surface and ground waters in headwater regions of central Europe ([10]). Low pH values cause the mobilisation of toxic aluminium, which reduces the biota in water courses and reservoirs. Fog and cloud waters have been recognised as important carriers for the deposition of water-borne pollutants in high elevation forests ([21], [40], [42]). The process of fog condensation on vegetation foliage and dripping to soil surfaces can significantly affect the water balance in mountain watersheds ([37], [19], [12], [22], [5]). Recently, fog collection technologies (simple devices based on erected nets facing oncoming winds and trapping water particles in the air) have been developed in many parts of the world to modify the water cycle and to support local water supplies ([30]). Fog has also been viewed as an important source of moisture in coastal ecosystems ([5]); however, effective fog/cloud water trapping by vegetation has been mainly reported in cloud forests ([37], [9], [8]). In general, higher elevation forests show relatively significant additional water yields due to the occurrence of fog ([12], [38], [11]), and Holder ([9]) has called the conservation of high elevation forests an important tool for water supply engineering.

Considering forests in Central Europe, Brechtel ([3]) reported an elevation of 700 m as the threshold of significant fog precipitation. Upland watersheds above 700 m are currently covered mainly by coniferous forests ([1]). Fog drip in short vegetation types (like grass) has been found to be much smaller than in forests, with deposition intensities of 0.018 -0.02 mm per hour in grasslands compared with 0.02-0.4 mm per hour in coniferous forests ([22], [8]). In addition to canopy structure, however, a number of variables such as wind speed, liquid water content and droplet sizes influence the deposition of fog/cloud precipitation ([40]).

In regions affected by polluted air (particularly emissions of sulphur and nitrogen) and by consequent acid atmospheric deposition, processes trapping fog from the atmosphere can intensify the decline of water environment and water resources ([36], [6], [32], [11], [33], [13], [15]). Therefore, land use and watershed management practices might influence the volumes and chemistry of cloud/fog precipitation as well as the recharge, quantity and quality of water resources ([13], [16]). Increasing inputs of acid substances from fog precipitation may result in lower pH, reduced contents of calcium and magnesium (and water hardness), and the mobilisation of aluminium in water environments ([24], [26], [15]). pH is one of the most important operational water quality parameters, with national guidelines for drinking water quality often suggesting an optimum pH in the range 6.5 to 8.5 ([41]). Without air pollution and subsequent acid rain, most lakes and streams would have a pH level close to 6.5 ([24]). However, in the headwaters of the European “Black Triangle”, in the late 1980s water pH rose to 4-5 and concentrations of aluminium increased to 1-2 mg per litre ([16]). The limit of aluminium in drinking water is 0.1 mg per litre for large treatment facilities and 0.2 mg per litre for small facilities ([41]). Therefore, in some water treatment plants, the increased acidity of raw water necessitated several additional investments ([18]). The aim of this study was to analyse the genesis of acid atmospheric deposition in forested headwater catchments of the Jizera Mountains (Czech Republic) with special attention on the role of fog/low cloud events, elevation and vegetation canopy.

  Material and methods 

Study site

This study was performed in the upper plain of the Jizera Mountains (50^o 40′ - 50^o 52′ N, 15^o 08′ - 15^o 24′ E), which has a humid temperate climate (Fig. 1). This 200 km² headwater area is located above 800 m (subarctic region of the Köppen climate zone, Dfc), the supposed threshold for the occurrence of significant fog/clouds in the Czech Republic ([29]). The mean annual precipitation ranges from 1290 to 1400 mm, the mean annual air temperature from 4 to 5 °C, and the average maximum snowpack depth is about 120 cm (the snow cover usually lasts from the beginning of November to the end of April - [35]).

Fig. 1 - Catchments of the drinking water reservoirs (Josefuv dul - JD, Souš - S) and the experimental basin (J-1).

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The risk of acidification in the Jizera Mountains is enhanced by the granite bedrock and shallow podzolic soils of a low buffering capacity. The upper watersheds are fully forested; however, during the 19^th century, the mixed forest of native tree species (Common beech - Fagus sylvatica, Norway spruce - Picea abies, and Common silver fir - Abies alba) was converted to spruce plantations (almost 90% of forest stands) of lower ecological stability ([15]).

Precipitation analyses

We analysed precipitation using the water balance method under a canopy proposed by Lovett ([23]) as the most appropriate approach for measuring fog and cloud water deposition in ecological studies. During the period 2011-2012, precipitation (including fog-drip) under the canopy of two mature spruce stands (plots 1 and 2, at elevations of 745 and 975 metres a.s.l. - Fig. 1) were collected by modified Hellmann rain gauges (area of 200 cm², plastic collectors with a shield against bird contamination). Sets of ten gauges were randomly installed in plots of 30 × 30 metres according to forest characteristics (Tab. 1) and the method proposed by Krečmer & Páv ([20]). The amount of fog drip was estimated by comparing the volumes of water collected by these sets of ten gauges with a single rain gauge installed in adjacent forest openings (i.e., “bulk”, located 30 to 50 metres away from the forest edge). According to Krečmer et al. ([19]), in spruce stands the stemflow of rainwater can be considered to be negligible. To test this assumption at our stands, stemflow in the investigated plots was collected by plastic tubing installed around the circumference of four selected trunks. Collected volumes of water were recorded at monthly intervals; the interception loss of the canopy (I) was calculated as (eqn. 1):

where P is the open field (gross) precipitation (mm), P_t is the throughfall under the canopy (mm), P_s is the stemflow, n is the number of months, m is the number of seasons.

To identify the effect of elevation on potential fog drip, twelve passive fog collectors were installed in vertical transect A of the Jizerka experimental catchment (J-1, area of 1 km², elevation gradient from 862 to 994 metres - Fig. 1), where mature spruce stands were harvested by clear-cutting. For each collector, the fog drip was generated using 400 metres of Teflon line (diameter of 0.25 millimetres, i.e., surface area index = 5) installed at a height of 1.7 metres above the ground, and collected in one-litre sample bottles at monthly intervals. Sample bottles were protected against direct rainfall access by a wide-brimmed cover that overlapped the fog collector at an angle of 34°.

The collected water samples (fog drip, throughfall and stemflow) were analysed by standard techniques in the Hydrobiological Laboratory Velký Pálenec (Charles University in Prague), including pH, conductivity, and contents of elements related to acid atmospheric deposition - sulphur (S-SO₄) and nitrogen (N-NO₃, N-NH₄). Concentrations of NO₃^- and SO₄^2- were determined by ion chromatography, and NH₄⁺ by the rubazoic acid method ([14]). Although bacterial activity may alter the chemical composition of water after collection (especially nitrogen - [7]), in our case the relatively cold mountain climate (subarctic region), relatively high concentrations of nitrogen, and low values of pH and dissolved organic carbon likely limited bacterial activity ([4]). The potential growth of algae was reduced by using dark sampling bottles and keeping samples in the dark during transport.

Precipitation deposition in watersheds

Findings worldwide ([19], [3], [5]) have shown that in mountain watersheds, fog drip deposition increases with elevation and canopy density. Therefore, the hypsometric method with spatial interpolation (using ArcGIS 10.2) was employed to approximate the spatial atmospheric deposition of fog water (P_fd), sulphur (S-SO₄) and nitrogen (N-NO₄, N-NH₄) in two watersheds containing drinking water reservoirs in the Jizera Mountains, Josefuv Dul and Souš (Fig. 1). To identify seasonal relationships between fog drip and elevation, data from the fog collectors was analysed separately for the summer (May-October) and winter (November-April) seasons. The occurrence of fog was also recorded at four nearby climate stations: Josefuv Dul (elevation of 745 m a.s.l.), Souš (772 m), and at the highest and lowest points in the Jizerka experimental basin (980 and 850 m - Fig. 1).

Several authors ([19], [22], [39]) have observed higher fog drip at the edges of forest stands, sometimes exceeding the fog deposition in a nearby stand by 3 to 15 times. However, in fragmented forest stands such as those in the Jizera Mts. Krečmer et al. ([19]) found increases of only up to 10% in border areas 10-20 metres wide. We therefore neglected the edge effect in this study.

The vegetation canopy was analysed by satellite (Landsat imagery, resolution of 30 × 30 m) and detailed ground investigations ([17]). Then, the canopy surface was categorised into groups of herbaceous plants and forest stands, and their delineation in watersheds was performed. In the Jizerka experimental catchment, ground-based canopy analyses were carried out for twenty selected plots (30 × 30 m). Twelve of these were instrumented by passive fog collectors (along the harvested transect A - Fig. 1) and two by sets of ten rain gauges installed under the canopy. The forestry characteristics (age, stocking, horizontal canopy density) were estimated by a standard inventory ([16]). The capacity for fog deposition was approximated by the fog-drip coefficient F_c according to the canopy surface estimates ([19]). The value of the coefficient F_c ranges from 0 to 1. eqn. 2 was used to calculate the seasonal volume of fog precipitation for a specified canopy, elevation and season. The relation is based on a simple linear hypsometric relation between altitude and the precipitation volume (eqn. 2).

where P_fd is the seasonal amount of fog drip (mm), a and a₀ are coefficients of the hypsometric relation derived for the individual season, E is the elevation (m), A_er is the effective receptor area (m²), and F_c is the fog drip coefficient.

Similarly, the hypsometric method was used to assess the water quality of the deposited fog. Concentrations of the acidic substances (sulphate, nitrate and ammonia) from the fog collectors and rain gauges were used to estimate seasonal amounts of deposited elements (eqn. 3):

where m is the seasonal amount of deposited elements (kg km^-2), b and b₀ are coefficients of the hypsometric relation derived for individual season and element, E is the elevation (m), A_er is the effective receptor area (m²), and F_c is the fog drip coefficient.

Standard descriptive statistics (including mean, median, maximum and minimum values, standard deviation - SD, standard error of mean - SEM, upper and lower 95% confidence limits), normality test, and one-way ANOVA were used to analyse the data and to identify relationships between the groups of data ([27]). The paired t-test comparing means of two groups of a variable was used to test differences between groups, and the correlation was tested by the Pearson’s coefficient.

  Results and discussion 

Effects of elevation and canopy

The correlation matrix of elevation E (m), sampled volume of fog drip V (ml), water pH, and contents of N-NO₃, N-NH₄, S-SO₄ (mg l^-1) is presented in Tab. 2. Though passive string collectors are limited in their ability to collect cloud/fog water, and their efficiencies depend on the droplet spectra and wind speed ([31], [2]), these data allow the effects of elevation to be assessed. It is evident that the groups of data are inter-correlated. With rising elevation, there was a significant increase of fog drip, followed by decreasing water pH. Contents of acidic substances (N-NO₃, N-NH₄, S-SO₄) were not well correlated with elevation, but in higher altitudes their deposition did increase with increasing volumes of precipitated water. There was a linear relationship between mean monthly fog drip and elevation in both summer and winter seasons with correlation coefficients r = 0.93 and 0.98, r_crit = 0.73 (n = 5, α = 0.1), a slope significantly different from zero (P = 0.0082 and 0.0033), and a non-significant departure from linearity (F = 39.3 and 73.75 > F_crit = 9.78). During the winter season (November-April), the load of fog precipitation exceeded the summer fog drip (May-October) by 23-50%.

In plot 2 during the warm season (May-October), the observed canopy storage capacity was 2.3 mm. For the seasonal rainfall of 683 mm (Tab. 3) and approx. 100 rainy days saturating the storage capacity, the total interception loss of rainwater by the spruce canopy could be estimated at about 230 mm. Therefore, in plot 2 the interception losses not affected by the fog drip could reach 34 % of the gross precipitation. Similarly, in a dense mature plantation of Norway spruce (canopy density 0.8-0.9) not affected by additional precipitation from fog or low clouds, Krečmer et al. ([19]) reported 37 % interception. The volume of stemflow was found to be negligible, which is consistent with the findings of Krečmer et al. ([19]) and Lovett & Reiners ([22]). One possible explanations include the centrifugal structure of the spruce canopy and rough stem-bark.

The observed mean interception loss in the summer season (May-October) was 196 mm (31 % of the gross precipitation) for plot 1, and 112 mm (16 %) for plot 2. These differences in the interception loss between the two spruce stands could be explained simply by the deposition of fog droplets: 37 and 120 mm per season (i.e., 0.2 and 0.7 mm daily).

In the Jizerka experimental catchment (plot 2), the content of sulphur and nitrogen were analysed separately for rain and fog water (Tab. 3). The passive fog drip collectors at different elevations showed increasing concentrations of sulphur and nitrogen with increasing elevation. In comparison with rainfall the collected fog precipitation had lower pH, likely because of the genesis of fog droplets in an atmosphere polluted by emissions of sulphur and nitrogen. Verhoeven et al. ([36]) reported higher acidity in fog versus rain in the Fichtelgebirge Mountains (Bavaria, Germany) due to an excess of strong mineral acids by anthropogenic emissions into the atmosphere.

At transect A (899-975 m a.s.l.) during the period 2010-2012, the average annual value of pH varied with elevation from 3.6 to 4.2, and the average annual concentration values (mg l^-1) of S-SO₄, N-NO₃, and N-NH₄ varied with elevation from 7.7 to 7.9, from 2.2 to 2.7, and from 6.2 to 7.2, respectively. These data correspond to the results of Pahl et al. ([28]), who reported higher amounts of dissolved chemicals (e.g., SO₄) at greater distances above the cloud base.

In the watersheds of the drinking water reservoirs (Josefuv Dul and Souš - Fig. 1), the estimates of the fog-drip coefficient F_c was consistent with standard forestry mapping (age classes and horizontal canopy density) for existing spruce plantations. As shown in Tab. 4, values of the F_c coefficient reflected the efficiency of the spruce canopy to collect fog droplets with a maximum (F_c = 1) in a close mature stand, and decreasing with opening of the canopy.

On the other hand, fog deposition depends on the frequency that fog occurs. The meteorological observation of fog events at elevations of 745, 772, 850 and 980 m a.s.l. (Fig. 1) showed a statistically significant relation between the number of foggy days and elevation (correlation coefficient r = 0.99, r_crit = 0.95, n = 4, α = 0.05 - eqn. 4).

where D is the number of foggy days in the summer (days), and E is the elevation (m).

In the winter, the number of foggy days exceeded summer values by 60 %, corresponding to Tolasz & Baštyrová ([35]). During temperatures below zero, the deposition of fog droplets falls to almost one-third of the potential, according to Lovett & Reiners ([22]).

Watershed deposition

The observed and approximated loads of fog water and acidic substances were used for deriving the coefficients in the hypsometrically based regression of eqn. 2 and eqn. 3 using the effective receptor area of the fog collector A_er = 0.022 m² and the fog drip coefficient F_c depending on the vegetation cover. All relations (Tab. S1, Tab. S2, and Tab. S3 in Supplementary material) were statistically significant, with correlation coefficients varying in absolute value from 0.87 to 0.98 (r_crit = 0.8114, n = 6, α = 0.05), slope different from zero (P from 0.01 to 0.0015), and a non-significant departure from linearity (F from 17.34 to 66.66, F_crit = 7.39).

In the catchments of Jizerka (J-1), Josefuv Dul (JD) and Souš (S), approximations of the data using eqn. 2 and eqn. 3 allowed to determinate the spatial distribution of values within catchments respecting local conditions such as the altitude and vegetation cover, resulting in spatial maps of the mean annual load of fog water (Fig. 2), sulphur (Fig. 3) and nitrogen (Fig. 4), and mean annual pH of fog water (Fig. 5).

Fig. 2 - Mean annual fog drip in the investigated watersheds (2011-2012).

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Fig. 3 - Mean annual load of sulphur by fog drip (2011-2012).

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Fig. 4 - Mean annual load of nitrogen by fog drip (2011-2012).

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Fig. 5 - Annual pH values of fog drip (2011-2012).

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The annual load of fog water varied from 7% to 8% of the mean annual gross precipitation (1324 mm, estimated by the hypsometric interpolation of the standard point rain gauges). Therefore, the input of fog precipitation in the annual water budget increased the water yield from 10% to 12% (by 867 mm mean annual runoff). On the other hand, the mean annual loads of sulphate and nitrate from fog deposition were 1975 and 1080 kg km^-2 in the watersheds of Josefuv Dul and Souš, i.e., 55% and 48% of the amounts of sulphur and nitrogen, respectively, registered in the bulk. Likewise, in the Fichtelberg Mountains (Bavaria, Germany), Wrzesinsky & Klemm ([42]) reported a significant role of fog precipitation in the water balance of forest stands at elevations above 800 m a.s.l. At a comparable elevation in the White Mountains (New York, USA), Miller et al. ([25]) reported that cloud deposition contributed 32% and 37% of the total atmospheric load of nitrogen and sulphur.

The spatial maps can also be used to indicate locations in the investigated watersheds vulnerable to higher inputs of fog water, which may in turn affect the trees and water quality within the watersheds. Generally, based on the spatial distribution of pH, nitrogen and sulphur, the higher altitudes of catchments are at higher risk, especially in combination with the occurrence of a mature coniferous forest. From the spatial distribution of the vegetation cover, it was possible to estimate the total inputs of fog water, sulphur and nitrogen, as well as water pH into the catchments, providing us with important information on the acidification of water courses and reservoirs in the water supply catchments Josefuv Dul and Souš (Fig. 3, Fig. 4, Fig. 5). Because of the very low buffering capacity of the soil and bedrock in these catchments, these acid atmospheric loads directly affect the quality of surface waters ([15]), and may require investments in water treatment ([18]). Those investments could be reduced, however, using forest management practices that respect conditions for the formation of fog precipitation and its negative role in mountain catchments stressed by acidification.

Uncertainties in estimates of atmospheric deposition

The estimated loads of fog water and acidic substances (sulphate, nitrate, ammonia) to the watersheds investigated here are affected by several uncertainties. Generally, in mountainous terrain the accuracy of measured precipitation amounts decreases with increasing wind velocity ([34]), with significant errors (above 5 %) when wind velocities are above 2 m s^-1. For the rain gauges installed in spruce stands (plots 1 and 2), the effect of wind was assumed to be negligible. Generally, errors in chemical analyses are considered to be on the order of 10%. The month-long storage of rain/fog water in gauges also introduces the risk of error, namely for nitrogen that can be altered by bacterial activity. Nevertheless, Cape et al. ([4]) reported negligible changes of NO₃ contents in longer sampled rainwater in cold mountain environments, finding that significant errors might occur only in high summer months (July, August), but not exceeding 20%.

The efficiency of passive fog collectors is on the order of 10-20% of the actual liquid water content in a cloud; the collection efficiency of a single string depends on the size spectrum of the cloud droplets and wind velocities ([31], [2]). However, our use of passive fog collectors was intended to just indicate the effects of elevation in the upper plain of the Jizera Mountains, assuming comparable wind speeds in the investigated transect (Fig. 1). The collected volumes of fog might also have been affected by the input of surrounding rainfall. However, the passive collectors at the Jizerka transect were protected against inputs of rainfall up to wind speeds of 2 m s^-1. At higher velocities (based on hourly means) and rainfall intensities above 5 mm day^-1, the sampled volumes of fog water were possibly increased by rainfall up to 10%. Considering the collection techniques used, estimates of the canopy structure, and data interpolation, we believe the accuracy of the estimated long-term loads of fog water and acidic substances presented here to be in the range of ± 20%. Further research is needed to evaluate the effects of particular meteorological situations (particularly wind speed and direction) on episodic loads of fog drip. However, this study provides initial data for evaluating the effects of possible scenarios of forestry practices on atmospheric loads to the water supply catchments of Josefuv Dul and Souš.

  Conclusions 

In the Jizera Mountains, the fog water deposition decreases the interception loss in spruce stands (Picea abies) at elevations above 700 m. However, measurable volumes of fog water sampled by passive collectors were detected at elevations above 900 m. Fog drip from the forest canopy was found to increase with elevation and canopy extent. In two mature spruce stands, located at elevations of 745 and 975 m, the observed summer interception losses were 196 mm (31%) and 112 mm (16%), respectively, and were significantly affected by fog drip at the higher elevation. The volumes of fog drip sampled by the passive collectors demonstrated the effects of elevation and seasonal variations. In the winter season (November-April), the fog deposition exceeded the summer values (May-October) by 23-50%.

In the catchments of the drinking water reservoirs Josefuv Dul and Souš, loads of fog water varied from 88 to 106 mm (i.e., from 7% to 8% of gross precipitation). Therefore, for the mean annual precipitation of 1324 mm (867 mm runoff), the additional input of fog to the annual water budget could have increased the mean annual water yield from 10% to 12%. The negative effect of fog precipitation on increasing acid deposition in the investigated watersheds was also demonstrated. The mean annual loads of sulphur and nitrogen from fog drip were 658 kg km^-2 and 216 kg km^-2 (i.e., 55% and 48% of the bulk amounts of sulphur and nitrogen, respectively).

Therefore, in headwater catchments stressed by acidification, the deposition of fog water can enhance the water yield and the recharge of water resources. At the same time, however, it can also affect the quality of surface waters, particularly in environments of a relatively low buffering capacity. Both these effects are related to the elevation and canopy structure, and could therefore be altered by forestry practices modifying the above-ground canopy.

  Acknowledgements 

This research was supported by the Earthwatch Institute (Oxford, UK, Project on Mountain Waters of Bohemia), the Grant Agency of the Czech Republic (GACR 526-09-0567 - CLIMHED), by the Czech Technical University in Prague (SGS 16/140/ OHK1/2T/11), and the Ministry of Education of the Czech Republic (INTER-EXCELLENCE: INTER-COST LTC 17006, 2017).

  References

  Supplementary Material

Tab. S1 - Coefficients a, a₀ (eqn. 2) and Pearson’s correlation coefficients (r) between the deposited fog water and elevation.
Tab. S2 - Coefficients b, b₀ (eqn. 3) and Pearson’s correlation coefficients (r) between the deposited acidic substances and elevation.
Tab. S3 - Regression coefficients k, q and Pearson’s correlation coefficients (r) between pH of the fog water and elevation.